Generally, just a diagnosis does not equal disability – we must present evidence of symptoms and limitations as well. There are some medical impairments, however, that are just so awful or deadly that SSA will fast-track an approval:
Compassionate allowance means a determination or decision we make under a process that identifies for expedited handling claims that involve impairments that invariably qualify under the Listing of Impairments
20 C.F.R. § 404.1602
Many of the cancers fall into this category. These are usually picked up at the Initial level, and can result in an approval within days. Sometimes they are missed by the Social Security Administration at the initial level, however, and they need to have that pointed out to them. If you have an impairment that is listed on the compassionate allowances list (CAL), you should notify SSA immediately and send them a copy of the medical evidence where the diagnosis was made. If your case is at the hearing level, write to the Administrative Law Judge and request a decision on-the-record (OTR), with the medical evidence of diagnosis and a reminder that the condition is on the CAL.
https://www.ssa.gov/compassionateallowances/conditions.htm