BACKGROUND: Focalnmethods to determine the source of recurrence are presented, testednfor reproducibility and compared to volumetric approaches withnrespect to the number of recurrences ascribed to the FDG-PETnpositive and high dose volumes.

MATERIAL AND METHODS:Six patients treated for hypopharyngeal squamous cellncarcinoma were extracted from archives. Inclusion criteria were:nFDG-PET/CT for primary radiotherapy planning and clinical completenremission followed by loco-regional relapse. CT scan at the time ofnrecurrence was also required. The recurrence volume was delineatednin the follow-up scans by a radiologist. Putative points of originn(PO) of the recurrence were determined by two strategies 1) definednby an oncologist or 2) as the center-of-volume (COV) of thenrecurrence. The most likely recurrence point of origin on thentreatment planning scan was also determined. All expert basednpoints of origin were repeated to estimate reproducibility. Thenrecurrence volume and PO were propagated to the treatment planningnscan using a rigid transformation. Relations of the PO to targetnvolumes, radiation doses and therapy-points-of-origin werenquantified. For the volumetric methods, the overlap of thenrecurrence volume and target volumes was used to determine thensource of the recurrence.

RESULTS: Allnrecurrences were located in-field, but the volumetric approachesntended to designate fewer recurrences in the PET positive volumen(25% for the 95% threshold, 95% confidence interval (CI):3-65%)nthan the observer-based methods (50% for the COV and both expertnevaluations on the recurrence scan, 95% CI: 16-84%). Thenreproducibility of the expert POs is better on the recurrence scannthan on the therapy scan.

CONCLUSION:Volumetric approaches favor large target volumes as thensource of the recurrence, thus underestimating the number ofnrecurrences originating in the PET positive volume. Expert basednand COV approaches on the recurrence scan are the most reproduciblenmethods to determine the PO.