Room: ePoster Forums
Purpose: Early lung SBRT dose-escalation trials utilized homogenous dose distributions and prescribed dose to isocenter. Currently, lung SBRT treatment is prescribed to 70-80% isodose line to the planning target volume (PTV) margin, providing heterogeneous dose distributions with tissue-heterogeneity corrections. While neglecting spatial dose distribution, calculated mean biological effective dose (BEDmean) equals the nominal BED, potentially underestimating biological effectiveness to the tumor and organs-at-risk (OAR). Our aim is to incorporate spatial dose distribution to calculate variable BED.
Methods: Annuli were generated inside and adjacent to the tumor, representing the spatial dose distribution. For n treatments and Î±/Î² = 10 Gy, cascade-BED is defined: BEDmean=Dmean[1.0 + Dmean/n(Î±/Î²)]Ï†i. Where, Dmean=mean dose of the annulus and dose shaper function (Ï†i)=[hotspots/prescribed dose] for the target annulus, such as Ï†i(target)=[D120%/D95%,â€¦,â€¦,D95%/D95% = 1.0]; Ï†i>1.0 for heterogeneous and Ï†i=1.0 for homogenous dose distributions. For intermediate dose-spillage, Ï†i(lung)=[D94%/D95%,â€¦,â€¦,D18.5%/D95%] was calculated with Î±/Î²=3 Gy. We analyzed 10 lung SBRT patients treated to 54 Gy in 3 fractions (prescribed to PTV margin) using highly-conformal non-coplanar VMAT plans via 6X-FFF and Acuros algorithm. Average tumor volume was 27.1 Â± 16.7 cc.
Results: Neglecting spatial dose distribution, nominal BED10 (151 Gy) can significantly underestimate the radiobiological effective dose to the GTV (240Gy, corresponding to <5.0 cc) and ITV (220Gy, corresponding to <12.0cc) by a factor of 1.6 and 1.5, on average, respectively. For normal lung (D94-D80 parameters) average BED3 values were higher by almost two times compared to normal lung, D50. However, normal lung volume receiving higher BED3 was minimal: 0.1cc (D94, 325 Gy), 2.5cc (D90, 305 Gy) and 14.3cc (D80, 260 Gy), on average.
Conclusion: Increased BED due to dose shaper, Ï†i could trigger bystander effects and potentially increase hypoxic tumor cell kill. However, mobile tumors adjacent to OAR need extra precision when generating tumor margin and delivering supra-heterogeneous dose distribution.