Quality assurance testing of complex radiotherapy treatments using Gafchromic EBT2 film

Full text: This study examined the clinical suitability of Gafchromic EBT2 film for radiotherapy quality assurance, for small, complex and dynamic treatments, in order to qualify the utility of EBT2 film dosimetry in situations that are challenging for other dosimetry techniques. Doses delivered to solid water during a stereotactic radiosurgery treatment (consisting of eight very small static fields) and RapidArc treatments were determined using EBT2 film. The doses measured in the small fields used in the stereotactic treatment were compared with Monte Carlo dose calculations, Uncertainties arising from the intra-film uniformity, inter-film reproducibility, development time, film orientation and water-equivalence of EBT2 film were also examined. For films placed orthogonal to the beam direction, the measured dose uncertainty can be reduced to less than 3% (one standard deviation). When comparing films irradiated with the very small stereotactic fields (down to 0.6 cm across) with Monte Carlo data, agreement within 2% was achieved. However, when the same films were compared with the treatment planning system's predictions, they differed by up to 5%. This discrepancy was attributed to the inherent uncertainty of the small-field ionisation chamber measurements on which the treatment planning calculations were based. (author)