PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): Isocenter-based prescription method in stereotactic body radiotherapy (SBRT) for early-stage non-small cell lung cancer (NSCLC) is the most widely used technique in Japan. Lately, however, it has been replaced by PTV-based prescription method to ensure PTV coverage. The aim of this study is to evaluate the impact of the biologically effective dose (BED) on the clinical outcomes by making a comparison between Isocenter- and PTV-based prescription methods.
Materials/Methods: We retrospectively reviewed patients with early-stage NSCLC (≤5 cm in diameter) who underwent SBRT at our institution from January 2010 to December 2016 and received follow-up of at least 6 months. A total dose of 48 Gy in four fractions was prescribed at the isocenter from 2010 to 2013 (Isocenter group), while that was prescribed to cover 95% of planning target volume (PTV D95) from 2014 to 2016 (PTV group). ITV was determined by using 4D-CT scan as the volume encompassing GTVs in all respiratory phases. PTV was generated from ITV with 5-8 mm margins in all directions. Local control (LC) was analyzed using Kaplan-Meier method and Cox proportional hazard model.
Results: A total of 152 patients and 154 tumors was included (Icocenter group: 92 patients and 93 tumors, PTV group: 60 patients and 61 tumors). The median calculated BEDs10 of the PTV D95 and the ITV D99 for the Isocenter group were 87.0 Gy (range, 57.4-97.2 Gy) and 91.4 Gy (76.7-101.4 Gy), respectively. For the PTV group, the corresponding doses were 105.6 Gy (105.6-105.6 Gy) and 149.1 Gy (114.8-157.6 Gy), respectively. Median follow-up periods for all tumors were 42 months (range, 6-90 months) in the Isocenter group and 23 months (6-41 months) in the PTV group. The 2-year LC rates were 84.5% in the Isocenter group and 95.8% in the PTV group (log-rank test, p = 0.105). In multivariate analysis adjusting for gender, histologic subtype, tumor size and BED, BED10 of PTV D95 (>105 Gy versus ≤105 Gy; HR, 3.10; 95% CI, 0.074-1.00; p = 0.049) and histologic subtype (SqCC versus Others; HR, 4.89; 95% CI, 1.91-12.43; p = 0.0013) were identified as independent predictive factors for LC. After stratification by tumor size, the PTV group (BED10 of PTV D95 >105 Gy) had better local control for tumors more than 2 cm in diameter compared with the Isocenter group (BED10 of PTV D95 ≤105 Gy) (2-year LC rate, 100% versus 75.9%; p = 0.030), while the difference was not significant for tumors 2 cm or less in diameter (2-year LC rate, 91.3% versus 92.8%; p = 0.87).
Conclusion: BED10 of PTV D95 >105 Gy is associated with improvement of local control for tumors more than 2 cm. PTV-based prescription method with 48 Gy in four fractions to cover PTV D95 showed excellent 2-year local control, although longer follow-up is needed to evaluate the long-term outcome.
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