SU‐E‐T‐386: Does the Couch Need to Be Included for Accurate IMRT Planning?

Purpose: To evaluate the effect of using couch structures in IMRT planningMethods: Three TG‐119 IMRT plans (HeadN four of the ten prostate cases included pelvic lymph node involvement. The film analysis (using in‐house software) required >95% points to pass a gamma criterion of 3%,3mm. Results: For the TG‐119 plans 99.1%±2.7% and 94.6%±19.9% of the points met the gamma 3%,3mm with and without the couch model respectively; the major contribution for the lower value (94.6%) was from the H&N plan for which only 85% of the points passed. Gamma results for clinical cases were: H&N (99.7%±0.9% and 97.4±7.9%), Prostate 6MV: (99.6%±1.8% and 94.7%±12.7%), Prostate 18MV results with and without the couch were nearly identical (99.8%±0.9% vs. 99.7%±1.1%, respectively). We also found the largest differences for posterior oblique beams traversing the couch rails: For 6MV prostate, there was no relative difference for beams striking the phantom only, 0.7% when treating entering through the couch, and 4.5% for beams traversing the rails. Conclusion: Including the couch structures is important for lower energies (6MV), especially for oblique beams traversing the couch rails. However, adjusting rail positions during delivery should improve agreement for algorithms which do not account for the couch.