Dosimetric verification using monte carlo calculations for tissue heterogeneity-corrected conformal treatment plans following RTOG 0813 dosimetric criteria for lung cancer stereotactic body radiotherapy

Purpose: The recently activated Radiation Therapy Oncology Group (RTOG) studies of stereotactic body radiation therapy (SBRT) for non-small-cell lung cancer (NSCLC) require tissue density heterogeneity correction, where the high and intermediate dose compliance criteria were established based on superposition algorithm dose calculations. The study was aimed at comparing superposition algorithm dose calculations with Monte Carlo (MC) dose calculations for SBRT for NSCLC and to evaluate whether compliance criteria need to be adjusted for MC dose calculations.

Methods and materials: Fifteen RTOG 0236 study sets were used. The planning tumor volumes (PTV) ranged from 10.7 to 117.1 cm(3). SBRT conformal treatment plans were generated using XiO (CMS Inc.) treatment planning software with superposition algorithm to meet the dosimetric high and intermediate compliance criteria recommended by the RTOG 0813 protocol. Plans were recalculated using the MC algorithm of a Monaco (CMS, Inc.) treatment planning system. Tissue density heterogeneity correction was applied in both calculations.

Results: Overall, the dosimetric quantities of the MC calculations have larger magnitudes than those of the superposition calculations. On average, R(100%) (ratio of prescription isodose volume to PTV), R(50%) (ratio of 50% prescription isodose volume to PTV), D(2 cm) (maximal dose 2 cm from PTV in any direction as a percentage of prescription dose), and V(20) (percentage of lung receiving dose equal to or larger than 20 Gy) increased by 9%, 12%, 7%, and 18%, respectively. In the superposition plans, 3 cases did not meet criteria for R(50%) or D(2 cm). In the MC-recalculated plans, 8 cases did not meet criteria for R(100%), R(50%), or D(2 cm). After reoptimization with MC calculations, 5 cases did not meet the criteria for R(50%) or D(2 cm).

Conclusions: Results indicate that the dosimetric criteria, e.g., the criteria for R(50%) recommended by RTOG 0813 protocol, may need to be adjusted when the MC dose calculation algorithm is used.