Room: Track 3
Purpose: Differences in calculated dose due to heterogeneity corrections are potentially meaningful in interstitial lung brachytherapy. We examine these differences and compare delivered plans to external-beam SBRT.
Methods: 10 image-guided brachytherapy ablation (IGBA) cases planned using the TG-43 dose calculation algorithm were re-calculated using the Elekta Advanced Collapse cone Engine (ACE). Median 24 Gy (18-26 Gy) in a single fraction was delivered. Differences in target coverage were quantified using CTV D90%, V100%, V150%, and mean. Differences in normal lung dose were assessed using D1cc, D2cc, and V7Gy. SBRT plans for 54 Gy in 3 fractions were generated using the brachytherapy CTV with a 6mm margin, and calculated using the Varian AcurosXB. SBRT target coverage was compared to IGBA. To account for the different fractionations, lung V20 Gy in equivalent dose in 2 Gy fractions (EQD2) was computed.
Results: The calculated CTV D90% was on average 7.2% greater when using the ACE algorithm versus the TG-43 formalism. Similarly, the mean differences in V100%, V150%, and average CTV dose were 0.8%, 3.2%, and 7.9% higher, respectively. D1cc, D2cc, and V7Gy of the normal lung were 8.8%, 8.6%, and 9.5 cc higher on average using ACE. Compared to the SBRT plans, IGBA cases calculated with ACE had a 20.5% greater D90% on average. Mean V100% was 1.9% less, while mean CTV dose was 206.3% greater. The EQD2 V20Gy for the IGBA plans was lower in all cases, with a mean of 64.6 cc compared to 247.4 cc for SBRT.
Conclusion: Incorporating tissue heterogeneity corrections results in potentially meaningful differences to the predicted target coverage and normal lung dose. The clinical impact of these differences warrants further study. Interstitial lung IGBA offers similar prescription target coverage to lung SBRT, and much greater mean dose in addition to improved sparing of the normal lung.