1143 - Rethinking Breast Irradiation Using Volumetric Modulated Arc Therapy with the Integration of Static Angle Ports and Dynamic Collimator

E. Hubley¹, B. Archibald-Heeren², T. Koivumäki³, B. Baron⁴, A. Billaudeau⁵, E. Costa⁵, A. S. Dirand⁵, L. El Moudden⁵, A. H. Josse⁴, A. Kärnä³, B. Koger¹, T. Nykanen³, D. Peurien⁵, V. Raatikainen³, A. Veret⁴, and J. Vu-Bezin⁵; ¹Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, ²Icon Cancer Centres, Sydney, NSW, Australia, ³Deparment of Medical Physics, Hospital Nova of Central Finland, Wellbeing Services County of Central Finland, Jyväskylä, Finland, ⁴Service de Physique Médicale, Département de Radiothérapie, Institut Curie, Saint-Cloud, France, ⁵Service de Physique Médicale, Département de Radiothérapie, Institut Curie, Paris, France

Purpose/Objective(s): Hybrid radiotherapy applied to breast cancer has shown to have a high potential to improve plan quality over static intensity modulated radiation therapy or volumetric modulated arc therapy (VMAT) alone. The unsuitability of treatment planning system (TPS) tools complicates the implementation of this technique and leads to cumbersome planning procedures. A new TPS tool (VMATp) that allows users to add integrated static angle ports and dynamically rotate the collimator during a VMAT arc, was tested. The MLC sequence for the arcs and ports and dynamic collimator were optimized concurrently. Materials/

Methods: Four institutions evaluated the quality of plans generated with VMATp compared to conventional VMAT for 24 left breast patients. Each institution selected at least 4 left breast/chest wall patients from their treated patients and replanned the treatment using VMATp. The dose prescription was 40.05 Gy in 15 fractions with minimum coverage of 38.05 Gy to 95% of all targets. All patients had deep inspiration breath hold CT-scan. Fourteen patients included nodal target volumes. The number of arcs, ports and their angles were adapted to each patient’s anatomy to balance target coverage and organs at risk (OAR) sparing. Static port angles were selected to replicate tangent angles to maximize benefit of irradiating the breast alone. Plan quality was assessed by 10 common OAR clinical goals as agreed by the four participating clinics prior to the study. Statistical significance between VMATp and VMAT plan quality was determined using a paired-sample nonparametric t-test (p=0.05).

Results: VMATp provided statistically significant improvements in OAR sparing over VMAT for 8/10 clinical goals. The volume of the 16 Gy isodose (V16Gy) in the heart and the left lung did not significantly change with the VMATp geometries. VMATp resulted in decreased mean heart dose by 22%, from 2.3±0.9 Gy to 1.8±0.6 Gy (p < 0.001). The right lung V5Gy decreased by 66% from 5.5±7.1 to 1.9±3.0% (p < 0.001) and the right lung mean decreased by 38% from 1.6±0.9 to 1.0±0.6 Gy (p < 0.001). The right breast mean dose decreased by 37% from 2.6±0.9 to 1.6±0.6 Gy and the V4Gy in the left lung by 16% from 50.1±8.3 to 43.0±8.4% (p < 0.001). The dose to 95% of the target volumes did not significantly change between the VMAT and VMATp except for the internal mammary chain (IMC) for which the dose was reduced by 1% (p=0.003). The IMC coverage was still clinically acceptable. When differentiating patients with and without nodes, all the above results remain significant. Moreover, the V16Gy in the left lung increased by 9% (p=0.026) for patients with nodes and decreased by 20% (p=0.002) for patient without nodes.

Conclusion: In this study, we show that for breast irradiation, VMATp improves OAR sparing while assuring target coverage.