Sci—Sat AM: Brachy — 11: Improving treatment planning for I‐125 lung brachytherapy using Monte Carlo methods

125I brachytherapy used in conjunction with sublobar resection to treat stage I non‐small cell lung cancer has been reported to improve disease‐free and overall survival rates compared with resection alone. Treatments are planned intra‐operatively using seed spacing nomograms or tables to achieve a prescription dose defined 5 mm above the implant plane. Dose distributions for patients treated with this technique at the Mayo Clinic Rochester were reanalyzed using a Monte Carlo (MC) calculation; significant differences were observed between the standard TG‐43 dose calculations and the actual dose delivered as determined by MC. This work investigates differences between TG‐43 calculated prescription doses and those calculated in more accurate models. Monte Carlo calculations are performed using the EGSnrc user‐code BrachyDose with a number of lung tissue phantom models including patient CT‐derived phantoms. Seed spacing nomograms using these models are recalculated by determining the dose to the prescription point using the activities per seed required to produce a prescription dose of 100 Gy with the TG‐43 point source formalism. Models using nominal density lung or CT‐derived density lung tissue result in a significant increase in dose to the prescription point (up to approximately 25%) compared to TG‐43 calculated doses. The differences observed suggest that patients routinely receive significantly higher doses than planned using TG‐43 derived nomograms. Additionally, deviation from TG‐43 increases as seed spacing increases. Media heterogeneities significantly affect dose distributions and prescription doses for 125I lung brachytherapy, underlining the importance of using model‐based dose calculation algorithms to plan and analyze these treatments.