Discussion
Metatarsal involvement amongst smallbone osteosarcomas is uncommon. 3 There are about 32 cases of osteosarcomas reported in the literature from 1940 to 2018 involving the metatarsal bones (Table 1) . According to a review article from the Mayo Clinic, the most common bone of the foot involved is the calcaneum. 6 While the incidence of osteosarcomas of the foot as a whole is around 0.2% to 2%,3 metatarsal involvement is documented in 0.5% of these patients. 7 However, a recent study depicted metatarsal involvement in 33% of all osteosarcomas of the foot. 8
Osteosarcomas at conventional sites tend to have a bimodal age distribution with respect to disease affliction. 9 Metatarsal osteosarcomas, however, are more common in an older age group. 4,10 Our patient is probably the second youngest reported case of metatarsal osteosarcoma in the literature. 11
Biscaglia et al propounded that osteosarcomas of the metatarsal were a distinct subgroup due to the rarity of occurrence, anatomical location, and prognosis. 4 This often led to misdiagnosis and subsequent inadequate or inappropriate surgery. In six out of the ten cases (60%) described in Table 1, an incorrect pretreatment diagnosis was made that led to treatment delay. None, except one patient, received neoadjuvant chemotherapy, which is currently the standard of care. The average duration from symptom onset to diagnosis was found to be 2 years. 4 However, in our case, the duration of symptoms was approximately 2 months.
Surgery for metatarsal osteosarcomas can be challenging, as the compartments of the foot are narrow spaces with poor demarcation. Limb salvage surgery in the form of metatarsectomy needs proper preoperative planning and execution. Neoadjuvant chemotherapy will serve to downstage the tumor within the fascial barriers of the metatarsal compartment.It has also been postulated that osteosarcoma of the foot may have a better prognosis and survival compared to other osteosarcoma subsites. 10 This can be extrapolated from the fact that the majority are found to be low grade, and despite a long delay in treatment, there was no rapid increase in size and/or metastatic spread. However, tumor grade remains an important factor affecting survival— patients with higher grade tumors have worse survival. 8
A number of differentials, including benign tumors, are to be kept in mind when diagnosing and treating such patients (Table 2) . The most common benign tumors affecting the metatarsal are giant cell tumors (GCT) followed by chondromyxoid fibroma. Osteosarcomas and Ewing sarcomas constitute the malignant tumors. 12 Occasionally, infections like osteomyelitis of the small bones may mimic malignancy. The absence of an extensive soft tissue component and/or calcifications with the presence of bony changes (like sequestrum) favors a diagnosis of infection/osteomyelitis. In addition, clinical findings like fever, skin redness, and presence of a painful swelling (especially after onset of fever) point to an inflammatory pathology rather than malignancy. Stress fractures rarely simulate tumors. MRI showing marrow and soft tissue edema with a visible fracture line points to the diagnosis.
A plane radiograph showing cortical bone destruction with a soft tissue component and calcification should be considered suspicious and must be thoroughly evaluated prior to surgical treatment. 13 In a young patient such as ours, the important differentials that need to be considered include Ewing sarcoma, chronic osteomyelitis, and eosinophilic granuloma, which can radiologically mimic osteosarcoma at this location.