Subcutaneous anterior abdominal wall myositis ossificans

Twenty-one-year-old woman, presented to the clinic with a history of a tender right lower quadrant mass, first noticed 8 months ago. The mass had been slowly increasing in size. The patient reported localized abdominal pain and tenderness aggravated by movement and deep breaths. The pain was not related to her menstrual cycle. She reported a history of excessive exercise but denied any trauma. The patient was healthy without significant past medical or surgical history. Physical examination revealed a tender right lower quadrant subcutaneous firm mass, measuring approximately 5cm, attached to muscles. The overlaying skin was normal without redness or warmth and there was no lymphadenopathy. The general examination was otherwise unremarkable. The biochemical and hematological laboratory results were normal. An initial ultrasound showed a heterogenous lobulated subcutaneous mass, measuring approximately 5.4 × 3 cm. Color Doppler imaging did not reveal internal vascular flow. The mass did not exhibit posterior acoustic shadowing or through enhancement ( Fig. 1 ). The findings were not specific, and further evaluation by cross-sectional imaging was recommended. Fig. 1 (A) Gray scale ultrasound images of the right lower quadrant abdominal wall, showing heterogenous lobulated subcutaneous mass, measuring 5.4 × 3 cm with no vascular flow on color Doppler (Not shown). No posterior acoustic shadowing or enhancement. Axial (B) and Sagittal (C) post contrast CT images, showing right lower quadrant heterogenous subcutaneous soft tissue mass with punctate non peripheral calcifications, abutting the right rectus muscle. The mass measures 2.5 × 3 cm and is associated with adjacent fast stranding and edema. Fig 1 (A) Gray scale ultrasound images of the right lower quadrant abdominal wall, showing heterogenous lobulated subcutaneous mass, measuring 5.4 × 3 cm with no vascular flow on color Doppler (Not shown). No posterior acoustic shadowing or enhancement. Axial (B) and Sagittal (C) post contrast CT images, showing right lower quadrant heterogenous subcutaneous soft tissue mass with punctate non peripheral calcifications, abutting the right rectus muscle. The mass measures 2.5 × 3 cm and is associated with adjacent fast stranding and edema. The patient underwent single-phase contrast-enhanced Computed Tomography (CT), which revealed a right lower quadrant heterogenous subcutaneous soft tissue mass with a few punctate nonperipheral calcifications, abutting the right rectus muscle, without a claw sign. The mass measured 2.5 × 3 cm and was associated with adjacent fast stranding and edema ( Fig. 1 ). No overlaying skin contact or skin thickening was observed. The right inferior epigastric vessels were patent and located medial to the mass. The differential diagnosis after the CT remained nonspecific and broad. To narrow the differential diagnosis, she was referred for a Pelvic Magnetic Resonance Imaging (MRI). The MRI revealed a round soft tissue mass with lobulated margins in the right lower anterior abdominal wall, not directly related to the rectus sheeth. The mass was predominantly of low signal on T1-weighted images (T1WI) and intermediate to high signal on T2WI. No bright signal spots on T1WI to suggest blood product or fat content. The mass measured 3 × 4 × 4.5 cm (AP x TR x CC). The mass demonstrates diffusion restriction and avid postcontrast enhancement ( Fig. 2 ). MRI was negative for ovarian endometriomas, uterine adenomyosis and deep pelvic endometriosis. Fig. 2 Axial MRI images of the lower abdomen showing a round soft tissue mass, with lobulated margins located at the right lower anterior abdominal wall. The mass is of intermediate to high signal on T2WI (A), predominantly intermediate to low signal on T1WI without bright signal spots on to suggest blood product or fat content (B), post contrast image shows avid heterogenous enhancement(C). Fig 2 Axial MRI images of the lower abdomen showing a round soft tissue mass, with lobulated margins located at the right lower anterior abdominal wall. The mass is of intermediate to high signal on T2WI (A), predominantly intermediate to low signal on T1WI without bright signal spots on to suggest blood product or fat content (B), post contrast image shows avid heterogenous enhancement(C). The constellation of imaging findings were nonspecific, offering a broad differential diagnosis. The differential diagnoses included: desmoid tumor, anterior abdominal wall endometriotic deposit, and Sarcoma. Each diagnosis of the differential has distinct management approaches based on the disease morbidity and mortality. Therefore, tissue diagnosis was required. The patient underwent an Ultrasound Guided biopsy and 5 core biopsies were obtained and sent for histopathology. The histopathology revealed fibroblastic/myofibroblastic proliferation with bone formation/ossification, chronic inflammation and focal fat necrosis ( Fig. 3 ). The tissue was positive for smooth muscle actin (SMA), negative for Desmin, CD34 and ALK-1. These findings suggested heterotopic ossification/myositis ossificans, traumatic or nontraumatic. Fig. 3 Low power view 20× (A). Shows fibroblastic/myofibroblastic proliferation, which is rich in vascular and inflammatory cells resembles nodular fasciitis with osteoblasts and osteoid. Higher power view 100× (B). Shows mixture of fibroblasts and osteoblasts along with erratic osteoid. Cells show mild degree of pleomorphism and brisk mitosis. Fig 3 Low power view 20× (A). Shows fibroblastic/myofibroblastic proliferation, which is rich in vascular and inflammatory cells resembles nodular fasciitis with osteoblasts and osteoid. Higher power view 100× (B). Shows mixture of fibroblasts and osteoblasts along with erratic osteoid. Cells show mild degree of pleomorphism and brisk mitosis. The patient received Ibuprofen for symptom control and is scheduled for a follow up CT is in 3 months, followed by clinic visit. The Follow-up CT showed a decrease in the size of the mass from 2.5 × 3 cm to 1 × 3 cm with an increase in the size and amount of intralesional calcifications, which were now peripherally located, demonstrating the zone phenomena typically seen in MO ( Fig. 4 ). The follow-up clinic visit, physical examination confirmed the decrease in size and further hardening of the mass, with the patient reporting improvement insymptoms. Fig. 4 Axial post contrast CT image (A) and Coronal bone window CT image (B), showing interval decrease in the size of the mass and further increase in the size and amount of intralesional calcifications/ossification demonstrating the zone phenomena. Three-dimensional reconstructed image of the mass (C), showing the ossification. Fig 4 Axial post contrast CT image (A) and Coronal bone window CT image (B), showing interval decrease in the size of the mass and further increase in the size and amount of intralesional calcifications/ossification demonstrating the zone phenomena. Three-dimensional reconstructed image of the mass (C), showing the ossification.

Exempted by King Saud University IRB, IRB Protocol Number : E-24-8549.

Obtained, however, no patient identifiers included in the Case report.

Early myositis ossificans can present a diagnostic dilemma, especially when occurring in an extra-skeletal location. Tissue biopsy plays a major rule in final diagnosis.

Myositis ossificans (MO) is a pseudo- inflammatory, benign bone and cartilage forming solitary tumor [ 3 ]. It predominantly arises in skeletal muscles, but also occurs around tendons, ligaments, joint capsules, facia and aponeuroses [ 4 ] A history of trauma is often present, but not necessary [ 3 ]. On pathology, MO demonstrate zone phenomena where ossification forms peripherally and progress in a centripetal pattern [ 5 ]. Similarly, imaging of MO typically shows peripheral rim of calcification on X-ray and CT [ 5 ]. MRI can demonstrate the zone phenomena as peripheral hyperintensity on the post contrast T1WI of the iso- to mildly hyperintense intramuscular lesion on T1 and T2WI in the acute phase and hypointense on subacute phase indicating mineralization [ 5 ]. The distinctive pattern of calcification/ossification is usually not apparent within the first 2 weeks [ 5 ]. In the case discussed, the mass is located in the subcutaneous tissue of the anterior abdominal wall, at the right lower quadrant, without involvement of the abdominal wall muscles or the rectus sheeth. Despite the relatively long history of the palpable mass (8 months), the initial ultrasound, CT and MRI did not show rim ossification, but revealed few punctate areas of calcification throughout the mass, a distribution atypical for MO. Given the patient's age, duration of symptoms and the lack of typical imaging findings, the differential diagnosis was kept broad, to include desmoid tumor and endometriosis. Since, the imaging features of this case were not typical for any of the other 2 differential diagnosis, as desmoid tumors are typically of low signal intensity on T1 and T2WI MRI images and abdominal wall endometriosis typically demonstrate foci of blood products and small cysts within the deposit, tissue diagnosis was essential. Tissue biopsy revealed findings of early myositis ossificans with predominant fibroblast and myofibroblast with minor component of osteoid formation. The typical radiological features of late MO is evident on follow up CT.

Myositis ossificans, also known as myositis ossificans traumatica or myositis ossificans circumscripta, is the most common form of Hetertopic ossification [ 1 ]. Histopathlogically, MO development has 3 stages. The first stage is defined by the presence of organizing granulation tissue with fibroblastic, osteoblastic tissue differentiation and osteoid formation. The second stage is characterized by the development of mineralized osteoid matrix with immature lamellar bone. These 2 stages last 3-4 weeks consecutively. The third stage is involving the maturation of immature bone into lamellar cortical and trabecular bone, usually occurring after 8 to 10 weeks [ 1 , 2 ]. Imaging features parallel these histopathological stages, ranging from an inflammatory change, enhancing mass, mass with peripheral punctate calcifications to the pathognomonic ossification rim [ 1 ]. The case presented in this review, had an atypical time line.

Available.