First of all, let’s look at the origin of the word chemotherapy in order to understand what it is:

Chemotherapy (n.): “treatment of diseases by chemical substances,” 1906, from German Chemotherapie, coined by German biochemist Paul Ehrlich (1854-1915), from chemo- + therapie. Especially of cancer from 1950s; short form chemo attested by 1977.

before vowels chem-, word-forming element denoting “relation to chemical action or chemicals,” from combining form of chemical (adj.), used to form scientific compound words from c. 1900. In 19c., chemico- was used.

therapy (n.)
1846, “medical treatment of disease,” from Modern Latin therapia, from Greek therapeia“curing, healing, service done to the sick; a waiting on, service,” from therapeuein “to cure, treat medically,” literally “attend, do service, take care of” (see therapeutic).

St. Jude Hospital tell us that:

Isotretinoin (Accutane) is a type of medicine drug called a retinoid. It acts on specific receptors of the cell nucleus to control cell growth. Isotretinoin may be used as a chemotherapy to treat certain types of cancer such as neuroblastoma. It is usually used as a treatment for severe acne. [1]

So, yes Accutane is a chemical substance used for the “medical treatment of disease”, namely cancer.