How Long Does LDRT Take to Work, and How Long Does Relief Last?

This is the question almost everyone asks after the last session: when will I know? Low-dose radiation therapy, or LDRT, does not work like a pain pill or a cortisone shot. Nothing feels different when you walk out of the treatment room. The effect builds slowly as the inflammation in the joint settles down, and that takes weeks.

When to Expect Results

Give it time. In the German patterns-of-care data behind the DEGRO guideline, a response to radiotherapy for knee arthritis was usually reached two to three months after treatment^[1]. US programs say the same thing in plainer terms: meaningful relief often takes about two months or longer, and the follow-up visit to judge the result is usually set at about three months^[2].

If it works for you, the relief is not measured in weeks. In a study of 1,037 knee arthritis patients, 57 percent of the people who responded still had notable pain relief more than a year later^[3].

Why the First Month Can Fool You

The best test of LDRT so far is a randomized, sham-controlled trial from South Korea, presented at ASTRO 2025. It enrolled 114 people with mild to moderate knee osteoarthritis. One group got sham treatment, one got a very low dose of 0.3 Gy in six sessions, and one got 3 Gy in six sessions, which is the standard six sessions of 0.5 Gy^[4].

Early on, people felt better in every group, including sham. That is a strong placebo effect and a normal part of arthritis trials. By four months, 70.3 percent of the 3 Gy group met the trial responder definition, compared with 41.7 percent of the sham group. The very low 0.3 Gy dose did not separate from sham^[4][5]. When the full trial was published in 2026, its authors described the result as a time-limited effect beyond that early placebo response, so it shows the treatment can work, not how long it lasts^[6].

So judging LDRT at two or three weeks tells you almost nothing. The dose matters too: the token dose did not work, and the standard dose did.

The Timeline, in Numbers

A large German series of 295 irradiated joints in 159 patients shows the shape of the response. On a 0 to 10 scale, median pain was 7 before treatment, 4 at six weeks, and 3 at twelve months, where it stayed at two years^[7]. Much of the relief arrives in the first weeks after the course, and it keeps improving for some time after that.

That matches the older knee data. In the 1,037-patient knee study, 79.3 percent reported at least slight pain relief immediately or within two months of finishing a series, using a loose definition that counted any slight improvement^[3]. Stricter definitions give lower numbers, which is one reason published response rates vary so much from study to study.

Reviews of the wider literature put pain relief at 60 to 90 percent of irradiated patients, but most of that literature is European observational work rather than sham-controlled trials, so treat the top of that range with care^[8].

How Long Does the Relief Last?

For the people it helps, LDRT tends to hold up well. In the 1,037-patient knee study, 57 percent of responders kept notable relief for more than a year. Two earlier German cohorts cited in the same paper put that figure at about 40 percent and 67 percent, so roughly half of responders are still better at a year or beyond^[3]. One caution about that study: the long-term numbers came from a mailed survey, and only 106 questionnaires could be evaluated, so the follow-up is thinner than the headline sample size suggests.

The German joint-by-joint series reported a good response in every group it analyzed for at least two years, though that cohort mixed several joints rather than knees alone^[7]. A separate cohort of 598 patients treated for painful joint and tendon problems shows how late the effect can arrive. Only 13.9 percent had a good response when treatment ended, but far more did at a median follow-up of 38 months, and the authors concluded that the delayed onset makes the long-term results better than the early ones. Read that one carefully, though: the cohort mixed arthritis with tendon problems, and tendon problems responded better than arthritis, so its headline figure is not a knee arthritis number^[9].

Relief does not last the same length of time for everyone, some responders relapse, and arthritis itself keeps progressing underneath. If pain comes back a year or two later, that is a conversation about a second course, not proof the first one failed.

What If Nothing Has Happened by Six Weeks?

Wait. Six weeks is too early to call it. In the German series, pain kept falling well after the six-week mark^[7], and the sham-controlled trial did not separate real treatment from sham until four months^[4].

If the response is incomplete once you have waited, a second series is a standard part of this treatment. Protocols differ on timing. One prospective study repeated the course of six sessions of 0.5 Gy after a minimum interval of eight weeks when pain relief was incomplete^[10]. The dose per session seems to matter too: in a series of 483 people with finger arthritis, about 70 percent improved, and those treated with 0.5 Gy per session did better than those given 1.0 Gy^[11]. In US practice the usual pattern is simpler: come back at about three months, and decide then^[2][8].

The Other Side: Two Trials That Found Nothing

You should know that two Dutch sham-controlled trials found no benefit at all. In the knee trial, 55 patients got six sessions of 1 Gy or sham treatment, and at three months the responder rates were 44 percent with LDRT and 43 percent with sham, which is no difference^[12]. The companion hand trial, and a joint long-term follow-up of both trials, also found no difference at six and twelve months^[13].

Researchers who work with LDRT have published criticisms of those trials: small samples, 1 Gy per session instead of the German standard of 0.5 Gy, only one course with no option of a second series, a three-month endpoint for a treatment known to act slowly, and patients with longstanding, advanced disease who had already failed other treatments^[14]. Those criticisms have merit, but they do not erase the results. The controlled evidence is split: one newer positive trial at the standard dose, two older negative ones, and a large body of observational data in between.

Stage of disease may matter, though the data conflict. The ASTRO trial team describes LDRT as best suited to mild to moderate arthritis where the joint structure is still preserved, since radiation will not regrow cartilage that is gone^[4]. The large German knee study, though, found that severe changes on X-ray were linked with more relief, not less, and that age, sex, and how long the pain had lasted did not predict who responded^[3]. Nobody can yet tell an individual patient in advance which group they will fall into.

How to Judge Your Own Course

• Do not score the first month. In the sham-controlled trial, every group felt better at first, and the real difference showed only at four months^[4].
• Expect gradual change. In the German series, median pain fell from 7 to 4 by six weeks and kept falling to 3 by a year^[7].
• Judge it at about three months. German guideline data put the usual response at two to three months, and US programs schedule the decision visit at about three months^[1][2].
• Track function, not just pain. Stairs, walking distance, and sleep tell you more over three months than a daily pain score does.
• Ask about a second series before giving up. One protocol repeats the course after at least eight weeks when the first response is incomplete, and US programs usually decide at about three months^[10][2].

References:
1. Ott OJ, et al. Guideline recommendations for the radiotherapy of painful degenerative skeletal disorders (DEGRO). Strahlentherapie und Onkologie. 2015;191:1-6 (response usually reached two to three months; gonarthrosis single doses 0.5 to 1 Gy, total 3 to 6 Gy). PDF
2. Low-Dose Radiation For Osteoarthritis. Franciscan Health (six treatments over two weeks; meaningful relief often at two months or longer; follow-up at about three months). Franciscan Health
3. Keller S, et al. Efficacy of low-dose radiotherapy in painful gonarthritis: experiences from German bicenter study. Radiation Oncology. 2013 (1,037 knee patients; 79.3% with at least slight relief within two months; 57% of responders with relief beyond one year; long-term follow-up by mailed survey, 106 questionnaires). Springer
4. Low-Dose Radiation Therapy Offers Substantial Relief to People With Painful Knee Osteoarthritis. ASTRO news release, 2025 (Kim BH, et al., randomized sham-controlled trial, 114 patients, 3 Gy in six fractions vs 0.3 Gy vs sham; responders at four months 70.3% vs 41.7%). ASTRO
5. ASTRO: Low-Dose Radiation Improves Clinical Outcomes in Mild-to-Moderate Knee Osteoarthritis. HealthDay via Drugs.com, October 2025. Drugs.com
6. Kim BH, et al. Single course low-dose radiation therapy in knee osteoarthritis (LoRD-KNeA): a randomized controlled trial. BMC Medicine. 2026 (responders at four months 70.3% at 3 Gy vs 41.7% sham; authors describe a time-limited treatment effect beyond an early placebo response). BMC Medicine
7. Hautmann MG, et al. Radiotherapy for osteoarthritis: an analysis of 295 joints treated with a linear accelerator. Strahlentherapie und Onkologie. 2020;196:715-724 (159 patients; median pain 7 before treatment, 4 at six weeks, 3 at twelve and twenty-four months). Springer
8. Low-Dose Radiation Therapy Making a Comeback for OA. Medscape, 2025 (review literature summarized as pain relief in 60% to 90% of irradiated patients; reevaluation at three months). Medscape
9. Juniku N, et al. Radiotherapy for painful benign skeletal disorders: results of a retrospective clinical quality assessment. Strahlentherapie und Onkologie. 2019 (598 patients with arthrosis and enthesopathies; good response in 13.9% at the end of treatment; median follow-up 38 months; better results for enthesopathies than arthrosis). Springer
10. Rogers S, et al. Prospective evaluation of changes in pain levels, quality of life and functionality after low dose radiotherapy for epicondylitis, plantar fasciitis, and finger osteoarthritis. Frontiers in Medicine. 2020;7:195 (six sessions of 0.5 Gy, repeated once after eight weeks when pain relief was incomplete). Frontiers
11. Donaubauer AJ, et al. Low dose radiation therapy, particularly with 0.5 Gy, improves pain in degenerative joint disease of the fingers: results of a retrospective analysis. International Journal of Molecular Sciences. 2020;21(16):5854 (483 patients; about 70% improved; 0.5 Gy per session better than 1.0 Gy). MDPI
12. No Benefit Found for Low-Dose Radiation Therapy in Treatment of Knee Osteoarthritis. Rheumatology Advisor, reporting Mahler et al., Annals of the Rheumatic Diseases. 2019 (55 patients; responders at three months 44% LDRT vs 43% sham). Rheumatology Advisor
13. Low-Dose Radiation Therapy for Knee and Hand Osteoarthritis Questioned. RheumLive, reporting van den Ende et al., Lancet Rheumatology long-term follow-up of the Dutch trials (no difference at six and twelve months). RheumLive
14. Low-dose radiotherapy of osteoarthritis: from biological findings to clinical effects, challenges for future studies. Strahlentherapie und Onkologie. 2022 (published criticisms of the Dutch trials: size, schedule, single course, endpoint timing, advanced disease). Springer