Stereotactic Body Radiotherapy (SBRT) for Oligometastatic Spine Metastases: An Overview

Abstract

The oligometastatic state is hypothesized to represent an intermediary state of cancer between widely metastatic disease and curable, localized disease. Advancements in radiotherapy have allowed for delivery of high precision, dose escalated treatment known as stereotactic body radiotherapy (SBRT) to targets throughout the body with excellent rates of local control. Recently, the first phase II randomized trial comparing conventional radiotherapy to comprehensive SBRT of oligometastatic disease demonstrated an overall survival and progression free survival advantage. The spine is a common site of metastasis, and a complex site for SBRT given the adjacent spinal cord and the tumor embedded within the bone tissue putting the patient at risk of fracture. Although there are expert spine SBRT guidelines for practice, there are as yet no reported randomized trials that proves superiority as compared to conventional radiation. The use of SBRT in patients with oligometastatic disease and spinal metastases is the focus of this review.

Keywords: oligometastases; outcomes; response assessment; spine metastases; stereotactic body radiotherapy (SBRT); toxicities.

Figure 1

A case presentation of a lady with invasive ductal carcinoma of the breast who was treated definitively with lumpectomy, adjuvant chemotherapy, and adjuvant radiotherapy. Shortly after completion of therapy, on re-staging investigations, she was found to have oligometastatic disease in the bones, specifically at C4, T3, T10, the left sacral ala, and right scapula. She received SBRT to each site and was started on hormonal therapy. At most recent follow-up 20 months later, she has not had progression of known disease nor interval development on new metastatic disease. (a) Posterior-anterior projection of pre-treatment bone scan demonstrating increased uptake within the right scapula, T3 and the right sacral ala. Subsequent images of axial slice of T2-weighted MRI demonstrating near complete marrow replacement of C4 (b), focal marrow abnormality in posterior T3 body (c), rounded focus centrally of the T10 vertebral body (d), and 13 mm lesion of left sacral ala (e).

Figure 2

A man with oligometastatic castrate-resistant prostate cancer with painful spine metastases. This man was treated to 24 Gy in 2 fractions. (a) Axial planning CT scan demonstrating T6 vertebral level with gross tumor volume (GTV), clinical target volume (CTV), and planning target volume (PTV) delineated with red, green, and blue lines, respectively. (b) Sagittal planning CT demonstrating T6 vertebral level with GTV, CTV, and PTV in red, green, and blue, respectively. (c) Dose distribution at the level of T6 with PTV (colorwash blue) and spinal cord planning organ at risk volume (PRV) in colorwash green. Demonstration of sharp-dose fall-off to respect critical structures while allowing coverage of the target volumes.