Using Patient As Control To Identify The Threshold Suv For Fdg-Pet Tumor Delineation For Radiation Therapy In Head And Neck Cancers

One standard methodology for clinical target volume (CTV) delineation in head and neck cancers using FDG-PET has been based on critical stand uptake value (SUV) thresholds from population studies. However, this threshold does not account for individual patient variations in normal glucose uptake. To address this issue, we have developed a method using SUVs of normal tissue equivalent to the diseased region for a patient specific SUV threshold value. Head and neck cancer patients with FDG-PET scans obtained under sedation 90 minutes after tracer injection were selected based on having primarily unilateral disease sites for retrospective study. These patients were treated by identifying nested clinical target volumes to treat the primary and boost regions simultaneously. CTV1 represented the tightest margin to the GTV and was used to identify the characteristic SUV values for the tumor region and the equivalent normal tissue. Typically, the volume with an SUV greater than 2.5 was identified in the planning stages, but the CTV1 was not identified based solely upon this information. The CTV1 was mirrored on the opposite side of the patient to identify equivalent normal tissue to calculate the normal SUV. A new CTV1 was then created based on the mean of the normal SUV plus two standard deviations. Mirror CTV1s were created for 10 patients, and any overlap was eliminated. Nine of the cases had less than 15% overlap between the CTV1 and mirror CTV1s. One patient had ∼29% overlap and a mirror CTV1 mean SUV of 2.66, indicating that the disease was too bilateral to use this method effectively. Another patient had high PET activity in the tonsils which extended into the mirror region and artificially increased the normal mean SUV. Of the remaining eight patients, the CTV1 SUV population mean was 3.44 ± 1.59, and the normal SUV population mean was 1.74 ± 0.62. The new SUV cutoff values ranged from 2.36 to 3.64, but the new CTV1s created using only the patient specific SUV cutoffs were smaller than the treated CTV1s for all cases by an average of 61.9 ± 16.5%. New CTV1s were largely contained within original CTV1s, however an average of 6.3 ± 9.4% of the new CTV1 volume was identified outside the original CTV1. The large standard deviation is due to one patient with an SUV threshold of 2.36 and 27.9% of the new CTV1 volume outside the original CTV1. This may be because new regions were identified using a threshold lower than 2.5. The range of SUV thresholds obtained by using a value two standard deviations above the equivalent normal tissue mean and the subsequent volume change in CTV1 indicates that patient specific information is important in tumor delineation. However, this method is limited to cases where there is primarily unilateral disease.