Letter to the Editor: “Second-look PET-CT following an initial incomplete PET-CT response to (chemo)radiotherapy for head and neck squamous cell carcinoma”

by R. Rulach, S. Zhou and C. Paterson

Second-look PET-CT following an initial incomplete PET-CT response to (chemo)radiotherapy for head and neck squamous cell carcinoma

Dear Prof Menu,

We read with interest the work of Prestwich et al on the use of FDG PET-CT scans for response assessment of head and neck squamous cell cancers (SCC) [1]. We agree with their conclusions that second look PET-CTs can spare unnecessary neck dissections. We recently conducted a retrospective review of the use of surveillance 2-[Fluorine-18]-fluoro-2-deoxy-D-glucose (FDG) PET-CTs after radiotherapy for head and neck SCC in the West of Scotland, between January 2013 and September 2016. Our rate of second look PET-CTs is 39/187 (20.9%) over 3 years, which is higher than the 40/562 (7%) reported in the data from Leeds [2].

There may be several reasons for this difference. Firstly, the West of Scotland cohort has a higher rate of oropharyngeal cancer (71% in Leeds, 82.3% in West of Scotland). Secondly, the West of Scotland data was collected from 2013 (the Leeds data collection period started in 2008) and so may reflect growing acceptance by multidisciplinary teams (MDT) to delay surgery after radiotherapy, embarking on a period of prolonged surveillance instead. Finally, in the West of Scotland, enlarged lymph nodes with absent FDG avidity were categorised as equivocal, whereas this would be classed as negative by the criteria used in Leeds. We suspect this would lead to less scans being labelled as equivocal and therefore less need for repeat PET-CTs. This highlights the differences between MDTs in how patients are selected for second look PET-CTs and the need for expert consensus to share best practice and optimise clinical decision making.

In our study, we stratified the oropharyngeal cancer sub-group by human papillomavirus (HPV) status and found that the positive predictive value for HPV positive disease of 3 month PET-CT was significantly different to HPV negative disease (20% and 81.8% respectively, two-sample proportion test p < 0.01), and our rate of complete response on follow—up imaging at 6 months was 67.6% [3]. We note that in the paper by Prestwich et al, the majority of late converting disease were oropharyngeal tumours.

Given the growing evidence of delayed responses on PET-CT in HPV positive oropharyngeal tumours, perhaps in the absence of clinical concerns, we should consider performing a single definitive PET-CT at 6 months after the completion of treatment, continuing with earlier PET-CT scans for HPV unrelated head and neck cancers? Although this apparent delay in knowing outcome from treatment may be anxiety provoking in some patients, the current uncertainty after initial PET-CT and need for repeated investigation is also difficult for many patients. Finally, further work is needed to identify early predictors of relapse (e.g., circulating tumour DNA, HPV titres) to combine with PET-CT data, to better identify those patients who would benefit from early surgical management.