Memorial Sloan Kettering psychiatrist Matthew Doolittle says that up to 20 percent of cancer patients develop post-traumatic stress syndrome (PTSD), an anxiety disorder that can cause feelings such as fear or helplessness. Avoiding situations that may trigger a traumatic response can interfere with a patient’s adherence to treatment and follow-up. Other symptoms, such as sleeplessness and irritability, can have an impact on a patient’s quality of life and ability to function on the job and at home.