Session Item

It is known that patients with HPV positive squamous cell carcinoma of the oropharynx (OPSCC) have a lower absolute risk of locoregional relapse (LRR) and a relatively higher rate of distant metastasis than HPV negative OPSCC patients. Therefore, it is not unlikely that the relapse pattern within different parts of the tumor/elective volumes also differs for this patient group. This could potentially influence how the distribution of radiation dose should ideally be optimized. The aim of the current work was to compare the relapse pattern in HPV negative and positive OPSCC patients, in relation to FDG-PET avid volumes and RT target structures at baseline. In addition, we wanted to see if the findings of a previous study in a mixed HNSCC population¹ – that most relapses originate within the FDG-PET avid volume – was valid also in the current population of OPSCC patients only.

Among 632 OPSCC patients treated 2009-2017, 40 patients with known HPV status and isolated LRR were included. Patients where journal charts indicated persistent disease after RT were excluded (independent chart review is ongoing). Oncologists contoured relapse volumes (RV) on relapse scans (CT, PET/CT or MR). The relapse scans and RV structures were deformably registered to the planning CT, and the point of origin (POI) was estimated using a nidus/centre of volume strategy¹. The relapse (POI) positions were analyzed in relation to the following volumes/targets: FDG-PET avid volume (GTV-PET), gross tumor volume (GTV) and high-dose clinical target volume (CTV1), all 66-68 Gy. Elective volume (CTV-E), 50-60 Gy. The relapse pattern was compared between HPV negative and positive patients with appropriate statistics (for details, see Table 1 caption).

Fifty-four RV:s were found in the 40 patients. Of these, 31 originated from GTV-PET, while the other RT targets (excluding any targets incorporated) harbored 7 (GTV), 9 (CTV1) and 5 (CTV-E) relapses. Two nodal relapses were found outside the RT target volumes (Table 1). No significant difference in relapse volume was found between HPV negative and positive patients (Table 1). The distribution of POI positions among HPV negative and positive patients are visualized in Figure 1. No significant difference in relapse pattern in relation to the target volumes was found (Fisher’s exact test, p=0.83).

The data do not suggest any difference in relapse pattern in relation to FDG-PET and RT target volumes between HPV negative and positive OPSCC patients. In agreement with previous findings, the majority of relapses were found in GTV-PET.

¹ Due et al. “Recurrences after intensity modulated radiotherapy for head and neck squamous cell carcinoma more likely to originate from regions with high baseline [18F]-FDG uptake”. Radiother Oncol 2014;111:360-5