[Spindle cell/sclerosing rhabdomyosarcoma: a clinicopathological study of 20 cases]

Objective: To study the clinicopathological features and immunophenotype of spindle cell/sclerosing rhabdomyosarcoma (SRMS) in adults and children, as well as its correlation with the expression and gene-mutations of MYOD1. Methods: Twenty cases of SRMS were collected at Xijing Hospital, Fourth Military Medical University from 2009 to 2019. These cases were evaluated for clinical, pathological, and immunohistochemical features. MYOD1 gene sequencing was performed on 12 cases with available tissue and sufficient DNA quantity using Sanger sequencing. Results: The 20 patients included 12 children and 8 adults, 11 males and 9 females, with an age range of 8 months to 85 years (mean 22 years). Most of them presented with a painless, progressively enlarged solid mass. The tumors occurred in head and neck (7 cases), abdominal and pelvic cavity (7 cases, including 4 in abdominal cavity, 2 in pelvic cavity, 1 in abdominal and left thoracic cavity), upper limb (5 cases, including 2 in left shoulder, 1 in right armpit, 1 in right humerus, and 1 in left forearm), and the back (1 case). The diameter of these tumors ranged from 2.5 to 20 cm, with a mean of 6.2 cm. Histologically, all of the tumors were mainly composed of spindle cells arranged in fascicles, and in 7 cases, at least in part, arranged in herringbone pattern, resembling adult fibrosarcoma. Foci reminiscent of interstitial sclerosing were presented in 4 cases, pseudovascular structures in 2 cases, loosely myxoid stroma in 4 cases, and varying degree of necrosis in 9 cases. A various number of spindled or polygonal rhabdomyoblasts were observed between spindle cells in 3 cases.Among them,16 cases showed spindle cell morphology, 2 cases showed scleroisng morphology, and 2 cases showed a hybrid phenotype of spindle, sclerosing and primitive undifferentiated areas. Immunohistochemically, the tumor cells were positive for desmin, Myogenin and/or MyoD1, but negative for CKpan, ALK1, CD34, EMA, HMB45, SMA, H-cald and S-100. Four cases (4/12) harbored a homozygous or heterozygous MYOD1 (p.L122R) mutation. MYOD1-mutant SRMS usually had diffuse and strong nuclear MyoD1 positivity. Follow-up was available in 12 cases, ranged from 1 to 51 months. At the end of follow-up period, 3 patients died of the disease, 3 patients developed local recurrences, 2 patients survived with disease. Conclusions: SRMS is a rare type of rhabdomyosarcoma, and more commonly occurs in the head and neck of children than adults. MYOD1-mutant SRMS usually had diffuse and strong nuclear MyoD1 positivity, frequently associated with a more aggressive behavior.