"highly malignant" is correct and can be used in written English.
It describes something that is very serious or severe. For example: "The tumor was found to be highly malignant, so aggressive treatment was necessary."
GIST is a malign disease that ranges from a curable disorder to a highly malignant disease.
This time, the surgeon describes a highly malignant brain tumor that will almost certainly grow back.
He told her she had what looked like a highly malignant tumor.
In extremely rare instances, hydatidiform moles develop into choriocarcinomas, which are highly malignant tumours.
Orbital Rhabdomyosarcoma is a highly malignant tumor predominantly affecting children.
Glioblastoma multiforme (GBM) is a highly malignant brain tumor.
Metastasis is a characteristic of highly malignant cancers associated with a poor clinical outcome.
Introduction: Melanoma is a highly malignant cutaneous tumor of melanin-producing cells.
Bone sarcomas in children and young adults are rare but highly malignant.
Atypical teratoid/rhabdoid tumors (ATRT) are highly malignant CNS tumors commonly diagnosed in infants.
Pancreatic ductal adenocarcinoma (PDAC) is a highly malignant disease with a very poor prognosis.
