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.