Chondromyxoid fibroma (CMF) is a very uncommon bone tumor of cartilaginous origin accounting for <1% of all bone tumors ^[1]. The males are affected slightly more commonly than females, usually in affect the second and third decade of life ^[2]. It generally affects the metaphysis of long bones of a lower limb and seldom involves the small bones. However, the involvement of the flat bones like ribs is infrequent [2, 3, 4].

A teenage female patient presented to our outpatient department with complaints of pain on the right side of the chest for 6 months. The pain was a dull aching, mild in intensity without any diurnal variation. She had no visible swelling in the chest and elsewhere in the body. Physical examination revealed mild tenderness on the lateral aspect of the sixth rib. Her blood investigation, including the inflammatory markers, was within the standard limit. The chest X-ray examination showed a nodular, hyperdense lesion on the lateral chest wall near the 5–7^th rib on the right side (Fig. 1a). The lesion was confirmed to be 4.4 × 3.9 × 2.7 cm, arising from the sixth rib with ground-glass matrix and cortical thinning with smooth margin, without any soft-tissue involvement on the computed tomography scan (Fig. 1b). Fine-needle aspiration and cytology were done, which displayed mostly benign spindle cells. After taking informed consent, enblock excision of the lesion was done (Fig. 1c and d), and excised part was sent for histopathological examination. The histopathological study showed a well-circumscribed tumor with reactive bone formation at the periphery (Fig. 2a) with lobules of chondromyxoid tissue separated by spindle cells (Fig. 2b) and stellate-shaped cells (Fig. 2c). The intervening spindle cell showed monomorphic oval to spindle nuclei without any atypia, mitosis, or necrosis (Fig. 2d). The post-operative period was uneventful, and she was discharged on the 7^th day. She was doing fine at her last follow-up at 1 year.

Clinically, CMF is symptomatic and diagnosed incidentally radiographically ^[5]. Sometimes the presentation can be associated with variables such as pain, swelling, restriction of movement, or pathological fracture. Pain is usually mild, intermittent, and dull aching type. The typical radiographic appearance of the tumor is a lytic, oval, and eccentric metaphyseal lesion with a long axis parallel to the bone having a sclerotic rim without calcification ^[6]. The form in the flat bones is atypical as the lesion extends throughout the entire diameter of the affected bone, expanding both surfaces and often making diagnosis difficult ^[7]. Magnetic resonance imaging can help delineate the extent of the disease ^[8]. A tissue biopsy will confirm the diagnosis ^[9]. Typical features of CMF will show lobules of spindle-shaped or stellate cells spread among myxoid and chondroid matrix separated by zones of fibrous tissue without calcification ^[10]. Differential diagnosis includes chondroblastoma, enchondroma, low-grade chondrosarcoma, and aneurysmal bone cyst, which can be ruled out with the classical histopathological picture of CMF [2, 5, 11]. CMFs of the small bones are managed with simple curettage. Recurrence can be minimized with phenol application and sometimes enblock resection along with bone graft or PMMA (polymethylmethacrylate) application ^[12]. Armah et al. have excised the sole CMF in ribs in their 15 cases series ^[10]. Similarly, Long et al. reported a case where they did a surgical excision of the second rib ^[13]. Yamamoto et al. reported a case of exophytic soft-tissue attached to a CMF of a rib ^[14]. The authors did an intraoperative frozen mass biopsy and curettage and cauterization of the bone. A frozen biopsy is vital when eccentric CMF is associated with a soft-tissue component, as in their case. Radiotherapy is sometimes considered in case of unresectable tumors; however, it predisposes to the risk of post-radiation sarcoma ^[12]. Malignant transformation of CMFs is unusual and reported to be 1–2% ^[15].

CMFs are rare benign tumors that may be clinically and radiologically indistinct from other lesions. Therefore, a histopathological study is critical for the accurate diagnosis of CMF. In flat tubular bones like the ribs, enblock resection is the mainstay of treatment.

CMF is a rare benign tumorous lesion that affects mostly elderly males. Ribs are infrequent location of presentation. A thorough workup is necessary in such a situation to rule out other severe conditions which can mimic CMF.