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A Multi-Isocenter IMRT Approach of Large Fields to Reduce the Hot Or Cold Spots Caused by Setup Error

B Liu

B Liu, S Zhou, S Song, Univ Southern California, Los Angeles, CA 90033

SU-E-T-613 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To reduce the hot and cold spots introduced by setup error of multi-isocenter IMRT for large fields.

Methods: A patient with entire peritoneal cavity as PTV for IMRT on Trilogy are studied, with prescription of 3000 cGy to the PTV and 3600 cGy to gross disease in 20 fractions. The PTV dimension on the Sup-Inf direction is 46 cm, thus two isocenters (one in the abdominal region and one in the pelvis) are used. Eight beams with gantry angles 320°, 280°, 240°, 200°, 160°, 120°, 80°, 40° are used for each isocenter, and the two groups of eight beams overlap for 13cm. We intentionally vary the inferior boarder of the abdominal fields and the superior boarder of the pelvis fields, so that there are 4 pairs of beams with different inferior boarders for the 8 abdominal fields and also 4 pairs of beams with different superior boarders for the 8 pelvis fields: the benefit of this beam setup is that only 2 out of the 8 beams will contribute to hot or cold spot when setup errors are introduced. Setup errors are simulated by shifting the isocenters in the Sup-Inf direction in the Pinnacle system

Results: Our method and IMRT without varying superior and inferior boarder give similar plans: V27s are 94.8% vs 95.1%; mean liver doses are 2720 cGy vs 2721 cGy; mean kidney doses are 1795 cGy vs 1774 cGy. However, for 0.3 cm inferior shift of the abdominal isocenter, hot spots are 3400cGy vs 3850 cGy; for 0.3 cm superior shift of the abdominal isocenter, the cold spot are 2700 cGy vs 2400 cGy.

Conclusion: Our IMRT approach for large field can significantly reduce the hot and cold spots introduced by setup error.

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