Background: Focal methods to determine the source of recurrence are presented, tested for reproducibility and compared to volumetric approaches with respect to the number of recurrences ascribed to the FDG-PET positive and high dose volumes.
Material and methods: Six patients treated for hypopharyngeal squamous cell carcinoma were extracted from archives. Inclusion criteria were: FDG-PET/CT for primary radiotherapy planning and clinical complete remission followed by loco-regional relapse. CT scan at the time of recurrence was also required. The recurrence volume was delineated in the follow-up scans by a radiologist. Putative points of origin (PO) of the recurrence were determined by two strategies 1) defined by an oncologist or 2) as the center-of-volume (COV) of the recurrence. The most likely recurrence point of origin on the treatment planning scan was also determined. All expert based points of origin were repeated to estimate reproducibility. The recurrence volume and PO were propagated to the treatment planning scan using a rigid transformation. Relations of the PO to target volumes, radiation doses and therapy-points-of-origin were quantified. For the volumetric methods, the overlap of the recurrence volume and target volumes was used to determine the source of the recurrence.
Results: All recurrences were located in-field, but the volumetric approaches tended to designate fewer recurrences in the PET positive volume (25% for the 95% threshold, 95% confidence interval (CI):3-65%) than the observer-based methods (50% for the COV and both expert evaluations on the recurrence scan, 95% CI: 16-84%). The reproducibility of the expert POs is better on the recurrence scan than on the therapy scan.
Conclusion: Volumetric approaches favor large target volumes as the source of the recurrence, thus underestimating the number of recurrences originating in the PET positive volume. Expert based and COV approaches on the recurrence scan are the most reproducible methods to determine the PO.