Feasibility of Left-sided Chest Wall Irradiation with Volumetric Modulated Arc Therapy

Abstract

To evaluate the merits and demerits of a Volumetric Modulated Arc Therapy (RapidArc) over a Forward Intensity Modulated Radiotherapy (F-IMRT) plan in the treatment of post mastectomy carcinoma of the left (LT) breast. In this study we compared RapidArc (RA) plan and F-IMRT plan for carcinoma of the LT Breast in ten patients who underwent mastectomy. PTV included chest wall (CW) and supraclavicular fossa (SCF). RA plans consisted of two partial arcs (310 deg-150 deg clock wise/ counter clock wise). F-IMRT was planned with a single isocenter using standard tangential beams for CW and direct anterior beam for SCF with additional field-in-field technique. The prescription dose to PTV was 50Gy/25 fractions at 98% coverage. Dose volume histograms were analyzed for the two modalities. Conformity index (CI) & Homogeneity index (HI) were compared between the two plans for each patient. For Organs at risk (OAR), 45Gy (V45), 20Gy (V20) & 5Gy (V5) volumes were compared for LT Lung, Heart and Normal tissue (Body minus PTV). Low dose volume (5Gy) was compared between the plans for RT Lung & RT Breast. The results were analyzed using paired-t-test and considered to be significant if p was < 0.05. The mean CI for RA plan was estimated to be 1.15 ±0.09 compared to F-IMRT plan of 2.14 ± 0.38 (p < 0.001). The mean HI obtained was better with RA plans (0.12 ± 0.05) in comparison with F-IMRT plans (0.14 ± 0.02) (p < 0.001). The mean percentage volume of LT Lung receiving doses of 45 Gy, 20 Gy, and 5 Gy in RA plans were 1.3 ± 0.25, 22.59 ± 3.92 and 74.15 ± 8.52 whereas in F-IMRT plan they were 13.15 ± 3.37, 30.2 ± 5.09 & 46.42 ± 5.56 respectively (p < 0.001). The mean percentage volume of Heart receiving 45Gy, 20Gy & 5Gy in RA plans was found to be 0.1 ± 0.07, 8.3 ± 6.11 and 71.32 ± 17.17 in comparison with F-IMRT plan of 6.23 ± 4.36, 12.22 ± 5.92 and 18.47 ± 7.970 respectively. The statistical difference for Heart V45 and V20 was p < 0.01 and for V5 was p < 0.001 between RA and F-IMRT plans. The maximum dose to RT Breast was found to be less in RA plan compared to F-IMRT plan, but the low dose volume (V5) showed 9% increase in RA plans. The same observation was found for RT Lung with 10% increase in 5Gy volume when compared to F-IMRT (p < 0.001). For normal tissue mean percentage volume receiving dose 45 Gy, 20 Gy, and 5 Gy in RA plans were 3.2 ± 0.89, 9.31 ± 2.6, and 24.02 ± 6.42 but for F-IMRT plans dose were 6.49 ± 1.16, 10.42 ± 2.28, and 14.47 ± 1.87, respectively (p < 0.001). RapidArc plans produced good conformity and homogeneity index with excellent dose coverage. RapidArc reduced the OAR volume receiving high doses (V45 and V20). F-IMRT scored in low dose volume (V5) which was more in RA plans due to multiple beams entries. RA plans provided reduced heart dose and improved lung sparing, at the cost of increased low dose volume in contra lateral breast & lung as compared to F-IMRT.