Proceedings of the 35th Annual ASTRO Meeting 281

Results: Median PSA at initiation of 3D-RT was 1.0 ng/ml (range: 0.1 - 69 nglml). Seventy-two percent (34/47) of patients had return of PSA to normal levels (< 0.4 @ml). Median time to PSA normalization was 4.2 months (range: 0.2 - 26.2 months). No patient in this group bas had clinical failure or risiig PSA, thus far. Of the 13 patients without PSA normalization, 1 has failed locally, 2 developed bony metastasis and 1 had both local and distant failure. The remaining nine have rising PSA’s, but no evidence of clinical failure (3 of 9 received hormonal manipulation at time of PSA rise). Gverall, 11 of those patients who failed had a pre- 3D-RT PSA greater than the median of 1.0 nglml(l1123 or 48% failure rate). Only 2124 (8%) patients with a pre-3D-RT PSA < 1.0 ng/ml eventually failed. Finally, there has been no failure in patients with initial Gleason’s score < 7.

Conclusion: Post-prostatectomy adjuvant 3D-RT appears an effective means of controlling local recurrences and residual prostate cancer. It also has been effective in preventing failures and normalizing PSA in patients treated for rising PSA alone. Early institution of post prostate&my 3D-RT may be indicated as evidenced by an increased risk of failure in those with a pre-RT PSA > 1.0 nglml.

1060

AN ESTIMATE OF THE: MARGIN REQUIRED WHEN DEFINING BLOCKS IN RADIOTHERAPY OF THE PROSTATE

Douglas Jones, B.Sc.' , Mark D. Hafermann, M.D.*, and John W. Rieke, M.D.’

I. Northwest Medical Physics Center, Lynnwood, WA 98036. 2. Virginia Mason Medical Center, Seattle, WA 98101.

- Purpose There are uncertainties associated with each step of the radiotherapy process and the identification, measure- ment and combination of these presents a challenging problem. In order to provide a rational basis for the margin to be put on the clinical target volume when using the box technique for radiotherapy of prostate cancer we have studied the variation in field placement and size in a large number of treatments.

Methods & Materials - Simulation and port films of 54 consecutive patients treated for primary localized prostate cancer were entered in the study. Forms were developed to collect the data and a well defined protocol for the review was established. A port film marker was used to define orientation and magnification factor of the films. The area of fields defined on simulation films was compared to that on port film to assess accuracy of block production.

Results - We have identified six categories of uncertainty, four occur before treatment and two during the course. (1) clinical uncertainty. The equivalent radius of boost fields showed a variation of 7 mm in the AP and 4 mm in the lateral fields. A reasonable estimate of this uncertainty is 3 mm. (2) An uncertainty in correlating a CT study to simulation films is estimated to be 2 mm in the AP and lateral dimensions and 3 mm superiorly. (3) The movement of the prostate in response to filling of the rectum and bladder was taken from data reported by Ten Haken et al* and amounts to 3 mm in tlfe AP, 0.3 mm in the lateral and 1.2 mm superiorly. (4) The difference in the blocks produced, compared to those des gned on the simulator film was determined to be systematically 0.7 mm smaller in equivalent radius with a standard deviat.on of 0.9 mm. (5) During treatment there will be movement of the prostate with respect to fixed reference points as des.cribed in (3) above. (6) The set variations were not significantly different when treating the pelvis or boosting the prostate and amounted to offsets defined by couch motions to achieve alignment <>f 1.6 mm UP, 0.3 mm RIGHT and 1.4 mm IN with standard deviations of 3 mm.

Conclusion - We consider it desirable to deliver at least 95% of the stated dose to the target in 95 of 100 treatments. The uncertainties have been combined using this criteria and added to the margin to allow for dose penumbra to arrive at rules for drawing blocks. These margins vary with direction, for example 20 mm at the superior end and 17 mm to the right side on an AI’ view.

* Ten Haken, R.K.; Ftarman, J. D.; Meimburger, D. K.; Gerhardsson, A.; hl< hhan, D. L.; Perez-Tamayo, C.; Schoeppel, 5. L.; Lichter, A. 5. Treatment planning issues relating to prostate movc.lllent in response to differential filling of the rectum and blacder. Int. J. Radiat. One. Biol. Phys. 20:1317-1324; 1’)‘ll.