To improve the objectivity of the integration of positron emission tomography (PET), we used the conformality index (CI) to measure the goodness of fit of a given PET iso-SUV (standardized uptake value) level with the GTV defined on PET (GTV_PET) and CT (GTV_CT). Twenty-two datasets involving 20 head and neck cancer patients were identified. GTV_PET and GTV_CT were delineated manually.An iso-intensity method was developed to automatically segment GTV_PET-ISO using (a) SUV and (b) maximum intensity thresholding (% Max), over a range of intensities. For each intensity, GTV_PET-ISO was compared to GTV_PET using the conformality index CI_PET (and, similarly, to GTV_CT using CICT). Comparing GTV_PET to GTV_PET-ISO vs comparing GTV_CT to GTV_PET-ISO, the average peak CI was 0.68 ± 0.09 vs 0.49 ± 0.12 (p<0.001), the optimum iso-SUV was 2.7 ± 0.7 vs 2.9 ± 1.0 (p=0. 253), and the % Max SUV was 21.8% ± 7.6% vs 23.8% ± 8.6% (p=0. 310), respectively. The radiation oncologist?s volumes corresponded to a lower iso-SUV (3.02 ± 0.58 vs 4.36 ± 0.77, p < 0.001) and lower % Max SUV (24.1 ± 9.1% vs 34.3 ± 11.2%, p<0.001) than those drawn by the nuclear medicine physician. Though manual editing may still be necessary, PET iso-contouring is one method to improve the objectivity of GTV definition in head and neck cancer patients. Iso-SUV?s can also be used to study the differences between PET?s role as a nuclear medicine diagnostic test versus a radiation oncology treatment planning tool.

Key words: radiotherapy; head and neck cancer; positron emission tomography; standardized uptake value; treatment planning.