Teaching Case

35-year-old female who presented to a physiotherapist with a 1-year history of chronic neck pain. The patient had a background of chronic headaches. On examination, she was noted to have a large, firm lump in the left posterior triangle of the neck.

Ultrasound of the neck showed a hypoechoic lesion with a fine echogenic margin. The lesion was not well appreciated with this modality despite being easily palpable (Figure 1).

CT with contrast demonstrated an 11cm lesion with diffuse amorphous calcification within the left paraspinal musculature. It was closely related to the underlying vertebral posterior elements (Figures 2a, 2b and 2c).

MRI showed that the lesion was arising from the spinous process of the 4^th cervical (C4) vertebra. It was hyperintense on STIR and T2 sequences and also displayed heterogeneous enhancement postcontrast (Figures 3a, 3b, 3c, 3d, 3e and 3f).

A technetium-99m bone scan was carried out to evaluate for metastatic disease. This confirmed a solitary lesion, which showed intense tracer uptake (Figure 4).

Parosteal osteosarcoma is a malignant bone tumour which arises from the most superficial layer of the periosteum [1]. It is uncommon and only represents 4% of all osteosarcomas [2]. This tumour type has a predisposition towards females in their 30s [3]. Patients most commonly present with a painless mass that slowly increases in size [1]. The converse was true in this case.

Parosteal osteosarcomas most frequently originate in the metaphysis of long bones, with the distal femur being the most common site, followed by the humerus and tibia. Cases have been reported where ribs, mandible, clavicle, tarsal bones, and the skull have been affected [2]. Osteosarcomas of any type originating from vertebrae are rare, comprising only 3%–5% of all osteosarcomas [4]. A literature search has not revealed any known cases of parosteal osteosarcoma originating from a vertebra in humans; however, a case has been reported in a canine subject [5].

On plain X-ray, parosteal osteosarcomas are often seen as a “large lobulated exophytic, cauliflower-like mass with central dense ossification adjacent to the bone” [6]. The “string sign” is seen in approximately 30% of cases and is where there a thin radiolucent line is seen to separate the tumour from the cortex [7]. This patient did not have plain imaging performed, but the mass appears cauliflower-like on cross-sectional imaging. On MRI, these tumours typically display mainly low signal intensity on both T1-weighted and T2-weighted imaging [6]. High signal intensity on T2-weighted sequences can be an indicator of a high-grade tumour [8]; however, this finding will be affected by necrosis or haemorrhage within the lesion [9]. This was the case with this patient, as despite the lesion being of high signal on T2, the pathology did not show any high-grade features.

With regards to differential diagnoses, myositis ossificans will tend to have more calcification peripherally, whereas parosteal osteosarcoma will have more denser calcification centrally [7]. Periosteal osteosarcomas will usually have an aggressive periosteal reaction, whereas parosteal osteosarcomas will demonstrate cortical thickening without an aggressive periosteal reaction [6].

Parosteal lipomas are more easily differentiated on cross-sectional imaging as a definite fat component will usually be seen [10].

Parosteal osteosarcomas tend to be low-grade and usually slow-growing. They are managed with surgical excision. Chemotherapy is rarely needed unless the tumour shows dedifferentiation [3]. This patient underwent complete resection of the tumour, with confirmation of the diagnosis on final pathology.

All patient data have been completely anonymised throughout the entire manuscript and related files.