There is a vast variety of tumor and tumor-like lesions affecting the foot and ankle, which can be classified as benign or malignant. This article provides an overview of the imaging characteristics of these soft tissue masses. (Table 1)

Table 1: Classification of Common (and Less Common) Soft Tissue Masses Around the Foot and Ankle.

1. Benign

1.1. Granuloma Annulare: An uncommon benign inflammatory dermatosis most often seen on the extremities of young children. The subcutaneous form is the most frequently observed by radiologists, typically presenting as a superficial mass. (Fig. 2).

Fig 2: Granuloma Annulare in a Four-Year-Old Boy: Radiography (A) shows a soft tissue mass without evidence of bone involvement or mineralization (green arrow). On ultrasound, it appears as an ill-defined solid mass (red line in B), hypoechoic compared to the surrounding fat, with an anechoic central area (yellow arrows). MRI typically reveals a mass with relatively low signal intensity on all pulse sequences, variable but generally well-defined margins, and extensive diffuse enhancement (not shown). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.2. Fibroma/Plantar Fibromatosis: Plantar fibromatosis (Ledderhose disease) is a benign fibroblastic proliferation of the plantar fascia. It occurs predominantly in the 4th–6th decade, it may be bilateral (20– 50%) and presents with multiple nodules (33%). There is an association with other fibrotic diseases such as palmar fibromatosis (Dupuytren’s disease) and penile fibromatosis (Peyronnie’s disease). It manifests as a palpable mass that is not painful unless very large. US may be sufficient for diagnosis (Fig. 3).

Fig 3: Fibroma / Plantar Fibromatosis: A 65-year-old man with melanoma and an incidental hypermetabolic lesion in the plantar fascia. Fibromas appear as fusiform hypoechoic lesions arising from the plantar fascia (A). On MRI, they are typically heterogeneous and hypointense relative to skeletal muscle on T1WI (B) and iso- to hyperintense compared to skeletal muscle on T2FS WI (C). This lesion exhibits uptake on FDG PET/CT (D). Histology (E) shows the fascia at the top (red arrow) and fibromatosis at the bottom (green arrow), with fascicles of cells oriented in three directions. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

 1.3. Neural lesions

1.3.1. Traumatic Neuroma: Traumatic neuromas are nonneoplastic proliferations of injured nerves that develop after trauma or surgery. They typically present with radiating pain without an associated palpable mass and are a frequent clinical diagnosis. Focal areas of perineural fibrosis around a common plantar digital nerve of the foot, usually caused by chronic repetitive stress, are known as Morton’s neuroma, with the third interspace being the most commonly affected site. In contrast, traumatic neuromas most often occur in the lower extremity following amputation (Fig. 4).

Fig 4: Morton Neuroma and Post-Amputation Neuroma: A 70-year-old man with Morton neuroma. On ultrasound (US), it appears as a well-circumscribed, ovoid, hypoechoic lesion (A). MRI can be useful when ultrasound findings are inconclusive, as it helps differentiate Morton’s neuroma from its primary differential diagnosis, bursitis. The lesion typically appears as a dumbbell- or ovoid-shaped structure with a low-to-isointense signal on T1WI (yellow arrows). Microscopic Appearance (D): Neural degeneration with epineural and endoneurial vascular hyalinization, along with perineural fibrosis surrounding a plantar digital nerve. (E, F) Another case involves a 56-year-old man who underwent first toe amputation (red arrow) and subsequently developed neuromas (blue arrows). Both Morton neuromas and post-amputation neuromas appear as solid nodules on MRI and demonstrate enhancement following contrast media administration. (Cases courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.3.2. Benign Peripheral Nerve Sheath Tumors

Classically, benign peripheral nerve sheath tumors (PNSTs) are divided into schwannomas and neurofibromas. They are rare in the foot and can occur at any age (Figs. 5, 6).

Fig 5: Schwannoma: The diagnosis of peripheral nerve sheath tumors requires identifying the direct continuation of a nerve into and out of a mass (tail sign). MRI is a valuable imaging modality for evaluating and detecting deep-seated schwannomas. On MRI, schwannomas appear as well-circumscribed, rounded lesions that exhibit a high signal on T2WI (A) and FS T2WI (B), as well as a low-to-intermediate signal on T1WI (C). Histological analysis confirms the diagnosis of schwannoma in this patient (D). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

Fig 6: Cystic Neurogenic Tumor: A 59-year-old man with tarsal syndrome. MRI images of a cystic neurogenic tumor of the foot (blue arrows) demonstrate an isointense signal relative to muscle on T1WI (A), high signal intensity on T2WI and FS T2WI (B and D) due to cystic degeneration, and heterogeneous enhancement post-contrast (C). The solid components enhance, while the cystic areas remain non-enhancing. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.4. Rheumatoid pathology

1.4.1. Rheumatoid Nodules: These are the most common extra-articular manifestation of rheumatoid arthritis. They are typically subcutaneous and occur at sites of repeated microtrauma. Only 1% of cases involve the foot, most commonly beneath the metatarsal head region. Diagnosis is suggested based on the clinical context and accompanying radiologic findings, such as bursitis and bone erosions (Fig. 7).

Fig 7: Rheumatoid Nodules: A 47-year-old woman with rheumatoid arthritis: Rheumatoid nodules appear as solid lesions on both magnetic resonance imaging (MRI) and ultrasound (US), sometimes exhibiting increased peripheral vascularization, detectable using Doppler US (D). MRI characteristics are variable and non-specific, typically appearing isointense to muscle on T1WI with a heterogeneous intermediate-to-high signal on T2WI. In this patient, MRI reveals a small nodule in the first toe, closely associated with the flexor tendon (yellow arrows). (E, F): Another rheumatoid nodule is seen in the same foot, located caudal to the fifth metatarsophalangeal joint (green arrow) and associated with characteristic adjacent submetatarsal bursitis (red arrows). The presence of erosions and synovitis is a key indicator for diagnosing rheumatoid nodules. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.4.2. Gouty Tophus: Gout is an inflammatory arthritis caused by elevated uric acid levels in the blood and affects 1–2% of adults in developed countries (Fig. 8).

Fig 8: Gout Tophi: A 46-year-old man with bilateral painful swelling of the first metatarsophalangeal joints. The diagnosis of gout can be made based on the presence of bony erosions with sclerotic margins and overhanging edges in characteristic locations (yellow arrows on X-ray). Gout tophi appear as juxta-articular soft tissue nodules (red arrows) and present as hypoechoic ovoid lesions on ultrasound (B). MRI shows amorphous and sometimes nodular regions of low signal intensity on T1WI, with variable intensity on T2WI (green arrow pointing to the tophus). Histological analysis (D) confirms the lesion as a gout tophus. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.5. Fat lesions: Lipomatosis and lipomas are well visualized using all imaging techniques. Spindle cell lipoma is a benign lesion in which mature fat is replaced by collagen-forming spindle cells. Its occurrence in the foot and ankle is extremely rare, with only a few documented cases. In most instances, radiologic findings overlap with those of liposarcoma, making a preoperative biopsy necessary (Fig. 9).

Fig 9: Lipomas: Lipomas and lipomatosis, characterized by excessive and diffuse fat deposition, exhibit a typical signal pattern on MRI and a density on plain films or CT similar to normal fat. We present two cases. 1) Spindle Cell Lipoma: A 56-year-old man with a painful nodule related to the Achilles tendon. This lesion appears hyperintense on both T1-weighted imaging (T1WI) (A) and T2-weighted imaging (T2WI), with signal loss on fat-saturated imaging (B). However, spindle cell lipomas contain non-fatty components with varying signal behaviors, some of which may enhance on post-contrast fat-saturated T1WI (red arrow). A biopsy confirmed the histological diagnosis of a spindle cell lipoma (D). 2) Soft tissue Lipomatosis (E, F): A 61-year-old woman with difficulty wearing footwear. MRI images show soft tissue lipomatosis (red circle) with characteristic fat signal intensity. (Cases courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.6. Vascular lesions: are classified as neoplastic tumors or non-neoplastic malformations, the latter being more common in the foot and ankle. They are further categorized as capillary, lymphatic, venous, arteriovenous, or combined subtypes and may contain fat, fibrous tissue, or smooth muscle, often conforming to adjacent structures. These lesions account for ~7% of benign soft tissue masses in the foot and ankle and typically appear before age 30. Though often asymptomatic, they can cause post-exercise pain due to vascular steal. A bluish skin discoloration is characteristic. Radiographs may show phleboliths, periosteal reactions, or cortical thickening.

Hematomas are more common than vascular lesions and are often secondary to trauma. Their composition ranges from fresh hemorrhage to chronic hemoglobin degradation products (Fig. 10).

Fig 10: Hematoma: 46-year-old man with a history of a sprain one month ago. The lateral radiograph shows focal soft tissue swelling (A). Ultrasound (B) reveals a well-defined fluid collection with heterogeneous content, consistent with a hematoma. MRI in the proton density (PD) weighted imaging (WI) sequence demonstrates high signal intensity (C) and FS PDWI hyperintensity (D), suggestive of an early subacute hematoma (yellow arrows). Bone edema (red arrows) is also visible, likely resulting from the previous trauma. (E, F) MRI performed four months later on the same patient shows a reduction in the affected area, indicating the resolution of most of the hematoma. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.7. Tenosynovial Giant Cell Tumor (TGCT): is a group of usually benign lesions arising from the synovium of joints, bursae, and tendon sheaths. Based on growth pattern and clinical behavior, they are classified as localized or diffuse types. TGCT is the unifying term adopted in the 2020 WHO Classification of Soft Tissue and Bone Tumors (5th ed.) (Fig. 11).

Fig 11: Tenosynovial Giant Cell Tumor (TGCT) in a 45-year-old woman: Radiography, often used as an initial diagnostic tool, reveal periarticular soft tissue swelling and increased density due to hemosiderin accumulation (red arrow in A). MRI is essential for diagnosis, evaluating disease extent, and guiding treatment planning. The diffuse type exhibits a heterogeneous appearance on T2WI, while the localized type appears isointense to surrounding muscles on T1WI and predominantly hyperintense on T2WI. In this case, the lesion presents a heterogeneous appearance on PD FS WI (B) and appears isointense to surrounding muscles on T1WI (C). Additionally, Echo-gradient sequences (D) demonstrate areas of low signal intensity due to the blooming effect, indicating hemosiderin deposits within the lesion. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.8. Bursitis and synovial cysts

1.8.1. Intermetatarsal Bursa/Bursitis

The intermetatarsal bursa is a normal synovium-lined cavity between the metatarsal heads. A small amount of fluid in the first through third intermetatarsal spaces is not necessarily indicative of bursitis. Chronic mechanical compression and irritation between the metatarsal heads, similar to Morton’s neuroma, are believed to contribute to its development. These conditions often coexist.

Adventitious Bursitis: Adventitious bursae form in adulthood at sites exposed to high pressure and friction, usually near bony prominences. In the foot and ankle, they commonly develop over the medial first metatarsal head and plantar to the metatarsal heads.

1.8.2. Ganglion Cysts: are common pseudotumoral lesions thought to result from myxoid degeneration of connective tissue. Macroscopically, they present as round, oval, or multilobulated cystic lesions filled with mucinous fluid. After the hand and wrist, the foot and ankle are the most frequent locations (Fig. 12).

Fig 12: Ganglion Cyst: Ultrasound typically reveals a well-defined, fluid-filled structure with an anechoic appearance and posterior acoustic enhancement due to its fluid content. In this case, internal septations are present, giving the cyst a multilocular appearance (A). No bone involvement or mineralization is observed (B). On MRI, ganglion cysts appear as well-defined, septated masses with signal intensities similar to water. They are typically hypointense on T1WI (C) and hyperintense on T2WI (D, E). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.9. Epidermoid Cyst: is a proliferation of keratin-producing epidermal cells within the dermis, often resulting from the traumatic implantation of epidermal tissue. In the foot, the most common location is the plantar or medial aspect of the first metatarsal head (Fig. 13).

Fig 13: Epidermoid Cyst in a 37-Year-Old Woman: Radiography reveals a soft tissue lesion adjacent to the proximal phalanx (yellow arrow in A), while ultrasound shows a well-defined, rounded hypoechoic mass with variable echogenic foci and posterior acoustic enhancement (B); a ruptured epidermoid cyst may exhibit lobulated contours and color Doppler signals, mimicking a solid mass. MRI shows a hypointense signal on T1WI (C) and a heterogeneous, predominantly hyperintense signal with linear hypointensities on T2WI (D). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

1.10. Infection (Abscess): An abscess is a focal fluid collection containing inflammatory cells, bacteria, and necrotic tissue debris, surrounded by hypervascular connective tissue. On ultrasound, it appears as an irregular, hypoechoic, centrally avascular fluid collection with a hypervascular rim. On MRI, it shows increased signal on fluid-sensitive sequences, decreased signal on T1WI, and peripheral enhancement on Gd-T1WI (Fig. 14)

Fig 14: Soft Tissue Abscess: 48-year-old diabetic man with a previous amputation of the left foot. Bone marrow signal abnormality in the third metatarsal phalanx related to osteomyelitis (red arrows) and a dorsal soft-tissue abscess, which is hypointense on T1-weighted imaging (yellow arrow in A), hyperintense on T2-weighted imaging (yellow arrow in B), and shows peripheral enhancement after gadolinium administration (yellow arrow in C). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

2. Malignant

2.1. Synovial Sarcoma: is the most common sarcoma in the foot and ankle, accounting for 18–25% of cases in this region. It primarily affects young adults (15 to 45 years). Despite its name, it originates from mesenchymal cells and is unrelated to synovial tissue. There are three subtypes: monophasic, biphasic, and poorly differentiated, with 90% showing keratin positivity. Lesions grow slowly (2–4 years) and are typically painful, unlike other sarcomas (Fig. 15).

Fig 15: Synovial Sarcoma: 67-year-old woman with synovial sarcoma of the left foot referred to our center. The MRI appearance of synovial sarcoma varies depending on the lesion's size and growth rate. Typically, it appears hypointense on T1-weighted imaging (A) and hyperintense on T2WI (B), with enhancement of viable tumor tissue on contrast-enhanced T1WI. These lesions may resemble cysts and often have well-defined borders. Therefore, when a cystic lesion is detected in an atypical location, contrast administration is essential to rule out a solid lesion. Larger lesions may be heterogeneous, exhibiting the characteristic "triple sign" on T2-weighted imaging, containing septa (resembling a grape cluster), and showing fluid levels in 10–25% of cases. In this patient, the lesion displaces tendons and obliterates fat (yellow arrows). PET-CT demonstrates pathological FDG uptake with an SUVmax of 2.9 (C). Histological analysis reveals spindle cell proliferation with a hemangiopericytic growth pattern, corresponding to FNCLCC grade 2 monophasic synovial sarcoma (D). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

2.2. Extraskeletal Chondrosarcomas: are rare, with the foot being an exceptionally uncommon location. They account for 2% of soft tissue sarcomas and only 1% of all chondrosarcomas. They can occur at any age but typically present around the fifth decade of life (Fig. 16).

Fig 16: Extraskeletal Chondrosarcoma: A 31-year-old man presented with a lump on the sole of his foot. Imaging revealed a soft tissue mass along the lateral border of the fifth metatarsal bone, with areas of chondroid matrix mineralization on the oblique plain film (red circle in A). Ultrasonography (B) demonstrated both soft tissue (orange arrow) and hard tissue components (blue arrow) within the lesion, characterized by an ill-defined hyperechoic pattern and acoustic shadowing. MRI findings resemble those of bone chondrosarcomas. Short-axis T1WI (C) and sagittal FS T1WI after contrast administration (D) show a predominantly low T1 signal, isointense to muscle (green arrow), with scattered areas of fat signal (red arrow). The lesion exhibits intense post-contrast enhancement (yellow arrow), with no evidence of peritumoral edema. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

2.3. Hidradenocarcinoma: are rare, aggressive cutaneous adnexal malignancies with sweat gland differentiation (Fig. 17).

Fig 17: Hidradenocarcinoma in a 27-Year-Old Woman: MRI typically reveals a well-defined subcutaneous mass with heterogeneous signal intensity. The lesion appears isointense on T1-weighted imaging (A) and demonstrates intermediate signal intensity compared to muscle on sagittal fat-saturated T2WI (B). On sagittal fat-saturated T1WI with contrast (C), the lesion exhibits heterogeneous enhancement (red arrow), with more intense peripheral enhancement. Ultrasound imaging (D) shows a solid dermal-hypodermal lesion, predominantly hypoechoic, with central hyperechogenicity and irregular, spiculated margins. PET-CT (E) reveals hypermetabolic activity (blue arrow). A biopsy confirmed the diagnosis of hidradenocarcinoma. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

2.4. Dermatofibrosarcoma protuberans: is a rare, low-grade soft tissue sarcoma arising from the dermal and subcutaneous tissues. It typically occurs in early to mid-adulthood, most commonly affecting the trunk and proximal extremities. Prognosis is generally excellent after complete resection, though recurrence is possible. Metastases are rare, primarily involving the lungs. Fibrosarcomatous transformation is uncommon (Fig. 18).

Fig 18: Dermatofibrosarcoma Protuberans: A 28-year-old man presented with an anterior ankle mass. On ultrasound (A), dermatofibrosarcoma protuberans appears as an oval, well-defined, predominantly hypoechoic lesion with posterior acoustic enhancement (yellow arrow). MRI shows the lesion with low signal intensity on sagittal T1WI (green arrow in B), higher signal intensity than fat on axial T2WI (red arrow in C), and hyperintensity on sagittal FS T2WI (red arrow in D), with gadolinium enhancement (not shown). (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).

2.5. Metastases: Metastasis to the foot and ankle are rare (less than 1%) and indicate poor prognosis. The most common primary sources are lung, colon, and genitourinary tumors. Myxofibrosarcoma (MFS) is a rare soft tissue tumor that predominantly affects adults, characterized by a high recurrence rate and a low incidence of distant metastases (Fig. 19).

Fig 19: Metastasis: This case involves an 81-year-old man diagnosed with myxofibrosarcoma of the thigh (red circle in A). A new lesion has developed on the foot, demonstrating enhancement on CT (yellow arrow) and uptake on PET-CT (green arrow). This lesion corresponds to a metastatic deposit from the primary myxofibrosarcoma. (Case courtesy of Hospital Clínico San Carlos, Madrid, Spain).