Letter: The Zap-X Radiosurgical System in the Treatment of Intracranial Tumors: A Technical Case Report

Abstract

To the Editor:
\nWe read with great interest the recent article by Pan et al1 about
\nnewly designed surgical robot for brain and head and neck radiosurgery,
\nthe Zap-X system (Zap Surgical Systems Inc, San Carlos,
\nCalifornia). There are described 2 patients treated with ZAP, the
\nfirst harboring a trigeminal schwannoma and the second a petroclival
\nmeningioma, treated with a radiation dose of 13 Gy at the
\n50% isodose line. Both cases experienced tumor reduction and
\nsymptom improvement during follow-up course, in the absence
\nof any complications.
\nAs the authors state, this is a small series. However,
\nthe extremely novel technology, as well as such encouraging
\npreliminary results, are of interest for the radioneurosurgical
\ncommunity and should be followed by other reports in due time.
\nSeveral aspects caught our attention and would deserve further
\ndiscussion.
\nThe marginal dose prescribed is a standard one for trigeminal
\nschwannomas2 and in the range of what has been published for
\nmeningiomas.3
\nThe prescription isodose line was 50%, which seems to be
\nsimilar to what one would consider using a Gamma Knife (GK,
\nElekta Instruments AB, Stockholm, Sweden).
\nThe illustration of the cases within the article, with the dose
\nfall out outside the target volume, suggest a steep gradient and
\nis extremely interesting as a concept and physical capacities of
\nsuch device. Such might be related to what has been previously
\npublished, a combination of lower beam energy (3 MV), more
\nsolid angle gyroscopic crossfire but also the accuracy related to
\nimage-to-image x-ray correlation technology.4
\nThe presence of structures at risk, in particular the brainstem,
\ndid not represent a problem per se in prescribing such optimal
\nmarginal doses, taking into account that the brainstem is
\nsuggested to support as high as between 12 and 15 Gy at
\nthe periphery, without a risk of complications within this
\narea. Neuroimaging during follow-up showed no oedema or
\nother radiation-induced changes. Nevertheless, it did suggest the
\npresence of intralesional changes (necrosis) for the trigeminal
\nschwannoma, which is in line with accurate radiosurgical dose
\ndelivered and associated benign tumor radiation-induced histological
\nchanges.
\nThese clinical and radiological outcomes, as well as the
\ntechnical capacities, would raise the question whether single
\nfraction radiosurgery performed with the ZAP system would
\ninduce a similar radiobiological effect as to what is currently
\ngenerated by other devices, including GK (Elekta Instruments
\nAB) or CyberKnife.5-7
\nThere are some technical aspects that we also wish to comment
\nconcerning the Zap-X technology. Some of the innovations
\nappears revolutionary and promise to enhance the role of radiosurgery
\nin neurosurgical practice while keeping this technique
\nclose to the culture and moral principles of Neurosurgery. The
\nZap-X maintains a robotic approach to dose delivery and imageguidance
\n(frameless radiosurgery) but with a simplified approach
\n(ie, the gyroscope instead of a robotic arm and a streamlined
\ncranial radiosurgery-centric planning and delivery without
\nthe complexity of multipurpose radiotherapy delivery systems).
\nInnovative online dosimetry provides automated stop if dose
\ndeviations occur during delivery. The system is self-shielded,
\nnamely a vault is not necessary, and the device can be installed
\nanywhere, possibly complementing a neurosurgical operative
\ncompound. The overall costs of the system promise tomake radiosurgery
\nmore affordable in a world with less economic resources
\ndestined to neurosurgery.
\nWe congratulate the efforts of Dr Adler’s team. We salute the
\