All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You have to consult a trustworthy Muslim doctor. If the fasting can delay your cure or increase your illness, then you should leave it. But you should make up the missed days of Ramadan after being cured. Allah Says (what means): {… but if any of you is ill or on a journey, the same number (should be made up) from other days.} [Quran 2: 184]
You are not asked to pay any expiation.
If you have no hope for a cure from your illness, then you should feed a poor person for everyday you missed fasting in Ramadan. Allah Says (what means): {… And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day].} [Quran 2: 184]
Imam Ad-Daaraqutni reported from Ibn ‘Abbaas in the explanation of this verse that he said: “This concession is only for an aged person who cannot fast or for a chronically sick person who has no hope for cure in the future.” [The chain is sound]
We ask Allah to grant you an early cure as He is All-powerful and Able to grant cure.
Allah knows best.