Purpose: To correlate site and diameter of brain metastases with hippocampal sparing in patients treated by RapidArc (RA) technique on whole brain with simultaneously integrated boost (SIB).
Methods and materials: An RA plan was calculated for brain metastases of 1-2-3 cm of diameter. The whole brain dose was 32.25 Gy (15 fractions), and SIB doses to brain metastases were 63 Gy (2 and 3 cm) or 70.8 Gy (1 cm). Plans were optimized and evaluated for conformity, target coverage, prescription isodose to target volume, homogeneity index, and hippocampal sparing.
Results: Fifteen brain lesions and RA plan were generated. Hippocampal volume was 4.09 cm³, and hippocampal avoidance volume was 17.50 cm³. Related to site of metastases, the mean hippocampal dose was 9.68 Gy² for occipital lobe, 10.56 Gy² for frontal lobe, 10.56 Gy² for parietal lobe, 10.94 Gy² for deep brain structures, and 40.44 Gy² for temporal lobe. The mean hippocampal dose was 9.45 Gy², 10.15 Gy², and 11.70 Gy² for diameter's metastases of 1.2 and 3 cm, respectively, excluding results relative to temporal brain lesions.
Conclusions: Location more than size of metastases can adversely influence the hippocampus sparing. Further investigation is necessary to meet definitive considerations.