Stereotactic ablative body radiotherapy (SAbR) for oligometastatic cancer

Abstract

The metastatic state of most solid cancers traditionally has been regarded as an incurable dissemination of disease, with treatment focused on delaying progression rather than eliminating all tumour burden. In this setting, local therapies including surgery and radiotherapy are directed at quality of life end points and not at improvement in survival. However, improvements in imaging and systemic therapy have highlighted populations of patients with lower burden of metastatic disease, termed "oligometastatic," who may present an exception. This condition is hypothesized to bridge the gap between incurable metastatic disease and locoregional disease, where miliary spread either has not occurred or remains eradicable. Consequently, elimination of such low-burden residual disease may "cure" some patients or delay further progression. Accordingly, use of local therapies with the intent of improving survival in oligometastatic disease has increased. Technological advances in radiation delivery with stereotactic ablative body radiotherapy (SAbR) in particular have provided a non-invasive and low-morbidity option. While observational studies have provided interesting preliminary data, significant work remains necessary to prove the merits of this treatment paradigm. This review discusses the data for the oligometastatic state and its treatment with SAbR, as well as challenges to its investigation.

Figure 1.

Use of stereotactic ablative body radiotherapy (SAbR) in a 64-year-old male with oligometastatic prostate cancer developing metachronously at a single left pelvic sidewall lymph node just above prior salvage prostate fossa radiotherapy (RT) field for biochemical recurrence after prostatectomy: (a) systemic scans including fluorine-18 fludeoxyglucose positron emission tomography demonstrated a single site of disease. (b) Prior salvage RT was directed to the prostate fossa alone with lymph node oligometastasis occurring immediately above the field. (c) Oligometastasis was treated with SAbR to 40 Gy in 5 fractions with daily CT imaging, use of stereotactic frame and careful constraints placed on adjacent bowel, sciatic nerve and any potential for overlap of incident beams with prior treated areas. Image fusion and composite plan including prior RT facilitated this treatment, which resulted in swift biochemical and radiographic response (currently without evidence of disease) without need for androgen deprivation therapy or grade 2 or higher toxicity.