This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Counteracting or preventing the formation of malignant tumors; anticancer.

adj. Inhibiting the development of a tumor.

n. Any substance that inhibits tumors.

adj. used in the treatment of cancer

"Tumor cell death caused by radiation can induce DC-mediated cytotoxic T-lymphocyte (CTL) responses and long-term antitumor immunity ["

antineoplastic   

anticancer    antigen-specific    antitumour    antihypertensive    immunosuppressive    cytotoxic    antidepressant    demulcent    antimicrobial    adjunctive