Osseous Pseudoprogression after Spine Stereotactic Radiosurgery

One of the issues we run into when assessing response to radiation therapy is pseudoprogression: Enlargement of the area of abnormality that is not truly progression. This phenomenon is best known in brain lesions following gamma knife therapy, but has also been seen in lung lesions after stereotactic body radiotherapy (SBRT).

In the spine, we can see pseudoprogression after spine stereotactic radiosurgery (SSRS) in bone lesions (osseous pseudoprogression, or OPP), as well as in the epidural soft-tissue components of bone lesions.

The take-away messages are:

• We have so far only seen OPP after single-fraction SSRS. This is likely due to the higher biological dose of single-fraction therapy.
• If you see an enlarging bone lesion on MRI performed within 3-6 months after single-fraction SSRS, you can't be confident that it represents true progression, because about 1/3 of these enlarging bone lesions will represent OPP.
• The only finding on conventional MRI that has been shown to be associated with OPP is tumor growth confined to the 80% iso-dose line and the slope of enlargement (earlier time to tumor enlargement).

References

• Amini B, Beaman CB, Madewell JE, Allen PK, Rhines LD, Tatsui CE, Tannir NM, Li J, Brown PD, Ghia AJ. Osseous Pseudoprogression in Vertebral Bodies Treated with Stereotactic Radiosurgery: A Secondary Analysis of Prospective Phase I/II Clinical Trials. AJNR Am J Neuroradiol. 2016 Feb;37(2):387-92.
• Bahig H, Simard D, Létourneau L, Wong P, Roberge D, Filion E, Donath D, Sahgal A, Masucci L. A Study of Pseudoprogression After Spine Stereotactic Body Radiation Therapy. Int J Radiat Oncol Biol Phys. 2016 Nov 15;96(4):848-856.