A Monte Carlo approach to lung dose calculation in small fields used in intensity modulated radiation therapy and stereotactic body radiation therapy

Aims: In the current study, the effect of electronic disequilibrium on lung dose with small photon beams was verified. Materials and Methods: The central axis absorbed dose in lung phantom was calculated by Monte Carlo (MC) method. The 6 and 18 MV photon beams of Varian Clinac 2100EX were simulated using MCNPX MC Code (Los Alamos national lab, USA). The MC model was used to calculate the depth doses water and low density water resembling the soft-tissue and lung, respectively. Four small field sizes including 0.5 cm (2) x 0.5 cm (2) , 1 cm (2) x 1 cm (2) , 2 cm (2) x 2 cm (2) , and 3 cm (2) x 3 cm (2) were used in this study. Results: Percentage of dose reduction in lung region relative to homogenous phantom for 6 MV photon beam were 44.6%, 39%, 13%, and 7% for 0.5 cm (2) x 0.5 cm (2) , 1 cm (2) x 1 cm (2) , 2 cm (2) x 2 cm (2) , and 3 cm (2) x 3 cm (2) fields, respectively. For 18 MV photon beam, the results were found to be 82%, 69%, 46%, and 25.8% for the same field sizes, respectively. The solid tumor dose inside lung was reduced considerably between 17% and 35% for 18 MV beam, while there was only 9% dose reduction for tumor dose for 0.5 and 1 cm field sizes. Conclusion: Our study showed that the dose reduction with small fields in the lung was very enormous. Thus, inaccurate prediction of absorbed dose inside lung and also lung soft-tissue interfaces with small photon beams may lead to critical consequences for treatment outcome.