SU-E-T-691: Comparison of 5-Channel Versus Single and 9-Channel Vaginal Cylinder Treatment Plans.

PURPOSE To compare the quality of 5-channel vaginal cylinder treatment plans for endometrial cancer to single channel and 9-channel plans. Using more channels tends to spare the rectum and bladder, while creating hot spots and longer planning and treatment times. The objective is to determine if the 5-channel plans offer a beneficial compromise to the single and 9-channel plans. METHODS Fifteen treatment plans from 3 patients were evaluated using the Oncentra MasterPlan (Nucletron inc.) treatment planning system. Each of the 15 plans was previously treated with 9 channels from a 35mm diameter multi-channel cylinder, using Oncentra's IPSA inverse planning tool. The prescribed dose was 500cGy × 5 fractions to the 5mm vaginal cuff rind. A 5-channel plan was created by removing the 2 channels closest to the bladder and the 2 closest to the rectum from the 9-channel plan, then proportionately adjusting the weights in inverse planning for the bladder, rectum, and D150, while keeping D95 within 0.1% (D95 is typically around 95%). The single channel plan was normalized to a 5mm distance from the cylinder wall. RESULTS When compared to single channel, the 5-channel plans had an average of 3.5% and 6.7% lower doses to the bladder and rectum, respectively. The V200 was lower in 7/15 plans (-0.2% average), and V150 was higher than single channel on 13/15 plans (+5.7% average). When compared to 9-channel, the 5-channel plans had lower dose to the bladder on 9/15 plans (-0.3% average), and a lower V150 on 9/15 plans (-0.6% average). The 5-channel rectum dose was higher on all plans (+4.4% average), and the V200 was higher on 8/15 plans (0.3% average). CONCLUSION The 5-channel vaginal cylinder plans offer a reasonable compromise between single channel and 9-channel plans.