Session Item

Despite development in imaging modalities, precise target definition remains a major challenge in head and neck cancer. In this study, tumor volumes defined from multimodal imaging with PET/MRI prior to surgery are compared with the actual pathological tumor volumes from surgical specimens. The mismatch between the volumes defined from imaging and pathology is estimated and the clinical impact of potential mismatch is evaluated.

Twenty-eight patients with head and neck squamous cell carcinoma were scanned on an integrated PET/MRI system, prior to surgery. Three GTV’s were delineated defined from the MRI (GTV_MRI), the PET (GTV_PET) and one by utilizing both anatomical images and clinical information (GTV_ONCO).  Surgical specimens were extracted en bloc and scanned with the same PET/MRI and co-registered to the patient images based on predefined anatomical landmarks. Each specimen was sectioned in blocks which were then sliced (4-µm thick) and stained with haematoxylin and eosin. All slices were digitalized, scanned and pathological tumor volume was delineated by a trained head and neck pathologist. Pathological tumor areas were interpolated to yield a 3D tumor volume (GTV_PATO), which was imported in Eclipse TPS (Varian Medical Systems, Palo Alto, CA) as a structure. The GTV_PATO was compared with the GTV’s defined from imaging. Additionally, the mismatch between GTV’s were assessed by dividing the GTV_PATO that is outside the imaging  GTV with the whole volume of the corresponding GTV.  To validate current practice of a 5 mm CTV expansion, a margin of 5 mm was added to the GTV_ONCO and the mismatch was re-assessed.

For consistency, only T-sites were included in this analysis (N=27). Further thirteen patients were excluded before the analysis, because fragmentation of the specimen during histologic processing or poor interpolation and insufficient number of points to form a structure in Eclipse. For the fourteen patients that were included in the analysis the mean volume of the GTV_ONCO was larger than the other GTV’s. In four patients the GTV_PATO was larger than the GTV_MRI and for one patient the GTV_PATO was larger than the GTV_PET. The mean mismatch of the GTV_PATO when evaluated to the GTV_PET, GTV_MRI and GTV_ONCO was 29.7%, 39.5% and 7.5% respectively (Table 1). However, after the addition of 5mm margin all GTV_PATO’s were encompassed in GTV_ONCO.

GTV was defined differently with different imaging modalities and a mismatch with the pathological tumor volume was observed in thirteen of the fourteen included patients. For all patients, 5mm margin appears sufficient to ensure that all of the pathological information will be included during treatment.