2705 - Dosimetric Comparison of Simultaneous Integrated Boost Using Field-in-Field IMRT vs. Hybrid VMAT vs. Ring Gantry VMAT for Left-Sided Breast Cancers

A. M. M. Faheem, G. Babu, K. R. Rajeev, Z. Gopalakrishnan, M. Sajeev, P. S. George, and B. S. Mathew; Regional Cancer Centre, Trivandrum, India

Purpose/Objective(s): This study aimed to compare the target coverage and organs-at-risk (OAR) sparing achieved with Field–in-Field IMRT (FiF), C-arm linear acclelerator hybrid-VMAT (hVMAT) and ring gantry-VMAT (rVMAT) for simultaneous integrated boost (SIB) in patients with left-sided breast cancer. The study was approved by the Institutional Review Board. Materials/

Methods: Fifteen patients with left-sided invasive breast cancers who received whole breast radiotherapy and photon tumor bed boost were included. The whole breast clinical target volume (wCTV) was contoured based on the ESTRO consensus guidelines version 1.1. The CTV boost volume for SIB (bCTV) encompassed the surgical clips/seroma. Plans for whole breast radiotherapy (40 Gy) + SIB of 8 Gy (0.53 Gy/day) over 15 fractions were generated using FiF, hVMAT and rVMAT for each patient. The target coverage parameters and doses to OAR were compared. The Anova test was used to determine the differences between the mean values of the three categorical groups and the post hoc Bonferroni test was used to compare two groups. A p value of 0.05 was considered significant.
Results: The mean bPTV dose, V95%bPTV and V98%bPTV were similar for all three techniques; rVMAT plans yielded superior conformity index (CI) compared to both hVMAT (0.8 vs 0.4; p<0.001) and FiF (0.8 vs 0.2; p<0.001). The CI of hVMAT was better than that of FiF (p<0.001). The homogenenity index (HI) for rVMAT, hVMAT and FiF were 0.06, 0.05 and 0.07 respectively and this was significantly better for rVMAT vs hVMAT ( p= 0.03) and FiF vs hVMAT (p=0.002). The wPTV coverage with regard to volume receiving 95% of prescribed dose was significantly better for rVMAT compared to FiF (98% vs 97%; p=0.03) but not hVMAT. The mean heart dose and V5Gy heart were not statistically different across the techniques; V30Gy heart was comparable for rVMAT and hVMAT but significantly worse for FiF (p<0.001). Ipsilateral lung V20Gy was significantly better for rVMAT compared to hVMAT (9.4%vs 14.5%; p=0.04) and FiF (9.4%vs 16.4%; p=0.003); hVMAT and FiF were comparable. rVMAT plans yielded the worst V5Gy of ipsilateral lung (p<0.001), contralateral lung (p<0.001) and combined lung mean dose (p<0.03); hVMAT and FiF were comparable. The contralateral breast mean dose was significantly higher for rVMAT (p<0.001); hVMAT and FiF were comparable. There were no significant differences with regard to volume of normal tissue in the high dose (98%) or low dose (2%) zone.
Conclusion: Ring gantry VMAT provided superior target conformity and homogeneity as well as heart and ipsilateral lung sparing. Nevertheless the significant low doses received by whole lung and contralateral breast are a concern. Hybrid-VMAT plans appear to provide almost similar PTV coverage-albeit with lesser CI and HI - and acceptable OAR sparing