Question: If the surgeon treats multiple lesions (in this particular case, two different lesions) during gamma knife surgery, may we report multiple units of 61793? Montana Subscriber Answer: Yes. When the neurosurgeon performs stereotactic radiosurgery/gamma knife surgery, you will report one unit of 61793 (Stereotactic radiosurgery [particle beam, gamma ray or linear accelerator], one or more sessions) for each lesion the surgeon targets. You may report 61793 for either cranial or spinal radiosurgery. In your case, you will report 61793 for treatment of the first lesion and a second unit of 61793 with modifier 59 (Distinct procedural service) appended to show that the surgeon treated lesions at different locations. Learn more: For complete information on reporting multiple units of 61793, see "If You-re Treating 61793 as a Once-and-Done Code, You-re Missing Out," Neurosurgery Coding Alert, Vol. 9, No. 5, pages 33-35.