Consideration of Time From Planning to Treatment for Frameless Fractionated Stereotactic Radiosurgery in Patients With Intact Large Brain Metastases

Abstract

Patients with large brain metastases may receive fractionated stereotactic radiosurgery (FSRS) if they are not surgical candidates yet still pursuing cancer-directed therapy. FSRS with limited margins can allow for optimal sparing of uninvolved regional brain tissue. However, due to required treatment planning process, there is often elapsed time between MRI brain and treatment start which may contribute to marginal misses. Growth rates of large brain metastases and implications for optimal margin to account for growth rate and time interval between planning MRI and FSRS are incompletely characterized.Patients treated at a single institution from 2018-2020 who received frameless FSRS on a Cobalt-60 treatment unit for large (> 2cm diameter) brain metastases were identified. Patients with a prior diagnostic brain MRI scan with technically similar parameters to a second MRI obtained for treatment planning were included. Resection cavities and lesions undergoing re-treatment after prior SRS were excluded. Brain metastasis volumes were obtained by contouring on both diagnostic and planning MRIs. Patient characteristics and treatment details were also collected. Analysis was conducted using paired Student's t test and Pearson's R and growth rate was determined using a linear model.Thirty-six patients with 54 brain metastases met inclusion criteria. The most common primary cancer sites were lung (36%), colorectal (14%), and skin (14%), and most common histologies were adenocarcinoma (53%) and melanoma (17%). Nearly half (47%) of patients were on systemic therapy at the time of FSRS and the majority (85%) received steroids prior to starting FSRS. Treatment dose ranged from 24-32.5 Gy in 3-5 fractions, with median of 30 Gy in 5 fractions with zero margin. Median time from diagnostic to planning MRI was 15 days (IQR: 11-21) and planning MRI to FSRS was 1 day (IQR 1-3). The median size of included brain metastases on planning MRI was 7.38cc (IQR: 5.09-11.55) compared to 6.03cc (IQR: 3.87-8.95) on prior diagnostic MRI (P < 0.001). Eight metastases (15%) decreased in size in the context of concurrent central nervous system-penetrant systemic therapy or steroids. Median calculated daily growth was 0.10cc (2.37%) per day (IQR: 0.02-0.21cc, 0.41-3.78%). There was no significant association between lesion growth rate and histology, intracranial location, steroid use, and baseline tumor size. A 1.0 mm, 1.5 mm, 2.0 mm, and 2.5 mm margin would have covered anticipated growth in > 90% of lesions over 3, 7, 10, and 14 days between planning and treatment, respectively.In the absence of expedited planning, FSRT for intact large brain metastases with limited margins may result in geometric under treatment. An adaptive margin strategy based upon initial tumor size, and time between planning imaging and treatment delivery may be afforded to maintain ideal conformity, properly targeting a growing target lesion while maximally sparing uninvolved regional brain.J. Chew: None. S. Sinha: None. S.J. Liu: None. S.E. Fogh: Independent Contractor; Accuray. L. Boreta: None. D. Raleigh: None. L. Ma: Patent/License Fees/Copyright; University of California Regents. S.E. Braunstein: Advisory Board; Radiation Oncology Questions, LLC.