An Investigation of GRID and Spatially Fractionated Radiation Therapy: Dosimetry and Preclinical Trial

Purpose: To develop and implement novel methods of extreme spatially fractionated radiation therapy (including GRID therapy) and subsequent evaluation in pre-clinical mice trials investigating the potential of novel radiation treatments with potential for promoting anti-cancer immunogenic response.

Methods: Spatially fractionated GRIDs were designed and precision-milled from 3mm thick lead sheets compatible with mounting on a 225 kVp small animal irradiator (X-Rad). Three pencil-beam GRIDs created arrays of 1mm diameter beams, and three “bar” GRIDs created 1x20mm rectangular fields. GRIDs projected 20x20mm fields at isocenter and beamlets were spaced at 1, 1.25, and 1.5mm, respectively. Output factors, peak-to-valley ratios, and dose distributions were determined with Gafchromic film. The bar GRID with 1mm beamlet spacing (50:50 open:closed ratio) was selected for the pre-clinical trial. Soft-tissue sarcoma (p53/MCA) was transplanted into C57BL/6 mice’s flanks. Four treatment arms were compared: unirradiated control (n=18), conventional radiation therapy (n=16), GRID therapy (n=17), and hemi-irradiation (n=17) where one-half of the beam was blocked. All irradiated mice received a single fraction of 15 Gy to irradiated regions. To date, this is the first study to compare GRID treatment against conventional RT at the same dose.

Results: Very high peak-to-valley ratios were achieved (bar GRIDs: 11.9±0.9, 13.6±0.4, 13.8±0.5; pencil-beam GRIDs: 18.7±0.6, 26.3±1.5, 31.0±3.3). Pencil-beam GRIDs spared twice the number of intra-tumor immune cells as bar GRIDs but left more of the tumor untreated (2-3% vs 14-17% area receiving 95% prescription, respectively). Penumbra was halved when GRIDs were 50% closer to treatment isocenter. The GRID selected for mouse trials was capable of sparing approximately 15% of intra-tumor CD8+ and CD4+ T cells. Preliminary results indicate mean times to tumor quintupling were: 12, 13, 14, and 20 days for unirradiated, GRID, hemi-irradiated, and conventional treatment groups, respectively. To date, all tumors have quintupled except for nine in the AP control group.

Conclusions: Peak-to-valley ratios with kV grids were substantially superior to MV grids, which historically achieve ratios between 2.5 and 6.5. In data collected to date, GRID and hemi-irradiation did not significantly delay tumor growth as compared to an unirradiated control (P = 0.122 and P = 0.2437, respectively, P-values from logrank analysis). Differences between GRID and hemi-irradiation were not statistically significant (P = 0.5257). To date, the AP control group has performed significantly better than all other groups (P<0.001). These results do not corroborate the success of hemi-irradiation in Markovsky et al. 2019. GRID treatments may be more effective if a substantially higher dose and/or multiple fractions were employed.