Whether someone has cancer or not, scans involving ionizing radiation (i.e., x-rays, radioactive tracers) pose a health risk because the effects of ionizing radiation are cumulative in the body. When the risk of cancer recurrence is elevated, or higher than normal for whatever reason in a patient, the risk of the patient developing cancer from whatever pre-existing health condition he or she has is higher than the risk of developing radiation-induced cancer. In this case, the risk-to-benefit (of scans involving radiation) for a cancer patient weighs in favor of the benefit. For someone who is not at an advanced risk for cancer, the risk-to-benefit of having scans involving radiation weighs more heavily on the risks rather than the benefits.
That being said...Magnetic Resonance Imaging (MRI's) and Ultrasounds (Sonograms) are totally safe! MRI's use radio-waves and magnetic fields, and ultrasounds are just mechanical sound waves.
Cancer was not necessarily hiding for 17 years. Please see next comment.
Radiation-induced cancer could have occurred and been developing for 17 years. Ionizing radiation breaks bonds in cellular DNA. Damaged DNA can cause cancer over time. Therefore, a different cancer or cancer in another part of the body 17 years later could also have been due to all the radiation from the scans. Earlier I had researched radiation-induced breast cancers (due to mammograms), and I was surprised to learn that there is a certain percentage of breast cancers (albeit small) that are, statistically, attributed to the cumulative effect of the radiation from mammograms, for example. (Because of this, there are centers that do breast MRI's instead of mammograms.) Statistics were taken in Nordic countries for years. In fact, Nordic countries have extensive medical data and subsequent studies, and this is known in medical research.
In the Chernobyl nuclear reactor disaster, the people in the immediate vicinity died in days, in weeks, or in months due to radiation burns. However, scientists who necessarily went there to try to contain further leaks after the initial explosion did not die on the spot or in a few months. They developed cancers several years later, and they knew they would, due to the cumulative effect of the radiation to which they had been exposed.