Sep 14, 2018
Eighteen centers employing various stereotactic radiosurgery (SRS) platforms participate in UK commissioning program
In intracranial SRS, the steeper dose gradient that Leksell Gamma Knife® provides, in comparison to other SRS platforms, not only ensures better sparing of organs-at-risk (OARs), it enables clinicians to adhere to a “zero margin PTV philosophy,” investigators maintained in the largest planning evaluation of SRS centers and platforms to date. The results of their study were published recently in Practical Radiation Oncology.
“The study, which focused on SRS of benign brain tumors, was the largest of its type ever undertaken and showed consistently lower doses delivered to normal brain with Leksell Gamma Knife,” says study co-author Ian Paddick, Consultant Physicist. “The volume of 50 percent of the prescription dose was up to three times higher with linac technology.”
In a UK national commissioning program, the 18 NHS centers currently performing SRS were asked to complete benchmark planning cases. The centers used a variety of SRS platforms from multiple manufacturers. In the study, four benign cases (vestibular schwannoma, larger VS, skull base meningioma and secreting pituitary adenoma) with contours were provided to the centers, which were instructed to plan the cases using their institutional protocols.
A total of 68 plans were submitted. Prescription doses, margins used, volumes and dose-volume parameters to targets and OARs were recorded and used as the data set for the evaluation. Most centers provided single-fraction treatment prescriptions.
According to the researchers, the linac submissions were highly variable for such parameters as selectivity, dose gradient, margins and prescription dose (particularly for the pituitary case).
“This can cause sites using a linac to irradiate larger volumes of healthy tissue when compared to CyberKnife and Leksell Gamma Knife,” says Peter Fröberg, Director Neuroscience Portfolio, Elekta. “When linac submissions were able to maintain low dose to OARs, it was almost always accompanied by a trade-off in plan quality to the target. Only Leksell Gamma Knife plans were able to avoid trade-offs between different dose objectives, such as between doses to OARs and general dose spillage.
“This is an important factor especially as we are dealing with patients who have benign disease,” Fröberg continues. “We especially don’t want to expose these patients to any additional radiation when treating a non-malignant disease.
Steep dose gradient facilitates maximum tumor targeting and maximum normal tissue sparing
In discussing the results, the investigators stressed that in intracranial SRS the entire brain surrounds the target, necessitating the placement of a steep dose gradient on the edge of the PTV to minimize the brain’s dose. They add that this is best achieved by having a steep dose gradient on both the inside and outside of the PTV:
“By including a margin around the CTV, a much higher dose is delivered to the target, but also a higher dose to the part of the brain that is now included within the PTV. For example, adding a 1 mm margin to an 8 mm diameter target almost doubles the irradiated volume, and will normally mean that more brain is being irradiated within the prescription isodose than tumor.
“The fixed geometry of [Leksell Gamma Knife] ensures spatial uncertainties are minimized and therefore lends itself to a zero margin PTV philosophy.”
Publication of this most recent SRS benchmarking study on benign brain tumors was preceded by the investigators’ benchmark planning studies for multiple brain metastases. In this study, researchers largely noted variations in clinical practice and priorities. However, most SRS platforms delivered dramatically different doses to normal brain, with Leksell Gamma Knife giving the lowest doses outside the target, especially for the smallest volumes or when margins were used on other technologies.
According to the authors: “The data will be used to advance standardization and quality improvement of national services and can provide useful guidance for centers worldwide.”
2. Eaton David J., Lee Jonathan, Paddick Ian, Stereotactic radiosurgery for multiple brain metastases: Results of multi-centre benchmark planning studies, Practical Radiation Oncology (2017), https://doi.org/10.1016/j.prro.2017.12.011