This depends so much on the individual, where he/she is at in the emotional healing process, and even the type of cancer. I am a therapist, and have a lot of experience with women who are sexual trauma survivors. Standard medical procedures, particularly those that are more invasive, can trigger high emotions and PTSD symptoms. Many cancer tests and procedures, especially those for breast, cervical, uterine cancer can be even more invasive, and may bring back memories, flashbacks, etc. Add the stress of having cancer to this, and one's emotional state may be much more fragile.
I don't know what the nature of your PTSD is, but you may want to speak with your treatment team, and let them know if any particular procedures may be emotionally difficult for you. I have a friend who is a rape survivor, and she reminds her gynecologist of this every time she has a pap smear in hopes that her doc will be sensitive to her angst.
If you don't have a therapist, you may want to find one. Cancer treatment is so emotionally taxing in and of itself, and if you are experiencing depression, you may need even more support.
Best of luck with everything, Dani.