The utility of [F]FDG PET in discriminating between local recurrence and inflammatory changes following SABR in primary lung cancer patients.

[PURPOSE] Stereotactic ablative radiotherapy (SABR) is used in patients with early-stage primary lung cancer who are not fit for surgery or who refuse surgery. Post-treatment, patients are monitored with surveillance CT, with [F]FDG PET-CT often used for further investigation if there is suspicion of recurrence on CT. This study investigated the utility of [F]FDG PET-CT in detecting recurrence in patients with suspicious CT findings.

[METHODS] This is a dual-centre retrospective study of 754 consecutive patients who received SABR for lung lesions in our institutions between 2012 and 2023. Seventy-four FDG PET-CT scans from 65 patients, performed at a median of 585 days after SABR were included. SUVmax and subjective score of likelihood of recurrence were recorded. Follow-up imaging, biopsy and resection histology, where available, were used to ascertain the patients' outcome.

[RESULTS] Out of 74 included FDG PET-CT scans, there were 11 local recurrences and 63 non-recurrences by reference standard. The SUVmax of the lesion post-SABR was significantly higher in patients who had recurrent tumour (median 4.3, range 2.2-16.1) compared to those who did not (median 3.3, range 1.4-6.4) (p = 0.011). Subjective scores of likelihood of recurrence performed better than SUVmax readings alone with positive and negative predictive values of 0.45 and 0.96, respectively, which are significantly improved to 0.94 and 1.00 when only including tumours with baseline SUVmax > 5.0.

[CONCLUSION] FDG PET is useful in guiding management of patients with suspected recurrence after SABR, particularly in cases where the tumour is FDG avid pre-treatment.