Radiation therapy treatments are almost always fractionated, meaning the total dose prescribed by the radiation oncologist is spread fairly evenly across multiple days, typically 25-38 treatments for most disease sites. And since patients are rarely treated on the weekend, it usually takes 1 to 2 months for a patient to complete treatment. I don't know that I would consider 1 to 2 months a short time. There are some exceptions, such as gamma knife for brain cancer, in which the entire prescribed dose is delivered all in one session.

Also, radiation therapy is often used to treat not-so-small areas (volumes). For example, mesothelioma cases often require irradiation of the entire thoracic cavity. And if that isn't a big enough target for you, well, total body irradiation is actually a pretty common modality for certain, less localized, cancers originating in the blood and bone marrow. Both external beam irradiation (for mesothelioma) and total body irradiation treatments are always fractionated, generally delivering no more than 2 Gy to a patient in a single day, to give healthy tissue time to recover between fractions. Having said all that, you are right that this is still quite unlike the conditions presented in the article. Perhaps you would prefer studies analyzing the increased exposure of long-haul international airline flight crews.

You cannot focus the radiation field perfectly. The whole body will be subjected to (non uniformly distributed) dose of maybe a permille of the reference dose in the irradiation center: there is both leakage from the treatment machine and scattered radiation from within the patients body.

Given that a typical treatment will be about 50Gy (distributed over many fractions), my estimate is for a few ten mGy (mSv) in total to almost every part of the body outside of the direct beam.

(I'm building dosimeters in my day job and work on medical accelerators all the time.)

Chest x-rays for example dose lots of tissue. CT scans are significantly 100-1000x higher than that and well above whole body average annual radiation exposure. https://en.m.wikipedia.org/wiki/CT_scan

Chest X-rays are a minuscule dose. They are in a similar realm to the dose you get from a long distance flight or 2 - especially if you eat bananas while you fly. Cardiac CT, chest/abdo/pelvis and PET scans are dose order of magnitude greater though.