Which of the following is not a malignant neoplasm of muscle tissue?

Adenocarcinoma is classified as a malignant neoplasm that arises from glandular epithelial tissue, rather than muscle tissue. This type of cancer originates in the cells that line the glandular organs, such as the breast, prostate, or lungs, and does not pertain to muscular origins.

On the other hand, leiomyosarcoma and rhabdomyosarcoma are both malignant neoplasms specifically derived from muscle tissue. Leiomyosarcoma develops from smooth muscle, while rhabdomyosarcoma arises from skeletal muscle. Therefore, identifying adenocarcinoma as the option that does not correspond to malignancies of muscle tissue is accurate.