Potential for dose escalation in the postprostatectomy setting with intensity-modulated radiation therapy: a dosimetric study using EORTC consensus guidelines for target volume contours.

PURPOSE Radiation therapy (RT) is delivered as adjuvant and salvage therapy after radical prostatectomy for prostate cancer. Interest in dose escalation in this setting may necessitate more advanced RT techniques, such as intensity modulation. This study was designed to compare intensity-modulated radiation therapy (IMRT) versus 3-dimensional conformal radiation therapy (3DCRT) planning. METHODS Twenty patients were identified who received post--radical prostatectomy RT with 4-field, 3DCRT plans to 68.4 Gy. Contours were revised to comply with the European Organization for Research and Treatment of Cancer consensus guidelines. The IMRT plans with 5 versus 9 coplanar fields were compared for 10 patients. Then the 9-field IMRT plans were compared to 3DCRT in all 20 patients. Differences in dose-volume histogram values were evaluated using 2-tailed paired t tests. Cone beam computed tomographic images were analyzed to evaluate rectum doses in the treatment position during the RT course. The IMRT plans to 72.0 Gy were compared to 3DCRT to 68.4 Gy to be able to consider potential use in dose escalation. RESULTS The 9-field IMRT plans (vs 3DCRT) improved bladder mean dose and volume receiving 65 Gy or more (V65), as well as rectum mean dose (31.6 Gy vs 36.1 Gy; P < .001), volume receiving 75% or more of the prescription dose (24.4% vs 31.0%; P < .001), and V65 (10.5% vs 20.0%; P < .001). Advantages of IMRT were at the cost of small increases in maximum point doses delivered to the bladder and rectum. Cone beam computed tomographic images (n = 132) were analyzed for 8 patients; rectum mean dose and V65 were also improved by IMRT on these scans. IMRT allowed increasing dose to 72.0 Gy with similar bladder and rectum mean doses, V65, and V40 compared to 3DCRT to a total dose of 68.4 Gy. CONCLUSIONS The IMRT improves dosimetric parameters for the rectum and bladder, which may allow dose escalation after radical prostatectomy. Future studies should determine whether these advantages translate into improved clinical outcomes for prostate cancer patients.