This is what they have to say about “safe” radiation:
Potential Long-Term Risks of Treating Brain Tumor with Proton Beam Therapy - After the cancer radiation treatment is completed (using the proton beam technique), long-term side effects can occur months or even years down the line. The risks vary depending on the amount of radiation used and the areas exposed to radiation. As the radiation techniques continue to improve and evolve, it becomes difficult to predict with accuracy how it will affect a patient in terms of post-treatment complications and side effects. While the advocates of proton beam therapy claim that the long-term risks of the treatment are low for brain tumor patients, it is vital for patients and their caregivers to have an understanding of the potential long-term dangers to the patient’s health.
Risk of developing secondary cancer:
There is some amount of risk that the patient may develop secondary cancer in or near the field of proton beam radiation. When healthy tissue is exposed to radiation, it may develop second cancer. The latest radiation techniques are specifically designed to limit such exposure. However, it may not always be possible to avert all exposure, and despite the best precautions, the patient may still develop second cancer.