Radiation Oncology Investzgations 1:71-75 (1993)

Role of Tertiary Collimation for Linac-Based Radiosurgery

Vernon Smith, pil. I ) . , Michael C. Schell, Ph. D . , and David A. Larson, M . D . , p h . ~ .

Department of Radiation Oncology, University of Califrnia, San Francisco, Califrnia

SUMMARY For the stereotactic irradiation of intracranial lesions with a linear accelera- tor, the use of a tertiary collimating system can be expected to confer several benefits in comparison to the use of the standard linac collimating jaw system. Moving the collimator closer to the patient should lead to reduced penumbra and reduced susceptibility to positioning error. In addition, the use of a circular rather than a square aperture should yield a more advantageous dose distribution for irradiation techniques which follow spher- ical geometry. An investigation using film dosimetry with a tissue-equivalent head phan- tom demonstrated that the tertiary system was superior to the standard jaw system as expected. Radiat Oncol Invest 1993;1:71-75. 0 1993 Wiey-Liss, Inc.

Key word: to normal tissue

radiosurgery , collimation, stereotactic irradiation, linac, dose reduction

INTRODUCTION There has been much recent interest in the use of linear accelerators for stereotactic external beam radiosurgery [ 13. Most institutions using this treat- ment modality have constructed special hardware [2] which attaches to the accelerator and allows a higher degree of precision to be attained than is possible with standard linac treatments. Some groups have used the standard accelerator jaws [3] which give square fields and are located at a large distance from isocenter. It is also difficult to adjust collimator jaws to a specific size with a precision of much better than -+ 1 mm. Others have employed a satellite collimator attached to a tray [4] which fits into the standard accessory tray holder of the accel- erator. Yet others have used a special purpose ter- tiary collimator assembly [2] which, taking advan- tage of the fact that only the cranium is being irradiated, can be much closer to isocenter than normal shielding blocks, and can be located pre- cisely using a dowel-pin mechanism. Table 1 gives an example of how penumbra size and set-up accu-

racy are influenced by differences in collimator placement. The table gives the calculated values, in millimeters, of the geometrical penumbra, and the positional errors at isocenter due to a 0.5 mm dis- placement of the X-ray target and to a 0.5 mm displacement of the collimator for four different placements of the collimator. These are for our own facility using the Varian Clinac-6/100 linac. The distances from isocenter correspond to the lower edges of the upper jaw, the lower jaw, a shielding block placed in the tray holder, and the insert for our radiosurgery collimator. This analysis predicts that employment of a tertiary collimator located close to the patient should result in significantly reduced penumbra and reduced susceptibility to po- sitioning errors in comparison to the use of the conventional jaw system or to a tray-mounted colli- mator. Most special purpose collimator systems use a variety of inserts giving a choice of circular aper- tures ranging in size from about 1 cm to several centimeters. A single arc of rotation with a circular collimator gives a high dose region (80-90% iso-

Received November 30, 1992; accepted January 25, 1993. Address correspondence/reprint requests to: Vernon Smith, Ph.D., Department of Radiation Oncology, Box 0226,

0 1993 Wiley-Liss, Inc. University of California, San Francisco, CA 94143.

72

Table 1. Effects of Collimator Placement

Smith et al.: Role of Tertkly Collimation

Upper Lower Tray Tertiary jaw jaw collimator collimator

Distance from isocenter (cm) 72 62 35 23

Geometrical penumbra for 2 mm focal spot (mm) 5. I 3.3 1 . 1 0.6

Positional error due to 0.5 mm displacement of X-ray I .3 0.8 0.3 0. I5

Positional error due to 0.5 mm displacement of collimator 1.8 I .3 0.8 0.65 target (mm)

Retainer

Mounting Plate Cerrobend

u Fig. 1. tactic radiosurgery designed by Lutz et al. [ 2 ] .

Tertiary collimator system for linac-based stereo-

dose) which has a spherical shape, and the addition of further arcs separated by 40-60" allows almost exact superposition of the high dose region from each. In contrast, a single arc of rotation with a square collimator gives a high dose region which has a cylindrical shape, and the addition of further arcs results in superposition of each cylinder with an angular offset between each which causes a smearing of the high dose region. Thus circular apertures should be more appropriate than square apertures for irradiation techniques which follow essentially spherical geometry. The purpose of this paper is to examine whether the predicted benefits of a tertiary collimation system are observed in practice.

MATERIALS AND METHODS Linac radiosurgery at the University of California at San Francisco [5] is performed with a Varian Clinac-6/ 100 linear accelerator fitted with a tertiary collimator system based on a design by Lutz et al. [2], as shown in Figure. 1. The collimator housing can accommodate various 10 cm thick Cerrobend inserts, each with a step-tapered hole of circular cross section, providing geometrical field sizes at isocenter ranging from 1.25 to 3 cm diameter. The lower end of the insert is located 23 cm from iso-

t VERT

Fig. 2. Treatment technique used for this study consisting of 5 arcs of 100" each with a 40" angular separation between each arc.

center. The three-dimensional dose distributions produced by the conventional jaw system and by the tertiary collimator system were investigated for a typical radiosurgery technique using film dosime- try. A RAND0 tissue-equivalent head phantom was modified so that X-ray film could be placed between transverse slices of the phantom at approx- imately 3 mm intervals. A total of 13 films were used for each set of irradiation conditions. The treatment technique which was chosen as a basis of comparison consists of 5 arcs of 100" each with an angular separation of 40" between arcs, providing a geometrical arrangement resembling the slices of an orange [3], as shown in Figure 2. Sets of mea- surements were made using the standard jaws with field sizes of 1 X 1 and 2 X 2 cm and using the tertiary collimator with field sizes of 1.25 and 2.0 cm diameter. In the interest of accuracy, the maxi- mum optical density on any of the films was limited to 1.8. This required the use of a 2 in. thick lead attenuator located just below the jaws of the accel- erator to reduce total dose delivered for the 5 arcs

Smith et al.: Rok of Tiv-tiary Collimtion 73

B AP h A

A-p /\I

I I

Fig. 3. Representative isodose curves (5, 10,20,50, 80, and 90%) for a transverse slice which is 3 mm inferior from isocenter for (A) the 2.0 cm diameter tertiary radiosurgery collimator and (B) a 2.0 X 2.0 cm standard linac collimator jaw setting.

100

90

80

70

60

50 2

3 40

30

20

10

0

9

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 <- SUP AXIAL SLICE NUMBER INF ->

Fig. 4. the 2.0 cm diameter tertiary radiosurgery collimator.

Representative plot, of area in cm2 of each isodose surface vs. transverse slice number, for

because the accelerator was limited to a minimum of 5 MU/degree (MU = Monitor Unit). A charac- teristic curve of optical density vs. dose was estab- lished for the Kodak XV2 film used for the study by irradiating a single film 6 times in different loca- tions, using the 3.0 cm radiosurgery collimator, with a range of doses covering the optical density region of interest. The non-linear response of the film with respect to dose was taken into account by using the characteristic curve to calculate optical density percentages corresponding to dose percent- ages of 90, 80, 50, 20, 10, and 5%. The densitom- eter attachment of a Therados RFA3 field analyzer

was used to draw lines corresponding to these opti- cal density percentages, thus yielding corrected iso- dose lines. Figure 3A,B shows representative iso- doses for the 2.0 cm diameter tertiary collimator and a 2.0 X 2.0 cm standard jaw setting, respec- tively. Both sets of isodoses are for an axial plane which is 3 mm inferior from isocenter (slice 8 of Fig. 4). The area contained within each isodose line was measured with a planimeter. An example of the results for the 2 cm diameter tertiary collimator is shown in Figure 4, which plots the area within each percentage dose line as a function of slice number. The volume enclosed within the three-dimensional

-0- 2.0 CM DIAM. TERTIARY COLLIMATOR lo00

- + 2x2 CM JAWS

loo s2 Y 3 9 10

1 5 10 20 50 80 90

PERCENT DOSE

Fig. 5. Cumulative dose-volume histograms for the 2.0 cm diameter tertiary collimator and for a 2.0 X 2.0 cm standard linac collimator jaw setting

dose surface was estimated based on the area of each slice and the known interslice spacings, and from this dose-volumes were calculated.

RESULTS AND DISCUSSION Figure 5 shows dose-volume histograms for the 2 cm diameter tertiary collimator and for a 2 X 2 cm standard jaw setting. The two curves are roughly comparable for the 80-90% dose level and for the 5% dose level, but the standard jaw configuration encloses significantly more volume for the interme- diate dose levels. A similar dose-volume histogram for the 1.25 cm tertiary collimator and a 1 X 1 cm standard jaw setting showed similar effects if the values were scaled to allow for the difference in aperture. In order to quantitate the differences be- tween the two types of collimation, an equivalent diameter for each dose-volume was obtained by calculating the diameter of the sphere having the same volume as the dose-volume. It was assumed that these equivalent diameters would vary approx- imately linearly with collimator aperture. Based on this assumed linear relationship, the apertures nec- essary to provide 80% dose coverage to spheres of 1.0 and 2.0 cm diameter were calculated for both types of collimation. The equivalent diameters of other dose percentages were then calculated for these apertures, again assuming a linear relation- ship. Finally, volumes for each dose percentage were calculated from these equivalent diameters. Table 2 is for a 1 .O cm diameter 80% dose reference condition and shows that for the same 80% volume the 50% and 20% dose volumes and equivalent

Table 2. 20% Isodoses for a 1.0 cm Equivalent Diameter 80% Isodose

Equivalent Diameters of 50% and

Tertiary square collimator jaws

Collimator dimension (cm) 1.06 1.10 50% volume (cc) I .30 1.98 50% equivalent diameter (cm) I .36 I .56 20% volume (cc) 5.95 8.50 20% equivalent diameter (cm) 2.25 2.53

Table 3. 20% Isodoses for a 2.0 cm Equivalent Diameter 80% Isodose

Equivalent Diameters of 50% and

Tertiary Square collimator jaws

Collimator dimension (cm) 2.01 I .95 50% volume (cc) 8.96 11.32 50% equivalent diameter (cm) 2.58 2.19 20% volume (cc) 33.60 43.96 20% equivalent diameter (cm) 4.00 4.38

diameters are significantly larger for the standard square jaws than for the circular tertiary collimator. The 50% volume is 52% larger and the 20% volume is 43% larger for the standard jaws than for the tertiary collimator. Table 3 is for a 2.0 cm diameter 80% dose reference condition and exhibits a similar relationship. The 50% volume is 26% larger and the

20% volume is 3 1 % larger for the standard square jaws than for the tertiary collimator.

CONCLUSIONS The results demonstrate that for a spherical target volume, in comparison with the standard linear ac- celerator jaw collimation system, a tertiary system with step-tapered circular collimators placed close to isocenter provides significantly reduced dose to normal tissue for the same lesion dose.

REFERENCES 1. Smith V, Larson DA, Schell MC: Linac-based stereo-

tactic irradiation-A brief survey. Centerline News1

Smith et al.: RoIe of Tertiary Collimation 75

(Varian Assoc) 1 2 : l 4 , 1990. 2. Lutz W, Winston KR, Maleki N: A system for stereotac-

tic radiosurgery with a linear accelerator. Int J Radiat Oncol Biol Phys 14:373-381, 1988.

3. Chierego G , Marchetti M, Avanzo RC, Pozza F, Co- lombo F: Dosimetric considerations on multiple arc ste- reotaxic radiotherapy. Radiother Oncol 12: 141-152, 1988.

4. Hartmann G: Cerebral radiation surgery using moving field irradiation at a linear accelerator facility. Int J Ra- diat Oncol Biol Phys 11:1185-1192, 1985.

5. Schell MC, Smith V, Larson DA: Evaluation of radio- surgery techniques with cumulative dose volume histo- grams in linac-based stereotactic external beam irradia- tion. Int J Radiat Oncol Biol Phys 20:1325-1330, 1991.