Room: Exhibit Hall | Forum 7
Purpose: For high dose fraction, standard linear-quadratic (LQ) model results in a curve that bends continuallyâ€“overestimating the biological effective dose (BED) and subsequently the local-control rates. However, experimentally measured dose-response curve is a nearly straight line on a log-linear plot. SBRS of solitary lung tumor with a single dose of 30Gy is gaining popularity in our clinic. However, using standard-LQ model results in 120 Gy BED10 and subsequently 98% local-control rate regardless of tumor sizeâ€“which is unrealistic. Aim of this report was to adopt LQ-L20Gy biophysical model in conjunction with tumor size-adjusted BED (s-BED) to meaningfully predict local-control rates in lung SBRS.
Methods: Ten early-stage NSCLC patients underwent SBRS of lung tumor with a single dose of 30Gy. Average tumor diameter was 2.7Â±0.8cm (ranged:2.0-4.2cm) derived from ITV on 4D-CT contouring. Patients were treated at Truebeam with VMAT using 6MV-FFF(1400MU/min) beam and AcurosXB-based algorithm for PTVD95=100%. Bipartite LQ-L20Gy model was applied with Î±/Î²=10Gy and transition dose (DT)=2Ã—Î±/Î² to straighten the curve. For single dose of d>DT, BED10(LQ-L20Gy)=[DT]+[(DTÃ—DT)/(Î±/Î²)]+[(Î³/Î±)Ã—(d-DT)];with (Î³/Î±)=1.0+[2Ã—DT/(Î±/Î²)].For dâ‰¤DT; the LQ-L20Gy converges to standard-LQ model. After that, s-BED=BED10-10Ã—L was defined for tumor diameter, L(centimeter)â€“considering that BED decreases approximately linearly with increasing tumor diameter. More accurate s-BED was calculated using PTV(D99). Local-control rates were calculated for both LQ and LQ-L20Gy models utilizing TCP=(EXP[s-BED-TCD50]/k)Ã·(1.0+EXP[s-BED-TCD50]/k), where parameters TCD50 and k defined the shape of the curve.
Results: For single dose of 30Gy, LQ(s-BED=90Gy) and LQ-L20Gy(s-BED=87Gy) models predicted 95% chance of 2-year actuarial local-control rate; compared to standard-LQ model that over predicted (98% TCP with 120Gy BED10) irrespective of tumor size. With s-BED, LQ model agreed with LQ-L20Gy in predicting TCP, on average within 2%.
Conclusion: Improved bipartite LQ-L20Gy model using s-BED could potentially better predict local-control rates for a single fraction lung SBRS. Clinical follow-up outcomes are warranted to further validate this biophysical model.