Six year old girl with a growing mass over mid back. History of trauma.
Posterior right chest wall FDG-avid mass (SUV max 9.4), mildly heterogeneous on CT. There is calcification near the periphery of the mass. This lesion extend closely to ribs, without costal destruction or periosteal reaction. Mild pleural reaction.
No other FDG-avid lesion.
Ewing’s family of tumors
Heterotopic ossification (HO) is characterized by bone formation in soft tissues, mainly striated muscles, tendons, ligaments, fasciaesand aponeuroses. In the literature, the term HO is often used interchangeably with the term myositis ossificans (MO). There are three subtypes of HO: post-traumatic, neurogenic and fibrodysplasia ossificans progressiva.
Post-traumatic HO is the most common subtype. It usually follows a trauma. However, history of a trauma may be absent in up to 40% of cases. It is benign reactive process. It may occur at any age, but there is a predilection for second and third decades. Most common locations include muscles of the thighs, buttocks and arms corresponding to muscles most commonly traumatized in teens and young adulthood.
Pathophysiology of heterotopic bone formation is poorly understood. Heterotopic ossification could be the result of the dysregulation of stem cells following tissue injury.
There is a predictable radiologic pattern for HO (see Table 1). Important key is the centrifugal calcification pattern (calcification appears in the periphery of the lesion) and is distinct of others sarcomas (osteosarcoma, chondrosarcoma, synovial sarcoma) which usually lack the zonal pattern of HO.
|| < 2-4 weeks
|| 4 weeks-6 months
|| > 6 months
Soft tissue mass
No or faint peripheral calcification
Periosteal reaction of adjacent bone may be present
Well defined peripheral calcification
Center of lesion isodense to normal muscle
| Densely calcified soft tissue mass with trabecular bone
Few authors reported myositis ossificans with PET-FDG. Most reports showed hypermetabolic masses with variable degree of calcifications. For example, Koob et al. described an intercostal lesion in a ten year old child with peripheral calcifications and SUV of 13. Other authors reported hypermetabolism in more mature lesions.
Fibrodysplasia ossificans progressiva is a hereditary autosomal dominant condition of progressive heterotopic bone formation. Individuals affected are usually normal at birth except for a malformation of the great toes, which is present in almost all cases. Typically, children affected will develop episodic painful inflammatory soft tissue swellings in the first decade. Subsequently, most of them transform in mature heterotopic bone. Heterotopic ossification progresses in characteristic anatomical pattern: axial to appendicular, proximal to distal, cranial to cephalad.
McCarthy et al.: Heterotopic ossification: a review. Skeletal Radiol. 2005 Oct;34(10):609-19. Epub 2005 Aug 25
Cabello et al. : Miositis osificante circunscrita en un niño de cuatro años. Rev Esp Med Nucl. 2008 Sep-Oct;27(5):358-62. Spanish.
Costelloe et al. : Musculoskeletal pitfalls in 18F-FDG PET/CT: pictorial review. AJR Am J Roentgenol. 2009 Sep;193(3 Suppl):WS1-WS13, Quiz S26-30
Tyler et al. : The imaging of myositis ossificans. Semin Musculoskelet Radiol. 2010 Jun;14(2):201-16
Koob et al. : Intercostal myositis ossificans misdiagnosed as osteosarcoma in a 10-year-old child. Pediatr Radiol. 2010 Dec;40 Suppl 1:S34-7
Clarençon et al. : FDG PET/CT findings in a case of myositis ossificans circumscripta of the forearm. Clin Nucl Med. 2011 Jan;36(1):40-2.
Agrawal et al. : [18F]Fluoride and [18F]fluorodeoxyglucose PET/CT in myositis ossificans of the forearm. Eur J Nucl Med Mol Imaging. 2011 Oct;38(10):1956
Govindarajan et al. : Myositis ossificans: the mimicker. BMJ Case Rep. 2013 Dec 10;2013. pii: bcr2013201477
Pignolo et al. : Fibrodysplasia ossificans progressiva: diagnosis, management, and therapeutic horizons. Pediatr Endocrinol Rev. 2013 Jun;10 Suppl 2:437-48
Colman et al. : Non-neoplastic soft tissue masses that mimic sarcoma. Orthop Clin North Am. 2014 Apr;45(2):245-55
Walczak et al. : Myositis Ossificans. Am Acad Orthop Surg. 2015 Oct;23(10):612-22