A 42-year-old man presented with a lump on the plantar arch of his foot. Was it a fibroma or sarcoma? Dr Sebastian Fung takes us through the diagnostic process.
A 42 year old man presented with an uncomfortable lump on the plantar arch of his foot. He had noticed that this lump was increasing rapidly in size and causing pain and presented to the doctor. The lump resulted in difficulty weight-bearing and ambulating. On examination, there was palpable, mildly tender solitary immobile 3-4cm lump with no transillumination. No disruption of the skin was evident. Calcaneal squeeze test was negative, and there was no tenderness at the calcaneal enthesis of the plantar fascia. Clinically this was thought to be due to either a fibroma or sarcoma.
On MRI, there is a lobulated 4cm fusiform soft tissue mass centred on the plantar aponeurosis central cord corresponding to the palpable lesion, located well distal to the calcaneal enthesis. This is isointense to muscle on T1, high signal on T2, and enhances avidly post-gadolinium. Internal low signal linear “septations” (‘Comb sign’) is noted, typically seen in plantar fibromata. There is no overt invasion of surrounding structures. The remaining plantar fascia is intact further posteriorly/proximally.
A PET CT was performed and shows avid uptake of FDG (Fluorodeoxyglucose) indicating rapid glucose metabolism and cell turnover within the lesion. This is typically seen in fast growing neoplasms, or infection.
This lesion is located in an anatomical location typical for a plantar fibroma, but is atypical in its enhancement and avid FDG uptake. However, although plantar fibromata are fibrous and non-malignant, they have proliferative phases which can exhibit enhancement characteristics similar to this lesion.
The diagnosis was confirmed histologically as a proliferative plantar fibroma, and the patient underwent surgery and radiotherapy.
This case illustrates the need to assess the anatomical origin or epicentre of a soft tissue mass as well as its imaging characteristics. On face value, the presence of a rapidly enlarging mass with enhancement on MRI and FDG avidity on PET CT would point to a highly vascularised and potentially malignant neoplasm. However, its classic location and appearance allowed a diagnosis of plantar fibroma to be preoperatively entertained.
Figures a,b – Sagittal T2 fat saturated image (Figure A) showing a fusiform high signal (bright) soft tissue mass within the plantar aponeurosis (red arrow) . This is similar in signal to muscle on T1 axial imaging (Figure B) but shows internal linear striations. Note that the lesion is contained within and centered on the plantar fascia (linear black structure). The round focus next to the lesion is a skin marker capsule.
Figure c,d – Axial T1 (figure c) and Sagittal ( figure d)T1 fat saturated sequences showing intense enhancement of the lesion (red arrows)
Figure E,F, G – PET CT images showing the mass with avid FDG uptake, which is an indicator of glucose metabolic activity
