hr. J. RadiarionOncology Bid. 6

@

Pergamon

Prew

Ltd..

1979

Phys..

Vol.

Pnnted

5, pp. 875.886

0360.3016/79/0601-0875/$02.00/O

in the U S.A.

Special Feature

STATIC

PION BEAM TREATMENT PLANNING OF DEEP SEATED TUMORS USING COMPUTERIZED TOMOGRAPHIC SCANS?

KENNETH R. HOGSTROM,Ph.D., ALFRED R. SMITH, Ph.D., STEVE L. SIMON, M.S., JOHN W. SOMERS, M.S., RICHARD G. LANE, Ph.D., ISAAC I. ROSEN, M.S., CHARLES A. KELSEY, Ph.D., CARL F. VON ESSEN, M.D., MORTON M. KLIGERMAN, M.D.,*

PETER A. BERARDO, Ph.D., and SANDRA M. ZINK, Ph.D.9

Static negative pion beam treatment of deep seated tumors is in progress at LAMPF.The influence of the physical principles of pions on treatment planning, e.g., an enhanced peak dose, multiple scattering, variations of beam quality, pion beam emittance, and a fixed vertical beam, are discussed. Computerized tomographic (CT) scan data have proven invaluable in providing quantitative information on inhomogeneities within the anatomy. The methods of designing collimation and compensating bolus for tailoring the pion beam to individual patient ports are described. An interim method, using dosimetry measurements in a water phantom and CT scan data to construct patient isodose distribution is outlined. Typical treatment plans for patients with abdomhml and head and neck cancers are presented. The method of X-ray simulation in verifying lwalization of CT-defined tumor volumes is demonstrated for the head and neck. Pion radiotherapy, dosimetry

Treatment

planning,

Computerized

INTRODUCTION The negative pi-meson (pion) radiotherapy program conducted jointly by the University of New Mexico Cancer Research and Treatment Center and the Los Alamos Scientific Laboratory at the Clinton P. Anderson Meson Physics Facility in Los Alamos (LAMPF), has proceeded to the treatment of large, deep-seated tumors with static peak pion fields. Lesions of the head and neck, brain, chest wall, lung, stomach, pancreas, bladder, prostate, and rectum, which previously had been identified as favorable sites for pion clinical trials,” have been planned for treatment. The purpose of this paper is to discuss the physical principles of static pion fields that are relevant to treatment planning and how they tire presently used in planning individual treatments.

tomography

scans, Inhomogeneity

corrections,

Pion

The physical properties of negative pions2.6*‘8V’9*2’.22 allow pion beams to be shaped in depth to fit individual tumor treatment volumes. The shape of the pion Bragg peak can be controlled by a dynamic range shifter,’ and the depth of penetration by beam tuning” and the use of bolus.14 The goal in treatment planning is to consider such physical properties and to provide the radiotherapist with the plan that best meets his requirements. This paper will discuss how treatment planning is presently accomplished with static pion beam therapy, using individual or multiport static pion beams. Static pion beams are characterized by collimation of a broad pion beam 10 the transverse projection (plane perpendicular to the incident beam) of the tumor treatment volume and control of the longitudinal (parallel to the incident tion of the LAMPF biomedical channel. Mike Paciotti has provided channel parameters for the catalogue of pion beam tunes used for treating patients. The authors also wish to thank J.M. Sala, M.D., Mirkutub M. Khan, M.D., and Charles J. Sternhagen, M.D., radiation oncologists: F.D. Mettler, M.D., diagnostic radiologist; and Yoshiaki Tanaka, M.D., Research Fellow in Radiology, of the Cancer Research and Treatment Center and the Department of Radiology, University of New Mexico, for their participation in the initial treatments involving static pion beams. Accepted for publication 30 December 1978.

tThese investigations were supported in part by U.S. Public Health Service Grants, No. CA-16127 and CA-14052 from the National Cancer Institute, Division of Research Resources and Centers; and by the U.S. Department of Energy. *Cancer Research and Treatment Center and Department of Radiology, University of New Mexico, Albuquerque, NM 87131, U.S.A. §Los Alamos Scientific Laboratory, Los Alamos, NM 87545, U.S.A. Reprint requests to Kenneth R. Hogstrom, Ph.D. Acknowledgements-The authors wish to thank Jim Bradbury and Jerry Helland for overseeing quality beam opera875

876

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beam) peak size and shape by use of a range shifter. The primary limitation of static pion beams is the constraint that their peak shape parallel to the beam remains constant throughout the transverse extent of the pion field. This constraint will be eliminated by the use of dynamic therapy.12 where the patient will be treated by a superposition of narrow .pion beams, each of which will be free to have a different peak shape, This will allow the pion peak to be shaped to tumor treatment volumes of varying thickness. Individual treatment plans based on the current sophistication of static pion beam treatment planning, but which do not necessarily represent the full potential of pion radiotherapy, are discussed. METHODS AND MATERIALS Use of CT scans in treatment planning Treatment planning for patients with and without the aid of computerized tomographic (CT) scans has made it apparent that adequate treatment plans cannot be developed without the use of CT scan data.-! CT scans allow a 3-dimensional tumor treatment volume to be uniquely prescribed by the radiotherapist and nearby critical structures to be placed in their proper perspective. A 3-dimensional description of the treatment volume from CT scans helps to minimize the dose delivered to normal tissue in several ways. First, information concerning the longitudinal dimension (parallel to direction of incident beam) of the treatment volume can be obtained only from CT scans. Although X-rays or other diagnostic methods may give the maximum longitudinal thickness of the tumor, the relevant quantity is the effective longitudinal thickness, which is obtained by integrating the linear stopping power of the tumor over the maximum longitudinal thickness. This effective thickness can be obtained from CT data4.” by relating the CT numbers to pion stopping powers, but not from diagnostic X-rays. Although with static pion radiotherapy the peak dimension in depth must remain the same across the transverse plane of the treatment volume, dynamic pion radiotherapy will permit variation of peak depth dose deposition. This requires knowledge of the longitudinal thickness of the tumor volume at each point in the transverse plane. Such information can be obtained only from CT scans, evenfor homogeneous tumor volumes. Optimizing the peak depth dose deposition in pion radiotherapy enables minimization of the relative dose to overlying and surrounding normal tissue. Figure 1 illustrates pion depth dose curves used to treat a 5-cm thick tumor. A S-cm modulated peak tEach patient is presently CT scanned with an EM1 5000 scanner. CT scan slices are taken in one cm increments

June 1979. Volume

01 0

5. Number

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6

I IO

I 15

DEPTH BELOW SKINkm) Fig. 1. By covering a 5-cm thick tumor with a 5-cm modulated peak rather than an S-cm modulated peak, surrounding normal tissue can be spared more adequately. This can be done clinically only when there is a quantitative description of the anatomy within the field, and CT scans presently are the only practical way of obtaining such information.

could be used, provided a quantitative description of overlying body tissues and the effective longitudinal tumor thickness is known. However, in the absence of such information, ,presently available only from CT scans, an g-cm peak might be required to ensure adequate coverage. Using a S-cm peak rather than an g-cm peak results in a 12% decrease in the entrance dose, as well as a decrease in irradiation of surrounding normal tissue. Another advantage of using a smaller spread peak width is the increased dose rate. The 5-cm peak has a dose rate approx. 17% greater than the g-cm peak. This represents a significant reduction in patient treatment time and discomfort during immobilization, as well as an economic and time gain, since pions are at a premium in both price and availability. Probably the most useful information obtained from CT scan? is the quantitative description of inhomogeneities. Dose distributions are affected both near and distal to inhomogeneities. The distal effect is the change in the pion stopping distribution caused by range changes and multiple scattering arising from inhomogeneities shadowing the stopping volume. This phenomenon can be controlled to some extent by use of bolus if a description of the inhomogeneities is available. The near effect is the change in dose at tissue-inhomogeneity interfaces caused by variations in the secondary particle emission and density effects. Little can be done to control this phenomenon. The presence of dense bone or air cavities is most likely to affect the linear penetration of the pion beam. Inadequate compensation for inhomogeneities in pion beams can cause a gross underdose to all or part of the tumor volumez3 or an overdose to nearby which encompass the diseased area.

Computerized tomographyscans in pion planning 0 K. R.

normal tissues. As shown by Fig. 2, ignoring a 2-cm bone inhomogeneity would cause an underdose to the distal edge of the tumor by 46% for pions, compared to only 7% for Vo.

The effects of inhomogeneities on the dose distribution of charged particles have been discussed elsewhere.7-9 The distal effect of simple inhomogeneities on pion beams has been calculated;” more significantly, measurements made at LAMPF14 indicate that use of paraffin bolus can correct adequately for simple inhomogeneities. Correction for smaller and more subtle inhomogeneities in the anatomy is a more difficult task and is under study; however regardless of the algorithm used to calculate bolus for such inhomogeneities, CT scans remain the only practical method presently available for obtaining quantitative input parameters for defining the anatomy. Considerations

in pion treatment planning

A catalogue of beam tunes has been generated for the treatment of deep-seated solid tumors with static pion beams. These beams have a penetration from IO to 28gmlcm’ in depth, with transverse dimensions ranging from approx. 5 to 20 cm. Table 1 shows those beam tunes which have been used on one or more patients and those which are being developed for patient use. The peak width in the longitudinal dimension can be varied from approx. 2 to 18 cm by

-

HOVOSENEWS

PHPNT3M

200

H~GSTROM et al.

877

Table 1. Clinical static pion beam tunes patient frequency-i Nominal transverse dimensions at 85% (cm*) Nominal Penetration (cm)

10x10

10 15

15x15

9x20$

3x35

U

U

II 2

2

21

4 15 8

U

-

28

U

U

U

-

2 -

tBased on 33 patients treated between 3 Oct. 1976 and 21 Dec. 1977. *Designed to abut to form large square field. PUsed for lung nodule irradiations. u = Under development.

the dynamic range-shifter,’ with a shape designed to give a particular biological effect across the spread peak. In selecting the proper beam tune, peak width, number of portals, direction of portals, collimation, and bolus configuration, the treatment planner must consider several principles regarding static pion beam treatments: 1. It is advantageous, in cases where critical structures are transversely adjacent to the tumor, to treat with the beam of the least penetration, as the penumbra becomes narrower in the peak region with decreasing pion momentum. This is illustrated in Fig. 3, which compares profiles for beams of 10, 15, and 21 cm penetration. 2. Beam flatness over relatively large fields can be best accomplished by abutting 2 smaller fields, as is shown in Fig. 4. This results from the emittance of pion beams; i.e., the incidence pion flux has a Gaussian type falloff. Pion fields abut nicely in the transCOMPARISON FOR

OF PENUMRRA PION

BEAMS

AT MIC

PEAK

OF DIFFERENT

PENETRATIONS

. .. . . . ..LOW (IO cm) ---MEDIUM (15cm) -HIGH (21 cm)

__~

I01

0

4

I2

8

DEPTH

I6

20

IN WATERfcm)

Fig. 2. This example illustrates how ignorance of a bone inhomogeneity underdoses the distal edge of the tumor by approx. 46% for the pion field, as opposed to only 7% for

WV._ 5 LO.

-8

-5

0

X PROFILE

5

8

km)

Fig. 3. Comparison of identically collimated beams with modulated peak widths of 8cm show that increased multiple scattering in the deeper penetrating beams leads to less sharp penumbras in the peak region.

878

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COMPARISON OF REAM PROFILES ABUTTED

Oncology

FOR A SINGLE

PION

0 Biology 0 Physics

PION FIELD

AND

FIELDS

IO0

SO ---SINGLE

40

-5

FIELD

-ABUTTED

FIELDS

0

5

Y PRDFILE

IO

(cm)

Fig. 4. A more uniform dose profile across a large treatment field can be obtained by abutting 2 pion fields as opposed to the collimation of a broad field. This example illustrates how dynamic pion treatments which will be a superposition of many abutted small fields can give a uniform dose across the tumor profile.

verse dimension because their penumbras are not so sharp as most conventional radiotherapy beams, thereby reducing the probability of hot or cold spots resulting from misalignment. 3. The low exit dose of pions allows the beam to be aimed directly at critical organs or structures. This generally is not possible with conventional radiation. 4. Directing the beam through the path of minimal inhomogeneities minimizes the importance of bolus correction. 5. Overlapping parallel opposed fields tend to flatten both physical and biologically equivalent dose. Because radiobiology has not yet correlated absorbed dose with equivalent dose, overlapping parallel opposed fields are the best way to flatten equivalent dose without biological models. This is so because not only is the total dose flattened, but also the fractional high linear energy transfer (LET) component, resulting in a spread peak with a reasonably constant LET distribution throughout. 6. The fixed nature of the pion beam as opposed to the rotational freedom of most megavoltage machines means that the patient must rotate for multiport irradiations.. Proper treatment planning, therefore, requires CT scans in each of the rotated positions for assurance of minimal changes in internal anatomy between the scanning and treatment positions. For example, normally it is assumed that internal anatomy does not shift significantly for head and neck patients; however, for abdominal tumors significant shifts require scans for each treatment direction. 7. Short term movement of tissues can occur during breathing. This is particularly important when treating ttn making this decision, sharp gradients (parallel to the incident pion beam) in the borders of the stopping distribution must be considered; these will tend to underdose

June 1979, Volume

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6

small lung lesions. Also, long term changes resulting from bowel gas need to be recognized. 8. With static beam treatment it is not possible to vary the longitudinal extent of the pion peak to conform to the prescribed tumor volume throughout its transverse extent. Therefore, the radiotherapist must prescribe how the pion peak is to fit in depth around the prescribed treatment volume.? 9. The patient’s physical condition must be considered. There isusually an optimal set of directions for the pion beam to enter relative to the patient, but because the patient must rotate physically, he must be physically capable of withstanding rotation into those preferred positions for treatment. This factor is emphasized because pion treatments presently take 5-10 times longer than exposure times for conventional therapy. The complexity of treatment planning for pions emphasizes the need for close radiotherapist, diagnostic radiologist, and medical physicist interaction. Presently, multiport treatments are limited to paralielopposed and abutted fields. As experience is gained, patient positioning and casting techniques are improved, CT scans become available at different rotations, and use of the computerized treatment planning code PIPLAN3,20,23becomes a standard practice, then more complicated multiport irradiations will be possible.

Collimator

design,

bolus

design,

and dosimetry

for

a

single pion port

After the radiotherapist has prescribed a treatment volume and approved the preliminary treatment plan formulated by the medical physicist, the design of bolus and collimation for each port and dosimetry measurements begins. Bolus that compensates for tissue inhomogeneities, patient skin contours, and tumor contours is presently designed from CT scans by use of a parallel beam bolus model. The model determines the thickness of paraffin at points in the plane normal to the incident beam, so as to place the pion peak at a designated depth assuming that the incident pions are parallel and do not undergo multiple scattering. Figure 5 shows the resulting bolus for one CT slice. The bolus thickness in approx. 1 cm steps across the tumor volume is calculated from the stopping power of the tissues, s(x), relative to the stopping power of water, so, by the line integral distal edge of treatment volume

J

R, /(cm) = 0.97

(s/so)dx ___

0.97 skin

’

the distal edge of the tumor volume as a result of multiple scattering and secondary particle fluxes being greater leaving the tumor volume than entering.

Computerized tomography scans in pion planning 0 K. R. H~GSTROMet al.

PlON

BEAM

BOLUS

DESIGN

A+= H

FROM

CT

CT

INCREMENTS

i HOUNSFIELD

R_

SC*NS

= PION

NO

OF ITH

ELEMENl

RANGE

1

0 97 = REUTIVE POWER

OF

STOPPING PARIFFIN

Fig. 5. Bolus is calculated from CT Hounsfield numbers to stop the distal edge of the pion peak along the posterior border of the tumor, thereby sparing the spinal cord.

where R, is the range of the pion peak in water in cm, 0.97 is the stopping power relative to water used for paraffin bolus, and dx is one CT pixel (1 mm). To first order, the values of (s/s,,) are related to the CT Hounsfield numbers, H, by the expression

879

where p is the linear attenuation coefficient of the tissue, ~~ is the linear attenuation coefficient in water, and H = 0 (500) for water (air). This formula assumes that the relative stopping power for pions is approximately equal to the relative electron densities and that the relative electron densities are approximately equal to the relative linear attenuation coefficients (plpo). The latter would be true if the CT scanner were operated at sufficient energy that Compton scattering was the dominating factor; however, CT scanners presently have an energy of about 80 KeV where photoelectric absorption is significant. Consequently, empirical models are being developed to relate CT measurements of pion stopping powers. The magnitude of resulting errors probably is not clinically significant except for compact bone; nevertheless, methods involving dual energy CT scans may reduce such errors.4 Initially, the inability to decode CT data tapes necessitated manual design of bulus from negative film and digital output. This was a tedious process limiting both bolus resolution and accuracy as well as the number of patients who could be prepared for treatment. A preliminary version of PIPLAN3*23 soon was developed which could read the CT data tapes and calculate patient bolus in a manner similar to the method described above. Parafin has been chosen as a bolus material because it is easily cast in 3 dimensions, has a stopping power near that of water, is inexpensive, and withstands frequent handling. Although paraffin (CH2) lacks oxygen, it still is essentially tissue equivalent so long as it intercepts only passing pions.

CT SCAN NO. 4 5 6

LEFT

7 6 9 IO II 12

I

INFERIOR

____

~O~JM&~CRIBED

TUMOR

_

COLLIMATOR APERTURE

Fig. 6. Individual patient collimators for each pion field are designed by plotting the lateral extent of the tumor volume for each CT scan and then constructing a collimator aperture which allows for the penumbra of the pion beam, placing the tumor treatment volume within the 80% contour.

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Collimators are designed directly from treatment volumes prescribed on each CT slice. As Fig. 6 shows, the transverse extent of tumor treatment volumes for each CT slice is plotted to scale on a transverse plane, providing the projection of the entire tumor treatment volume. The collimator aperture lies approximately 1.5 cm outside the boundary of the tumor treatment volume so that the 80% isodose contour will coincide with the prescribed volume. The collimator thickness is governed by the and may vary from 2.5 to beam momentum” 5.0 cm of lead alloy. A typical patient bolus and collimator after fabrication are shown in Fig. 7. Presently, patient isodose contours are not calculated directly, but are estimated from dosimetry measurements that simulate the patient set-up. The geometry for dosimetry in a water phantom has the same air gap between collimator and water as that between collimator and bolus for the patient treatment and an identical quadrapole-to-tumor distance (QTD). Dosimetry scans include a central axis depth dose scan, a transverse scan at midpeak, and a peak dose calibration for each static beam pion port. A typical set of these measurements for an AP port of a pancreas tumor appears in Fig. 8. The collimator design can be verified by plotting the transverse projection of the tumor volume on the transverse isodose plot. The 80% contour should closely follow the prescribed tumor volume, as seen in Fig. 8. The ability of the treatment plan to cover the tumor

June 1979, Volume

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volume in the longitudinal direction is evaluated by constructing isodose plots on selected CT scans. To first order, the dose at any point within the patient is estimated by D(x, y, 2) = d(z)f(-G Y), where d(z) is the central axis depth dose evaluated at z, the effective distance below the bolus of a point in tissue, and f(x, y) is the dose relative to central axis from the transverse scan evaluated at (x, y), the location of a point in tissue relative to the central axis. Presently, the bolus model and isodose calculations ignore beam divergence, multiple scattering, and secondary emissions; therefore, the resulting dose distribution may deviate slightly from the most clinically desired distribution, especially at points where sharp gradients in the pion stopping distribution occur. Limitations of the parallel bolus model have been demonstrated experimentally in earlier works.14 In addition, the reconstruction of isodose contours from measured dose distributions does not account precisely for patient geometry and inhomogeneities. The effect of these inaccuracies is believed to be clinically insignificant in most cases, and in uivo dose measurements have upheld this belief to date.” Nevertheless, the calculation of dose distributions allows redesign in cases where the treatment plan is not clinically acceptable. This capability is expected to improve with an updated version of PIPLAN,3.23 presently being implemented.

Fig. 7. A typical patlent collimator and bolus after fabrication. Air hardening case to stabilize the bolus on the patient.

foam has been used in this

Computerized tomography scans in pion planning 0 K. R. H~GSTROMet a/.

881

SETUP:

!.5cm J ______-

DOSIMETRY CENTRAL

_____

,OOQTD

SCANS’ AXIS

DEPTH

TRANSVERSE

DOSE

ISODOSE PLOT

AT

Z=lOOcm

I00

DEPTH CALIBRATION

X-AXIS

IN WATER (cm) AT DOSE PEAK DOSE /No OUTPUT

IMU

*002755 RAD INo IMU DOSE/No MU CONSTANCY BLOCK DOSE/No

FACTOR =

I

I MU

0G6

Fig. 8. Dosimetry measurements which simulate patient setups are presently measured for each patient field. These measurements include a central axis depth dose curve, a transverse 2-dimensional scan at mid-peak, and a calibration which allows calculation of an output factor for that particular field relative to the daily constancy block calibration. A plot of the 80% isodose line (isodose lines at 95%, 90%) 80%, 70%, . . . 10%) of the transverse scan compared to the prescribed 80% treatment volume shows that the collimator was properly designed.

RESULTS Patient treatment plans , After reviewing a patient’s history with the diagnos-

tic radiologist, the radiotherapist gives to the physics treatment planning section a series of CT scans spaced normally 1 cm apart; these include the definition of a tumor treatment volume. Lines delineating the volume define the prescribed dose level (normally 80%). These lines include margins, when allowed, to account for unknown errors in the present treatment planning methods. Specific instructions include where to place extra pions in areas where the peak width exceeds the longitudinal width of the tumor treatment volume. Maximum tolerable dose levels to critical organs and structures are specified. Based on consultations with the medical physicist and previously discussed properties of static pion fields, the therapist indicates the preferred number and direction of pion fields. The treatment planning section then designs the physical aspects of the treatment; the goal of treatment planning is to deliver a dose that is as uniform as possible to the

tumor treatment volume, while minimizing nearby critical organs and structures. Patient 1: Adenocarcinoma

Treatment began with was added later. Oblique out because of a lack of shift in internal anatomy

dose to

of the pancreas

a single AP port: a PA port and lateral ports were ruled knowledge (CT data) of the

upon patient rotation. The resulting isodose curve for the single AP field is shown in Fig. 9; the anatomy was duplicated from the CT scans. There are several advantages of a single AP treatment. There are no gross inhomogeneities or irregular skin surfaces in the field for such a treatment. A 15-cm penetrating beam can be used, and the penumbra will be relatively sharp. The dose to the spinal cord will be less than 20% and will not limit the tumor dose. An isodose plot of the parallel-opposed, anteriorposterior/posterior-anterior (AP/PA) treatment plan is shown in Fig. 10. The advantages of APIPA treatment are a flatter physical and biologically equivalent dose over the tumor volume and a reduced skin dose.

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June 1979, Volume

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Fig. 9. Isodose contours modified by the bolus from the dosimetry measurements of Fig. 8 have been superimposed on the anatomy for the central CT slice of the pancreas case. Note how a single AP pion port can give uniform dose to the tumor treatment volume, with clinically small doses to the liver, kidney, and spinal cord.

I

BOLUS

Fig. 10. Isodose curves for an AP/PA treatment

I

plan of a pancreas

tumor using pions.

Computerized tomography scans in pion planning 0 K. R. H~GSTROMet al.

Disadvantages include a cord dose increase from approx. 20 to 50%, uncertainty in correcting for the vertebral body in the PA port, and a slightly wider penumbra, as the PA port requires a beam of greater penetration than the AP port. Both treatment plans are acceptable, since they cover the tumor adequately, give a minimal skin dose, and give less than tolerance doses to the kidneys, liver, and spinal cord. A conventional treatment plan using 10 MV X-rays, seen in Fig. 11, can be compared with the pion treatment plans. The pion treatment plan gives less dose to healthy tissue lateral to the tumor treatment volume. In particular the dose to the colon is reduced from approx. 60 to 40% and the liver dose is reduced from approx. 50% to 20%. Cord dose is identical for, both treatment plans; however, the relative biological effectiveness (RBE) of the pion dose in the peak region is greater than in the plateau so that pions should deliver less isoequivalent dose to the cord. The pion treatment plan, on the other hand, does not give as uniform coverage across the tumor treatment volume, and the skin dose is greater for the anterior and posterior surfaces. Of course a 4-field box technique with pion would reduce skin and cord dose. Patient 2: Carcinoma

of the hypopharynx

The series of CT scans delineating the prescribed tumor treatment volume for carcinoma of the hypopharynx is shown in Fig. 12. The patient had a tracheostomy, which presented no difficulty in the treatment procedure: therefore, the treatment plan was typical of head and neck treatments. Parallel-

opposed bilateral fields are used normally, except in cases where the tumor is wrapped around the spinal column. If the tumor is not symmetric, as is the case here, then the lateral fields are rotated to minimize cord dose. A 20” rotation was required with this patient. The advantage of parallel opposed treatments in head and neck patients lies predominantly in the ability to deliver a uniform total and biologically equivalent dose. A beam tune with 15cm penetration normally is used for head and neck treatments. This tune has reasonable beam edges, although unavoidable air gaps between the collimator and patient tend to degrade the penumbra. The parallel opposed treatment also equalizes skin dose. In this patient, skin dose was significant because the tumor volume was near the skin. However, in cases where the tumor volume is deep within the head or neck, the skin dose is reduced to approx. 50%, compared to single port irradiations, where the dose is approx. 75% on the entrance side and 20% on the exit side. Figure 13 shows the constructed isodose contours for an anatomical section at midfield. The tumor treatment volume is almost completely covered by the 80% dose level. The cord dose at this level is between 50 and 20%. and the skin dose is between 50 and 80%. The high dose anterior to the treatment volume and the underdose to the left posterior extent is a result of the anterior extent of the treatment volume in the adjacent superior CT slice and the inability to collimate across such steep tumor gradients in the transverse dimension. This CT slice indicates the need for replanning if the low dose to 10 MV X RAY

Fig. 1 I. Conventional

treatment

883

plans for the pancreas

tumor

using 10 MV

X-rays.

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tion is used, however, to verify the treatment volume defined from CT data, and involves the following steps. First the patient is oriented in his scanning position by the use of a Lightcast shell in which he was scanned. Then, accurate field marks are constructed on the patient. For head and neck patients this is a 2-step process. The patient is aligned in a perfectly lateral position in one half of the whole body Lightcast, which is embedded in a secondary polyurethane foam support cast. Lateral positioning is accomplished by minimizing mandible shadowing using lateral X-rays, as shown in Fig. 14. The entire cast is then fixed at an angle of rotation of 20”, since the patient, not the beam, must be rotated in pion therapy. The field is then aligned by translating a field template until X-ray radiographs, using the orthodiagraphic projection technique, verify the correct location relative to anatomical landmarks. The oblique lateral film in Fig. 15 verifies the location of the field with respect to the mandible and spinal column. Failure to position this field accurately either would result in an overdose to the cord or underdose to the posterior edge of the tumor, and would cause the bolus to correct improperly for inhomogeneities and beam shaping.

Fig.

12. Carcinoma of the hypopharynx ment volumes marked on the

with

tumor

treat-

CT scans.

the left posterior extent is not clinically acceptable. Geographical miss in pion therapy is more likely than in conventional radiation, where field marks are made during simulation on a machine similar to the treatment machine. With pions the CT scan is, in effect, the tumor defining device, as treatment volumes are extracted from this information. Simula-

DISCUSSION The physical principles and methods used in the initial stages of static pion beam treatment have been presented. After the patient’s disease is diagnosed and treatment volumes are constructed from CT scans by the diagnostic radiologist and radiotherapist, the radiotherapist and medical physicist plan for the optimum beam tunes, direction of ports, collimators, and bolus. After dosimetry measurements are made, reconstructed isodose contours are superimposed on the anatomy and reviewed by the radiotherapist. The treatment fields are simulated in the patient before treatment to assure delivery of the proper treatment. The importance of CT scans in pion radiotherapy PIONS

Fig.

13.

Isodose curves superimposed on a central CT slice of carcinoma of the hypopharynx using parallel opposed pion fields.

Computerized tomography scans in pion planning 0 K. R. H~GSTROMet al.

Fig. 14. True lateral alignment of head and neck patients is ensured by minimizing mandible shadowing (i.e., the 2 mandibles appear as 1) on lateral radiograph taken with vertical X-ray beam to simulate the pion beam.

been discussed. CT scans allow the radiotherapist to define 3-dimensional tumor treatment volumes. The ability to calculate bolus and dose distribution depends heavily on the quantitative description of the anatomy and 3-dimensional treatment volumes, which can be depicted accurately only by CT scans, an important consideration for any type of charged particle therapy. Patient examples show typical isodose distributions presently used in pion radiotherapy. Of course, as CT scans with the patient rotated become available and with improved dose calculational methods, more sophisticated multiport treatment plans are expected; these will give better isodose distributions. Coupled with a probable therapeutic gain for pions,” this promises the usefulness of pions in treating tumors that are not successfully treated by conventional radiotherapy.

885

Fig. IS. The technique of orthodiagraphic projection is used in accurately localizing the field. In this case the superior CT slice, denoted by the superior solder wire, is localized with respect to the mandible. The continuous solder wire denotes the entire pion field.

has

Experience gained from static beam treatment planning has defined clinically needed research goals. The development of dynamic treatments is in progress,‘* as this mode of treatment will allow pion beams to be shaped to fit arbitrary 3-dimensional tumor volumes. The complexity of pion beam treatment plans emphasizes the need of in oiuo dosimetry. Methods of in uioo dosimetry and treatment volume visualization are being studied with patients.13 Presently, in uiuo measurements generally agree to within 10% of predicted dose. Nevertheless, pion beam treatments are becoming more complex, and monitoring devices will be needed routinely to verify individual treatment plans.

REFERENCES Amols, H.I., Liska, D. J., Halbig, J.: Use of a dynamic rangeshifter for modifying the depth dose distributions of negative pions. Med. Phys. 4: 404-407, 1977. Armstrong, T.W., Chandler, K.C.: Monte Carlo calculations of the dose induced by charged pions and comparison with experiment. Rad. Res. 52: 247-262,

1972.

Berardo,

P.A.,

Zink,

S.M.:

CT data and pion treatment

planning at Los Alamos (abstr.), Med. Phys. 5: 326, 1978. 4. Chen, G.T.Y., Lymen, J.T., Riley, J.: Conversion of CT data to relative stopping power for charged particle radiotherapy (abstr.), Med. Phys. 4: 358, 1977.

5. Cohen, M.: Gamma rays: wCo teletherapy units. BE J. Radiol., Suppl. 11, Central Axis Depth Dose for Use in Radiotherapy: 53-61, 1972.

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6. Dicello, J.F.: Dosimetry of pion beams. In Particle Radiation Therapy: Proceedings of an International Workshop, Key Biscayne: American College of Radiology, ed by Smith, V., Philadelphia, American College of Radiology, 1975, pp. 155-186. 7. Goitein, M.: The measurement of tissue heterodensity to Znt. J. Radial. guide charged particle radiotherapy. Oncol. Biol. Phys. 2 (Suppl. 2): 27-33, 1977. 8. Goitein, M., Chen, C.R.Y., Ting, J.Y., Schneider, R.J., Sisterson, J.M.: Measurements and calculations of the influence of thin inhomogeneities on charged particle beams. Med. Phys. 5: 265-273, 1978. 9. Goitein, M., Suit, H.D.: The influence of tissue inhomogeneities on the dose distribution of charged particle beams. In Particle Radiation Therapy ; Proceedings of an International Workshop, Key Biscayne: American College of Radiology, ed. by Smith, V., Philadelphia, American College of Radiology, pp. 37-150, 1975. 10. Hamm, R.N., Wright, H.A., Turner, J.E.: Effects of tissue inhomogeneities on dose patterns in cylinders irradiated by negative-pion beams. Phys. Med. Biol. 21: 982487.1976. 11. Hills, J., Hendee, W.R., Smith, A.: Converting CT numbers to stopping powers for pion therapy treatment planning (abstr.), Med. Phys. 5: 325, 1978. 12. Hogstrom, K.R., Smith, A.R., Berardo, P.A., Zink, S.M., Paciotti, MA., Helland, J.A.: Comparison of static and dynamic pion beam treatment modes at LAMPF (abstr.), Med. Phys. 5: 338, 1978. 13. Hogstrom, K.R., Smith, A.R., Somers, J.W.: In uioo dosimetry for negative pion therapy (abstr.), Znt. J. Radiat. Oncol. Biol. Phys. 2: (Suppl. 2) 135, 1977. 14. Hogstrom, K.R., Smith, A.R., Somers, J.W., Lane, R.G., Rosen, I.I., Simon, S., Kelsey, C.A.: Measurement of the effect of inhomogeneities and compensating bolus in clinical pion beams. Med. Phys., 6: 26, 1979.

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15. Kligerman, M.M., Knapp, E.A., Petersen, D.F.: Biomedical program leading to therapeutic trials of pion radiation at Los Alamos. Cancer 36: 1675-1680, 1975. 16. Kligerman, M.M., Sala, J.M., Wilson, S., Yuhas, J.M.: Investigation of pion treated human skin nodules for therapeutic gain. Znt. J. Radiat. Oncol. Biol. Phys. 4: 263-265, 1978. 17. Paciotti, M.A., Bradbury, J.N., Wetland, J.A., Hutson, R.L., Knapp, E.A., Rivera, O.M., Knowles, H.B., Pfeuffer, G.W.: Tuning of the 1st section of the biomedical channel at LA&@% IEEE Trans. Nucl. Sci. w-22 (3): 178b1789, 1975. 18. Perris, A.B., Smith, F.A., Perry, D.R.: Charged particle emission from the capture of negative pions: energy spectra, LET distribtmtion, and W-value. Pitys. Med. Biol. 23: 217-234, 1978. 19. Raju, M.R., Richman, C.: Negative pion radiotherapy: Physical and radiobio@ieal aspects. Cur. Top. Radiat. Res. Qtly 8: 1%233, 1972. 20. Shlaer, W.J.: F%‘LAN, a treatment planning code for pion radiation therapy. Los Aiamos Scientific Laboratory LASL Rupt LA 5433-M!?, Los A’lamos, NM 87545, 1973. 21. Smith, A.R., Rosen, I.I., Hogstrom, K.R., Lane, R.G., Kelsey, C.A., Amols, H.I., Richman, C., Berardo, P.A., Helland, J.A., Kittell, R.S., Paciofii, M.A., Bradbury, J.N.: Dosimetry of pion therapy beams. Med. Phys. 4: 408-413, 1977. 22. Turner, J.E., Dutrannois, J., Wright, H.A., Hamm, R.N., Baarli, J., Sullivan, A.H., Berger, M.J., Seltzer, S.M.: The computation of pion depth-dose curves in water and comparison with experiment. Rad. Res. 52: 229-246, 1972. 23. Zink, S.M., Berardo, P.A., Hogstrom, K.R., Smith, A.R.: Three-dimensional pion dose calculation using CT data (abstr.), Med. Phys. 5: 325, 1978.