The construct of “rare cancers” has beneficially helped to focus care provision within Comprehensive Cancer Centres and enabled greater research into these tumours. The communication of rarity to the patient can nonetheless heighten existential distress and challenge adaptive coping. The psychological threat of cancer is exacerbated by any difficulty in obtaining confidence with histology and molecular pathology, treatment uncertainty, unclear prognosis, the need for second and third opinions, and a clinician’s direct statements of uncertainty. Indeed, uncertainty in the oncologist can be conveyed unwittingly to the patient with unintended detrimental effect. Communication about rare cancers needs to convey confidence about the management plan to allay anxiety and ameliorate suffering in the patient and their relatives.