Dosimetric impact and modeling of prone breast positioning device and couch structures.

In prone breast radiation, as the medial tangential beam usually passes through the immobilization board and couch, it is necessary to quantify the attenuation effect and the potential skin dose enhancement from these external structures. The prone breast board studied consists of an insert on which the contralateral breast rests and a board base indexed to the couch. Two different Varian couch systems were also studied. Transmission factors (TF) of the board were measured using a Farmer chamber at 4 cm depth. Couch TFs were measured using a thimble chamber centered in a cylindrical phantom. A custom support model was created in the treatment planning system (TPS). TFs were then computed in the TPS for comparison. Selected clinical plans were recomputed in the TPS incorporating external structures for target coverage evaluation. The correction for the attenuation effect in the TPS was also demonstrated. Skin dose effects were evaluated using a Markus parallel plate chamber with a 1 mm buildup cap. Measured insert TFs ranged 0.976 to 0.983 for 6 MV and 0.990 to 0.999 for 23 MV. Board base TFs ranged 0.979 to 0.985 for 6 MV and 0.989 to 0.998 for 23 MV. TPS values agreed within 0.9% and 0.5% for the insert and board base, respectively. Assigned Hounsfield units (HUs) providing the best agreement were 200, -100, and -900 for the insert, the board "base shell" and "base inside," respectively. Varian Exact Couch and Exact IGRT Couch TFs varied with respect to couch angle, with minimum values of 0.837 and 0.956, respectively, for 6 MV. The clinical treatment volume (CTV) and whole breast receiving 95% of the prescription dose (CTV-V95 and WB-V95) of selected patients demonstrated reduced coverage due to attenuation of external structures. Close proximity to the base increased skin dose by up to 25% to 30%. Contacting the insert increased skin dose by 65% to 93% for 6 MV and 117% to 157% for 23 MV, respectively. Results have shown reduced coverage by attenuating external structures. Proper modeling of immobilization devices and couch structures in the TPS should be implemented for accurate dose calculation. Increased surface doses were observed due to direct contact to the insert or close proximity to the base. Further study is required to quantify such a skin dose enhancement effect and its correlation to clinically apparent skin effects and toxicity.