Distribution of brain metastases: implications for non-uniform dose prescriptions

Objectives: The aim of this study was to determine the disease-specific distribution of brain metastases and, using radiobiological modelling, estimate how these anatomical tendencies might be exploited when delivering prophylactic whole-brain radiotherapy for small cell lung cancer in complete remission. Methods: Disease-specific brain metastasis atlases were created by mapping brain metastases to a standard image set from a database of patients who were to receive external beam radiation therapy. The specific diseases investigated included lung (both small cell and non-small cell), breast, renal and gynaecological cancers as well as melanoma. Radiobiological modelling was used to estimate how much improvement, in terms of the metastasis-free rate at 3 years, might be possible with non-uniform dose distributions if there are spatial biases in the incidence of micrometastases from small cell lung cancer. Results: For lung and breast cancer, there was an increased probability of cerebellar metastases compared with what would be predicted based solely on brain volume. This trend was not evident for renal cancer, gynaecological malignancies or melanoma. Conclusion: Radiobiological models suggest that if there is a non-uniform distribution of microscopic brain metastases in patients with small cell lung cancer, higher population-based metastasis-free rates might be achievable with non-uniform irradiation compared with the same integral whole-brain dose delivered as a uniform prescription.