Dosimetric Verification Using Monte Carlo Calculations for Tissue Heterogeneity-corrected Conformal Treatment Plans for Lung Cancer Stereotactic Body Radiotherapy

The recently activated RTOG studies on stereotactic body radiation therapy (SBRT) of non-small cell lung cancer (NSCLC) require tissue density heterogeneity correction (e.g., RTOG protocol 0813). The high and intermediate dose compliance criteria for each protocol were established based on superposition-algorithm dose calculations. The aim of this study was to compare superposition-algorithm dose calculations with Monte Carlo (MC) dose calculations for SBRT NSCLC treatment and to evaluate whether compliance criteria need to be adjusted for MC dose calculations. Fifteen RTOG 0236 DICOM-RT study sets with planning target volumes (PTV) and normal structures were used. The PTV volumes range from 10.7 to 117.1 cm3. SBRT conformal treatment plans were generated using CMS XiO treatment planning software with superposition algorithm to meet the dosimetric high and intermediate compliance criteria recommended by the RTOG 0813 protocol. The plans were recalculated initially using identical beam angles and weights using the MC algorithm of a CMS Monaco treatment planning system. Tissue density heterogeneity correction based on Hounsfield units from each DICOM-RT study set was applied in both calculations. Comparisons between the superposition and MC calculations were performed by evaluating the high and intermediate dose criteria: ratio of prescription isodose volume to the PTV, ratio of 50% isodose volume to the PTV, maximum dose 2 cm from PTV in any direction, and percent of lung receiving 20 Gy or more. The MC recalculated plans that did not meet the dosimetric criteria were re-optimized and the final results were evaluated. In the plans generated with the superposition calculations, 12 cases met all the dosimetric criteria, 2 cases did not meet both the criteria for R50% and D2cm, and 1 case did not meet the criteria for R50%. Overall, the dosimetric quantities of the MC calculations have larger magnitudes than those of the superposition calculations. On average, R100%, R50%, D2cm, and V20 increased by 9% (minimum, -8%; maximum, 29%), 12% (minimum, 0; maximum, 28%), 7% (minimum, 3%; maximum, 13%), and 18% (minimum, 11%; maximum, 29%), respectively. In the MC recalculated plans, 8 cases did not meet the criteria for R100%, R50%, or D2cm. After re-optimization with MC calculations, 3 cases did not meet both the criteria for R50% and D2cm, and 2 cases did not meet the criteria for R50%. The results for our particular MC dose calculation method indicate that the dosimetric criteria, e.g., the criteria for R50%, recommended by RTOG 0813 protocol, may need to be adjusted when MC dose calculation algorithm is employed.