Comparison of MyoD1 immunostaining of pediatric tumors using frozen or paraffin-embedded sections

Distinction of rhabdomyosarcomas (RMS) from other tumors can be difficult, requiring use of monoclonal antibodies against unique muscle proteins. In p revious studies using cryostat sections, we found that MyoD1, a transcripti on factor that activates the expression of muscle-specific genes, is highly expressed in rhabdomyosarcoma and useful in diagnosis. Recent studies usin g paraffin sections demonstrated poorer results, leading us to compare MyoD 1 staining using dual paraffin versus frozen sections of tumor tissue. We p erformed immunostaining on companion frozen and paraffin sections of 69 ped iatric tumors (40 rhabdomyosarcomas and 29 putative non-rhabdomyosarcomas [ nRMS]), using similar dilutions of antiMyoD1 monoclonal antibody 5.8A. Twen ty-four of 40 (60%) frozen rhabdomyosarcomas and 14 of 40 (35%) paraffin-em bedded rhabdomyosarcomas were nuclear positive for MyoD1. Of the 29 putativ e nonrhabdomyosarcomas, six of 29 (21%) and three of 29 (10%) were nuclear positive, using frozen and paraffin sections, respectively. These positive cases included one ectomesenchymoma, one embryonal sarcoma of liver, one ne urofibroma, three undifferentiated sarcomas, and one Wilms' tumor. In retro spect, these nine nuclear-positive cases appear to have myogenic potential. In six frozen and 12 paraffin RMS sections and in nine frozen and 15 paraf fin putative nRMS sections, only nonspecific cytoplasmic staining was obser ved. We conclude that MyoD1 immunostaining of small round cell tumors is mo re reliable and sensitive using frozen sections, despite its potential appl icability in paraffin sections.