Imaging superiority of [68Ga]-FAPI-04 over [18F]-FDG PET/CT in alveolar soft part sarcoma (ASPS)

Abstract

Several studies have indicated the superiority of [Ga]-FAPI04 over [F]-FDG PET/CT imaging in lesion detectability, diagnosis, and staging [1, 2]. Here, we report a 47-year-old man diagnosed with alveolar soft part sarcoma (ASPS) by tissue biopsy of the left pubis. [F]-FDG and [Ga]-FAPI04 PET/CT imaging was performed with a 2-day interval for adequate staging and evaluation. Multiple lesions in several organs were identified with mild uptake of [F]-FDG and moderate-to-high uptake of [Ga]-FAPI-04 ([F]-FDG and [Ga]-FAPI-04 PET maximum intensity projection, A and L respectively). [Ga]-FAPI-04 production process and PET/ CT acquisition were consistent with the former article [3]. SUVmax of representative lesions in scalp, lung, lymph node, liver, and bone on [F]-FDG and [Ga]-FAPI-04 were 2.1, 0.5, 3, 4, and 4.8 and 9.3, 2.3, 7.3, 6.5, and 12.7, respectively ([F]-FDG fused image, B–F; axial CT image, G–K; [Ga]FAPI-04 fused image, M–Q). Scalp lesions were identified on [Ga]-FAPI-04 PET/CT but ignored on [F]-FDG PET/CT due to its relatively low uptake of [F]-FDG and poor contrast caused by high glucose metabolism of the brain (B, M). Also, more liver metastases were found out as high lesion uptake and low liver-parenchyma uptake (SUVmax 1.5) on [ Ga]FAPI-04 PET/CT, which outperformed [F]-FDG exhibition (E, P). The histopathology results exhibited as hematoxylineosin staining original magnification ×40 and ×200, R, S; strong positive ASPSCR-transcription factor E3(TFE3) expression on immunohistochemistry ×100, T. ASPS is a rare soft tissue sarcoma, which is relatively slowgrowing and associated with a high rate of metastases at presentation. Final diagnosis is mainly based on pathological results, especially TFE3 molecular testing by immunohistochemistry [4]. Metastatic state values in the clinical management of ASPS, patients with no metastases are subjected to surgical resection, while systemic treatment is suitable for metastases. CT and MRI features of ASPS were well described in former studies, but local imaging restricted metastasis detection [4]. Whole-body [F]-FDG PET/CT is supposed as an auxiliary tool for the staging of most cancer including sarcoma. However, the pseudo alveolar pattern of ASPS caused by necrosis of the centrally located cells in the nests restricted its glucose metabolism; thus, slight [F]-FDG uptake existed in these patients and parts of metastases were missed. Kratochwil et al. [5] demonstrated that sarcoma exhibited the highest [Ga]-FAPI-04 SUVmax among 28 different kinds of cancer which might be attributed to an abundant interstitial component, and [Ga]-FAPI-04 could reflect cancer progression as well; thus, lesions presented a moderate-tohigh uptake of [Ga]-FAPI-04 in this case. High uptake of [F]-FDG in the brain and liver was sometimes recognized as limitations for small-metastasis detection, while low uptake of [Ga]-FAPI-04 was an advantage [1]; therefore, more lesions were identified in this case with lower background interference of [Ga]-FAPI-04 PET/CT. As the first case concerning [Ga]-FAPI-04 PET/CT imaging in ASPS, this report demonstrated that [Ga]-FAPI-04 worked better than [F]-FDG in lesion detection and staging of ASPS due to its higher sensitivity and better image contrast. This article is part of the Topical Collection on Image of the month.