What Are the Optimal SFRT Geometric Parameters? Examining through an In-Vitro Study
Abstract
Purpose
We investigated optimal geometric parameters for spatially fractionated radiotherapy (SFRT) through in-vitro experiments.
Methods
Two cancer cell lines (SCC-2 and SCC-6) were used, and irradiation was performed with 10 SFRT patterns delivered on a TrueBeam LINAC using 6 MV beams and Millennium-120 MLC. This consisted of high-dose diameters of 0.5, 1.0, 1.5, and 2.0 cm and a center-to-center (CTC) spacing of 1.0, 1.5, 2.0, and 2.5 cm. These cell lines were uniformly irradiated at 2, 4, 6, and 8 Gy to derive α and β values. For each pattern, cells were irradiated to maximum doses of 5, 10, and 15 Gy. The Linear-Quadratic (LQ) model was fitted to each pattern to derive α and β values. Evaluation of the patterns was based on the surviving fraction (SF) at equivalent uniform dose (EUD) of 4, 6, and 8 Gy, corresponding to the clinically relevant EUD range for SFRT treatments.
Results
The SCC-2 was more radio-resistant (α = 0.0058 Gy-1, β = 0.0155 Gy-2) than SCC-6 (α = 0.0297 Gy-1, β = 0.0366 Gy-2). For the SCC-2 cell line, the most optimal pattern was a high-dose diameter of 0.5 cm and a CTC of 2.5 cm, with SF(4Gy) = 0.541, SF(6Gy) = 0.281, and SF(8Gy) = 0.117. For the SCC-6 cell line, the best SFRT pattern was a high-dose diameter of 0.5 cm and a CTC of 2.0 cm, with SF(4Gy) = 0.455, SF(6Gy) = 0.176, and SF(8Gy) = 0.047.
Conclusion
Clinically, SFRT treatments with a cylindrical dose distribution utilize a high-dose diameter of 1.0-2.0 cm and CTC of 2.0-2.5 cm. These results suggest that keeping CTC to 2.0-2.5 cm but decreasing the high-dose diameter to 0.5 cm may be clinically beneficial. However, decreasing the high-dose diameter and increasing CTC may pose a challenge in achieving an EUD of 4-8 Gy.