Popliteal Amyloidoma

A 63-year-old woman with a history of a resected popliteal mass years ago, is referenced to our institution for a 6 cm slow-growing mass, located in the left popliteal fossa, associated with pain and local functional disturbance.
Firstly, an ultrasound was requested, followed by an MRI and a PET-CT scan.

Ultrasound revealed a solid hypoechogenic lesion in the left popliteal fossa, with lobulated well-defined contours and no evident vascularization.
Magnetic Resonance Imaging (MRI) better evaluated the extent of the tumour, demonstrating 3 individualized lesions along the popliteal fossa, the bigger located in the supra-genicular plane. All of them revealed lobulated well-defined contours, separated from the adjacent structures without infiltration/invasion of them, only exerting mass effect. All lesions demonstrated iso-intensity to muscle on T1-weighted images, heterogeneous markedly hypo-intensity on T2-weighted images, no restriction on diffusion-weighted images, some oedema represented by discrete hyper-intensity on T2-SPAIR images, and a slight homogeneous enhancement after gadolinium administration on T1-3D Fat Suppression dynamic acquisition.
PET-CT demonstrated some punctiform dispersed calcifications within the lesions, a density similar to muscle, and a Standardized Uptake Value (SUV) of 4,49. No other lesions were documented.

Amyloidosis includes a group of diseases where there is extracellular deposition of amyloid, a unique proteinaceous insoluble material, resistant to proteolysis.[1,2] The deposition may be systemic, organ-limited or localized, and the disease can be hereditary or acquired.[3] Classification is based on the type of protein fibril (25 types already identified), the most common being amyloid light chain (AL type) and serum amyloid A (AA type).

The rarest presentation is in the form of a tumoural mass of amyloid deposition without systemic amyloidosis, called amyloidoma, which can be present in many anatomic sites.[2] In the soft-tissue subgroup, amyloidomas of the extremities are considered exceptional. The largest series of soft-tissue amyloidomas included 14 cases, none of which in the limbs.[4]
Clinically, they present as a slow-growing soft-tissue tumour, with local mass-effect in the adjacent structures, which can be associated with pain, functional disturbance and even vascular compromise.[5] These findings, however, are non-specific.
In the popliteal fossa, the origin of an amyloidoma is thought to be the result of chronic synovial inflammation, with consequent deposition of AA amyloid type in a synovial cyst similar to a Baker’s cyst.[5]

Attending to its rarity, imaging characteristics are not well known, with only a hand-full of case reports describing its signal on MRI.[3,5,6] On CT, amyloidomas frequently reveal foci of punctiform calcifications, which can increase the suspicion of an amyloidoma.[7] On T1W the signal is iso-intense to muscle, on T2W the signal is described as hypo-intense, with mild inhomogeneity, and on post-contrast T1W the tumour mildly enhances homogeneously.[3,5,6] However, at least one case report of a thigh amyloidoma revealed a heterogeneous lesion with areas of hyper-intensity, both on T1W and T2W,[1] which makes the signal characteristics not specific for diagnostic purposes. Regarding its borders/contours, a range of presentations from well-defined, regular or lobulated, to infiltrative and irregular have been described.[1,3] Regarding PET-CT, some case reports have described SUVs under 5,[8] while others mentioned SUVs over 5.[9,10]

Treatment of choice is based on surgical excision, with no adjuvant therapy, since is a benign non-metastatic tumour. This principle is also valid for local recurrences.[1]

Peripheral soft-tissue amyloidoma is a rare localized presentation of amyloidosis, usually with local pain and functional disability, with no typical imaging findings, being diagnosed only after a biopsy, and treated with surgical excision.

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