Stereotactic radiosurgery alone for multiple brain metastases? A review of clinical and technical issues

Abstract

Over the past three decades several randomized trials have enabled evidence-based practice for patients presenting with limited brain metastases. These trials have focused on the role of surgery or stereotactic radiosurgery (SRS) with or without whole brain radiation therapy (WBRT). As a result, it is clear that local control should be optimized with surgery or SRS in patients with optimal prognostic factors presenting with up to 4 brain metastases. The routine use of adjuvant WBRT remains debatable, as although greater distant brain control rates are observed, there is no impact on survival, and modern outcomes suggest adverse effects from WBRT on patient cognition and quality of life. With dramatic technologic advances in radiation oncology facilitating the adoption of SRS into mainstream practice, the optimal management of patients with multiple brain metastases is now being put forward. Practice is evolving to SRS alone in these patients despite a lack of level 1 evidence to support a clinical departure from WBRT. The purpose of this review is to summarize the current state of the evidence for patients presenting with limited and multiple metastases, and to present an in-depth analysis of the technology and dosimetric issues specific to the treatment of multiple metastases.

Keywords: brain metastases; multiple metastases; radiosurgery; stereotactic radiosurgery.

Fig. 1

Gamma Knife Icon system with a retractable 90 kV CBCT unit mapped in submillimeter stereotactic coordinates for treatment setups. Shown is the starting position of the CBCT arm for online imaging acquisitions. This picture is provided by Dr Dheerendra Prasad, Rosswell Park, Department of Radiation Oncology.

Fig. 2

The Cyberknife M6 which is equipped with the InCise2 high definition MLC (leaf width = 3.85 mm at 80 cm SAD) allowing for a maximum clinical field size of 11.5 x 10 cm². Picture provided by Dr Lei Wang of Stanford University, Department of Radiation Oncology.

Fig. 3

An example of a digitally controlled SRS/SRT linear accelerator system (Varian Truebeam Stx), which is equipped with a 120-leaf high-definition MLC and a kilo-voltage on-board imager. The minimum MLC leaf width is 2.5 mm for the central 32 leaf pairs (8 cm in field width) for the purpose of delivering SRS/SRT treatments.

Fig. 4

A typical dose distribution and resulting statistics for a single target shown in sagittal view. Blue colorwash = target volume; isodose lines: gray = 30 Gy, red = 20 Gy (prescribed dose); yellow = 16 Gy; light blue = 12 Gy; pink = 5 Gy. VMAT was planned using a 2-arc RapidArc Varian linac with HD 120 MLC. Plans were generated such that the target volume receiving 20 Gy was 98%.

Fig. 5

Sample dose distribution for a 9-target case shown in axial view (2 separate slices). Blue colorwash = target volume; orange colorwash = brainstem. Isodose lines: gray = 28 Gy, red = 18 Gy (prescribed dose); green = 12 Gy; blue = 9 Gy; pink = 5 Gy. VMAT was planned using 4 arcs RapidArc on a Varian linac with HD 120 MLC.