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Volume and Location Dependence On the Difference Between Monte Carlo and Pencil Beam Dose Calculations for Lung Stereotactic Body Radiation Therapy

T Zhuang*, T Djemil, P Qi, A Magnelli, K Stephans, G Videtic, P Xia, The Cleveland Clinic Foundation, Cleveland, OH

SU-E-T-486 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Stereotactic body radiotherapy has been an efficacious treatment modality for early stage non-small cell lung cancer. The accuracy of dose calculations is in question due to the presence of inhomogeneity. It was required in several clinical trials to calculate dose without heterogeneity correction. However, to better correlate the outcomes with the planned dose, accurate dose calculation with heterogeneity correction is highly desirable.

Methods: We compared the recalculated dose with Monte Carlo (MC) algorithm to the original Pencil Beam (PB) calculations for clinical lung SBRT plans. Thirty-one clinical plans that followed protocol guidelines were retrospectively investigated. Dosimetric parameters D1, D95 and D99 for the PTV and D1 for organs at risk were compared. Correlations of mean lung dose and V20 of lungs between two calculations were investigated.

Results: Compared to the PB calculations without heterogeneity correction in clinical plans, we found that in terms of D95 of PTV, (1) the two calculations resulted in similar D95 for edge tumors with volumes greater than 25.1cc; (2) an average overestimation of 5% in PB calculations for edge tumors with volumes less than 25.1cc; and (3) an average overestimation of 9% or underestimation of 3% in PB calculations for island tumors with volumes smaller or greater than 22.6 cc, respectively. With heterogeneity correction, the PB calculation resulted in an average reduction of 23.8% and 15.3% in D95 for island and edge lesions respectively compared to the MC calculation. For organs at risks, no clinical meaningful differences were found among all the comparisons. Excellent correlations for mean dose and V20 of lungs were observed between the two calculations.

Conclusions: Using a single scaling factor to account for the differences in using heterogeneity correction may not be sufficient. To understand dose-response relation in Lung SBRT, accurate dose calculation such as the Monte Carlo algorithms is highly recommended.

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