Dosimetric Parameters Associated With Pathologic Response in Patients Undergoing Preoperative Spatially Fractionated Radiotherapy (GRID) for Soft Tissue Sarcoma.

Purpose/objective(s) Spatially fractionated radiotherapy, or GRID radiotherapy, is a technique utilizing multiple beamlets of high dose radiotherapy in a heterogeneous grid pattern to treat tumor non-uniformly with megavoltage X-rays while maintaining normal tissue tolerance. We sought to characterize associations between GRID dosimetric parameters and pathologic response in patients undergoing preoperative GRID radiotherapy for high grade soft tissue sarcoma. Materials/methods We analyzed 26 patients with high grade soft tissue sarcomas undergoing GRID radiotherapy. Dosimetric parameters included mean dose to GTV, maximum dose to 0.01 cc, maximum dose to 0.03 cc, D5, D10, D20, D50, D90, D100, V5, V10, V20, V50, V90, V100, total area under the curve, maximum tumor length, maximum tumor width, tumor volume, equivalent uniform dose to GTV, minimum surface to tumor distance, maximum surface to tumor distance, maximum tumor depth, maximum dose outside GTV, maximum dose to skin, number of beamlets, peak to valley ratio, and peak to peak distance. Results The dosimetric parameters most closely associated with pathologic response were maximum dose to 0.01 cc and maximum dose to 0.03 cc (Spearman coefficient of 0.685, P-value of 0.01). Mean dose to GTV, V90, and V100 showed weak associations that weaker associations that were not significant (Spearman coefficient of 0.442, 0.510, and 0.504 with P-values of 0.08, 0.09, 0.09). Conclusion This study showed an association between the dosimetric parameters maximum dose to 0.01 cc and maximum dose to 0.03 cc and pathologic response in patients undergoing preoperative GRID radiotherapy for high grade soft tissue sarcoma.