Mary how is Rick doing? He's right at the midpoint. Responding well I hope.
Here's a reference to different radiation levels from different devices (PET isn't listed, nor is dental)
http://www.radiologyinfo.org/en/pdf/x-ray_safety.pdf PET scans safety:
PET scanning is non-invasive, but it does involve exposure to ionizing radiation. The total dose of radiation is small, however, usually around 7 mSv. This can be compared to 2.2 mSv average annual background radiation in the UK, 0.02 mSv for a chest X-Ray, up to 8 mSv for a CT scan of the chest, 2-6 mSv per annum for aircrew, and 7.8 mSv per annum background exposure in Cornwall (Data from UK National Radiological Protection Board).
Because the half-life of 18F is about two hours, the prepared doses decay significantly during the working day. If the FDG is delivered to the scanning suite in the morning, the specific activity falls during the day, and a relatively larger volume of radiopharmaceutical must be injected in later patients to deliver the same radioactive dose.
Here's a link to dental x-ray safety for those interested - the amount of radiation is pretty small. I personally think that the lead aprons were mandated by some risk management attorney to the ADA. It's really a joke. Especially after you've had 60-80 cGy of ionizing radiation under a LINAC (How come they don't give you a lead apron there !?!?!). But if you're paranoid - go for it. I support paranoia in all its forms;-) They should give flight crews lead aprons. They get more radiation being at high altitude. At least they banned smoking on most flights now...
By the way those big LINACS, the ones using klystron tubes (20MeV) can put out nuetrons as well as ionizing radiation. I think that most H&N cases are treated on a 6 MeV machine because of the depth and beam geometry.
http://hps.org/hpspublications/articles/dentaldoses.html It does help to be in the business. I was treated on a LINAC from a company I used to work for (in the development group - Siemens Medical Labs) and my therapist was training at the company during the same I was working there. She's moved on to bigger and better things - She runs the Gamma Knife now at UCSFCCC. She shared with me, one time, that it's really hard on them to have otherwise healthy people come and have the living crap kicked out of them by the radiation and watching the deterioration. I visit her every now and then to encourage her that the stuff really works sometimes. So bring your therapist some chocolate chip cookies!
I'm in the radiology business doing mainly PACS but an occasional mammography, MRI, CT and lots of ultrasound systems(I am very familiar with ALARA - It has to be in the user manual as part of the labeling mandated by the FDA) FDA 510K Pre-Market Notification (PMN).
In California the Sherman Act regulates x-ray installations as well as the FDA, including non-medical uses.
