Halakhah for Ketubot 154:16

The second question concerns treating a patient who would not survive the year if untreated, but the treatment involves significant risk. A patient can live only for two or three months without treatment; moreover, the treatment available carries significant risk so that the patient may die earlier if he undergoes the treatment. On the other hand, if it is successful, he may live a normal life. Regrettably, I cannot find a clear source to resolve this issue. However, a reasonable approach would be to say that a person is halachically required to seek medical care only when there is a known and proven treatment available. The fact that a treatment is significantly risky may lead one to conclude that undertaking such a treatment may be forbidden. Given these conditions, it would be preferable to abstain from intervention, leaving the patient in the hands of the Healer of all flesh.