Endorectal Brachytherapy Boost After External Beam Radiation Therapy in Elderly or Medically Inoperable Patients With Rectal Cancer: Primary Outcomes of the Phase 1 HERBERT Study

Abstract

To evaluate the toxicity and efficacy of the combination of external beam radiation therapy (EBRT) followed by high-dose-rate endorectal brachytherapy (HDREBT) boost in elderly and medically inoperable patients with rectal cancer. A phase 1 dose-escalation study was performed. Treatment consisted of EBRT (13×3Gy) followed by 3 weekly brachytherapy applications 6weeks later. The HDREBT dose started at 5Gy per fraction, increasing with 1Gy per fraction if dose-limiting toxicity (DLT, defined as grade ≥3 proctitis <6weeks after HDREBT) occurred in ≤2 patients per dose level. The primary endpoint was the maximum tolerated dose, defined as 1 dose level below the dose at which 3 patients experienced DLT. Secondary endpoints were toxicity, clinical tumor response, freedom from local progression, and local progression-free and overall survival (L-PFS and OS). Thirty-eight patients with a median age of 83years were included in the study. Thirty-two were evaluable for DLT and late toxicity and 33 for response evaluation. Maximum delivered dose was 8Gy per fraction, resulting in a recommended dose of 7Gy per fraction. Response occurred in 29 of 33 patients (87.9%), with 60.6% complete response (CR). The L-PFS and OS rates were 42% and 63%, respectively, at 2years. Patients with CR showed a significantly improved L-PFS (60% at 2years, P=.006) and a trend in improved OS (80% at 2years, P=.11). Severe late toxicity occurred in 10 of 32 patients. We found that HDREBT after EBRT results in a high overall response rate, with improved L-PFS for patients with a CR. The high observed rate of severe late toxicity requires further evaluation of the risks and benefits of an HDREBT boost.