The patient went on to have a resection.

Pathology  

Mediastinal mass and right lung, en block resection of mediastinal mass and right pneumonectomy specimen showing:

1. residual viable high-grade embryonal rhabdomyosarcoma with treatment effect.
2. tumor measures 13 cm in maximum dimension
3. approximately 40% of the tumor is viable.
4. bronchial resection margin is negative for tumor.
5. station 11 lymph nodes are negative tumor.
6. tumor extends right up to the inked pericardium but does not extend through it.
7. the pulmonary vein margin is negative.

The portion of the tumor which is viable shows a mixture of cell types. There is the high-grade spindle cell component that was seen on the initial biopsy. There are other areas that show transformation into bizarre pleomorphic tumor cells many of which have large amounts of eosinophilic cytoplasm and could be classified as anaplastic rhabdomyosarcoma which may be attributed to treatment effect. Some areas show more differentiated rhabdomyoblastic cells. Abnormal normal mitotic figures are readily identifiable. 

There is strong staining of the tumor cells with myoglobin and desmin. The myoglobin strongly stains the areas of the better-differentiated cells and shows focal staining in the more spindled primitive areas. The pattern is similar for the desmin stain. The O13 stain shows stronger staining in the more primitive spindled areas and weak to no staining in the better differentiated areas.