GammaTile® Brachytherapy as Spatially Fractionated Radiation Therapy for Postoperative Brain Cancer: A Paradigm for Precision Intracranial Radiotherapy.

Purpose: Tile-based radiation therapy (TBRT) delivers adjuvant radiation to brain tumor beds using evenly spaced cesium-131 (Cs-131) sources in collagen tiles. This study evaluated TBRT as spatially fractionated radiation therapy (SFRT) across tumor sizes and source counts. Methods and Materials: Dose distributions were generated from postoperative CT scans for 18 patients treated with TBRT, using 10-32 implanted Cs-131 sources per case (median, 20). The resection cavity (RC) was contoured on CT with registered postoperative T1-weighted MRI guidance. CTV_5mm was defined as the RC plus a 5 mm expansion excluding skull and surgical tract, and CTV_5mm_Eval as CTV_5mm minus the RC. Both volumes were analyzed using SFRT dose metrics from the Radiosurgery Society SFRT/Flash working group white paper, and planar high-dose core number density (HCND_A) was used to characterize TBRT spatial fractionation.

Results

The mean dose indices of D5 (dose covering 5% of the target volume), D10, D50, D90, D95 and D100 were 175.8 ± 43.3, 148.8 ± 35.1, 86.7 ± 18.5, 56.1 ± 10.5, 50.2 ± 9.0 and 33.4 ± 7.2 Gy for the CTV_5mm_Eval, respectively. The heterogeneity indices, represented by the ratio of D5/D95 and D10/D90, were 3.5 ± 0.6 and 2.7 ± 0.4, respectively, which were in the range of the linac-based Lattice therapy. HCND_A was a constant, TBRT delivered one high dose core for every 1 cm2 of tumor bed. All dose metrics in both the CTV_5mm and CTV_5mm_Eval were largely consistent across all plans.

Conclusions

TBRT produced heterogeneous dose distributions comparable to conventional SFRT. A 60 Gy TBRT nominal prescription dose approximated the SFRT D90, or valley dose of the 5 mm expansion target volume, suggesting TBRT may provide an SFRT platform for tile-accessible cancers.