In relation to whether a person who dies from cancer being considered a martyr, the scholars have mentioned that there is nothing explicitly narrated in regards to it. However, it is a possibility.
Shaikh Bin Baz (d. 1420 H.) – may Allah have mercy upon him – was asked:
“Is the one who dies from cancer considered a martyr?”
“I do not know of anything in relation to this. I do not know of anything that was reported which indicates that. It comes in the Hadith that the one who dies under debris, or drowns, or burns, or dies of an abdominal disease, or the one who dies fighting in the path of Allah, or the one who dies of the plague [are martyrs] … As for dying from cancer being a means of martyrdom, then it is a possibility, and Allah knows best. I do not know of anything clear on the matter. As for dying from the plague, then it is well-known and whoever dies from it dies as a martyr” End translation. [Fatawaa Nur ‘ala Darb tape (no. 397)]
Likewise, Shaikh Sulaymaan ar-Ruhayli (may Allah preserve him) said that he does not know of anything explicitly on it. However, some scholars have included the one who dies of stomach cancer as being a martyr due to the generality of the Ahaadith which mention abdominal disease, as seen here:
This above view was also mentioned by Shaikh Bin Baz (may Allah have mercy upon him) in another fatwa [see here] and Shaikh Abdul-Hameed al-Hajuri (may Allah preserve him).
And Allah knows best
Faisal bin Abdul Qaadir bin Hassan