PATIENT: Buddy, an 8-year-old, male-neutered Labrador Retriever
LESION: C3 Mass Lesion
BACKGROUND: Buddy initially presented for suspected neck pain in late April 2017. He was reluctant to rise and his head carriage was low. An MRI on 5/5/2017 revealed a large ovoid mass within the spinal canal overlying the body of the third cervical vertebra (Figure 1). The mass was located to the right of the spinal cord, occupied at least 2/3 of the cross-sectional area of the spinal canal, and caused marked displacement of the cord to the left with severe spinal cord compression.
There was also moderate moth eaten lysis of the right pedicle of the C3 vertebra. The mass was thought to be a primary vertebral body tumor (e.g., osteosarcoma) vs. round cell neoplasia (e.g., lymphoma) vs. other neoplasia. At the time of treatment, Buddy was unable to walk, very painful, but still eating and drinking.
TREATMENT: CyberKnife utilizes hundreds to thousands of small beams of radiation to target a tumor, which are administered from up to 1200 different angles around a patient’s body (Figure 2), thus “painting” on a dose of radiation with submillimeter accuracy. This means that a clinically insignificant dose of radiation is received by the healthy tissue surrounding a tumor, making radiation side effects minimal to absent (Figure 3).
CyberKnife’s capability to deliver a very large dose of radiation in 1-3 treatments (equivalent to 3-4 weeks of daily treatments on a conventional linear accelerator) results in a patient undergoing anesthesia fewer times and completing a radiation protocol in one week or less.
Buddy was determined to be an optimal candidate for CyberKnife given the location of the tumor and the proximity to the spinal cord. The goal of treatment was to shrink the tumor, thereby decompressing the cord and allowing him improved mobility and quality of life. Without a biopsy and definitive diagnosis, it was difficult to predict the tumor’s response to radiation and overall prognosis, however, the radiation treatment protocol would remain the same regardless of histopathology. Because of the aggressive nature and rapid progression of the tumor, there was concern that the spinal cord could be damaged beyond repair due to constant compression by the tumor. Therefore, 3 fractions (treatments) of CyberKnife therapy were administered over 3 consecutive days instead of distributed over several days. If he did not show improvement relatively quickly (as hoped because rapidly growing tumors respond more quickly to radiation therapy), surgical decompression may have become necessary.
RESULTS: By the last day of treatment, Buddy was lifting his head and seemed more comfortable. Three days after treatment, he was brighter, wagging his tail, and could get up on his own and take a few steps. Over the next several days, Buddy walked outside on his own with minimal sling support. At home the next week, Buddy was getting back to normal – sniffing the ground and moving his neck normally, getting in and out of the car without trouble, and even sneaked upstairs to everyone’s surprise. By the end of the next month, Buddy had a complete resolution of his clinical signs associated with the tumor. A follow up MRI revealed complete resolution of the C3 spinal canal mass (Figure 4) with no progression of the focal area of lysis noted in ventral C3. Buddy was able to discontinue his steroid medication earlier than expected, and has had no recurrence of his clinical signs.
RELEVANCE: The optimal situation to utilize CyberKnife stereotactic radiation therapy is in the bulky disease setting, meaning that tumors are visible and “targetable”. After treatments have been completed, the tumors will typically shrink over time. The sensitivity of the tumor to radiation will ultimately dictate its response and how quickly it will shrink. Given the significant response to radiation therapy (complete resolution in 1 month), Buddy’s tumor was most likely a round cell neoplasia, e.g., lymphoma, plasma cell tumor, etc., rather than vertebral body osteosarcoma or other neoplasia.