Schedule in Cf-252 neutron brachytherapy: complications after delayed implant therapy for cervical cancer in a phase II trial.

The objective of this study was to review severe complication frequency in a protocol study using a defined prescribed dose combined with fractionated whole pelvis radiotherapy to 40-45 Gy. The method used a dose of Cf neutrons to 35 Gy equivalents (relative biological effectiveness or RBE adjusted) to a total tumor dose of 80 Gy-eq in one to four implant sessions. Compliance was excellent, and most patients received two implants to 35 (0.4) (SE) Gy-eq in two sessions plus external radiation to a total point A or paracervical region dose of 80 (0.3) Gy-eqs. In patients who received delayed implants, the severe complication rate (pelvic necrosis, fistulas) was significantly greater (40% versus 3%). We postulate that neutron brachytherapy caused tumors to regress rapidly and completely, which allowed the neutron dose to adjacent radiosensitive organs (bladder, rectum, sigmoid colon, and bowel) to become excessive. The delayed Cf implant apparently contributed to the greater risk for normal tissue complications.