An Innovative VMAT Technique for Left-sided Breast Cancer Patients With Postmastectomy Radiotherapy (PMRT) Evaluated by Ratio of Heart Volume in Tangent Line (RHVTL)

Abstract For patients with left-sided breast cancer (LBC), postmastectomy radiotherapy (PMRT) has been shown to improve the overall survival and many advanced planning techniques was adopted in PMRT. We aim to use an innovative VMAT technique to enhance the conformity of PTV and reduce the scattering dose of surrounding OARs, thereby reducing the long-term toxicity of the heart as well as ipsilateral lung (IL). The study further analyzes the more appropriate treatment planning techniques for personalized LBC patients with PMRT. 35 LBC patients were retrospectively selected undergoing PMRT. The PTV included lymph nodes, chest walls, excluding internal mammary nodes, where 95% of PTV receiving the prescription dose of 50Gy (2Gy/fraction) with three different techniques, VMAT, IMRT, Hybrid VMAT. Furthermore, the ratio of Heart Volume in Tangent line and heart volume (RHVTL) was proposed to evaluate the relative antonymy position between patient's heart and PTV, which hypothetically represents the complexity of treatment planning. The data from this study showed that for LBC patients undergoing PMRT, the CI from VMAT was 0.85 (IMRT and H-VMAT were 0.77 and 0.83), the heart D mean was 502.9cGy (IMRT and H-VMAT were 675.6cGy and 687cGy) and the V20 of IL was 21.3 as the lowest of the three techniques, but the dose of the contralateral breast (CB) and contralateral lung increased noticeably. In H-VMAT and IMRT, the mean heart dose was significantly related to RHVTL, with R-values of 0.911 and 0.892 respectively, while the values in VMAT was 0.613, thus the VMAT technique was relatively unaffected by the difficulty of treatment plan. For RHVTL values exceed than 0.06, the mean heart dose under VMAT technique raised by 98.7cGy compared to the RHVTL value of less than 0.06, but H-VMAT and IMRT increased by 233cGy and 261.58cGy individually. This study illustrates that separated fields and adjacent fields in VMAT technique obtained the optimal conformality and lowest doses of heart in three techniques for LBC with PMRT. Thus, based on the results of our preliminary study, the VMAT technique is highly recommended when RHVTL is exceeded 0.06.