What you are inquiring about is known as "spontaneous remission" (WIKI):
...unexpected improvement or cure from a disease which usually is taking a different course.
See the rest of that article for some additional summary information concerning frequency, causes of this phenomenon, and more references.
As for more sources...
- Answering a religious claim to healing vs. one from alternative medicine, HERE is a referenced list of types of cancer known to spontaneously remit (See the page for expanded references):
- adult T-cell leukaemia/lymphoma (Takezako et al., 2000)
- adult T-cell leukaemia (Murakawa M et al., 1990)
- oesophageal leiomyosarcoma (Takemura et al., 1999)
- lung cancer following myxoedematous coma (Hercbergs, 1999)
- hepatocellular carcinoma (2 cases; Magalotti et al., 1998)
- non-small-cell lung cancer (Kappauf et al., 1997)
- lung metastases from primary uterine cancer (Mastall H, 1997)
- liver cancer (Van Halteren HK et al., 1997)
- pleural and intrapulmonary metastases from renal carcinoma (Lokich J, 1997)
- squamous cell lung cancer (Schmidt W., 1995)
- bladder cancer (Hellstrom PA et al., 1992)
- intrahepatic, peritoneal and splenic metastases after hepatectomy for hepatocellular
- carcinoma (Terasaki et al., 2000)
- disappearance of lung metastases from hepatocellular carcinoma (Toyoda et al., 1999)
- large-cell and polymorphic lung cancer with extensive metastatic disease (Kappauf H. et al., 1997)
- metastatic malignant melanoma (Hurwitz PJ. 1991); several similar cases cited in the literature
- HERE is a 1978 article examining spontaneous cases, which states:
There is indisputable scientific evidence of a spontaneous regression of cancer in several cases.
Appendix One presents review articles of spontaneous regression of cancer that have appeared in the medical literaure over the last 100 years.
So, yes, there are definitely cases. It appears that the mechanism is unknown and that it may occur more/less frequently depending on the type of cancer as well.