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Chest Wall Dosimetry Using Non-Coplanar Volumetric Modulated Arc Radiotherapy for Lung Stereotactic Ablative Radiotherapy

S Yu

Amy S Yu*, P Maxim , M Gensheimer , Stanford University, Stanford, CA


SU-I-GPD-T-585 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The risk of chestwall toxicity and rib fracture of lung stereotactic ablative radiotherapy (SABR) is increased for cases in which 30 cc or more of the chestwall received 30 Gy (V30). The purpose of this study is to evaluate the chestwall dosimetry and the plan quality of coplanar vs non-coplanar volumetric modulated arc radiotherapy (VMAT) for SABR in which the tumors are located adjacent to the chestwall.

Methods: Five lung cancer SABR VMAT plans in which the chestwall received 30 Gy exceeded 30 cc or more were re-planned with non-coplanar VMAT with two opposite 15-degree couch kicks. Treatment plans using the same energy were generated with two partial arcs to avoid direct beam entrance through the contralateral lung. Ninety five percent of the target was covered with the prescription dose. Dosimetric parameters of chestwall (V40 and V30), lung (V20, V10, V5 and mean dose), integral dose, conformity index ([Volume of the target that is covered by the 100% isodose line]2/target volume/100% isodose volume), the gradient index (50% isodose volume/100% isodose volume) and monitor unit (MU) for each plan were used to evaluate the plan quality.

Results: The volumes of chestwall receiving 30 and 40 Gy were reduced on average by 29% and 29%, respectively. The differences in lung mean dose were less than 0.2 Gy. There was no significant different for combined lung V20, V10, and V5. On average, integral dose decreased 10% and number of MU increased 29% for non-coplanar plans. The conformity indexes were 0.89±0.03 and 0.91± 0.02 (mean±SD), and gradient indexes were 3.79±0.51 and 3.72±0.39 for the planar and non-coplanar plans, respectively.

Conclusion: We have demonstrated that non-coplanar VMAT plans with reasonable couch kick to avoid potential collision can reduce the V30 and V40 of chestwall without compromising the lung dose and improve the plan quality.

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