Air Expander Implant Can Compromise Target Coverage and Dose Homogeneity of Post-Mastectomy Radiation Therapy

Abstract

AeroForm® (AirXpanders, Inc., Palo Alto, CA) is a carbon dioxide-based, patient-controlled tissue expander that enables patients to undergo more convenient and rapid tissue expansion compared to traditional saline expanders. It was approved by the FDA for use in post-mastectomy reconstruction at the end of 2016, and has been increasingly seen in women requiring post-mastectomy radiation therapy (PMRT). The presence of the AeroForm®, however, presents a dosimetric challenge in the delivery of PMRT given the presence of high density stainless steel within the expander. In this study we explore the ability of the treatment planning system (TPS) to predict dose in the presence of an Aeroform®, and confirm the accuracy of our TPS by performing in vivo dose measurements in a patient receiving PMRT. External beam radiation planning was performed in treatment planning software. A 3D field-in-field treatment plan using an electron-photon match medially to cover the internal mammary nodes was created and calculated using both Acuros XB (AXB) and anisotropic analytical algorithm (AAA). A material override for high density material was incorporated into the AXB algorithm to account for areas of saturation on our planning CT scan due to the expander. In vivo dose measurements were performed using optically stimulated luminescent dosimeters (OSLD, nanoDot™, Landauer) placed at designated points correlating to cold spots seen in the TPS on our patient. We observed significant areas of dose reduction within our target volume using the AXB algorithm in our treatment planning system (TPS). This was not apparent when using AAA. In vivo measurements confirmed our observed loss of target volume coverage lateral to the AeroForm® due to attenuation of the tangent photon beams by the implant device. Loss of target volume coverage medially was less pronounced as this area was covered separately by an electron field in our treatment plan. We observed a 6% difference (170 vs 159.5cGy, prescribed dose 200cGy) between AXB calculated dose with delivered dose. We found that increasing bolus from 0.5 to 1 cm only modestly improved coverage at the skin in our TPS, however did not overcome attenuation by the expander. AXB predicts dose more accurately than AAA in Aeroform® patients due to the presence of both high density stainless steel and air within the expander. The Aeroform® tissue expander, however, significantly attenuates incoming photon tangent beams. The significant cold spots witnessed both medially and laterally to the expander may be overcome with additional boost fields, however, are not resolved with the addition of bolus.