Study Recommends Lower Radiation Dose for Spinraza Scans

Preforming an imaging scan that uses less radiation can be used to plan the administration of Spinraza (nusinersen) in people with spinal muscular atrophy (SMA), without sacrificing confidence in treatment planning, a study shows.

The study, “Radiation dose reduction for CT-guided intrathecal nusinersen administration in adult patients with spinal muscular atrophy,” was published in Scientific Reports.

Spinraza was the first approved SMA treatment. Because the medication cannot cross the blood-brain barrier, it must be delivered by an injection directly into the fluid that surrounds the brain and spine (an intrathecal injection). This is most commonly done by means of a lumbar puncture, also called a spinal tap.

The blood-brain barrier is a semi-permeable membrane that protects the brain and spinal cord against the external environment, and is a major barrier for the efficient delivery of certain therapeutics.

Intrathecal injection requires detailed knowledge of the layout of the bones and soft tissue at the injection site, especially in SMA, in which there are often abnormalities in the formation of the spine. This is generally accomplished using computed tomography (CT) scans.

CT scans require exposing the patient to radiation. While the risk from a single scan is minimal, Spinraza must be administered routinely over the course of a person’s life. This requires continual exposure to radiation, which is a safety concern because it raises risks such as a higher risk of cancer.

Researchers reviewed images for 64 CT-guided Spinraza injections that were done at their institution between September 2017 to October 2019. The injections were given to 13 adults with SMA — eight with SMA type 2 and five with SMA type 3 — seven males and six females, ranging in age from 6 to 46.

Prior to May 2019, CT scans at the researchers’ institution were done using a standard radiation dose of 133–200 milliamperes. An updated protocol put into place in May 2019 used a lower radiation dose (20–67 mA), along with more advanced image reconstruction software to try to make up for the resultant lower quality.

Forty-four of the analyzed scans were done with the old protocol, and the remaining 20 were done with the low-radiation protocol. All 13 people received at least one Spinraza injection using the low-radiation protocol, with no apparent complications. For both protocols, only one injection attempt was necessary for the majority of cases, and there were no significant differences in average time taken to perform a lumbar puncture.

“[T]he successful intrathecal drug administration itself can be considered as evidence for a sufficient image quality and appropriate planning also when using low-dose MDCT [multi-detector CT] scanning,” the researchers wrote.

The 64 scans were given to two neurologists, who were blinded to which protocol was used to generate them. The neurologists were asked to rate the images based on various factors, including their overall quality and how confident they would be doing treatment based on these scans.

The researchers then compared scores on scans done via the two protocols. Images done with the low-radiation protocol were rated as significantly lower overall quality. Critically, however, there were no differences in terms of ability to view bone and soft tissue, or in the neurologists’ confidence in doing lumbar punctures based on the images. In other words, though the low-radiation scans did not have a high-quality image, they were just as usable as the scans from the standard protocol.

The study suggests that a low-radiation imaging protocol could be used to minimize the risks associated with radiation in people with SMA who are being treated with Spinraza, without sacrificing the utility of the images obtained.

“We encourage other centers to consider low-dose imaging protocols as an option for [multi-detector CT]-guided [lumbar puncture] in diagnostic and therapeutic management,” the researchers wrote.