Whole-brain radiation therapy for brain metastases: detrimental or beneficial?

How many brain metastases can be treated with stereotactic radiosurgery before the radiation dose delivered to normal brain tissue rivals that associated with standard whole brain radiotherapy?

INTRODUCTION

Clinical trial data comparing outcomes after administration of stereotactic radiosurgery (SRS) or whole-brain radiotherapy (WBRT) to patients with brain metastases (BM) suggest that SRS better preserves cognitive function and quality of life without negatively impacting overall survival. Here, we estimate the maximum number of BM that can be treated using single and multi-session SRS while limiting the dose of radiation delivered to normal brain tissue to that associated with WBRT.

METHODS

Multiple-tumor SRS was simulated using a Monte Carlo - type approach and a pre-calculated dose kernel method. Tumors with diameters ≤36 mm were randomly placed throughout the contoured brain parenchyma until the brain mean dose reached 3 Gy, equivalent to the radiation dose delivered during a single fraction of a standard course of WBRT (a total dose of 30 Gy in 10 daily fractions of 3 Gy). Distribution of tumor sizes, dose coverage, selectivity, normalization, and maximum dose data used in the simulations were based on institutional clinical metastases data.

RESULTS

The mean number of tumors treated, mean volume of healthy brain tissue receiving > 12 Gy (V12) per tumor, and total tumor volume treated using mixed tumor size distributions were 12.7 ± 4.2, 2.2 cc, and 12.9 cc, respectively. Thus, we estimate that treating 12-13 tumors per day over 10 days would deliver the dose of radiation to healthy brain tissue typically associated with a standard course of WBRT.

CONCLUSION

Although in clinical practice, treatment with SRS is often limited to patients with ≤15 BM, our findings suggest that many more lesions could be targeted while still minimizing the negative impacts on quality of life and neurocognition often associated with WBRT. Results from this in silico analysis require clinical validation.

Tumor Primary Site and Histology Subtypes Role in Radiotherapeutic Management of Brain Metastases

Randomized controlled trials have failed to report any survival advantage for WBRT combined with SRS in the management of brain metastases, despite the enhanced local and distant control in comparison to each treatment alone. Literature review have revealed important role of primary histology of the tumor when dealing with brain metastases. NSCLC responds better to combined approach even when there was only single brain metastasis present while breast cancer has registered better survival with SRS alone probably due to better response of primary tumor to advancement in surgical and chemotherapeutic agents. Furthermore, mutation status (EGFR/ALK) in lung cancer and receptor status (ER/PR/HER2) in breast cancer also exhibit diversity in their response to radiotherapy. Radioresistant tumors like renal cell carcinoma and melanoma brain metastases have achieved better results when treated with SRS alone. Secondly, single brain metastasis may benefit from local and distant brain control achieved with combined treatment. These diverse outcomes suggest a primary histology-based analysis of the radiotherapy regimens (WBRT, SRS, or their combination) would more ideally establish the role of radiotherapy in the management of brain metastases. Molecularly targeted therapeutic and immunotherapeutic agents have revealed synergism with radiation therapy particularly SRS in treating cancer patients with brain metastases. Clinical updates in this regard have also been reviewed.