Post-operative Stereotactic Radiosurgery to Brain Metastases Cavity: A Large, Observational Single-Centre Series From the United Kingdom

Abstract

Introduction Stereotactic radiosurgery (SRS) is widely regarded as the standard of care after the resection of brain metastases in order to reduce local cavity recurrence risk. The objective of this study was to explore the reproducibility of published outcomes for patients receiving post-operative stereotactic radiosurgery (cavity SRS) in a National Health Service (NHS) setting for a non-selective series of patients. For our service, the median interval between surgery to cavity SRS (cSRS) is eight weeks, whereas similar timelines have been found to have a deleterious impact on survival in the published literature. Materials and methods This retrospective cohort study analysed outcomes for 100 consecutive cSRS patients treated between 2015 and 2019 at a Northern English cancer centre. A case note review was conducted, with collection of primary tumour, disease extent, SRS treatment details and outcome data. Results Median survival for all primaries was 16 months, with renal, melanoma and breast having optimum survival at 28, 26 and 17 months, respectively. Local relapse was seen in 6/100 patients, with a further 6/100 patients having leptomeningeal disease. Radionecrosis was rare (3/100). Dose prescription, size of PTV and number of metastatic sites did not produce a statistically significant impact on survival times. The detrimental impact of delay from surgery to SRS beyond 56 days reported by others was not evident in our series (p-value 0.786). Conclusion SRS to the surgical cavity after the resection of brain metastases in eligible patients produces favourable outcomes and demonstrates outcomes comparable to the world literature. Our study does not demonstrate a significant drop in survival with delay beyond eight weeks to cSRS, which may reflect a different cause of delay in our NHS setting than in other healthcare systems.

Keywords: brain metastases; cavity radiosurgery; overall survival; postoperative radiosurgery; stereotactic radiosurgery; stereotactic radiotherapy (srt).

Figure 1. Pre- and post-operative imaging scans for a typical cavity SRS patient.

(A) Pre-operative T1-weighted post-contrast diagnostic MRI scan, (B) post-operative T1-weighted 3D fast gradient echo sequence post-contrast planning MRI scan, (C) planning CT scan with 1 mm slice separation. In all the images, the tumour cavity outline (extended for pre-operative dural contact) and the planning target volume (PTV), grown with a 2 mm margin, are shown in red; the brainstem and planning organ at risk volume (PRV), grown with a 1 mm margin, are shown in green.

Figure 2. Dose distributions for a typical cSRS patient with a left-sided cavity and a right-sided intact metastasis, treated concurrently.

(A) The left-sided cavity with the planning target volume (PTV) (blue) grown with a 2 mm margin from the clinical target volume (CTV) (red). The orange contour indicates the position of the lesion on the pre-operative MRI scans. The prescription isodose of 24 Gy in three fractions is shown in green. (B) The right-sided intact met with the PTV (red), grown with a 1 mm margin from the CTV (blue). The prescription isodose of 21 Gy in one fraction is shown in green. The intact met was treated concurrently with the cavity. cSRS: cavity stereotactic radiosurgery.

Figure 3. Overall survival curves for different patient characteristics: (A) presence of extracranial metastases; (B) post-surgery residuum; (C) GTV size over 20 cc; (D) time to cSRS after surgery of greater than eight weeks; (E) primary diagnosis; (F) age group.

GTV = gross tumour volume, cSRS = cavity stereotactic radiosurgery.

Figure 4. Number of sites of metastatic disease for patients with each primary diagnosis. Primary diagnosis of the 100 patients in the study cohort was breast (n = 20), lung (n = 38), renal (n = 12), melanoma (n = 12) or other (n = 18).

Patients with only one site are those whose metastatic disease is confined to the brain.

Figure 5. The relationship between GTV volume and dose prescription for cavity SRS.

GTV = gross tumour volume, SRS = cavity stereotactic radiosurgery.