Journal of the ICRU Vol 7 No 2 (2007) Report 78 Oxford University Press
doi:10.1093/jicru/ndm031
9 QUALITY ASSURANCE A comprehensive quality assurance (QA) program for a proton-beam therapy facility consists of procedures that ensure a consistent and safe fulfillment of the dose prescription as well as minimal radiation exposure to the personnel and the public. Practical implementation of a QA program depends on the details of the proton accelerator and the selected beam-delivery technique. In many cases, QA checks will be relevant only to the particular equipment or to the implemented technology. Very often the institution that has acquired a new proton-beam therapy facility plays the dual role of both manufacturer and user, and therefore the proposed checks are a result of research and development, and could be modified with accumulated experience. The QA measurements described below follow the steps of the proton-therapy procedure and focus on dose delivery and treatment planning. Commissioning and validation of proton-beam delivery and treatment-planning systems (TPSs) include machine-specific beam-data acquisition, data entry into the treatment-planning system, validation of the calculations, development of operational procedures and constancy checks, as well as training of all staff concerned with the operation of the system. New types of proton-beam-delivery systems might also require biological assessment as a part of commissioning (Kagawa et al., 2002; Pedroni et al., 1995), whereas preclinical testing of commercially available facilities is usually limited to physics and dosimetry acceptance checks (Kooy, 2002). The data obtained during the commissioning and validation process are used later as benchmarks and thresholds for periodic QA checks. Acceptance, commissioning, as well as periodic engineering and maintenance checks of proton accelerators and multiple treatment room switchyards are not discussed, as the acceptance and quality control of particular proton accelerators and switchyards with vacuum beam lines, focusing, bending, and steering magnets, power supplies, and cooling equipment are usually based on the recommendations of the manufacturer and might not be relevant to another type of machine.
9.1
PROTON-BEAM DELIVERY SYSTEMS
Only a few publications and internal reports are available, which describe commissioning and periodic QA checks of different components of the beamdelivery systems (Chu et al., 1993; JASTRO, 2004; Moyers, 1999; Pedroni et al., 1995; Schreuder, 2002). The nozzle of a proton-beam-delivery system using dual scattering foils consists of a fixed section that contains various components for beam shaping and beam monitoring, and a movable snout that permits positioning of the patient-specific devices, such as aperture and compensator/bolus (see Section 3). The snouts are designed to minimize radiation leakage to the patient, and a part of quality control is focused on the radiationprotection issues. The integrity and quality of alignment of hardware components should be checked through the analysis of depth –dose curves and lateral profiles that are measured in a water phantom. Although measured results are needed to verify performance specifications and provide benchmarks for treatment-planning software (Moyers, 1999), the use of Monte Carlo simulation (Paganetti et al., 2004) can also support dosimetry efforts in helping to define safety tolerances and to design QA procedures. The Monte Carlo simulation of an isocentric-gantry treatment nozzle described by Kooy (2002) and by Paganetti et al. (2004) allows the sensitivity of proton-dose distributions in a water phantom with respect to various beam parameters and geometrical misalignments, to be studied, and, as a result, the tolerance levels for these parameters might be established. Figure 9.1, for example, shows the high sensitivity of the Bragg peak to beam misalignment. Depending upon the amount of material in the beam, the pristine Bragg curve is characterized by either a low-energy tail or possibly by a secondary Bragg peak at a different depth. The upper part of Fig. 9.1 shows the effect on the depth –dose distribution of a beam scraping a frame used to support the first scatterrer, whereas the lower part shows the proton energy distribution at a water phantom surface. On the basis of measurements and results of simulation, it is possible to calculate tolerances for the appropriate
# International Commission on Radiation Units and Measurements 2007
PRESCRIBING, RECORDING, AND REPORTING PROTON-BEAM THERAPY
An important part of quality-control procedures is related to beam monitoring with multi-wire and multi-segment ionization chambers that control deviations in beam position, check the beam size and its uniformity, and control the dose delivered to the patient during the treatment. Usually, ratios of signals between various detectors are used to trigger safety control interlocks and terminate beam delivery. Tolerance values for beam termination should be specified to ensure proper settings of range-shifter devices, selection of scattering-foil positions, and rotational velocity of modulator propellers. The calibration of the primary dose monitor should be based on ionization-chamber measurements in a water phantom, and dose calculations using an established dosimetry code of practice (see Section 4). The monitor calibration is performed on a daily basis at many proton-therapy facilities. For daily checks of scanned-beam systems, an ionization chamber is positioned in the reference volume (often a 10 10 10 cm3 cube) within a water phantom, which is irradiated so as to deliver a dose of 1 Gy to the reference volume (Coray et al., 2002). Figure 9.2 shows an example of the relative doses measured daily in a homogeneous volume. The effect of the energy and the gantry angle on dosimetry should also be checked periodically. The relation between the proton-beam energy and the monitor-chamber response should be frequently checked (Coray et al., 2002). If a rotating gantry is used, multiple irradiations from different directions are often carried out with the patient in the same position. The monitor calibration value should be corrected at each gantry angle if the value varies as a function of gantry angle. The stability of the monitors with gantry angle and reproducibility of the beam from
Figure 9.1. An example of the effect of beam misalignment on the pristine Bragg peak and on the proton spectrum (Paganetti et al., 2004; reproduced with permission).
operational parameters for beam delivery and action levels that should be used in periodic checks. Characterization and periodic stability checks of the mechanical and radiation isocenter for proton gantries are important in order to facilitate better patient alignment from all beam directions. A procedure to determine the shape and size of the proton gantry isocenter to within 0.2 mm is given by Moyers and Lesyna (2004). Periodic QA checks for wobbling and scanning delivery techniques are similar to those for the passive beam scattering technique. However, because these delivery systems are dynamic and accurate delivery of the planned dose depends on the accurate deposition of individually weighted pencil beams, additional QA methods are required for scanned-beam systems. Measuring procedures for the determination of the shape, position, and direction of individual pencil beams must be developed, together with methods for assessing the homogeneity of the dose and shape of complex fields, which can be delivered with such systems (Chu et al., 1993; JASTRO, 2004; Pedroni et al., 2005).
Figure 9.2. Dose measured daily in the center of a reference volume in a scanning beam (n is the number of data points) (Coray et al., 2002; reproduced with permission).
136
QUALITY ASSURANCE
the accelerator should be verified as a part of periodic QA checks (JASTRO, 2004; Moyers, 1999).
for QA of radiotherapy TPSs. This report and the recently published report from the IAEA, TRS 430 (IAEA, 2004), deal with an image-based threedimensional TPS and can be generally applied to TPSs used in planning radiotherapy with protons. However, emphasis should be given to specific issues not covered by TG 53 (Fraass et al., 1998) or TRS 430 (IAEA, 2004). The TPSs used at proton centers with a passive scattering beam-delivery system are mostly based on scatter-convolution algorithms (Moyers, 1999). This method employs as input data the measured depth –dose tables, off-axis profiles, and output factors for each energy and modulator combination. During commissioning, the dosimetric characteristics of the proton beams, such as depth doses, off-axis profiles, fieldsize factors, modulation factors for different field sizes, penumbra sizes, and beam ranges, should be measured and recorded for validation of the TPS and also stored in a database to be referenced in future periodic QA checks (JASTRO, 2004; Moyers, 1999). Again, for scanning systems, the beam data-acquisition procedure is somewhat different, and requires the measurement of depth –dose curves and lateral profiles of pencil beams, requiring small-field dosimetry equipment with a good spatial resolution (Pedroni et al., 2005). TG 53 (Fraass et al., 1998) and TRS 430 (IAEA, 2004) do not cover the calibration of computed tomography (CT) images and conversion of CT Hounsfeld units to proton stopping powers. The uncertainty in the range of protons due to inaccurate calibration of CT images and beam hardening artifacts is estimated by Schaffner and Pedroni (1998) to be 1.1 percent (1 SD) of the total range in soft tissue and 1.8 percent (1 SD) in cortical bone for the CT scanner that they used. Given the importance of the accuracy of the CT Hounsfield values, it is strongly recommended that regular checks of the consistency of the CT Hounsfield values be performed. A proton-beam TPS usually has automated features to design aperture shapes from target-tissue outlines and to design compensator/boluses that provide distal-edge target coverage. Boluses that control penetration of the beam in non-uniform phantoms relative to the shape of the distal side of a specified target-tissue region are designed using preset margins. User-specified amounts of bolus expansion to account for lateral proton scatter and patient-position uncertainty are employed in bolus design (see Section 6). Tests using actual tissue samples to simulate typical treatments are performed as a final test to verify the design of a bolus intended to compensate for
9.2 PATIENT POSITIONING AND IMMOBILIZATION Accurate positioning of the patient in the proton beam requires location of the patient on the support system and precise operation and correct interaction of two systems: one that performs and controls the movements required to place the patient in the prescribed position, and another that verifies the position of the patient relative to the beam axis (see Section 7). The geometric/positioning measurements are performed to quantify the accuracy of the patient-positioning devices and the patient alignment with respect to the nozzle and gantry. The periodic checks of coincidence of the proton-beam isocenter and the patient setup-laser positions can be performed using a phosphor imaging plate (IP) (Terunuma et al., 2003). When radiation is applied to an IP, an image of the radiation field is temporarily stored in the IP. The IP is then exposed to the patient position laser. As a result, the radiation field produces a ‘positive’ image while the patient setup laser creates a ‘negative’ image of its position. The advantages of this method are direct measurements in a short time, with high resolution. The periodic QA checks of stereophotogrammetric positioning systems (Jones et al., 1995) and digital x-ray imaging systems deal with the calibration of charge-coupled device (CCD) cameras and verification of the alignment of the axial x-ray imaging system (Schreuder, 2002; 2004). If automatic positioning based on comparison of digitally reconstructed radiographs (DRR) and x-ray data is employed, a check of the accuracy of the positioning algorithm should be done (JASTRO, 2004; Lesyna, 2004). Patient positioners can also include highprecision robotic systems (de Kock, 2002; Schreuder, 2004) that are capable, in combination with gantry angulations, of locating the treatment isocenter at any point within the patient so as to direct the beam from any angle through that point. The accuracy of robotic patient positioning depends mainly on the capability of the robot control and safety software rather than on the mechanical accuracy of the couch, and therefore periodic quality control involves a substantial amount of software and safety checks (de Kock, 2004). 9.3
TREATMENT-PLANNING SYSTEMS
The report of Fraass et al. (1998) contains recommendations from AAPM Task Group 53 (TG 53) 137
PRESCRIBING, RECORDING, AND REPORTING PROTON-BEAM THERAPY
Figure 9.3. Verification of patient treatment in a scanning beam. The calculated orthogonal dose profiles (solid curves) and the measured data points (crosses) are shown (Coray et al., 2002; reproduced with permission).
the tissue irregularities (Miller et al., 1999; Schaffner and Pedroni, 1998). Patient-specific periodic QA checks for a beamdelivery system using passive-scattering techniques include portal calibrations and verification of beam-shaping devices (apertures and compensators/boluses) (Heese et al., 2002; iTL, 2001; JASTRO, 2004; Kacperek, 2003; Moyers, 1999). The TPS calculates the dose per monitor unit (MU) at the reference point in the patient but at present the dose per MU is individually verified in a phantom for each patient treatment portal at many facilities (iTL, 2001; JASTRO, 2004; Moyers, 1999). Precision in manufacturing the bolus directly affects the distal part of the depth – dose distribution, whereas the errors in the patient aperture impinge on the lateral-dose distribution. Although milling machines provide high manufacturing precision, the manufactured beam-shaping devices for each patient should be checked against TPS data files by comparing aperture shapes and bolus thicknesses at pre-selected points. Improvement in quality control of dose delivery can be achieved through the use of proton radiography that provides verification in the relative range of protons in vivo in the patient (Schneider and Pedroni, 1995). The use of PET monitoring to check proton range, dose localization, and stability of the treatment during different fractions has been investigated (Enghardt et al., 1999; 2004; Hishikawa et al., 2002; Parodi et al., 2002; 2007a; 2007b; 2007c). Real-time PET imaging could be considered a potentially useful tool for QA in proton therapy (see Section 7.4.2). The TPS described by Lomax et al. (2004) calculates the dose distribution for a spot scanning-beam technique from a superposition of individual pencil beams, taking into account the density information from the corresponding CT slices. An empirical model of the pencil beam is used that takes into account the attenuation of the primary protons, effects of multiple Coulomb scattering, and losses
due to nuclear interactions. The model parameterization data are stored in lookup tables, and the therapy planning can predict absolute doses. There are no patient-specific devices; therefore the individual information on each patient treatment is in the control-point sequence (Chu et al., 1993; Lomax et al., 2004) that is used to calculate dose distributions in a water phantom. Patient-specific QA checks include manual calculations of the MU, range checks and, most importantly, independent dose calculations based on the control file itself, which is directly compared with that calculated by the TPS. In addition, as a weekly QA check, a field is quasi-randomly selected and the dose distribution checked in a water phantom with an array of ionization chambers. The thickness of the water column can be set to the required depth of measurement. The water phantom is irradiated, using the same control-point sequence as for the patient treatment. The measured dose profiles should be then compared with the dose distribution recalculated by the TPS using a homogeneous medium instead of the patient CT data. Figure 9.3 shows two orthogonal dose profiles taken during a routine patient verification (Coray et al., 2002). The solid line represents the calculated profile; the crosses show the measured doses. As a quality check, the routine dosimetry with ionization chambers should agree with the expected dose from the treatment plan within the user-established tolerances. Verifications of dose delivery for a dynamic treatment technique using a fluorescent screen and CCD camera (Boon et al., 2000) have shown good sensitivity with .1 mm spatial resolution that allowed the detection of deviations of a few percent from the calculated dose distribution. If respiratory-synchronized irradiation is employed, CT images used for treatment planning must also be taken in respiratory-gating mode. The same specifications for irradiation gating, synchronizing timing and thresholds, should be applied for acquisition of treatment-planning CT images. As 138
QUALITY ASSURANCE Table 9.1. Quality-assurance procedures for passive beam-delivery systems. Daily checks † † † †
Aperture alignment; room lasers, interlocks; communication; patient-positioning system Depth–dose and lateral profiles (range, entrance dose, uniformity of range modulation and Bragg-peak width, flatness, symmetry) Dose monitor calibration, check of MU value under standard condition Individual patient treatment calibration and range checks
Weekly checks † † † †
Patient-positioning and imaging systems Beam-line apparatus Respiratory-gating equipment Dose delivered to randomly selected patients (comparison of planned dose distributions to those measured in a water phantom)
Annual or scheduled inspection checks † x-ray patient positioning and alignment systems † CT Hounsfield number calibration † Comprehensive tests of therapy equipment W
monitor chambers, timers, beam-delivery termination and control interlocks, stray radiation exposure to patients, gantry isocenter, depth– dose and lateral profiles, baseline data for daily QA checks
Table 9.2. Additional quality-assurance procedures for scanning beam-delivery systems. Daily checks † † † †
Dose rate and monitor ratios for the pencil beam Performance of the beam-position monitors Depth–dose curve of a pencil beam in a water phantom Calibration of the primary dose monitor
Weekly checks † Qualitative three-dimensional check of the outline and range of the dose distribution for one patient’s irradiation field in a water phantom Half-yearly checks † Calibration of the primary dose monitor and the phase space of the beam tunes Annual or scheduled inspection checks † Check of the beam characteristics W calibration of the whole dosimetry system, performance of the scanning system in terms of dose linearity and dose-rate dependence
an example, the following issues should be investigated for respiratory-synchronized irradiation to serve as benchmarks for periodic QA checks (JASTRO, 2004):
9.4
EXAMPLES OF PERIODIC CHECKS
Each proton facility will have different QA requirements, and the items listed in Tables 9.1 and 9.2 are guidelines and suggestions and do not describe all tests that must be made at any particular facility. In Table 9.1, the procedures for passive scattering beam-delivery systems (iTL, 2001; JASTRO, 2004; Kacperek, 2003; Moyers, 1999; Schreuder, 2002) are listed, and in Table 9.2 additional procedures for spot-scanning beam-delivery systems (Chu et al., 1993; Coray et al., 2002) are given. The procedures for scanned beams are given mostly in terms of dosimetric issues, as the check of dose delivery is the most important task.
(1) differences between observed respiration signal and actual organ movement; (2) phase uncertainty at CT scanning for treatment planning to make reference images; (3) setting of the threshold level of the extent of the movement; (4) movement of organ during allowed period for irradiation.
139
doi:10.1093/jicru/ndm031
9 QUALITY ASSURANCE A comprehensive quality assurance (QA) program for a proton-beam therapy facility consists of procedures that ensure a consistent and safe fulfillment of the dose prescription as well as minimal radiation exposure to the personnel and the public. Practical implementation of a QA program depends on the details of the proton accelerator and the selected beam-delivery technique. In many cases, QA checks will be relevant only to the particular equipment or to the implemented technology. Very often the institution that has acquired a new proton-beam therapy facility plays the dual role of both manufacturer and user, and therefore the proposed checks are a result of research and development, and could be modified with accumulated experience. The QA measurements described below follow the steps of the proton-therapy procedure and focus on dose delivery and treatment planning. Commissioning and validation of proton-beam delivery and treatment-planning systems (TPSs) include machine-specific beam-data acquisition, data entry into the treatment-planning system, validation of the calculations, development of operational procedures and constancy checks, as well as training of all staff concerned with the operation of the system. New types of proton-beam-delivery systems might also require biological assessment as a part of commissioning (Kagawa et al., 2002; Pedroni et al., 1995), whereas preclinical testing of commercially available facilities is usually limited to physics and dosimetry acceptance checks (Kooy, 2002). The data obtained during the commissioning and validation process are used later as benchmarks and thresholds for periodic QA checks. Acceptance, commissioning, as well as periodic engineering and maintenance checks of proton accelerators and multiple treatment room switchyards are not discussed, as the acceptance and quality control of particular proton accelerators and switchyards with vacuum beam lines, focusing, bending, and steering magnets, power supplies, and cooling equipment are usually based on the recommendations of the manufacturer and might not be relevant to another type of machine.
9.1
PROTON-BEAM DELIVERY SYSTEMS
Only a few publications and internal reports are available, which describe commissioning and periodic QA checks of different components of the beamdelivery systems (Chu et al., 1993; JASTRO, 2004; Moyers, 1999; Pedroni et al., 1995; Schreuder, 2002). The nozzle of a proton-beam-delivery system using dual scattering foils consists of a fixed section that contains various components for beam shaping and beam monitoring, and a movable snout that permits positioning of the patient-specific devices, such as aperture and compensator/bolus (see Section 3). The snouts are designed to minimize radiation leakage to the patient, and a part of quality control is focused on the radiationprotection issues. The integrity and quality of alignment of hardware components should be checked through the analysis of depth –dose curves and lateral profiles that are measured in a water phantom. Although measured results are needed to verify performance specifications and provide benchmarks for treatment-planning software (Moyers, 1999), the use of Monte Carlo simulation (Paganetti et al., 2004) can also support dosimetry efforts in helping to define safety tolerances and to design QA procedures. The Monte Carlo simulation of an isocentric-gantry treatment nozzle described by Kooy (2002) and by Paganetti et al. (2004) allows the sensitivity of proton-dose distributions in a water phantom with respect to various beam parameters and geometrical misalignments, to be studied, and, as a result, the tolerance levels for these parameters might be established. Figure 9.1, for example, shows the high sensitivity of the Bragg peak to beam misalignment. Depending upon the amount of material in the beam, the pristine Bragg curve is characterized by either a low-energy tail or possibly by a secondary Bragg peak at a different depth. The upper part of Fig. 9.1 shows the effect on the depth –dose distribution of a beam scraping a frame used to support the first scatterrer, whereas the lower part shows the proton energy distribution at a water phantom surface. On the basis of measurements and results of simulation, it is possible to calculate tolerances for the appropriate
# International Commission on Radiation Units and Measurements 2007
PRESCRIBING, RECORDING, AND REPORTING PROTON-BEAM THERAPY
An important part of quality-control procedures is related to beam monitoring with multi-wire and multi-segment ionization chambers that control deviations in beam position, check the beam size and its uniformity, and control the dose delivered to the patient during the treatment. Usually, ratios of signals between various detectors are used to trigger safety control interlocks and terminate beam delivery. Tolerance values for beam termination should be specified to ensure proper settings of range-shifter devices, selection of scattering-foil positions, and rotational velocity of modulator propellers. The calibration of the primary dose monitor should be based on ionization-chamber measurements in a water phantom, and dose calculations using an established dosimetry code of practice (see Section 4). The monitor calibration is performed on a daily basis at many proton-therapy facilities. For daily checks of scanned-beam systems, an ionization chamber is positioned in the reference volume (often a 10 10 10 cm3 cube) within a water phantom, which is irradiated so as to deliver a dose of 1 Gy to the reference volume (Coray et al., 2002). Figure 9.2 shows an example of the relative doses measured daily in a homogeneous volume. The effect of the energy and the gantry angle on dosimetry should also be checked periodically. The relation between the proton-beam energy and the monitor-chamber response should be frequently checked (Coray et al., 2002). If a rotating gantry is used, multiple irradiations from different directions are often carried out with the patient in the same position. The monitor calibration value should be corrected at each gantry angle if the value varies as a function of gantry angle. The stability of the monitors with gantry angle and reproducibility of the beam from
Figure 9.1. An example of the effect of beam misalignment on the pristine Bragg peak and on the proton spectrum (Paganetti et al., 2004; reproduced with permission).
operational parameters for beam delivery and action levels that should be used in periodic checks. Characterization and periodic stability checks of the mechanical and radiation isocenter for proton gantries are important in order to facilitate better patient alignment from all beam directions. A procedure to determine the shape and size of the proton gantry isocenter to within 0.2 mm is given by Moyers and Lesyna (2004). Periodic QA checks for wobbling and scanning delivery techniques are similar to those for the passive beam scattering technique. However, because these delivery systems are dynamic and accurate delivery of the planned dose depends on the accurate deposition of individually weighted pencil beams, additional QA methods are required for scanned-beam systems. Measuring procedures for the determination of the shape, position, and direction of individual pencil beams must be developed, together with methods for assessing the homogeneity of the dose and shape of complex fields, which can be delivered with such systems (Chu et al., 1993; JASTRO, 2004; Pedroni et al., 2005).
Figure 9.2. Dose measured daily in the center of a reference volume in a scanning beam (n is the number of data points) (Coray et al., 2002; reproduced with permission).
136
QUALITY ASSURANCE
the accelerator should be verified as a part of periodic QA checks (JASTRO, 2004; Moyers, 1999).
for QA of radiotherapy TPSs. This report and the recently published report from the IAEA, TRS 430 (IAEA, 2004), deal with an image-based threedimensional TPS and can be generally applied to TPSs used in planning radiotherapy with protons. However, emphasis should be given to specific issues not covered by TG 53 (Fraass et al., 1998) or TRS 430 (IAEA, 2004). The TPSs used at proton centers with a passive scattering beam-delivery system are mostly based on scatter-convolution algorithms (Moyers, 1999). This method employs as input data the measured depth –dose tables, off-axis profiles, and output factors for each energy and modulator combination. During commissioning, the dosimetric characteristics of the proton beams, such as depth doses, off-axis profiles, fieldsize factors, modulation factors for different field sizes, penumbra sizes, and beam ranges, should be measured and recorded for validation of the TPS and also stored in a database to be referenced in future periodic QA checks (JASTRO, 2004; Moyers, 1999). Again, for scanning systems, the beam data-acquisition procedure is somewhat different, and requires the measurement of depth –dose curves and lateral profiles of pencil beams, requiring small-field dosimetry equipment with a good spatial resolution (Pedroni et al., 2005). TG 53 (Fraass et al., 1998) and TRS 430 (IAEA, 2004) do not cover the calibration of computed tomography (CT) images and conversion of CT Hounsfeld units to proton stopping powers. The uncertainty in the range of protons due to inaccurate calibration of CT images and beam hardening artifacts is estimated by Schaffner and Pedroni (1998) to be 1.1 percent (1 SD) of the total range in soft tissue and 1.8 percent (1 SD) in cortical bone for the CT scanner that they used. Given the importance of the accuracy of the CT Hounsfield values, it is strongly recommended that regular checks of the consistency of the CT Hounsfield values be performed. A proton-beam TPS usually has automated features to design aperture shapes from target-tissue outlines and to design compensator/boluses that provide distal-edge target coverage. Boluses that control penetration of the beam in non-uniform phantoms relative to the shape of the distal side of a specified target-tissue region are designed using preset margins. User-specified amounts of bolus expansion to account for lateral proton scatter and patient-position uncertainty are employed in bolus design (see Section 6). Tests using actual tissue samples to simulate typical treatments are performed as a final test to verify the design of a bolus intended to compensate for
9.2 PATIENT POSITIONING AND IMMOBILIZATION Accurate positioning of the patient in the proton beam requires location of the patient on the support system and precise operation and correct interaction of two systems: one that performs and controls the movements required to place the patient in the prescribed position, and another that verifies the position of the patient relative to the beam axis (see Section 7). The geometric/positioning measurements are performed to quantify the accuracy of the patient-positioning devices and the patient alignment with respect to the nozzle and gantry. The periodic checks of coincidence of the proton-beam isocenter and the patient setup-laser positions can be performed using a phosphor imaging plate (IP) (Terunuma et al., 2003). When radiation is applied to an IP, an image of the radiation field is temporarily stored in the IP. The IP is then exposed to the patient position laser. As a result, the radiation field produces a ‘positive’ image while the patient setup laser creates a ‘negative’ image of its position. The advantages of this method are direct measurements in a short time, with high resolution. The periodic QA checks of stereophotogrammetric positioning systems (Jones et al., 1995) and digital x-ray imaging systems deal with the calibration of charge-coupled device (CCD) cameras and verification of the alignment of the axial x-ray imaging system (Schreuder, 2002; 2004). If automatic positioning based on comparison of digitally reconstructed radiographs (DRR) and x-ray data is employed, a check of the accuracy of the positioning algorithm should be done (JASTRO, 2004; Lesyna, 2004). Patient positioners can also include highprecision robotic systems (de Kock, 2002; Schreuder, 2004) that are capable, in combination with gantry angulations, of locating the treatment isocenter at any point within the patient so as to direct the beam from any angle through that point. The accuracy of robotic patient positioning depends mainly on the capability of the robot control and safety software rather than on the mechanical accuracy of the couch, and therefore periodic quality control involves a substantial amount of software and safety checks (de Kock, 2004). 9.3
TREATMENT-PLANNING SYSTEMS
The report of Fraass et al. (1998) contains recommendations from AAPM Task Group 53 (TG 53) 137
PRESCRIBING, RECORDING, AND REPORTING PROTON-BEAM THERAPY
Figure 9.3. Verification of patient treatment in a scanning beam. The calculated orthogonal dose profiles (solid curves) and the measured data points (crosses) are shown (Coray et al., 2002; reproduced with permission).
the tissue irregularities (Miller et al., 1999; Schaffner and Pedroni, 1998). Patient-specific periodic QA checks for a beamdelivery system using passive-scattering techniques include portal calibrations and verification of beam-shaping devices (apertures and compensators/boluses) (Heese et al., 2002; iTL, 2001; JASTRO, 2004; Kacperek, 2003; Moyers, 1999). The TPS calculates the dose per monitor unit (MU) at the reference point in the patient but at present the dose per MU is individually verified in a phantom for each patient treatment portal at many facilities (iTL, 2001; JASTRO, 2004; Moyers, 1999). Precision in manufacturing the bolus directly affects the distal part of the depth – dose distribution, whereas the errors in the patient aperture impinge on the lateral-dose distribution. Although milling machines provide high manufacturing precision, the manufactured beam-shaping devices for each patient should be checked against TPS data files by comparing aperture shapes and bolus thicknesses at pre-selected points. Improvement in quality control of dose delivery can be achieved through the use of proton radiography that provides verification in the relative range of protons in vivo in the patient (Schneider and Pedroni, 1995). The use of PET monitoring to check proton range, dose localization, and stability of the treatment during different fractions has been investigated (Enghardt et al., 1999; 2004; Hishikawa et al., 2002; Parodi et al., 2002; 2007a; 2007b; 2007c). Real-time PET imaging could be considered a potentially useful tool for QA in proton therapy (see Section 7.4.2). The TPS described by Lomax et al. (2004) calculates the dose distribution for a spot scanning-beam technique from a superposition of individual pencil beams, taking into account the density information from the corresponding CT slices. An empirical model of the pencil beam is used that takes into account the attenuation of the primary protons, effects of multiple Coulomb scattering, and losses
due to nuclear interactions. The model parameterization data are stored in lookup tables, and the therapy planning can predict absolute doses. There are no patient-specific devices; therefore the individual information on each patient treatment is in the control-point sequence (Chu et al., 1993; Lomax et al., 2004) that is used to calculate dose distributions in a water phantom. Patient-specific QA checks include manual calculations of the MU, range checks and, most importantly, independent dose calculations based on the control file itself, which is directly compared with that calculated by the TPS. In addition, as a weekly QA check, a field is quasi-randomly selected and the dose distribution checked in a water phantom with an array of ionization chambers. The thickness of the water column can be set to the required depth of measurement. The water phantom is irradiated, using the same control-point sequence as for the patient treatment. The measured dose profiles should be then compared with the dose distribution recalculated by the TPS using a homogeneous medium instead of the patient CT data. Figure 9.3 shows two orthogonal dose profiles taken during a routine patient verification (Coray et al., 2002). The solid line represents the calculated profile; the crosses show the measured doses. As a quality check, the routine dosimetry with ionization chambers should agree with the expected dose from the treatment plan within the user-established tolerances. Verifications of dose delivery for a dynamic treatment technique using a fluorescent screen and CCD camera (Boon et al., 2000) have shown good sensitivity with .1 mm spatial resolution that allowed the detection of deviations of a few percent from the calculated dose distribution. If respiratory-synchronized irradiation is employed, CT images used for treatment planning must also be taken in respiratory-gating mode. The same specifications for irradiation gating, synchronizing timing and thresholds, should be applied for acquisition of treatment-planning CT images. As 138
QUALITY ASSURANCE Table 9.1. Quality-assurance procedures for passive beam-delivery systems. Daily checks † † † †
Aperture alignment; room lasers, interlocks; communication; patient-positioning system Depth–dose and lateral profiles (range, entrance dose, uniformity of range modulation and Bragg-peak width, flatness, symmetry) Dose monitor calibration, check of MU value under standard condition Individual patient treatment calibration and range checks
Weekly checks † † † †
Patient-positioning and imaging systems Beam-line apparatus Respiratory-gating equipment Dose delivered to randomly selected patients (comparison of planned dose distributions to those measured in a water phantom)
Annual or scheduled inspection checks † x-ray patient positioning and alignment systems † CT Hounsfield number calibration † Comprehensive tests of therapy equipment W
monitor chambers, timers, beam-delivery termination and control interlocks, stray radiation exposure to patients, gantry isocenter, depth– dose and lateral profiles, baseline data for daily QA checks
Table 9.2. Additional quality-assurance procedures for scanning beam-delivery systems. Daily checks † † † †
Dose rate and monitor ratios for the pencil beam Performance of the beam-position monitors Depth–dose curve of a pencil beam in a water phantom Calibration of the primary dose monitor
Weekly checks † Qualitative three-dimensional check of the outline and range of the dose distribution for one patient’s irradiation field in a water phantom Half-yearly checks † Calibration of the primary dose monitor and the phase space of the beam tunes Annual or scheduled inspection checks † Check of the beam characteristics W calibration of the whole dosimetry system, performance of the scanning system in terms of dose linearity and dose-rate dependence
an example, the following issues should be investigated for respiratory-synchronized irradiation to serve as benchmarks for periodic QA checks (JASTRO, 2004):
9.4
EXAMPLES OF PERIODIC CHECKS
Each proton facility will have different QA requirements, and the items listed in Tables 9.1 and 9.2 are guidelines and suggestions and do not describe all tests that must be made at any particular facility. In Table 9.1, the procedures for passive scattering beam-delivery systems (iTL, 2001; JASTRO, 2004; Kacperek, 2003; Moyers, 1999; Schreuder, 2002) are listed, and in Table 9.2 additional procedures for spot-scanning beam-delivery systems (Chu et al., 1993; Coray et al., 2002) are given. The procedures for scanned beams are given mostly in terms of dosimetric issues, as the check of dose delivery is the most important task.
(1) differences between observed respiration signal and actual organ movement; (2) phase uncertainty at CT scanning for treatment planning to make reference images; (3) setting of the threshold level of the extent of the movement; (4) movement of organ during allowed period for irradiation.
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