In! J. Radiation Oncology Bml. Phw Vol. Prmted I” the U.S.A. All rights reserved.

22, pp

175-180 Copyright

??Technical Innovations

COMPUTER

and Notes

CONTROLLED

PAVEL V. HOUDEK, PH.D. MARKOE,

STEREOTAXIC

,* JAMES G.

HOWARD J. LANDY, M.D. ARNOLD M.

0360.3016/92 $5.00 + 0, 0 1991 Pergamon Press plc

M.D.

SCHWADE, M.D.

RADIOTHERAPY

,* CHRISTOPHER F. SERAGO,

,t VINCENT PISCIOTTA, M.S. ,* ALAN A.

JOANNE L. BUJNOSKI, D.O.

LEWIN, M.D.

SYSTEM

,* XIAODONG Wu,

M.S.

PH.D. ,*

,* ANDRE A. ABITBOL, M.D.

,* EVELYN S. MARIENBERG,

M.D.

,*

,*

,*

JEFFREY A. FIEDLER, M. S . * AND MURRAY S. GINSBERG* University of Miami School of Medicine, Miami, FL, U.S.A. A computer-controlled stereotaxic radiotherapy system based on a low-frequency magnetic field technology integrated with a single fixation point stereotaxic guide has been designed and instituted. The magnetic field, generated in space by a special field source located in the accelerator gantry, is digitized in real time by a field sensor that is a six degree-of-freedom measurement device. As this sensor is an integral part of the patient stereotaxic halo, the patient position (x, y, z) and orientation (azimuth, elevation, roll) within the accelerator frame of reference are always known. Six parameters - three coordinates and three Euler space angles - are continuously transmitted to a computer where they are analyzed and compared with the stereotaxic parameters of the target point. Hence, the system facilitates rapid and accurate patient set-up for stereotaxic treatment as well as monitoring of patient during the subsequent irradiation session. The stereotaxic system has been developed to promote the integration of diagnostic and therapeutic procedures, with the specific aim of integrating CT and/or MR aided tumor localization and long term (4- to 7-week) fractionated radiotherapy of small intracranial and ocular lesions. Stereotaxic radiotherapy, Computer-controlled procedures, CT simulator.

stereotaxic

guide,

Integration

of diagnostic

and therapeutic

of patients with small volume brain lesions would not benefit from the capabilities of new diagnostic technologies unless more precise systems for delivery of radiotherapy were devised and made available.

INTRODUCTION

Although precision stereotaxic radiotherapy has been practiced since the early 1950s (9), it has always been a very special treatment technique; only uniquely designed equipment, such as @%Zo“gamma-knife” irradiators (7, 10) or research particle accelerators (6), have been used, and only specific brain lesions, such as arteriovenous malformations, have been treated. During the last decade, however, stereotaxic radiotherapy has evolved from an exotic to a more common treatment modality because the proliferation of highly accurate CT diagnostic methods into medical practices stimulated the need for more precise therapeutic techniques. In particular, it was recognized that while brain lesions as small as 2-3 mm could be localized in nearly every medical imaging department, they could not be optimally treated with correspondingly small radiation fields because, in the majority of radiotherapy departments, the errors associated with data transfer and patient setup were commonly greater than the lesion itself. It became apparent that the majority

In the early 198Os, several linac-based stereotaxic systems designed to facilitate the coordination of diagnostic and therapeutic procedures were developed and implemented (2, 5, ll), and basic dosimetry of small radiation fields addressed (4). The superior versatility of iso-centritally mounted linacs contributed to the refinement of stereotaxic treatment techniques and promoted a departure from static-port irradiation, commonly used with “gamma knives” and research accelerators, to versatile moving-

Presented in part at the 32nd Annual Meeting of the American Society for Therapeutic Radiology and Oncology, Miami, FL, U.S.A., 15-20 October, 1990. *Dept. of Radiation Oncology. TDept. of Neurosurgery.

*Dept. of Medical Instrumentation. Reprints requests to: Pave1 V. Houdek, Ph.D., Department of Radiation Oncology (D-31), University of Miami School of Medicine, P.O. Box 016960, Miami, FL 33101, U.S.A. Accepted for publication 14 April 199 1.

Consequently, to provide patients with optimal treatment, the development of radiotherapy stereotaxic systems became essential, specifically systems that would use generally available medical linacs and permit accurate integration of diagnostic and therapeutic procedures, including target localization, treatment planning, patient set-up and initial irradiation, and repetitive treatment (5).

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beam techniques that included single-arc (5), multi-arc (1, 2), and dynamic treatments (11). No advances, however, have yet been made in the automation of patient set-up techniques. The alignment of the patient target point, usually defined as the center of the lesion, with the accelerator coordinate system is still carried out manually using a patient-phantom assembly initially developed for neurosurgical procedures (8) and later modified for use in radiation therapy (2, 5). Hence, the setup procedure remains the most time-consuming and difficult part of stereotaxic treatment and thus the tendency to minimize the total number of patient setups, and so the number of treatment sessions, prevails. Consequently, the vast majority of patients currently undergoing stereotaxic radiotherapy are treated in a single treatment session. Although the wealth of data suggests that a single fraction regimen is adequate for treatment of AV malformations, conventional cancer radiotherapy experience has firmly established that multifraction time-dose regimens are biologically superior to single fraction highdose treatments. Therefore, we have concentrated our efforts on the development of a linac-based stereotaxic radiotherapy system that would integrate diagnostic and therapeutic procedures and facilitate rapid and accurate patient setup, and consequently the delivery of long term fractionated radiotherapy for small intracranial and ocular lesions.

Fig. 1. Stereotaxic guide; top: assembled - note the field sensor phantom attached to the halo ring; bottom: clockwise from top base assembly, halo ring, and gyro mechanism.

MATERIALS The computer-controlled stereotaxic radiotherapy system described here incorporates five major technologies: stereotaxic guide, computer controlled low-frequency magnetic field measuring tool (space digitizer), treatment planning computer, CT simulator, and medical linear accelerator. The first two are those most relevant to this paper.

relatively high transmission of diagnostic and therapeutic beams. Halo screws and inserts are made of titanium, which was found to produce less streaking on CT images than stainless steel. It is also a magnetically inert metal and hence MR images of patients with an affixed halo can be safely obtained.

The stereotaxic guide The stereotaxic guide is of a modular design and consists of base assembly, gyro mechanism, and a halo ring, as shown in Figure 1. The halo provides a bone fixation link between the patient and the guide. Through the gyro mechanism, it is coupled with the base assembly, which is pegged into the CT simulator or accelerator couch. This gyro-base link is designed as a single point of fixation connection with a quick-release lock that facilitates fast patient disengagement from the base assembly/couch, essential in case of a medical emergency. The guide is fabricated from Delrin AF* plastic produced from oriented teflon fibers and acetal resin. Delrin was judged superior to other CT and MRI compatible materials, including nylon and carbon-fiber based materials, because of its dimensional rigidity, abrasion resistance, and

The space digitizer The space digitizer? is a low-frequency magnetic field measuring device consisting of field sensor, field source, computer interface, and controlling computer. The sensor is a six degree-of-freedom measuring device that digitizes the field produced by the source in real time. Consequently, information concerning sensor position (x, y, and z coordinates) and orientation (azimuth a, elevation e, and roll r), defined in Figure 2, is instantly available. By rigidly attaching the sensor to the patient halo and making the source an integral part of the accelerator, as shown in Figure 3, patient head position and orientation within the accelerator frame of reference are always known. The six parameters are transmitted six times per second, via the interface, to the controlling computer for analysis and archiving. At the time of patient setup, the data are

*Commercial 19020.

Plastic & Supply Corp., Cornwells

Heights,

PA

tPolhemus,

Inc., Colchester,

VT 05446.

Computer controlled stereotaxic Rx system 0 P. V.

ELEVATION Y

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HOUDEK

et al.

Fig. 4. Patient undergoing CT aided target localization: field sensor phantom affixed to the halo ring.

ROLL Y

Note the

. METHODS

Fig. 2. Definition of the accelerator coordinate system; positional coordinates (x,y,z) and Euler orientation angles (azimuth, elevation, roll).

compared with the stereotaxic parameters of the target point. During treatment, they are correlated with preset patient motion limits. For diagnostic procedures, that is, during CT I MR scanning, the field sensor phantom, Figure 4, is used. This phantom is also fabricated from Delrin AF and has a titanium pin embedded in the center of its tip, the geometry of which corresponds to that of the real sensor used in the accelerator room during patient set up.

Fig. 3. Patient in treatment set-up: Note the field source located

in the accessory mount and the field sensor attached to the halo ring.

AND RESULTS

Although the methodology described herein refers to specific equipment used in our institution, the system can be employed with other commercially available accelerators, CT scanners, or treatment planning computers. Target localization The halo ring is affixed onto the patient’s cranium under local anesthesia by using titanium screws and a fastening tool with a preset maximum limit of 6 inch lbs torque. If possible, the halo is placed parallel to the base of skull and not coinciding with any axial target planes because this simplifies patient set-up. The sensor phantom is rigidly attached to the halo and the patient is locked into the stereotaxic guide on the CT simulator couch, as demonstrated in Figure 4. Adjustments to x, y guide position and orientation settings for azimuth and elevation are used to make the patient as comfortable as possible. Readings on the associated scales of the guide are noted. The CT simulator gantry laser is preset to the upright position and the titanium tip of the sensor phantom, which is used as a reference point, is set to coincide with this laser light. The laser is then moved to the 90” and 270” positions, where its projections onto the halo are marked. A series of images, with and without contrast, is obtained using a 1 x 1 X 2 mm voxel size and a 256 X 256 data acquisition matrix. The data are stored on hard disk, which is configured to function as a file server for both CT simulator and treatment planning computer. Treatment planning The coordinate system used for treatment planning purposes is identical to that of the CT simulator and the accelerator, with the origin always being either at the isocenter of the gantry (accelerator) or the scanning aperture (CT simulator), as illustrated in Figure 2. CT images are retrieved from the file server, a 3-D treatment plan is

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Patient

Fig. 5. Rectilinear transformation of the patient stereotaxic point T to the accelerator isocenter I.

target

computed, and the stereotaxic coordinates of target point T(x,, yr, zr) and reference point P(x,, yp, zp) are obtained (Fig. 5). Subsequently, the data transfer from the file server to the CT simulator computer is performed. Then the patient is locked into the stereotaxic guide on the CT simulator couch and setup as already described using the reference point (sensor phantom tip) and laser marks on the halo. A CT cut through the target point T, which usually coincides with the center of the lesion, is obtained and subtracted from an image through the same slice acquired during the localization procedure. A satisfactory result confirms that patient position within the scanner coordinate system is the same as it was during the original localization procedure. Under computer control, the laser is then moved to identify the anterior and left and right lateral projections of the target point T on the patient’s skin. Patient set-up and initial irradiation The patient is disconnected from the base assembly and with gyro mechanism attached is moved to the accelerator room, where the patient is locked into the base assembly on the accelerator table. The space digitizer field source, mounted on a shadow tray, is placed in the gantry accessory mount. The field sensor is attached to the patient halo ring. The coordinate system of the low-frequency magnetic field is assigned to coincide with the coordinate system of the accelerator, with its origin at the isocenter. The tip of the field sensor, which is the reference point P (Fig. 5), is positioned at the accelerator isocenter, using the field cross-hair with the gantry in upright and both lateral positions. Verification readings ( x = y = z = 0) are then obtained on the system controlling computer. To set up the patient for treatment according to the previously determined treatment plan, the target point T must be placed at the isocenter. Since, however, the coordinates of P and T are known, this task is accomplished by rectilinear transformation of the coordinate system by: x = (xp x,), y = (yp - yr), and z = (zp - zr), as shown in Figure 5. *Clinac 25OOC, Varian Associates, Inc., Palo Alto, CA 94303. tCT-SIM/0600, Medical High Technology International, Inc.,

Volume 22, Number 1, 1992

Using the described stereotaxic system and predetermined values of x, y, and z, patient setup is now performed rapidly and accurately by simply maneuvering the accelerator couch with the patient on it until the desired x, y, and z readings, defined above, are obtained. For fine adjustment of position along the x and y axes, the guide, rather than the accelerator couch, is used. Once patient setup is complete, the target point T coincides with the accelerator isocenter. At this time the base line values of patient azimuth a, elevation e, and roll r, defined in Figure 2, are also obtained and stored in a patient set-up file in the controlling computer. The setup is verified in the standard manner with port films. The coincidence of field and laser lights with the anterior and lateral projections of the target point provides additional evidence of setup correctness. To establish base line values for monitoring of patient motion during the treatment session, the field source is removed from the accessory mount, placed into the film cassette holder below the table, and the essential six readings are obtained and stored. Any change in these values during treatment is indicative of patient motion relative to the couch. The reason for conducting this procedure is to measure the degree of patient movement that the stereotaxic device’s tolerances permit. It is noted that all stereotaxic equipment allows some limited patient motion. During treatment, the patient data file is continuously updated and incoming coordinates and angles compared with base line values. Treatment can be stopped if any difference between a reading and its corresponding base line value exceeds the established motion limits. The data are stored in a patient treatment file in the controlling computer. Repetitive treatment Patient setup and motion monitoring during all subsequent irradiation sessions are significantly simplified by using the described system and previously established patient setup and treatment files. The setup file is recalled, and the accelerator couch and/or guide are maneuvered until the original x, y, z, a, e, and r readings are obtained. The treatment file is then retrieved and a new data set is acquired during treatment. Since the data for all irradiation sessions are stored, they can be evaluated whenever desired. System accuracy The overall precision of the described radiotherapy technique has been determined to be ? 1.5 mm using the computer controlled stereotaxic system, an accelerator* having isocenter tolerance of 2 2.0 mm, and CT simulator? and treatment planning computer$ with a common file server. The resolution of the space digitizer is 0.5 mm for position and 0.1” for orientation. Its accuracy is 0.8 mm Clearwater, FL 34620. $Theraplan L, Theratronix,

Kanata, Ontario K2K 2B7.

Computer controlled stereotaxic Rx system 0 P. V.

root-mean-square (RMS) in position and 0.5” degrees RMS in orientation. These manufacturer-provided specifications were confirmed under laboratory conditions. Digitizer stability and measurement reproducibility were also found satisfactory. The precision of this stereotaxic technique was derived as follows. A sheet of film* was cut to match a cross-sectional contour through a Rando head phantom, and then placed between two 0.25 in. plastic plates of similar shape. This “sandwich” was then inserted into the phantom. The target, which was marked on the film, was simulated by a 1 cm diameter cavity machined in the plates. The halo was affixed to the phantom. Target localization, treatment planning, and phantom setup including target irradiation, using transverse and sagittal arcs, were performed. The film was then developed. The difference between the centers of the target and the dose distribution did not exceed 1.5 mm. Considering the tolerances of the technologies involved, and also possible brain motion of up to 3 mm (3), this result is satisfactory. Further description of the method used can be found in literature (12).

DISCUSSION

AND CONCLUSIONS

In contrast to Europe and Japan, there is no single company in the U.S. which offers all three major pieces of radiotherapy equipment: simulator, treatment planning computer, and accelerator. Consequently, the integration of diagnostic and therapeutic procedures, carried out on such equipment in a typical radiation oncology clinic, is difficult at best. In reality, no technology has been offered by any major corporation for this purpose despite the fact that accelerators, simulators, and computers are being continuously improved. The stereotaxic radiotherapy system described in this paper is an example of the type of technology that is essential for rapid and accurate procedure integration in brain radiotherapy. This system, a combination of a stereotaxic guide and

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electromagnetic measurement technology, not only promotes the correspondence of diagnostic and therapeutic procedures, but also partially automates patient setup for radiation treatment and facilitates continuous patient motion monitoring during the therapy session. Full automation, including corrections for patient movement or treatment interruption, is feasible. The major advantage of this system, in comparison to those commercially available at present, is that it was conceived specifically for application of multi-fraction and variable target volume stereotaxic regimens. The major disadvantage arises from the fact that as any other electromagnetic technology, it might be affected by background noise and/or by field metallic distortion. Therefore, time-consuming initial calibration has to be performed in the accelerator room. Field distortion should not be perceived as a limitation of this technology because the space digitizer is designed to perform accurately under the conditions that are more extreme than those in the accelerator room. Specifically, this technology was developed and is being used as a helmet-mounted sight for military fighter aircraft. The stereotaxic radiotherapy program at the University of Miami, initiated in 1982, has been carried out on commercially available equipment including both conventional and CT simulators, different treatment planning computers, and linacs with 4, 6, 10, and 24 MV x-ray beams for single and multi-arc treatment techniques as well as staticport irradiation. Only multi-fraction radiotherapy regimens have been used (13). The stereotaxic guides employed were designed and fabricated at the University of Miami. The system introduced in this paper has been in use for a period of approximately 1 year. We conclude that the described system is suitable for stereotaxic radiation oncology applications, specifically for integration of CT aided target localization with long term fractionated radiotherapy of small intracranial lesions.

REFERENCES 1. Chierego, G.; Marchetti, C.; Avanzo, R. C.; Pozza, F.; Colombo, F. Dosimetric considerations on multiple arc stereotaxic radiotherapy. Radiother. Oncol. 12:141-152; 1988. 2. Colombo, F.; Beneditti, A.; Pozza, F.; Avanzo, R. C.; Marchetti, C.; Chierego, G.; Zanardo, A. External stereotactic irradiation by linear accelerator. Neurosurgery 16: 154160; 1985. 3. Greitz, T.; Bergstrom, M.; Boethius, J.; Kingsley, D.; Ribbe, T. Head fixation system for integration of radiodiagnostic and therapeutic procedures. Neuroradiology 19: 1-17, 1980. 4. Houdek, P. V.; Van Buren, J. M.; Fayos, J. V. Dosimetry of small radiation fields for lo-MV X-rays. Med. Phys. 10: 333-336; 1983. 5. Houdek, P. V.; Fayos, J. V.; Van Buren, J. M.; Ginsberg, M. S. Stereotactic radiotherapy technique for small intracra*Kodak XV, Eastman Kodak Co., Rochester,

NY.

nial lesions. Med. Phys. 12:469-472; 1985. 6. Kjellberg, R. N.; Hanamura, T.; Davis, K. R.; Lyons, S. L.; Adams, R. D. Bragg peak proton beam therapy for arteriovenous malformation of the brain. N. Engl. J. Med. 309: 269-274; 1983. 7. Larsson, B .; Lid&, K.; Sat-by, B. Irradiation of small structures through the intact skull. Acta Radiol. Oncol. Radiat. Phys. Biol. 13:512-534; 1974. 8. Leksell, L. A stereotaxic apparatus for intracerebral surgery. Acta Chir. Scandinav. 99:229-233; 1949. 9. Leksell, L. The stereotaxic method and radiosurgery of the brain. Acta Chir. Scandinav. 102:316-319; 1951. 10. Lunsford, L. D.; Maitz, A.; Lindner, G. First United States 201 source cobalt-60 gamma unit for radiosurgery. Appl. Neurophysiol. 50:253-256; 1987. 11. Podgorsak, E. B.; Olivier, A.: Pla, M.; Lefebvre, P.; Hazel,

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stereotactic radiosurgery. Int. J. Radiat. Oncol. Biol. Phys. 14:115-126; 1988. 12. Serago, S. F.; Lewin, A. A.; Houdek, P. V.; GonzalezArias, S.; Hartman, G. H.; Abitbol, A. A.; Schwade, J. G. Stereotactic target point verification of an x ray and CT localizer. Int. J. Radiat. Oncol. Biol. Phys. 20:517-523; 1991.

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13. Schwade, J. G.; Houdek, P. V.; Landy, H. J.; Bujnoski, J. L.; Lewin, A. A.; Abitbol, A. A.; Serago, C. F.; Pisciotta, V. J. Small-field stereotactic external-beam radiation therapy of intracranial lesions: fractionated treatment with a fixedhalo immobilization device. Radiology 176563-565; 1990.

Computer controlled stereotaxic radiotherapy system.

A computer-controlled stereotaxic radiotherapy system based on a low-frequency magnetic field technology integrated with a single fixation point stere...
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