Integrating The Boost: Development Of A Hybrid Volumetric Modulated Arc Based Simultaneous Integrated Boost Class Solution For Breast Cancer

Purpose: We describe the development of a simultaneous integrated boost (SIB) class solution employing a hybrid Volumetric Modulated Arc Therapy (VMAT) for adjuvant treatment of breast cancer where conformal avoidance of normal organs from low dose is prioritized. Methods: A field in field tangent pair was used to irradiate the whole breast while the VMAT SIB was used to deliver a boost to the tumor bed with a margin of 5 mm. Hybrid VMAT SIB was planned in 25 patients using two short arcs of 30 degrees each offset by 5–10 degrees from the medial and lateral tangent angles respectively. SIB dose of 8 Gy was prescribed in addition to the whole breast dose of 40 Gy in 15 fractions. Plans were compared against a standard sequential mini-tangent boost plan (SEQ) delivering a dose of 12 Gy in 4 fractions using the Wilcoxon Signed-Rank test. Mean probability of developing grade 2 or more fibrosis was estimated using the NTCP formula provided by Hammer et al. Results: VMAT SIB significantly improved conformity as compared to the SEQ plan (0.51 vs 0.35, p < 0.01). Target volume coverage was similar between the two techniques. The mean dose to the contralateral breast, contralateral lung, and heart was increased by 0.13 Gy, 0.02 Gy, and 0.03 Gy respectively which was clinically insignificant. The mean probability of developing Grade 2 fibrosis at 5 years was similar between the two techniques (2.5% vs 2.7% p = 0.19). The class solution achieved consistent plans across a wide range of sites and target volume ranges (35.38–247.10 cm 3 ). The average total MU requirement was significantly reduced (593 vs 848.6, p < 0.01). Conclusion: A class solution for VMAT SIB was developed which satisfied the planning objectives. This solution will be used for a prospective randomized trial investigating a one-week hypofractionated schedule of radiotherapy.