Comparison of Measured and Calculated Out-of-Field Doses in a Paediatric Anthropomorphic Phantom / Out of the Body Scatter Contribution Evidence

Abstract

Abstract. A pediatric anthropomorphic phantom was irradiated using a standard 3DCRT treatment plan with and without including the couch in the Treatment Planning System (TPS; Varian Eclipse 13.5, AAA algorithm) calculations. The 2D scatter dose distributions at several planes away from the isocenter were measured using a Gafchromic EBT3 radiochromic film, and compared with the calculated ones at several distances from the isocenter. The out-of-field dose distributions (from TPS and EBT3) differ considerably, particularly where the cranio-caudal beam has left the body but is still near the surface of the phantom and near the couch interception. Further investigation was conducted to estimate the impact of the beam angle on the dose distribution. A 5x5 cm 6MV photon beam was used to irradiate a fiber carbon slab (couch top material) at different angles of incidence and the scattered radiation was measured with an ionization chamber placed 20 cm away from the isocenter. The results showed a 5% increase in scattered radiation between the angles of 0 0 and 85 0 . This is an indication that scatter from the couch must be taken into account when estimating out-of-field dose distributions and that the TPS, even when the couch is included in the calculation, is not sufficiently accurate to predict these doses.