Impact of magnetic resonance imaging on computed tomography-based treatment planning and acute toxicity for prostate cancer patients treated with intensity modulated radiation therapy

Purpose: To evaluate rectal and bladder dosimetric and clinical acute toxicity endpoints for prostate patients treated with intensity modulated radiation therapy (IMRT) using magnetic resonance images (MRI) registered to computed tomographic (CT) simulation images versus CT alone. Materials and Methods: The charts of 155 consecutive prostate cancer patients at our institution from 2004 to 2008 were reviewed. A cohort of 15 IMRT treatment plans was created to compare dosimetric endpoints for CT-MRI vs CT alone. A subsequent clinical comparison involved 81 patients (CT-MRI [n = 28] vs CT alone [n = 53]). Acute genitourinary and rectal toxicity rates for the CT-MRI and CT cohorts were compared; also, univariate and multivariate regression analyses were performed using all major demographic, disease, and treatment factors as covariates. Results: Contoured prostate volumes were 43.0 vs 55.7 cm³ (P < .001, n = 15) for CT-MRI vs CT definition, with significant reductions in all bladder dose endpoints and rectal V20, V30, and V70. Grades 0, 1, and 2 toxicity rates for CT-MRI (n = 28) vs CT (n = 53) were, respectively, 25%, 25%, and 50% vs 8%, 21%, and 72% (acute genitourinary [GU], P = .024) and 39%, 29%, and 32% vs 32%, 28%, and 40% (acute rectal, P = .495). On univariate regression, only MRI use and International Prostate Symptom Scores reached significance for acute GU toxicity. On multivariate regression, age, prostate volume, and MRI use reached statistical significance for acute GU toxicity. No factor reached significance for rectal toxicity. Conclusions: This study demonstrates a statistically significant reduction in clinical acute GU toxicity with the clinical implementation of MRI in the treatment planning process.