Case Report: PET-avid compensatory tensor fascia lata hypertrophy mimicking metastatic disease in a patient with a chronic Achilles tendon rupture.

In this case report, we describe an unusual case of PET-avid muscle enlargement in the right tensor fascia lata (TFL) muscle mimicking metastatic disease during lung cancer staging. The patient, a 67-year-old woman, had a history of left Achilles tendon rupture, which had been managed non-operatively. A PET/CT scan revealed focal fluorodeoxyglucose (FDG) uptake in a markedly expanded right TFL muscle. The presence of concurrent ipsilateral pelvic lymphadenopathy on the scan initially raised concern for muscle metastasis in this context. However, subsequent MRI of the hip showed diffuse muscle enlargement without a discrete mass, favoring benign muscle hypertrophy, particularly given the presence of a concurrent chronic Achilles tendon rupture that altered the patient's gait. A follow-up surveillance PET/CT and subsequent CT scans obtained over several years showed stability of the muscle abnormality, supporting the diagnosis of benign muscle hypertrophy and preventing an unnecessary biopsy. This case highlights the importance of correlating imaging findings with prior biomechanical injuries that can impact muscle architecture, particularly in a malignancy workup where FDG uptake in muscle may be misleading.