While certain objective factors may contribute to evaluating the unbearable aspect of suffering, experiencing suffering as unbearable is a subjective and personal issue for the patient which depends on their personality, their pain threshold and their very own views and values.
The issue of prospect for improvement of the suffering is a medical issue, but it must also be taken into consideration that the patient has the right to refuse treatment for pain, or even palliative care, especially when such treatment entail side effects or implementation procedures that he or she finds unbearable. A thorough discussion about this between the doctor and the patient is necessary.
Given the variability of these notions depending on the person concerned, the opinion of an independent doctor is mandatory in addition to that of the treating doctor.