SU‐E‐T‐581: Correction Factors for Secondary Monitor Unit Calculation in Stereotactic Lung Radiotherapy

Purpose: Secondary MU calculation is an important verification step of treatment planning. Use of simple secondary MU program is limited in thoracic SBRT due to inaccurate modeling of the dose in low density medium ‐ lung. The goal of this study was to quantify these inaccuracies and develop correction factors to improve the accuracy of the secondary MU calculation. Methods: We estimated the correction factors to account for the inaccurate modeling of the scatter by the Diamond (K&S)MU verification program by comparing the MU generated by the program to the pinnacle TPS using simulated water phantom with lung heterogeneities. The lung and tumor densities were assigned bulk relative densities of 0.33 and 1.0. Field size and depths of lung and tissue traversed were varied to establish a clear trend. Five patients were selected whose treatment MU calculations deviated by more than 5% and the developed correction factors were applied. Results: Comparison of MU in phantom indicated that the errors due to lack of re‐buildup ranged from −.5% to −4.0% for 4 cm × 4 cm and −3.3% to −4% for 10 cm × 10 cm. Inaccuracies due to lack of proper forward scatter modeling increased with field sized −.5% to −3.0% and in general increased with increasing distance of lung traversed. Application of these correction factors to the five patients' verification results reduced the discrepancy from an average of 8.1% to 6.3%. Conclusion: The most significant correction factors were due to inaccurate modeling due to (lack of sufficient) re‐ buildup and forward scatter. Correction factors can improve the usefulness of secondary MU program.