Conformal mixed neutron and photon irradiation in localized and locally advanced prostate cancer: Preliminary estimates of the therapeutic ratio

Purpose: To determine the incidence of chronic toxicity and the probability of biochemical and histologic complete response among patients with nonmetastatic prostate cancer, treated with three dimensional (3D) conformal mixed neutron and photon irradiation. Methods and Materials: Between November 1991 and December 1994, 151 patients with prostate cancer were entered in three prospective dose-finding studies of conformal mixed neutron and photon irradiation. Patients with low stage, low to intermediate grade prostate cancer (T1-2NXM0, Gleason Score less than or equal to 7) received 38 Photon Gy (PhGy) plus 9 (51 patients) or 10 (53 patients) Neutron Gy (NGy) to the prostate and seminal vesicles. Forty-seven patients with locally advanced prostate cancer (T3-4 NO-1 MO and/or Gleason Score greater than or equal to 8) received 15 NGy + 18 PhGy to the prostate and seminal vesicles and 9 NGy + 18 PhGy to the pelvic lymph nodes. Results: The median follow-up was 16 months (range: 3-30 months). There was no Grade 3-5 GI or GU toxicity recorded. At 20 months, the actuarial rates of Grade 2 GI morbidity were 6 and 29% for the 9-10 and 15 NGy protocols, respectively (p = 0.07). At 20 months, the incidences of Grade 2 GU morbidity were 4 and 16%, respectively (p = 0.08). Stiffness in flexing or abducting the hips was seen in 20 and 42% of patients receiving 9-10 and 15 NGy, respertively (p = 0.01). Potency was maintained in 65% of all patients. Among patients with an initial PSA less than or equal to 10, 100% had a 12-month PSA <2 and 78% <1 ng/ml. Negative postradiation biopsies were seen in 30% of patients at 6 months, 79% at 12 months, and 84% of patients at 18 months. Conclusion: The use of conformal mixed neutron and photon irradiation has been well tolerated with no severe bladder or rectal complications observed. However, because of the enhanced toxicity seen with 15 NGy, the current maximum dose levels of neutron irradiation have been limited to 11 NGy.