LDRT for Sacroiliac (SI) Joint Pain

If your pain sits low in your back, near the dimples above your buttocks, your sacroiliac joint may be the cause. Low-dose radiation therapy (LDRT) is an established treatment for arthritis in the knee, hand, and shoulder. But the SI joint is a different structure, and the direct evidence is thin. This page lays out exactly what is known.

Your SI Joint, in Brief

The sacroiliac joint sits where the sacrum meets the ilium, one on each side of your pelvis. It transfers weight from your upper body to your legs, and it barely moves, 2 to 4 millimeters in any direction^[1]. Its upper half is packed with fibrocartilage and has no synovial cavity; only the lower part has hyaline cartilage and a true joint space^[2]. The SI joint is a common pain source: it may account for up to a quarter of low back pain, and osteoarthritis is a frequent contributor to SI joint dysfunction^[1][3].

Why the Diagnosis Has to Come First

SI joint pain is easy to mistake for other problems. It can feel like hip pain, sciatica, or a lumbar disc issue, and diagnosing it is genuinely difficult even for specialists^[3]. Imaging alone rarely settles it. The standard approach combines a cluster of provocation tests, such as FABER, pelvic gapping, and Gaenslen's, with a diagnostic injection: if numbing the joint stops the pain, the joint is the source^[4]. Do not skip this step. Treating one spot with radiation only makes sense once that spot is confirmed.

What Has Actually Been Published on the SI Joint

Almost nothing. A search of PubMed, including the German term "Iliosakralgelenk," turns up no case report or study of low-dose radiotherapy specifically for SI joint pain. Nearly all published LDRT research covers the knee, hand, shoulder, hip, and elbow. The largest recent retrospective series treated 598 patients across seven diagnoses (heel, shoulder, hand, elbow, outer hip, knee, and hip joints) with no SI joint patients at all^[5]. About 62 percent of those patients reported a good response after a median follow-up of about 38 months^[5].

The Broader Evidence It Borrows From

In 2026 the American Radium Society published appropriate-use criteria for LDRT in osteoarthritis. A multispecialty panel reviewed 44 studies and reported overall response rates of 60 to 90 percent for pain reduction and better mobility^[6]. That guidance covers osteoarthritis broadly. The published abstract does not mention the SI joint, and the review only admitted studies with at least 50 patients^[6]. The broader evidence is also contested: a 2025 systematic review and meta-analysis of 12 studies with 1,750 participants found no significant benefit of LDRT over sham treatment for pain or function^[7]. So the SI-joint-specific evidence does not exist, and even the borrowed evidence is still being argued over.

Why the SI Joint Is Different

The SI joint is not a smaller knee. Half of it is fibrocartilage with no synovial cavity^[2], and it barely moves^[1]. LDRT's proposed anti-inflammatory effect has been studied almost entirely in synovial joints like the knee and hand. Whether the same effect happens in a joint that is half fibrous and mostly still is untested. That does not rule it out. It means no one has published the test.

What This Means for You

Some programs do offer LDRT for SI joint pain. In Germany, where the treatment has been used for decades, low-dose X-ray radiotherapy ("Röntgenreizbestrahlung") is listed among standard options for sacroiliitis^[8]. If a center offers it to you, ask three plain questions:

• How many patients with SI joint pain have you treated?
• What results did you see, and how did you measure them?
• Was the SI joint confirmed as the pain source before treatment?

Then talk with your doctor, ideally the one who made the SI joint diagnosis, before deciding. The bottom line: German clinical practice supports trying it, the borrowed OA research is encouraging but contested, and SI-joint-specific evidence does not yet exist. You would be in thinner-evidence territory. You should know that going in.

References:
1. Sacroiliac Joint Pain. StatPearls. StatPearls
2. Anatomy, Abdomen and Pelvis, Sacroiliac Joint. StatPearls. StatPearls
3. Sacroiliac Joint Injury. StatPearls. StatPearls
4. Differential Diagnosis of Posterior Buttock Pain: A Conceptual Review Based on Topographic Localization of Pain, Is It Really the Sacroiliac Joint? PMC. Full text (PMC)
5. Juniku N, et al. Radiotherapy for painful benign skeletal disorders: Results of a retrospective clinical quality assessment. Strahlentherapie und Onkologie. 2019;195(12):1068-1073. Full text (PMC)
6. Dove A, et al. American Radium Society (ARS) Appropriate Use Criteria (AUC) for Low-Dose Radiotherapy for Treatment of Osteoarthritis. American Journal of Clinical Oncology. Published online May 25, 2026, ahead of print. PubMed
7. Hammadeh M, et al. Efficacy, safety, and pain management of low-dose radiation therapy in osteoarthritis: a comprehensive systematic review and meta-analysis. Rheumatology International. 2025;45(9):210. PubMed
8. Sakroiliitis: Ursachen, Symptome, und Behandlung. Onmeda (German health portal). Onmeda