Evaluation of Full scatter convolution algorithm based Treatment Planning System performance in the presence of inhomogeneities using three-dimensional film dosimetry

Introduction: Inclusion of inhomogeneities such as air-filled cavities in the head and neck treatment fields may result in potential dosimetric disagreement which was caused by electronic disequilibrium. Most of treatments planning systems (TPS) are not able to predict dose distribution of inhomogeneous regions accurately. EBT2 films are used frequently in radiotherapy quality assurance testing to evaluate the performance of dose delivery systems and TPS. The development and usage of 3D film dosimetry method may be useful. Materials and Methods: 9 pieces of EBT2 films were embedded in a rectangular inhomogeneous head and neck phantom. Films were exposed with 6 MV X-ray photons with parallel-opposed fields and three approximate small field sizes were examined. A homemade computer code was developed in MATLAB for creation of 3D dose map of irradiated films and calculation of enclosed volumes which were surrounded by isodose lines of films. Calculated dose volumes should be compared with Dose Volume Histogram (DVH) which was derived from TPS. Results: our results showed significant differences between the results of the film dosimetry and TPS. Maximum difference of calculated and measured values was observed in volumes surrounded by 95% isodose curves and 3x3cm2 field size (P=0.035) and minimum difference was observed in volumes surrounded by 95% isodose curves and 5x5cm2 field size (P=0.047). Conclusion: The results of this study demonstrated overestimated results in volumetric calculation of TRGRT dose calculation algorithm in the presence of inhomogeneities and approxi small fields sizes. Also, these results confirmed that utilization of an accurate and comprehen method such as 3D film dosimetry can be useful for TPS verification.