Post-Radiation Sacral Insufficiency Fracture

Occasionally, an ill-defined region of moderate to intense metabolic activity will develop in the sacrum (unilateral or bilateral) in patients previously treated with pelvic radiation (e.g. gynecologic malignances, colorectal cancer, anal cancer, prostate cancer). While such uptake often represents osseous metastatic disease, it occasionally represents a post-radiation sacral insufficiency fracture.

Such fractures can occur as early as 3-months post-therapy, usually in patients with a clinical susceptibility (e.g. osteoporosis, long-term steroid therapy). They are typically moderate to intensely avid, becoming non-avid as they heal.

When the PET abnormality is accompanied by the characteristic CT appearance of such fractures, the diagnosis can be made with confidence. 

When the CT appearance is non-diagnostic, however, the diagnosis can only be suggested, with metastatic disease still a possibility:

“Ill-defined intense uptake is noted within the left sacrum (SUV 3.8). While this may represent an insufficiency fracture associated with prior radiation therapy, the lack of a well-defined fracture line on the CT images raises the possibility of an osseous metastasis. MRI or follow up may be of diagnostic value in this case.”