What is the expected timeframe for the development of radiation myelitis and therapies that have helped with neurologic symptoms?

The incidence and the timeframe of the development of radiation myelopathy are influenced by total radiation dose, radiation dose per fraction, time between courses of radiation, and associated chemotherapy or immunotherapy. Older age, the presence of diabetes, and previous exposure to radiation are also risk factors. Radiation myelopathy is generally a subacute to chronic progressive myelopathy, with a variable latency period, generally occurring between 6-24 months after treatment. The timing can be accelerated by a number of factors such as providing a 'top off dose' after the spinal cord has received what is considered a tolerance dose of 45-50 Gy at standard fractionation of 1.8-2 Gy/fraction. The length of the cord irradiated can also influence the onset and incidence of spinal cord injury.

Symptoms of radiation myelopathy include paresthesia (100%), motor weakness (70%), radicular pain (50%), and loss of bladder and/or bowel control (40%).

The damage in radiation myelopathy is considered irreversible, treatment is usually supportive. However, some patients have derived a short-term benefit from steroids which may be related to the edema and inflammation which is usually associated in the initial phase of damage.

Agree with Dr. @Dr. First Last and would add that timeline for the development of radiation myelopathy can be shorter with re-irradiation when compared to de novo treatment.

I have found this paper and a more recent publication helpful in designing my dose/fractionation when offering re-treatment over the spinal canal.

In the setting of SBRT re-irradiation, this HyTEC reference offers valuable guidance for re-irradiation constraints (Section 7, Table 4).

I agree with Dr. @Dr. First Last, and can add that there is not much that appears to help with radiation myelopathy. There are a couple of options outside of steroid therapy.

Hyperbaric oxygen (HBO) appears to be of limited help in studies with rats, and case reports in humans have shown some benefit. Studies of brachial plexopathy have shown some benefit, but it was limited (Pritchard et al., PMID 11230889). I think it is unlikely to hurt, but likely will be of limited benefit.

Pentoxifylline and vitamin E, sometimes administered with clodronate may be of some help. In randomized studies, it significantly reduced post-radiation breast fibrosis (Jacobson et al., PMID 22846413), and it has been shown to increase the alpha-beta ratio in rats (Kim & Park, PMID 19771248). However, a placebo-controlled randomized study of pentoxifylline, vitamin E, and clodronate to treat radiation plexopathy failed to show a benefit (Delanian et al., PMID 31987975). I think it may need to be used concurrently to gain benefit, but there may be some benefit if administered a sign of first symptom.

Sorry, I don't think there is a good treatment option once radiation myelitis is identified, but you can consider hyperbaric oxygen and/or pentoxifylline with vitamin E; just don't expect much.