Metabolic Response Assessment Following Stereotactic Ablative Radiotherapy (SABR) in oligo lung metastases - How early we get to see!

Background: FDG PET-CT is a well established cancer imaging tool for diagnosis, staging and response assessment. It is also well known that, post chemotherapy or radiotherapy, metabolic response precedes morphologic response which is based on size criteria. However, earliest time needed for the maximum metabolic response post radiosurgery has never been assessed. Present study evaluates the trend in metabolic regression post SABR with serial FDG PET-CT scans in patients with oligo lung or mediastinal metastases. Materials and Methods: 5 patients with oligometastatic lung tumors (Primaries: Colorectal cancer - 2, Breast - 2 and Head and neck cancer- 1) were planned for SABR by Deep inspiratory breath hold technique using Active breath controller & risk fractionated protocol. Doses delivered were in the range of 110-116gy biologic equivalent dose (BED). All patients underwent diagnostic pre- and post-treatment PET-CT scans. Post SABR PET-CT scans were timed serially at 48hrs, 7, 30, 60 and 90 days post treatment. PET-CT scans were reviewed in order to determine the pre- and post- treatment maximum standardized uptake value (max SUV) of the lesion, including “complete resolution” of FDG-avidity. Corresponding morphologic variations of the target lesions were studied on the corresponding CT images.

Results: All the 5 patients showed serial regression in metabolic activity, with maximum regression in metabolic activity occurring at 1week post SABR. The observed metabolic regression was between 40-54% (median - 50%) at 48 hours and 67-82% (median 80%) at 1week post SABR. During the evaluation period, there was no significant change in size of the lesion. After 1 week of SABR, there was no further decrease in metabolic activity, most probably due to initiation of inflammation.

Conclusions: In cases of recurrent solitary lung or mediastinal metastases, the metabolic response precedes morphologic response, with maximum response seen at 1 week post SABR. This is the first study reported in literature which looked into the trend of metabolic regression following SABR.