Chondroblastoma (CB) is a rare benign bone tumor which commonly involves epiphysis or apophysis of long bone in adolescents [1], [2]. It accounts for 1–2 % of all benign bone tumors [2]. It is considered to arise from the secondary ossification centers [2]. Foot (tarsal bones) is an uncommon site for CB, yet it is the most common site of involvement after long bones and its frequency ranges from 3 to 16 % [3], [4], [5], [6]. Most of the patients present with pain which is accompanied by swelling and limited mobility at the adjacent joint [2], [3], [4], [6]. Radiologically, CB presents as an expansile destructive cartilaginous lesion in epiphyseal location with sclerotic borders and punctate calcifications [4], [7], [8]. Grossly, tumor is pink, gray to tan, brown in color with “wet sawdust” appearance [1], [8]. Microscopically, tumor is composed of sheets and nests of round to polygonal cells having convoluted nuclear membranes. Areas of chondroid matrix and chicken-wire calcification are also seen [1], [6], [8]. Aneurysmal bone cyst (ABC)-like changes have been reported in 25–57 % cases [3], [6], [9], [10]. Although characteristic histological and immunohistochemical features of CB are well-described in the literature, but the diagnosis becomes challenging for histopathologists in small biopsy samples [11], [12]. Various benign and malignant bone tumors with chondroid differentiation and/or multinucleated giant cells are included in the differential diagnosis of CB such as giant cell tumor (GCT), chondromyxoid fibroma (CMF), ABC, clear cell chondrosarcoma (CCC), chondroblastoma-like osteosarcoma (CBLOS) [11], [12], [13]. Recently, H3K36M and H3G34W mutations have been described as the molecular signatures of CB and GCT, respectively [12], [14], [15], [16], [17]. Subsequently, H3K36M immunohistochemical (IHC) stain has emerged as a highly sensitive and specific markers which helps in discriminating CB from its mimics [12], [14], [18], [19], [20], [21]. In addition, H3G34W which is expressed in GCT, the closet mimic of CB, is always negative in CB [18].

Surgery is the mainstay of treatment. Common surgical treatment options include curettage with bone graft, curettage with filling of methyl methacrylate and curettage alone. En-bloc resection and amputation are performed for aggressive tumors [2], [3], [8]. Recurrence rate ranges from 2.5 to 15 % [3], [22], [23], [24].

The distinction of CB from its mimics is important because of differences in prognosis and treatment options, therefore, we aimed to describe the clinicopathological features and frequency of H3K36M and H3G34W IHC stains in CB at an unusual location i.e., foot.