dimensional film dosimetry with GAFCHROMIC films for quality assurance and dosimetric verification of 3D conformal radiotherapy in the presence of heterogeneities

Introduction: The presence of heterogeneities such as air-filled cavities in the head and neck treatment fields region, may result in potential dosimetric disagreement because the losses of charged particle equilibrium. Most of treatments planning systems are not able to predict dose distribution of inhomogeneities region accurately. Therefore, dose calculation algorithms need to model electron transport accurately. GAFchromic films are used in routine radiotherapy quality assurance procedure to perform comprehensive dosimetry. The development and using 3D dosimetry may be useful. Materials and Methods: several pieces of GAFchromic films were embedded in an in-house manufactured rectangular inhomogeneous head and neck phantom. A 6 MV photon beam from a Siemens Primus linear accelerator, equipped with the standard multileaf collimator (MLC) was assessed. Different approximately small field sizes were examined. A homemade computer code was developed for reconstruction of 3D dose map of irradiated Gafchromic. The measured data by film were compared to the similar data calculated by TPS in dose- volume histogram (DVHs). Results: The data calculated by TPS compared with the measure data based on 3- dimensional film dosimetry. Our results showed significant differences between the results of the film dosimetry and TPS. Maximum difference of calculated and measured values observed for volume enclosed in 95% isodose curves and 3x3cm2 field size (P=0.035). For this field size the dose calculated by TPS, after the air cavity differed from the film measurements on average by 42%. The disagreement between measured and calculated data was increased with decreasing of field size. Conclusion: our results demonstrated this TPS based on full scatter convolution algorithm doesn’t have acceptable performance in heterogeneous regions and approximately small fields and overestimated dose at some areas like air cavity or interface of air/tissue. Also our results confirmed that utilization of some accurate and comprehensive method such as 3D film dosimetry can be useful beside other methods of QA methods.