Y. A. Walter, C. J. Wang, D. Speir, B. Broekhoven, T. Jacobs, A. Hubbard, J. P. Dugas, and H. T. Wu; Willis Knighton Cancer Center, Shreveport, LA
Purpose/Objective(s): High fractional doses and steep gradients used in stereotactic body radiation therapy (SBRT) impose a need for effective motion management strategies. Robust immobilization systems and delivery techniques such as dose repainting and intrafraction repositioning, paired with a standard 5.0 mm planning target volume (PTV) margin have been used to guarantee adequate target coverage. The purpose of this study is to assess the dosimetric and clinical robustness of our current protocols by measuring the effect of intrafraction patient motion on plan dosimetry and local control. Materials/
Methods: Data were collected for 75 patients treated with lung SBRT between 2019 and 2023. Prescription doses were 50-60 Gy to 95-99% iPTV volume in 5 fractions. Treatments used volumetric modulated arc therapy (VMAT) with 4-D cone beam CT-guided repositioning between symmetric repaintings. Intrafraction repositioning shifts were collected for 373 fractions. Simulated dose perturbations were calculated for twelve cases, for which total 3D repositioning shifts averaged 3.0 mm or more (3.0-5.6 mm). Total dose perturbations were calculated by applying shifts to half of each respective fraction and to entire fractions to simulate executed repositioning between repaintings and worst-case scenarios, respectively. 98%-100% internal gross tumor volume (iGTV) coverage doses and volume of lung receiving =5 Gy (V5, lung-iGTV) and =13.5 Gy (V13.5, lung-iGTV) were calculated. Local control data for 28 patients with total 3D repositioning shifts averaging =3.5 mm (3.5-8.2 mm) were collected. The median follow-up interval was 13 months (4-40 months). Results: 36 patients (48%) had average 3D repositioning shifts =3.5 mm. Ten (13%) had total shifts >4.0 mm, and four (5%) had average shifts =5.0 mm. Worst-case perturbed doses for average shifts =3.5 mm resulted in a mean 7.30% ± 5.14% decrease in iGTV minimum dose (range +0.16% to -13.03%). There was no significant change in lung V5 and V13.5. In one case, the worst-case perturbation resulted in 100% iGTV coverage below prescription dose. However, this case had the largest average and single-fraction displacements of 5.6 mm and 8.6 mm, respectively, and was the only case which used a 3.0 mm PTV margin. This was also the only case for which local control was not achieved. The overall local control rate was 96% (1 failure) for cases with average 3D repositioning shifts =3.5 mm. Conclusion: Results show that current clinical protocols are robust against positional uncertainties. All but one case maintained 100% iGTV coverage above the prescribed dose, indicating that our standard 5.0 mm iPTV margin is adequate for maintaining iGTV coverage. Furthermore, our >95% local control rate for cases requiring large average repositioning shifts indicates excellent clinical outcomes even in worst-case scenarios, which may corroborate safe reduction of standard planning margins.
