Reply to 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 Robin J.D. Prestwich, Sriram Vaidyanathan, Jim Zhong, Andrew F. Scarsbrook

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

Dear Editor,

We would like to thank Rulach et al for their useful comments. It is certainly very interesting to note the different rate of second look PET in our experience (7%) and the West of Scotland data (20.9%). We would agree with regard to the potential suggestions made to account for this difference. Indeed, a second look PET-CT has become more common in the latter part of our reported experience (15 second look PET-CT performed out of 254 (6%) patients in the series 2008-2014 compared with 34/305 (11%) from 2015 onwards), likely to reflect growing confidence in PET-CT as a response assessment tool coupled with emerging data over that time regarding the limited positive predictive value of PET-CT. A further potential reason is the timing of the initial response assessment PET-CT. Our approach has been to perform this PET-CT scan at 16 weeks post-treatment, compared with the 12 week timepoint in their series; it is possible this delay may have reduced the false positive rate of the PET-CT.

The differentiation drawn between human papilloma virus (HPV) related oropharynx cancer and non-HPV related disease is important. Much of the current data regarding test characteristics of response assessment PET-CT is based upon series dominated by HPV-related oropharyngeal carcinoma and it is not clear that this can be extrapolated to the non-HPV cohort.

The difficulty of comparison between series using differing PET-CT response assessment criteria also highlights the importance of establishing optimal interpretive criteria for PET-CT response assessment reporting.