Short tangential arcs in VMAT based breast and chest wall radiotherapy lead to conformity of the breast dose with lesser cardiac and lung doses: a prospective study of breast conservation and mastectomy patients.
Volumetric modulated arc therapy (VMAT) is modern rotational intensity modulated therapy used for treatment of several sites. The study aimed to analyze partial tangential arc VMAT treatment planning and delivery, including analyzing the cardiac and contralateral breast doses resulting from this technique. A total of 153 consecutively treated breast cancer (conservation as well as mastectomy) patients were taken for this dosimetric study. All patients were planned using partial arc VMAT in the Monaco treatment planning system using two partial arc beams. All patients were divided into seven different categories: (1) all the patients in the study, (2) left sided whole breast and chest wall patients, (3) left Chest wall patients, (4) left whole breast patients, (5) right sided whole breast and chest wall patients, (6) right chest wall patients, and (7) right whole breast patients. We evaluated each treatment plan for PTV coverage and doses to OARs. SPSS version 16.0 software was used for statistical analysis. There were 91 left sided and 62 right sided breast cancer patients in the overall analysis. The percentage of PTV volume receiving 95% of the prescription dose (PTV V95%, mean ± SD) varied in the range of 91.2 ± 5.2-94.8 ± 2.1% with mean dose of 92.4 ± 5.2% for all cases. The (mean ± SD) cardiac dose for all the patients was 289 ± 23 cGy. The (mean ± SD) cardiac doses were higher for left sided patients (424 ± 33.8 cGy) as compared to right sided patients (123.9 ± 80 cGy) (p < 0.001). Cardiac mean doses were higher with arc angles >30° versus 30° (324.5 ± 247.1 vs. 234.4 ± 188.4 cGy) (p = 0.001). Similarly contralateral breast mean dose was higher with arc angles >30° versus 30° (126 ± 115 vs. 88.6 ± 76.1 cGy) (p = 0.001). However cardiac V20, V30 and V40 Gy did not exhibit any statistical difference between the two groups (p = 0.26, 0.057 and 0.054 respectively). This is the first large study of its kind that assesses the dosimetric outcome of tangential partial arc VMAT treatments in a large group of mastectomy and breast conservation patients. Our study demonstrates the efficacy of this technique in dose coverage of PTV as well as in minimizing dose to OARs. Further, based on our results, we conclude that the arc length for the bi-tangential arcs should be 30° since it helps to achieve the most optimal balance between target coverage and acceptable OAR doses.