engleski

Out-of-field dose measurements in radiotherapy - activity of EURADOS WG 9: Radiation protection in medicine

Introduction: The motivation of the EURADOS Working Group 9: Radiation protection in medicine was to assess un-due, non-target patient doses in radiother-apy and the related risks of secondary malignancy. Improvements in cancer treatment have increased survival times and thus increased incidence of second cancer may be expected in the future. In addition, increased whole body exposure may result from developments in radiotherapy. This means that radiotherapy clinics will need to simulate their treatments in order to estimate and minimize doses to healthy tissues and organs. The proposed work of this WG is designed to generate a robust dataset of out of-field dose measurements which can be used for the development and validation of dose algorithms. In this part of the work results of photon doses will be present. Experimental: The first objective was to check and select a set of three kinds of passive photon dosimeters TLD, RPL and OSL. Dosimeters were calibrated in a ref-erence clinical linear beam in a water tank at a reference facility at the CEA LIST/LNE LNHB, Saclay. Radiation qual-ities of 6, 12 and 20 MV were used. Profile and depth dose ion chamber measure-ments were also made to provide reference values. In the second part, the measurements were extended to BOMAB-like phantoms (approximately simulating the human body). In the experiments the following prostate treatments were simulated: Volumetric Modulated Arc Therapy (VMAT, 6 MV), Tomotherapy (6 MV), IMRT (6 MV and 18 MV), 5-field conformal RT (15 MV) and 4-field con-formal RT (6 MV and 18 MV). Irradiations were performed in two cen-tres, University Hospital of Santa Chiara, Pisa, Italy and Centre of Oncology M. Skłodowska-Curie Memorial Institute, Krakow, Poland. The experiments include a comparison of doses (beam profiles) in different positions in the BOMAB phan-tom, a comparison of different treatment modalities in two clinical centres and a comparison of dose profiles resulting from the different treatment techniques. Results and Discussion: Comparison of out-of-field doses for different modalities in the two clinical centres shows that dif-ferences in out-of-field doses for the same Planning Treatment Volume (PTV) can be even a factor of 4. Examples of dose profiles are given in Fig 1. a) for 6 MV IMRT (Pisa) and b) for 6 MV VMAT (Pisa). It is clear that the shape of the TP curves corresponds to the results of pe-ripheral doses measured with dosimeters (TLD, IFJ, Krakow and RPL, RBI, Za-greb), but generally dosimeters show larger doses then the TPS as distance from isocentre increases. This fact reveals im-portance of further investigations of out-of -field dose measurements in phantoms.

radiotherapy; out-of-field doses; solid state dosimeters

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