A 48-year-old man was referred for plain radiography and MRI with history of a painless swelling in distal right arm gradually enlarging in size over the past 2 months. He had undergone an ultrasound guided biopsy elsewhere which proved to be inconclusive on histopathology. Surgical excision was planned.
Plain radiographs revealed soft tissue swelling along the anteromedial aspect of distal right arm with no internal calcification or underlying bony changes. MRI showed a relatively well defined mass lesion with predominant hypointense signal on all sequences located within the brachialis muscle in distal arm. The lesion showed heterogeneous enhancement on post contrast images. Irregular areas of T1W hypointense/STIR hyperintense signal and contrast enhancement were also seen in subcutaneous soft tissues along medial aspect of the mass suggestive of inflammatory changes due to previous biopsy.
The lesion was also seen abutting the anterior margin of distal right humerus. No abnormal marrow signal changes were however noted within the humerus. The neurovascular structures were displaced anteriorly by the mass. No definite extension of the mass beyond muscle margin was seen. A possibility of an intramuscular fibrous soft tissue tumour was put forth based on MRI findings. Histopathology revealed a nuchal-type fibroma.
Nuchal type fibroma (NTF), classified as a type of benign fibrous proliferation, is a rare tumour-like process that usually occurs in the nuchal and interscapular regions [1, 2]. Although the process is considered benign, it has a potential for recurrence. A male predominance has also been reported. Histopathologically NTF is composed of paucicellular collagenous connective tissue with entrapped adipocytes, nerves and blood vessels. Extension into skeletal muscle is seen in some cases. NTFs are named so due to the fact that they mimic the thick collagenous tissue normally residing in the nuchal area [1]. Extranuchal sites of occurrence have been described in case studies. The most common location for extranuchal NTF was the upper back, but facial, chest wall, extremity and other locations have been described [1, 3]. Extranuchal NTFs also show histology similar to the nuchal sites [1].
MRI is an invaluable tool in the evaluation of soft tissue tumours. Fibrous tumours in general show hypointense signal on T1 and T2 weighted images. T2 weighted images in particular have been shown to reveal information related to histology. Fibrous tumours that have low cellularity and dense collagenous components usually show lower signal intensity on T2 weighted images. Cellular tumours tend to appear hyperintense on T2 weighted images and are associated with a higher risk of recurrence. Most fibrous soft tissue tumours also show enhancement on post contrast images [2]. Calcification in a soft tissue mass may also result in hypointense signal on MRI. Calcification can however be reliably excluded on conventional radiography or CT. Any underlying bony changes are also well seen on plain radiographs or CT.
Our case was peculiar due to the fact that the soft tissue tumour with hypointense signal on all sequences was entirely intramuscular, located within the brachialis muscle in distal arm. Calcification and underlying bony changes were excluded using plain radiography. Based on MRI features a diagnosis of a fibrous soft tissue tumour was made and possibilities of extra-abdominal desmoid tumour and desmoplastic fibroblastoma were put forth based on propensity for muscle involvement [2, 4]. At surgery a firm ovoid soft tissue mass was excised from within the brachialis muscle and histopathology revealed features consistent with a NTF. Although cases of NTF involving adjacent skeletal muscle have been described [1, 3], NTF occurring primarily in an intramuscular location has not been previously reported to the best of our knowledge.
Nuchal type fibroma within the brachialis muscle in distal arm.
According to the MRI and plain radiograph data of the right distal forearm (brachialis region) provided by the patient, the following observations can be made:
In summary, imaging suggests a relatively dense fibrous lesion within the brachialis muscle, with no notable calcification or signs of bony destruction.
Based on the patient’s age, lesion location, low MRI signal characteristics, and histological tendencies, the possible diagnoses include:
These tumors share the common feature of being rich in collagen fibers with low cellularity, leading to markedly low signal on T2-weighted sequences. Although Nuchal Type Fibroma most frequently appears in the posterior neck region, there are rare case reports describing it in the brachialis muscle, with similar histologic features.
Based on the patient’s histological findings (abundant collagen fibers with minimal cellular components, and no other distinct pathological changes), the final diagnosis in this case is:
Nuchal Type Fibroma (NTF), in a rare intramuscular presentation.
This diagnosis is supported by:
Postoperative rehabilitation should focus on restoring limb function and preventing muscle strength loss. It can be divided into the following phases:
Overall, adherence to the FITT-VP principle (Frequency, Intensity, Time, Type, Volume, Progression) is recommended to ensure a gradual and individualized approach.
This report provides a reference medical analysis based on existing imaging and clinical information. It is not a substitute for an in-person evaluation by a qualified medical practitioner. If you have any doubts or experience changes in your condition, please seek medical advice promptly and follow the treatment plan recommended by your specialist.
Nuchal type fibroma within the brachialis muscle in distal arm.
