Rectal toxicity and rectal dosimetry in low-dose-rate (125)I permanent prostate implants: a long-term study in 1006 patients

Objective To describe the acute and late rectal toxicity in 1006 prostate brachytherapy patients implanted 1998–2003. To determine whether rectal dose–volume histogram as well as patient and treatment factors were associated with rectal toxicity. Methods and materials Median followup was 60.7 months. Rectal dosimetry was calculated as dose–volume histogram of the rectum using Day 28 CT-based dosimetry and expressed as volume of the rectum in cc receiving 50%, 100%, and 150% of the prescription dose (VR50cc, VR100cc, and VR150cc, respectively). Univariate and multivariate analyses were performed to examine the influence of patient, implant, dosimetry, and learning curve factors on the development of acute and late toxicities using a modified Radiation Therapy Oncology Group (RTOG) scale. Acute toxicity was analyzed using logistic regression and late toxicity using Cox proportional hazards regression. Analysis of variance was used to examine the association between rectal toxicity and rectal dose. Results Rectal dosimetry in 93.5% and rectal toxicity in 96.2% have been recorded. Median VR100 = 1.05 cc. Late RTOG Grades 0, 1, 2, 3, and 4 were recorded in 68%, 23%, 7.3%, 0.9%, and 0.2% patients, respectively. On multivariate analysis, acute RTOG ≥2 rectal toxicity was associated with urinary retention (p = 0.036) and learning curve (p = 0.015); late RTOG ≥2 was associated with the presence of acute toxicity (p = 0.0074), higher VR100 (p = 0.030) and learning curve (p = 0.027). Conclusions Late rectal RTOG ≥2 rectal toxicity in this cohort was 8%. Increased VR100, presence of acute rectal toxicity, and learning curve were associated with higher rate of late RTOG ≥2 toxicity. Severe late rectal toxicity after prostate brachytherapy was rare.