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Introduction The most common dosimetry systems for verification of advanced radiation therapy technique delivery are 2D arrays of ionisation chambers/diodes. Their main drawback is relatively low resolution, especially when measuring highly modulated absorbed dose distributions containing numerous irregular fields. In contrast, radiochromic film has superior spatial resolution while retaining absorbed dose measurement accuracy. In order to exploit this advantage, an in-house radiochromic film dosimetry methodology was developed and implemented. Materials and methods The dosimetry system used consists of radiochromic films (GafChromic EBT 3), flatbed scanner (Epson Expression 10000XL) and film analysis software (radiochromic.com). The methodology was developed following the protocol by Mendez et al. Films from the same batch were used, minimising sensitivity fluctuations. Scans were acquired in reflection mode with fixed image acquisition parameters and resolution. Films were scanned prior and post irradiation correcting for scanner lateral response and film inhomogeneity. To achieve darkening saturation, irradiated films were scanned after a fixed 3-day interval. Film dose calibration curves were created for 0- 5Gy in 0.5Gy steps. Absorbed dose distributions were measured using films positioned at the isocentric plane of IBA MultiCube phantom and irradiated by the Siemens Oncor Expression 6MV photon beam. They were analysed in comparison to absorbed dose distributions calculated using the Elekta Monaco Monte Carlo based treatment planning system together with those acquired with IBA Matrixx 2D ionisation chamber array in equivalent geometry. The methodology accuracy was tested analysing absorbed dose calibration curves, noise, geometric accuracy, absorbed dose linearity. Additionally, Gamma index passing rates were assessed using 3%/3mm, 3%/2mm (reference) and 2%/2mm dose difference/distance to agreement criteria. Absorbed dose distributions of increasing complexity were analysed, from rectangular fields to single and multiple dose level head and neck (h&n) IMRT plans. Results In-house methodology results show that the mean absorbed dose error of radiochromic film measurements is under 2% and geometric accuracy under 1mm, making it suitable for advanced radiation therapy technique verification. Over 95% of points passed the 3%/3mm and 3%/2mm Gamma index criteria for all analysed absorbed dose distributions using either film or the 2D array. As expected, due to its higher resolution, when applying the stricter 2%/2mm criterium, film pass rates become lower than the ones obtained using the 2D array, falling with increasing plan complexity with largest discrepancies found at field edges and high dose gradients of multiple dose level h&n IMRT plans Conclusion The developed radiochromic film methodology is shown to be equivalent to 2D array methodology at reference level. With stricter criteria applied, better spatial resolution of film allows for identification of errors arising from imprecise multileaf collimator motion and positioning, leaf edge and penumbra modelling, particularly in high dose gradient areas. Such errors mean that in clinical cases, while the calculated h&n IMRT dose distribution may seem optimal, the one patient actually receiving may have large errors affecting tumour control probability and/or organ at risk sparing. These errors are not detectable using 2D array equipment, justifying the use of radiochromic film for advanced technique delivery verification. |
