Small cell osteosarcoma: cytopathologic characteristics and differential diagnosis.

Small cell osteosarcoma may present a challenging primary diagnosis on cytologic assessment owing to its rarity and its morphologic similarity to other small round blue cell tumors. Five cases of small cell osteosarcoma from our cytopathology archives were identified and reviewed and cytologic features elaborated. Three cases were fine-needle aspirations from bony lesions in the classic location for osteosarcoma (2 distal femur and 1 proximal tibia), and 2 aspirations were from metastases. Common cytomorphologic features included relatively small to intermediate cell size, high nuclear/cytoplasmic ratios, round nuclei, minimal anisonucleosis, finely granular nuclear chromatin, fine cytoplasmic vacuoles, and only rare osteoid. Small cell osteosarcoma shares many of the well-described cytomorphologic features of classic osteosarcoma, but the relatively small cells, round hyperchromatic nuclei, and scant osteoid constitute the common denominator. Correlation with radiographic findings and ancillary tests can aid in definitive diagnosis.