Simultaneously integrated boost to multiple brain metastases during whole brain radiation therapy-hippocampal avoidance

Purpose: To evaluate the feasibility of delivering a radiosurgery-quality integrated boost to visible brain metastases combined with hippocampal sparing whole brain radiotherapy using helical tomotherapy. Materials and Methods: Ten patients were planned on helical tomotherapy using MR-CT fusion-defined target and normal structure contours. Individually contoured hippocampi were used as conformal avoidance structures and limited to a mean-NTD2 of 6 Gy, where NTD₂ denotes the normalized total dose delivered in 2 Gy fractions assuming an α/β-ratio of 2 Gy. A whole brain dose of 32.25 Gy and simultaneous boost doses to brain metastases of either 63.0 Gy (> 2.0 cm) or 70.8 Gy (< 2.0 cm) to be delivered in 15 fractions were prescribed. Plans were generated for a 2.5- and 1.0-cm field width (FW). To assess plan quality, the prescription isodose to target volume (PITV) ratio, target coverage, conformation number, and homogeneity index were computed. Results: Comparing treatment plans using a 1.0-cm FW to those using a 2.5-cm FW, the mean PITV ratio improved from 1.75 ± 0.61 to 1.48 ± 0.37, the whole brain homogeneity improved by 32% (p < 0.0004), and the mean dose in 2.0-Gy fraction equivalents received by the hippocampi varied from 5.8 ± 1.9 to 5.9 ± 1.3 Gy₂. Conclusions: Delivery of radiosurgery-quality dose distributions to multiple brain metastases using helical tomotherapy is possible even when combined with whole brain radiation therapy and conformal avoidance of the hippocampi.