Teaching Case

A 29-year-old female with a history of osteoblastoma of the right radius that was previously treated with an intralesional curettage was presented to the orthopaedic surgeon with recurrent pain during movement of the wrist three years after the operation.

X-Ray imaging revealed a new occurrence of osteolytic lesions around the operated area. Magnetic resonance imaging (MRI) showed a local recurrence of several newly-formed lesions in the distal meta-diaphysis, which were hypointense on T1-weighted images (T1W) and hyperintense on T2-weighted images (T2W) with centrally enhanced contrast uptake. Radiologic features of the lesions were suspected to be either a dedifferentiation from osteoblastoma to a low-grade sarcoma with skip lesions, or a local recurrence of the osteoblastoma with surrounding osteoblastomatosis (Fig. 1, 2, 3). Positron Emission Tomography and Computerised Tomography (PET/CT) was performed for staging and showed metabolic activity only in the lesion of the distal radius (SUV max. 4,2) (Fig. 4). The patient was treated with a wide surgical resection of the distal radius, a reconstruction with ulna transposition and an arthrodesis (Fig. 5). Histological results identified the lesion as a grade 1 osteoblastoma-like osteosarcoma (OBLOS).

OBLOS is a rare variant of conventional osteosarcoma (OS) and represents 1,1 % of all osteosarcomas [1, 2]. It affects young adults with a slightly higher male-to-female ratio (1,2:1) [1, 3]. It causes chronic progressive pain in the affected area accompanied by swelling and a reduced mobility of the adjacent joint [1, 3-5]. OBLOS is hard to distinguish from a benign osteoblastoma (OB) because of their clinical resemblance and histological and radiological appearance. Unlike OB, OBLOS has the capability to metastasize and represents a high mortality risk [3]. Frequent location of OBLOS is the metaphysis of long bones, mostly the tibia, followed by the vertebrae, hands, feet and the femur [1].

Conventional radiologic imaging methods generally display the lesions as osteolytic, with thinned surrounding cortex and cortical destruction or as well-defined lesions, including a sclerotic rim [1, 3]. An MRI scan is a reliable diagnostic tool for assessing the extent of the intramedullary and soft tissue involvement. The lesions are hypointense on T1W and iso- or hyperintense on T2W, with substantial bone marrow oedema. The scans tend to show incoherent contrast enhancement centrally. However, MRI scans are inconclusive, since benign lesions may appear aggressive as well [1, 3, 6]. Histologically, OBLOS, may closely resemble osteoblastoma, sometimes with the only difference being increased mitotic activity (featuring atypical mitoses) and permeative growth in the former (OBLOS) but not the latter (osteoblastoma). On a molecular genetic level, more than 90% of osteoid osteomas and osteoblastomas but no osteosarcomas have been shown to harbour FOS or FOSB gene rearrangements [7, 8].

In this case, histopathological evaluation of the resected distal radius revealed seven separate foci of osteoblastic proliferation, morphologically resembling multiple nidi of osteoid osteoma/osteoblastoma, but focally displaying permeative growth and increased mitotic activity (Fig. 6), including some atypical mitoses. Molecular genetics was also performed and revealed no rearrangements in the FOS or FOSB genes. Hence, the histological diagnosis favoured a grade 1 osteoblastoma-like osteosarcoma (OBLOS) over osteoblastomatosis.

Prognosis of OBLOS is unfavourable because of its capability to metastasize, mostly in the lung, occasionally in the skeleton [4, 9, 10]. A wide surgical resection with chemotherapy is normally the treatment of choice [9, 11].

We report a case of OBLOS to emphasize, how difficult is its differentiation from a benign lesion. Incorrect diagnosis can lead to inadequate surgical treatment and a local recurrence. Clinicians should provide frequent follow-up visits.

Written informed patient consent for publication has been obtained.