Whole Brain Radiotherapy With Hippocampal Avoidance and Simultaneously Integrated Brain Metastases Boost: A Planning Study

Purpose: To evaluate the feasibility of using tomotherapy to deliver whole brain radiotherapy with hippocampal avoidance, hypothesized to reduce the risk of memory function decline, and simultaneously integrated boost to brain metastases to improve intracranial tumor control. Methods and Materials: Ten patients treated with radiosurgery and whole brain radiotherapy underwent repeat planning using tomotherapy with the original computed tomography scans and magnetic resonance imaging-computed tomography fusion-defined target and normal structure contours. The individually contoured hippocampus was used as a dose-limiting structure (<6 Gy); the whole brain dose was prescribed at 32.25 Gy to 95% in 15 fractions, and the simultaneous boost doses to individual brain metastases were 63 Gy to lesions ≥2.0 cm in the maximal diameter and 70.8 Gy to lesions <2.0 cm. The plans were generated with a field width (FW) of 2.5 cm and, in 5 patients, with a FW of 1.0 cm. The plans were compared regarding conformation number, prescription isodose/target volume ratio, target coverage, homogeneity index, and mean normalized total dose. Results: A 1.0-cm FW compared with a 2.5-cm FW significantly improved the dose distribution. The mean conformation number improved from 0.55 ± 0.16 to 0.60 ± 0.13. Whole brain homogeneity improved by 32% (p <0.001). The mean normalized total dose to the hippocampus was 5.9 ± 1.3 Gy₂ and 5.8 ± 1.9 Gy₂ for 2.5- and 1.0-cm FW, respectively. The mean treatment delivery time for the 2.5- and 1.0-cm FW plans was 10.2 ± 1.0 and 21.8 ± 1.8 min, respectively. Conclusion: Composite tomotherapy plans achieved three objectives: homogeneous whole brain dose distribution equivalent to conventional whole brain radiotherapy; conformal hippocampal avoidance; and radiosurgically equivalent dose distributions to individual metastases.