The plural of cancer es cancersSo it is inappropriate to leave it unchanged.
Examples such as the following can be found in the media: «This treatment does not work for most cancers», «The Patient Advocate requests that the origin of several cancers in Palencia be investigated» or «There are many cancers that are detected at time”.
When this voice forms the plural —either because it refers to to the development of the disease in several peopleeither because it is about various types of cancer—, the general rules of words are followed flat or acute ending in r, to which is added the ending -eslike in the martyrs, the craters o the leaders. Only the esdrújulas, like the managersremain unchanged.
Consequently, the appropriate in the previous sentences would have been “This treatment does not work in most cancers”, “The Patient Ombudsman requests that the origin of several cancers in Palencia be investigated” and “There are many cancers that are detected at weather”.
See also
➤ World Cancer Day, writing keys