Radiological Kinetics of Brain Metastases and Clinical Implications for Patients Treated With Stereotactic Radiosurgery

Growth exponents reflect evolutionary processes and treatment response in brain metastases

Tumor growth is the result of the interplay of complex biological processes in huge numbers of individual cells living in changing environments. Effective simple mathematical laws have been shown to describe tumor growth in vitro, or simple animal models with bounded-growth dynamics accurately. However, results for the growth of human cancers in patients are scarce. Our study mined a large dataset of 1133 brain metastases (BMs) with longitudinal imaging follow-up to find growth laws for untreated BMs and recurrent treated BMs. Untreated BMs showed high growth exponents, most likely related to the underlying evolutionary dynamics, with experimental tumors in mice resembling accurately the disease. Recurrent BMs growth exponents were smaller, most probably due to a reduction in tumor heterogeneity after treatment, which may limit the tumor evolutionary capabilities. In silico simulations using a stochastic discrete mesoscopic model with basic evolutionary dynamics led to results in line with the observed data.

Brain Metastasis Growth Kinetics: A Novel Prognosticator for Stereotactic Radiotherapy

AIMS

The rate of size change in brain metastasis may have clinical implications on tumour biology and prognosis for patients who receive stereotactic radiotherapy (SRT). We analysed the prognostic value of brain metastasis size kinetics and propose a model for patients with brain metastases treated with linac-based SRT in predicting overall survival.

MATERIALS AND METHODS

We analysed the patients receiving linac-based SRT between 2010 and 2020. Patient and oncological factors, including the changes in sizes of brain metastasis between the diagnostic and stereotactic magnetic resonance imaging, were collected. The associations between prognostic factors and overall survival were assessed using Cox regression with least absolute selection and shrinkage operator (LASSO) checked by 500 bootstrap replications. Our prognostic score was calculated by evaluating the most statistically significant factors. Patients were grouped and compared according to our proposed score, Score Index for Radiosurgery in Brain Metastases (SIR) and Basic Score for Brain Metastases (BS-BM).

RESULTS

In total, 85 patients were included. We developed the prognostic model based on the most important predictors of overall survival: growth kinetics, i.e. percentage change in brain metastasis size per day between the diagnostic and stereotactic magnetic resonance imaging (hazard ratio per 1% increase, 1.32; 95% confidence interval 1.06-1.65), extracranial oligometastatic diseases (≤5 involvements) (hazard ratio 0.28; 95% confidence interval 0.16-0.52) and the presence of neurological symptoms (hazard ratio 2.99; 95% confidence interval 1.54-5.81). Patients with scores 0, 1, 2 and 3 had a median overall survival of 44.4 (95% confidence interval 9.6-not reached), 20.4 (95% confidence interval 15.6-40.8), 12.0 (95% confidence interval 7.2-22.8) and 2.4 (95% confidence interval 1.2-not reached) years, respectively. The optimism-corrected c-indices for our proposed model, SIR and BS-BM were 0.65, 0.58 and 0.54, respectively.

CONCLUSIONS

Brain metastasis growth kinetics is a valuable metric for survival outcomes of SRT. Our model is useful in identifying patients with brain metastasis treated with SRT with different overall survival.

Contemporary practice patterns of stereotactic radiosurgery for brain metastasis: A review of published Australian literature

There has been a shift in the management of brain metastasis (BM), with increasing use of stereotactic radiosurgery (SRS) and delaying/avoiding whole-brain radiotherapy (WBRT), given the concern regarding the long-term neurocognitive effect and quality of life impact of WBRT. It is, however, unclear as to the contemporary practice pattern and outcomes of SRS in Australia. We conducted a literature search in PubMed and MEDLINE using a series of keywords: 'stereotactic', 'radiosurgery' and 'brain metastases', limiting to Australian studies, which report on clinical outcomes following SRS. Eight studies - one randomized trial and seven retrospective cohort studies - were identified and included in this review. A total of 856 patients were included, with the most common primary tumour types being melanoma, lung cancer and breast cancer. Approximately half of the patients had solitary BM, while 7% had 10 or more BM lesions. SRS is not routinely given in combination with WBRT. The 6-month intracranial control and 1-year intracranial control following SRS were reported in the range of 67-87% and 48-82%, respectively, whereas the 1-year overall survival and 2-year overall survival were reported in the range of 37-60% and 20-36%, respectively. There are limited data reported on SRS-related toxicities in all included studies. Overall, despite increasing use of SRS for BM, there is a low number of published Australian series in the literature. There is a potential role for establishing an Australian clinical quality registry or collaborative consortium for SRS in BM, to allow for systematic prospective data collection, and benchmarking of quality and outcomes of SRS.