Improvement of prostate treatment by anterior proton fields.

PURPOSE: We performed a treatment planning study to demonstrate the potential dosimetric benefits of anterior-oriented fields for prostate irradiation by proton beam. A novel in vivo beam range control method shows millimeter accuracy, suggesting that such fields could be safely used to spare the rectum given the sharp distal penumbra of protons. METHODS AND MATERIALS: Ten prostate patients treated with water-filled endorectal balloon were selected. Bilateral fields were planned following the conventional treatment protocol. Three anterior-oriented fields (0, +30, -30°) were planned, with the range compensators manually adjusted to improve rectal sparing. Dose distributions to the clinical target volume, rectum, anterior rectal wall (ARW), bladder, bladder wall (BW), and femoral heads were compared for: A) equally weighted bilateral fields, B) a single straight anterior field, and C) two equally weighted anterior-oblique fields.
RESULTS: The anterior-oriented fields required much less beam energy, ∼10 cm water equivalent path length less than lateral fields. For ARW, the V(95%) for Plans A, B, and C were 39%, 8%, and 6%, respectively; the corresponding V(80%) were 59%, 27%, and 26%, respectively (p = 0.002 when Plan A was compared with B or C). Plan B irradiated a larger volume of BW than did Plan A by 3% at V(95%), 11% at V(80%), and 16% at V(50%) (p = 0.002), whereas Plan C differs little from Plan A for BW at these dose levels. The femoral heads received ∼40% of the prescription dose in Plan A, but negligible dose in Plans B and C.
CONCLUSIONS: Compared to lateral fields, anterior-oriented fields can significantly reduce dose to the ARW, particularly at high dose levels. These fields alone, or in combination with lateral fields, allow for the possibility of either reducing treatment toxicity at current prescription doses or further dose escalation in the treatment of prostate cancer.