The use of dose volume constraints to achieve rectal isotoxicity: A phase II prostate cancer radiation therapy dose escalation study

Abstract

4628 Background: The primary limiting factor for prostate cancer irradiation is the risk of rectal toxicity. Rectal toxicity is associated with the dose and volume of the rectum irradiated. This dose escalation study has the unique feature of using dose volume histogram (DVH) constraints to determine the prescribed dose. Methods: From 1999–2002, 331 patients with clinical stage T1-T3N0M0 prostate cancer were prospectively treated with 3D conformal adaptive RT. A patient-specific confidence-limited planning target volume was constructed based on multiple CT scans. For each case, the rectum (rectal solid) was contoured in its entirety. The rectal wall was defined using a 3-mm wall thickness (median volume: 29.8 cc). The prescribed dose level was chosen using the following rectal wall dose constraints: 1) <30% of the rectal wall volume can receive >75.6 Gy. 2) <5% of the rectal wall can receive >82 Gy. The risk of chronic toxicity (NCI Common Toxicity Criteria 2.0) was assessed for the different dose levels prescribed. Median follow-up was 1.6 years. Results: 34 patients (10%) experienced grade 2 chronic rectal toxicity (9% rectal bleeding, 6% proctitis, 3% diarrhea, 1% rectal pain) at a median interval of 1.1 years. Nine patients (3%) experienced grade ≥3 chronic rectal toxicity (1 grade 4) at a median interval of 1.2 years (table). The 3-year rates of grade ≥2 and grade ≥3 chronic rectal toxicity were 20% and 4%, respectively. There was no significant difference by dose level in the 2-year rate of grade ≥2 chronic rectal toxicity. These rates were 27%, 15%, 14%, 17% and 24% for the ≤72, 73.8, 75.6, 77.4, and 79.2 Gy dose level groups respectively (p = 0.27). Conclusions: High dose levels (79.2 Gy) can be safely delivered if selected DVH constraints are met. The prescribed dose should be based on DVH information (toxicity risk) and the benefit of a specific dose level. No significant financial relationships to disclose.