SU‐FF‐T‐637: Investigation of Dose Accuracy in XiO Superposition Convolution Dose Calculation for Linacs with Jaws and MLCs

Purpose: To investigate the dose delivery accuracy using Linacs with MLCs as lower‐jaws planned with the CMS‐XiO superposition convolution dose calculation algorithms. Method and Materials: Siemens Primus accelerators with multi‐leaf collimators as lower jaws and Varian Trilogy accelerators with tertiary collimators were used in this study. Delivered dose were measured with a calibrated 0.125 cc ion chamber in a 30×30×15 cm³ solid water phantom for a series of specially‐designed T‐shaped MLC fields with different field‐aperture‐opening ratios (FAOR) and several clinically used 3DCRT MLC‐blocked fields were compared with dose calculations using the XiO superposition convolution method, Clarkson and hand calculations with combined or separate collimator scatter and phantom scatter factors. Several block‐equivalent‐square (BES) values, including XiO convolution back‐projected‐BES, Clarkson BES, geometrically‐estimated BES, were utilized in the hand‐calculations to compare as independent checks. Monte Carlo simulations were performed to verify the dose calculation and to study the sources of scattering contribution. Results: Up to 8.0% discrepancies were found between XiO superposition convolution calculation and measurements for the Siemens accelerator while good agreement (within 1.0%) was obtained for the Varian accelerator for various FAOR. An average 1.8% difference between Siemens and Varian Linacs was noted with the FAOR >1/3, possibly resulted from the “imaginary” lower jaws in the XiO convolution algorithm for Siemens. Monte Carlo simulations confirmed these results. Hand calculations using a combined collimator scatter and phantom scatter factor with a XiO back‐projected‐BES can estimate the delivered dose accurately (0.12% of measured values) including extreme cases for Siemens (FAOR<1/3). Conclusions: A 1.8% correction should be applied to Linacs that use MLCs to replace a pair of jaws in the XiO superposition convolution algorithm when FAOR is >1/3. For FAOR <1/3, the XiO superposition convolution should not be used while hand calculations can be used by combined collimator scatter factors with XiO back‐projected‐BES.