SU-E-T-709: Dosimetric Study On Volumetric Modulated Arc Therapy for Left Breast Using Four Partial Arc Technique

Abstract

Purpose: To evaluate the dosimetric characteristics of Volumetric Modulated Arc Therapy (RapidArc) plan for the treatment of carcinoma of the left breast using four partial arcs. ' Methods: RapidArc (RA) plans with four partial arcs were created for five Lt.breast patients for a dose regimen of 50.4Gy in 28 fractions. Computed tomographic images of slice thickness 3 mm were acquired and used for planning. The planning was done in Eclipse Planning system, platform 8.6. Among the four arcs used, the gantry angles for first two arcs were 310 ‐ 130 degrees (CW and CCW) and 310 ‐ 179.9 degrees (CW and CCW) for the other two arcs. The CTV was expanded I cm beyond body artificially and assigned soft tissue HU to incorporate skin flash. Results: The mean homogeneity index for the CTV was found to be 1.18 with a standard deviation of +/− 3.02%. The mean conformity index for CTV was 0.8955 with a standard deviation of +/− 4.6%. The average maximum dose and average mean dose to the heart was 4107.3cGy and 1568.9cGy respectively. The average V20, V25 and V30 for Lt.lung were 22.8%, 8.4% and 4.77% respectively. The average maximum spinal cord dose was 7.61Gy with a standard deviation of +/−6.3%. Conclusion: It is hard to achieve high conformality and homogeneity for breast treatment using Tangential IMRT fields. But, the RA plan showed superior target coverage with good sparing of heart and Lt. Lung. This four arcs technique nicely curves the 70% isodose line away from the lung while delivering homogeneous dose to the CTV. The analysis performed in this article were physical based, an elaborate biological based evaluation studies have been initiated.