Isolated Solitary Asymptomatic Skeletal Muscle Relapse of High-Grade Lymphoma Detected on Surveillance ¹⁸F- Fluorodeoxyglucose Positron Emission Tomography-Computed Tomography

A 26-year-old woman was diagnosed with Stage IB diffuse large B-cell lymphoma (DLBCL) involving neck nodes. She underwent four cycles of R-CHOP chemotherapy followed by involved field radiotherapy. End of the treatment ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography-computed tomography (PET-CT) showed complete metabolic response (CMR). Surveillance contrast-enhanced ¹⁸F-FDG PET CT was done after 6 months. [Figure 1, a-e] Maximum intensity projection PET image (a) showed a focal area of increased ¹⁸F-FDG uptake in the right abdominal region (arrow). Transaxial CT (b) and PET-CT (C) and coronal CT (d) and PET-CT (e) images showed an isoenhancing intensely ¹⁸F-FDG-avid nodular space-occupying lesion (arrow) in mid-part of the right erector spinae muscle, measuring 3.1 cm × 2.6 cm × 4.1 cm in size (SUV max 13.5). No other lesions were seen in the rest of the body. PET-CT-directed ultrasound-guided biopsy from the lesion showed DLBCL. The patient received three cycles of next line chemotherapy with gemcitabine and carboplatin. PET-CT was done for interim response assessment [Figure 1, f-j], which showed the complete resolution of the erector spinae lesion, suggesting CMR. She then underwent autologous stem cell transplantation. Post-transplant, she is doing fine at 1-year follow-up.

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Figure 1 Survillance PET maximum intensity projection image (a) showed a focal area of increased 18F-FDG uptake in the right abdominal region (arrow). Transaxial CT (b) and PET-CT (c) and coronal CT (d) and PET-CT (e) images showed an isoenhancing intensely 18F-FDG-avid nodular space-occupying lesion (arrow) in mid-part of the right erector spinae muscle, measuring 3.1cm×2.6cm×4.1cminsize(SUV max 13.5). No other lesions were seen in the rest of the body. PET-CT done for interim response assessment (f-j), showed complete resolution of the erector spinae lesion, suggesting CMR

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DLBCL is a high-grade B-cell lymphoma and is the most common variant of non-Hodgkin's lymphoma.[1] While extranodal involvement in DLBCL is relatively common, skeletal muscle involvement is quite rare with the reported incidence of 0.1%–1.4%.[2] In most cases, skeletal muscle involvement is associated with other sites of involvement as well.[3] While 30%–40% of patients with DLBCL will relapse, isolated skeletal muscle relapse is extremely rare,[4] as was seen in the present case. ¹⁸F-FDG PET-CT is now an integral part of the management of patients with lymphoma, especially DLBCL.[5] Interestingly, at times, it can uncover rare sites of disease involvement or relapse,[6] as in the present case. However, there are other differentials of the PET-CT findings in this case such as malignant fibrous histiocytoma or high-grade sarcoma.[7] Therefore, biopsy confirmation is always warranted.