Development of a Modelling to Correlate Site and Diameter of Brain Metastases with Hippocampal Sparing Using Volumetric Modulated Arc Therapy

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.