Room: AAPM ePoster Library
Purpose: with MV electrons is a popular technique for superficial lesions in radiation therapy. Usage of small sized electron cutouts compromises the lateral charge particle equilibrium and mandates cutout factor (CF) measurement for accurate dosimetry. Traditionally, cutout factor is measured as the ratio of charge collected at dmax with and without the cutout in water or water equivalent phantoms. It requires phantom setup with ion chamber, rotation of the collimator to align the largest opening of the cutout, dose delivery and calculations. In this study, we introduce an efficient alternative technique for cutout factor calculation.
Methods: Monte Carlo (eMC) algorithm in Eclipse 15.5 was used to calculate the ratio of dose with and without cutout in a water phantom. We measured CFs for five different cutouts including two irregular and three circular cutouts with 2.2 cm, 3.1 cm and 4 cm diameter at two different SSDs (100 cm and 110 cm) to validate our results. For the MC study, circular blocks were added as predefined shapes in Eclipse. For irregular cutouts, we used a transparent sheet and freehand tool to reproduce the cutouts shape in Eclipse.
Results: good agreement was observed between measured and calculated values. For 6 MeV electrons, the difference (mean ± std) between measured and calculated CF values were 2.6% ± 2.1% and 2.8% ± 2.0% at 100 cm and 110 cm SSD respectively. The corresponding values for 15 MeV electrons were 1.6% ± 1.0% and 1.8% ± 1.2%.
Conclusion: an alternative for CF measurement, we have introduced an efficient calculation method using eMC model in Eclipse. The small differences between measured and calculated CF values could be a result of imperfect cutout shape transfer into TPS and uncertainties associated with measurement process.