ESTRO 2022 - Abstract Book

S376

Abstract book

ESTRO 2022

OC-0433 Positive predictive value of post radiotherapy FDG PET-CT is affected by treatment and HPV-status

S. Zhou 1 , C. Chan 2 , H. Dyab 3 , R. Rulach 1 , F. Hendry 3 , C. Maxfield 3 , M.F. Hendry 3 , A. James 1 , C. Lamb 1 , D. Grose 1 , C. Wilson 1 , S. Schipani 1 , Y.C. Lau 4 , C. Paterson 1 1 Beatson West of Scotland Cancer Centre, Clinical Oncology, Glasgow, United Kingdom; 2 University Hospital Ayr, General Medicine, Glasgow, United Kingdom; 3 NHS Greater Glasgow and Clyde, Radiology, Glasgow, United Kingdom; 4 Royal Alexandra Hospital, Medicine, Glasgow, United Kingdom Purpose or Objective The high negative predictive value (NPV) of post-chemoradiation (CRT) PET-CT to guide the need for neck dissection (ND) in HNSCC was shown by the PET-Neck study. The positive predictive value (PPV) of this investigation remains under scrutiny with increasing evidence that it is affected by several factors. Materials and Methods Patients with node-positive HNSCC treated with radical ((C)RT) between January 2013 and July 2019 were identified from the PET-CT database. PET-CT responses were classified as complete (CR), incomplete (ICR) or equivocal (EQR) in keeping with definitions from the PET-Neck study, by radionucleotide radiologists. Patient demographics and clinical outcomes were obtained from electronic patient records. A true positive PET-CT was defined as a pathologically positive ND, or relapsed disease in the neck/distally during follow- up. Results 434 HNSCC patients were identified, patient and tumour characteristics are detailed in Table 1 . Median time from end of RT to PET scan was 93 (IQR 97-101) days. Median follow-up was 42.7 (IQR 30.8-60.0) months. 343 (79%) patients received CRT and 91 (21%) had RT alone. In the CRT group (n=343), 212 (61.8%) achieved a CR; 131 (38.2%) less than CR with 39 cases of recurrence, thus the PPV of PET-CT for CRT was 29.8% (95% CI 22.1-38.4), see Table 2 . In the RT-only group (n=91), 51 (56%) achieved a CR; 40 (44%) a less than CR with 22 cases of recurrence. Hence the PPV of PET-CT was 55.0 (95% CI 38.5-70.7) -significantly higher than the PPV of the CRT group, p=0.005. When stratifying for HPV status, 310 (71.4%) patients had HPV-positive and 124 (28.6%) HPV-unrelated HNSCC. Of the 310 HPV-positive group, 190 (61.3%) patients achieved a CR; 120 (38.7%) less than CR with 30 cases of recurrence. Thus the PPV of PET-CT was 25% (95CI% 17.4-33.7). In the HPV-unrelated HNSCC group (n=124), 73 (58.9%) achieved a CR; 51 (41.1%) less than CR with 31 cases of recurrence. The PPV of PET-CT was 60.8% (95%CI 46.1-74.2). This is significantly higher than the PPV of the HPV-positive group, p<0.001. Of the 120 HPV-positive patients who achieved a less than CR; 76 (24.5%) achieved EQR with 10 cases of relapse; and 44 (14.2%) ICRs, of which 20 patients had recurrence. PPV for PET-CT was 13.2% (95%CI 6.5-22.9) and 45.5% (95%CI 30.4-61.2) for EQR and ICR groups, respectively. Within the HPV-positive cohort, the PPV was significantly lower for the EQR compared to the ICR group, p<0.001. PPV for HPV-positive group was significantly lower than that of the HPV-unrelated group for both EQR and ICR groups, p=0.013 and p=0.038, respectively. The aim of this study was to assess the PPV of PET-CT stratified by treatment modality and tumour HPV-status.