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

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