Case Number : Case 2258 - 7 February 2019 Posted By: Raul Perret

Blunt anded nucleus and perinuclear clear halo, make me think of subcutaneous leiomyosarcoma. 

My possibilities are Leiomyosarcoma, Sinovial Sarcoma and least likely MPNST

Waiting for IHC and more comments.

Leiomyosarcoma is my first thought

The cells, even though showing nuclear hyperchromasia, are pretty uniform ovoid and focal "herringbone" arrangement. increased mast cells are noted. I am in favor of monophasic synovial sarcoma given the location of around knee. I will order some stains including especially Keratin, EMA and TLE1. of course, add other spindle cell tumor markers like SMA, DESMIN, CD34 etc.. if you have molecular check t (X;18) translocation. 

Leiomyosarcoma (spindle cell variant).

MyoD1 positivity and focal Myogenin positivity...Spindle Cell Rhabdomyosarcoma?

Yes, Spindle cell Rhabdomyosarcoma after IHC.

I´ve never seen one in real life. Nice case. Thanks

Every day learning from you guys. Nice case, thanks for sharing.

Yes, this was a case of spindle cell rhabdomyosarcoma. These rare neoplasms have a dual peak incidence in infants and adults. Most tumors that develop during infancy have a  good prognosis, are usually localized in the head and neck or the paratesticular area and frequently have NCOA2 or VGLL2 réarrangements. On the other hand, adults usually present aggressive tumors that frequently harbour MYOD1 mutations. More importantly, no matter the age of presentation, MYOD1 mutated tumors seem to have an overall worse prognosis . 

Morphologically, spindle cell RMS frequently looks like leiomyosarcoma because of its pink colour at low power and fascicular disposition of cells. However, unlike LMS, spindle cell RMS tends to be more cellular and fascicles rarely intersect at right angles. Do keep in mind that some cases are extremely hypercellular (blue tumors) and others can be largely sclerotic, sometimes with a pseudovascular morphology. The latter would be considered in the spectrum of sclerosing rhabdomyosarcoma, a morphological variant that is very closely related to spindle cell RMS.

Finally, do not forget that to confirm the diagnosis, appart from the morphology, you need the expression of Myogenin or MYOD-1. The latter is usually expressed more diffusely than the former like depicted in this case.

Have a great day

Thanks Raul. Great case and many learning points!