Methodological Aspects of Measuring Metabolic Tumor Volume in Patients with Diffuse B-Cell Large Cell Lymphoma by PET/CT with 18F-FDG

Introduction: Metabolic Tumor Volume (MTV) and Total Tumor Lesion Glycolysis (TLG) are used in ¹⁸F-fluorodeoxyglucose Positron-Emission Tomography combined with Computed Tomography (¹⁸F-FDG PET/CT) as functional markers, indicating tracer uptake in whole tumor. MTV and TLG are not yet engaged in clinical practice, because volume measurement accuracy depends on the selected measurement method, and optimal MTV and TLG segmentation is not established.
Purpose: Assessment of accuracy of metabolic tumor volume measurement ways, using ¹⁸F-FDG PET/CT in patients with Diffuse Large B-Cell Lymphoma (DLBCL).
Material and methods: Baseline ¹⁸F-FDG PET/CT performed in 21 patients with DLBCL. Tumor Volume (TV), measured on contrast enhanced CT (CTTV), considered as reference. While measuring MTV, we aimed to achieve a 1:1 ratio between CTTV and MTV. If anatomical and metabolic tumor contours matched, MTV was considered true (MTV_true). Overall MTV and TLG measurements are 254. Tumor contouring, using relative thresholds, was made around Standardized Uptake Value (SUV_max) of tracer in tumor and intact liver.
Results: On CT, the size of the lesions varied from 24 to 241 mm, M = 103.4±62.3, Me = 93 (48.5–155.5). In 15 (71.4 %) foci, hypodense areas of necrosis were determined. PET imaging revealed high tracer uptake in all foci of varying degrees of intensity. SUV_max values in tumors ranged from 5.8 to 30.5, Me = 20.4 (17.3–23.2). No significant correlation was found between the size of the foci and SUV_max (ρ = 0.17, p = 0.4744). The best accuracy in measuring MTV was observed when several thresholds were applied: VOI_PERCIST, VOI_2.5, VOI_auto-contour, as well as VOI_{10 %}, VOI_{15 %} and VOI_{20 %}. The optimal absolute threshold values were SUV values in the range from 2 to 3, and relative values in the range from 10 % to 20 % of the SUV_max in the tumor. MTV underestimation was more often determined using relative cut-off indices in the range of
25–50 % and SUV thresholds ≥ 5.3.
Conclusion: The results obtained showed that the use of a single threshold value for MTV segmentation in patients with DLBCL is not advisable. The choice of the MTV measurement method should be carried out individually for each lesion, taking into account its shape, size and structure, as well as the intensity and uniformity of tracer uptake in the tumor and adjacent anatomical structures.

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