CASE REPORT Fibroma of the patellar tendon sheath—a rare case in a young boy Khushboo Pilania 1 & Bhavin Jankharia 1 & Fayaz W. Memon 2 Received: 18 December 2018 /Revised: 1 February 2019 /Accepted: 3 February 2019 # ISS 2019 Abstract Fibroma of the tendon sheath is a rare benign fibrocollagenous soft tissue tumor, arising predominantly from the synovium of tendon sheath. Fibromas occur most commonly in relation to the tendons of the fingers, hand, and the wrist. Fibromas related to large joints are rather rare and though amongst the large joints, the knee is a common site. Fibromas of the patellar tendon sheath, specifically, are very rare. To the best of our knowledge, only three cases of fibromas arising from the patellar tendon sheath have been reported in the relevant English medical literature. Herein we describe the fourth biopsy-proven case of fibroma of patellar tendon sheath in a 6-year-old boy. Keywords Fibroma . Patellar tendon . Infrapatellar mass Introduction Fibroma of the tendon sheath is a rare benign fibrocollagenous soft tissue tumor, arising predominantly from the synovium of tendon sheath [1, 2] but occasionally also from the joint capsule [3]. It usually presents as a slow-growing painless mass [3–5]. Fibromas occur most commonly in relation to the tendons of the fingers, hand, and the wrist [6]. Fibromas related to large joints are rather rare and though amongst the large joints, the knee is a common site [2, 4, 7–11]. Fibromas of the patel- lar tendon sheath, specifically, are very rare. To the best of our knowledge, only three cases of fibromas arising from the pa- tellar tendon sheath have been reported in the relevant English medical literature [9, 10, 12]. Herein we describe the fourth case of fibroma of patellar tendon sheath. We also summarize the literature review with clinical, imaging, and histologic findings of fibromas in this rare location. Case report A 6-year-old boy presented with a hard painless swelling in the left knee for 7–8 months. There was no history of signif- icant trauma. On physical examination, there was a hard, non-tender swelling in the infrapatellar region of the left knee. The muscle bulk and power around the knee was normal. There were no signs of neurological or vascular compromise. Laboratory da- ta were unremarkable. Conventional radiograph of the left knee joint showed a soft tissue shadow in the infrapatellar region (Fig. 1). A plain MRI of the left knee was subsequently done, which showed a well-defined soft tissue tumor, measuring 3 × 3 cm, in the Hoffa’ s fat pad, closely adherent to and infiltrating the overlying patellar tendon. The lesion showed thick peripheral T1 isointense rim with central hypointensity (Fig. 2). The lesion on fat-saturated T2-weighted images showed a thick peripheral iso- to hyperintense rim with central fluid-like hy- perintense signal intensity (Figs. 3 and 4). Contrast was not given to the patient. On MRI, there was suspicion of patellar tendon fibroma and CT-guided biopsy was done. Histopathological analysis showed a markedly hypocellular spindle cell lesion with scattered fibroblasts in * Khushboo Pilania pilania.khushboo@gmail.com Bhavin Jankharia bhavin@jankharia.com Fayaz W. Memon drfayazmemon@yahoo.com 1 Picture This By Jankharia, Bhaveshwar Vihar, 383 Sardar V P Rd, Opposite Vanita Vishram, Mumbai, Maharashtra 400004, India 2 Grant Government Medical College & Sir J J Group of Government Hospitals, Mumbai, Maharashtra, India Skeletal Radiology https://doi.org/10.1007/s00256-019-03178-0