Out-of-field contributions for IMRT and volumetric modulated arc therapy measured using gafchromic films and compared to calculations using a superposition/convolution based treatment planning system

Dose outside of the treatment plane diminished to 1% of the prescribed dose, this for open fields, IMRT and rotational treatments alike. Noteworthy was an increase at about 20 cm from the central plane in IMRT, and in a more limited fashion for volumetric modulated arc treatment. In open fields this was not observed. Treatment planning systems were good at determining the out-of-field doses of single field treatments. In complex plans the TPS underestimated the dose to the patient. At distances greater than 20 cm from the field edge, these systems did not predict any dose. The Peridose program performed well in the case of classical treatments. In the case of IMRT treatments, the overall evolution of the dose as a function of the distance to the field was well-modeled. However, an over estimation of the order of 60-80% was observed, leaving the possibility for a corrective factor based on a point measurement. Dose levels over the whole body were of the order 100 mGy or higher over a complete treatment for the more complex treatments. Neutron dose levels were of the order single digit mSv for 10 MV treatments. For 18 MV the level of neutron contribution was in agreement with recent publications, corroborating reports that the dose from neutrons is lower than previously reported.