Is it possible for doctors to accidentally overdose someone on radiation therapy?

Yes. Radiation has a prescription and dose that is variable depending on the cancer and stage. It certainly can be given wrong or errors made. However having side effects or complications does not automatically mean there was an overdosage. That's because many parts of the body are sensitive to radiation even at appropriate doses. Therefore side effects are treated all the time.
Unfortuantely yes. It should never happen, but it has happened, and it will happen again. As a discipline we are rather concerned and careful to avoid this, but we are human, and humans make mistakes. There have been unfortuante episodes reported in the new york times in the last 3 years. Some emphasize the rarety, others the harm, we all need to be aware, vigilant and careful.
Yes. But it is extremely rare, there is lots of quality assurance and double and triple checking to assure correct dosing.

Related Questions

How do doctors stop the radiation therapy from bouncing around inside your body and hitting other organs?

Image guided. Excellent question. Direct radiation is best delivered by image guidance, and tracking limit collateral damage. Your question is about scatter radiation (radiation bouncing around inside the body); unfortunately, there is no good way of blocking or stopping scatter radiation in the body.
They account for it. Scatter radiation can and does occur, it can be as high as 10-15% of the delivered dose. That scatter must be accounted for in the planning and is mapped in a device called the dose-volume histogram.
They can't! Scatter occurs. It is repaired 99.999% this is why you are in the room alone. No need ti expose those that do not need to be reated. Radiation repair enzymes are ubiquitous and effective most of the time. This is not something to worry about, or there would be no success stories, and there are legion.

Why would my doctor not recommend radiation therapy after a lumpectomy?

Low risk to recurr. There is a good chance the type of disease was dcis (ductal carcinoma in situ). This is a pre-invasive disease (not cancer yet). Radiation can be avoided in the most favorable group of patients with dcis since chance of recurrence is very low. Typical conditions met are: 1) large clear margins of resection was obtained, 2) the disease is not too large (size), 3)not too aggressive (low-grade).
Ask your doctor why? This depends on your condition and age. Assuming you have cancer of the breast there are some (by no means all) radiation doctors who may not recommend it if it is early stage and you are over 70 and have good receptors (er and pr) so that you can take a hormone. Best bet is to get a second opinion and likely they won't agree.
It's the right thing. In 1970, no one had lumpectomy (partial mastectomy) and everyone had "radical mastectomy. Today many womaen can avoid mastectomy. The standard is to get roughly 5 weeks of adjuvant radiotherapy to the involved breast. Why because studies proved that no radiotherapy resulted in breast relapse and breast loss. Some women are now choosing to have both breasts removed.
??? You need an opinion from a radiation oncologist so you can adequately understand why you do or don't need treatment. Unless that recommendation was made by someone who specializes in radiation treatments I would hesitate to accept it.
Lumpectomy radiation. Almost all patients who have a lumpectomy need radiation afterwards because of the risk of residual microscopic cancer. A couple of exceptions are in situ cancer that is very small, low grade, and widely clear margins; or invasive cancer that is very small, er +, age over 70, and margins over 2 mm.

Can doctors use radiation therapy on the whole body to kill small cancer metastases?

No. The only setting where total body radiation is used is in the context of preparing a patient for a bone marrow transplant. Radiation is really a local treatment that is most commonly used to treat a particular mass or region but not so much the whole body.
Yes. It has been more common in the past to use whole body radiation to perform preparation of patient for bone marrow transplant. But these days chemotherapy is more common and effective. Whole body skin radiation can be used in widespread t cell lymphomas but there are easier alternatives. For most cancers whole body radiation is not feasible or advisable.
Not used anymore. But it used to be tried as part of lymphoma therapy as far back as the 1930's in very low doses. It was never proven to be beneficial. Half-body radiotherapy was used to treat pain in prostate cancer in the 1980's. Both have fallen out of favor because there are alternative better ways with fewer side effects.