Optimization of treatment parameters for post mastectomy electron therapy of defected chest-wall

Abstract

Background: Electrons have an advantage of steep dose fall-off after R80. The unique curved geometry of the chest-wall and the special isodose shapes of electrons make complexities to chest-wall electron therapy. Bolus material has been applied to electron fields for shaping the coverage of planned volume and compensating for surface irregularities. Electron bolus conformal therapy methods are not yet vastly available; therefore, this study tries to extrapolate suggestions for conventional usage of bolus and different electron beam energies. Methods: Computer plans for 10 post-mastectomy patients with at least 1cm elevated or depressed surgical defects were designed. Bolus thicknesses of 5, 10, and 15 mm and electron energies of 9, 12, 15, and 18 MeV were applied to the clinical target volume. Average doses to the surface, clinical target volume (CTV1), ipsilateral lung and heart were calculated by the planning system and then analyzed. Findings: High energies of electrons and thicker bolus reduced the risk of CTV1 underdose. Bolus increases surface dose whereas decreases mean dose to CTV. Nine MeV electrons and bolus was unsuitable for this type of treatment. In most treatments the dose increase to lung was statistically significant (P < 0.01) with higher energies of electrons and lower thicknesses of bolus. This was not observed for the heart. Conclusion: Careful design of bolus thickness is important when planning with lower energy electrons. This study suggests that planning with thicker bolus material and higher energy selection has advantage of dose conformity with conventional methods of electron therapy in a defected surface. © 2016, Isfahan University of Medical Sciences(IUMS). All rights reserved.