Purpose: To define the "ideal margins" to be used for the delivery of six-field conformal radiotherapy for localized prostate cancer.
Methods and materials: For a typical patient, 3-D based 6-field conformal treatment plans were generated using uniform margins ranging from 0.5-2.5 cm (in 0.25 cm increments). In a step-wise fashion the minimum margins required to encompass the gross tumor volume within the 90% isodose shell were identified. Additional margins were then added to account for extracapsular penetration, setup and patients movement error as well as for organ movement. Assumptions about the relative tolerance of surrounding normal tissues were also incorporated into the final decisions regarding margins.
Results: For the various areas of interface, between the prostate and surrounding normal tissues "ideal margins" varied from 0.75-2.25 cm.
Conclusion: The use of nonuniform "ideal margins" appears to insure adequate coverage of the tumor, while minimizing the volume of surrounding dose limiting normal tissues irradiated. This approach should in theory improve the tumor control and complication probabilities compared to using conventional treatment techniques and to using a 6-field conformal technique with uniform margins.