gms | German Medical Science

Objective: FET PET has been proposed to provide diagnostic information on brain metastases in addition to MRI imaging. For example, the selection of biopsy targets most likely yielding a valid diagnosis could be supported by pre-biopsy FET PET imaging. Moreover, the differentiation of post-radiation or post-resection tissue changes from tumor recurrence could be improved by FET PET. We aimed at better understanding the relation between FET PET imaging (as measured by standardized uptake values, SUV) and potential (i.e., MRI contrast enhancing) tumour tissue properties determined histologically.

Methods: Patients underwent FET PET and MRI (T1 ± Gd-DTPA) ahead of surgery for brain metastasis. Imaging data were fused (iPlan cranial/Brainlab). FET uptake was quantified as SUV, and a tumour-to-background-ratio of 1.6 or more was defined as PET-positive. Tissue samples were obtained during surgery and assigned to three groups (1. PET- and MRI-positive; 2. PET-positive and MRI-negative; 3. PET-negative and MRI-positive). Tissue samples were processed immunohistechemically and labelled for Ki-67/Mib-1 to determine proliferation (Ki-67 labelling indices). In addition, necrosis, vital tumour and brain parenchyma ratios were quantified for each tissue sample based on histology.

Results: 15 patients were enrolled (origin of neoplasm: four melanoma; four upper/lower GI; four NSCLC, one SCLC; one breast; one renal). 27 samples were obtained. For five samples, histology revealed parenchyma only (including all three groups). One sample was completely necrotic. The tumour ratio within the specimen increased with PET positivity, as did the tumour proliferation(Figure 1 [Fig. 1], left and right). The necrosis ratio within the specimen decreased with PET positivity (Figure 2 [Fig. 2]). Samples from PET- and MRI-positive locations (group 1)had the highest tumour ratio and proliferation index (57.1 and 65.8%, respectively), which were lower ingroup 3 (50.2 and 63.7%) and lowest in group 2 (34.7 and 57.5%).

Conclusion: In this preliminary study, FET PET provided diagnostic information on brain metastases. There is a trend towards tissue containing more vital, proliferating tumour and less necrotic material with increasing SUV TBR. This confirms that in brain metastasis treatment, FET PET can be helpful in guiding selection of biopsy targets and in differentiationof actual tumor progression from treatment-associated (necrotic) tissue changes, i.e., pseudoprogression.