Differences in gross tumor volumes for pancreatic cancer: a comparison of ungated positron emission tomography and contrast-enhanced four-dimensional computed tomography.

PURPOSE: We assessed differences in gross tumor volumes (GTVs) for pancreatic cancer between respiratory-ungated positron emission tomography (3D-PET) and contrast-enhanced four-dimensional computed tomography (CE-4DCT).

MATERIALS AND METHODS: We evaluated the GTVs in 21 patients. The sum of the GTVs, which was individually delineated by observers 1 and 2 on the CE-4DCT images from all respiratory phases, was used as GTV-4DCT. The GTVs on the 3D-PET images were extracted with three thresholds: 20%, 30%, and 40% of maximum activity concentration (GTV-n%). We selected one of the GTV-n%, which mostly resembled GTV-4DCT in size, as GTV-PET. Differences in the GTVs were analyzed.

RESULTS: Median values of GTV-4DCT for observers 1, 2, and GTV-PET were 55.0 mL, 45.7 mL, and 14.6 mL, respectively. GTV-PET was smaller than GTV-4DCT for observers 1 and 2 (p < 0.01 each). Differences of median values of maximum diameters between GTV-4DCT and GTV-PET were 1.7-1.8 cm, 1.4-1.6 cm, and 1.9-2.1 cm in the left-right, anterior-posterior, and craniocaudal directions, respectively.

CONCLUSION: GTV-PET based on 3D-PET images was smaller than GTV-4DCT for pancreatic cancer. When we refer to 3D-PET images without CE-4DCT images, we need to pay attention to the above-mentioned finding to contour the GTV.