Hello all. In view of the current list activity re: the "cyberknife," I wished to provide a condensed response made to a similar inquiry.
I can tell you that, from prior experience, if JHH felt the "cyberknife" were a significant advance, JHH would have it. Unfortunately, the published dose distribution (dosimetry) of this device does not show an improvement, and is, in fact, virtually identical to that of the dedicated radiosurgical linear accelerator. I have met with the inventor of the device, Dr. Adler, and personally discussed the issue of identical dosimetry. The distribution is so similar as to be indistinguishable because both devices use circular collimators, the metal tubes that shape the beam, and both devices use very similar computer programs (algorithms) to calculate the radiation dose during the treatment planning. Importantly, for both devices, the steps for the confirmation of the positioning of the patient prior to treatment are in place.
The JHH system creates the "virtual skull xray" or "digitally reconstructed radiograph" (DRR) that is created from the data acquired at the time of the simulation. The DRR allows confirmation of the exact, correct positioning of the patient immediately before treatment is initiated by comparison to orthogonal (right angle: antero-posterior and lateral) radiographs using the accelerator as the xray source. The cyberknife uses fluoroscopy, a different method. Finally, the "cyberknife" is not an improvement in "accuracy" over the dedicated linear accelerator. The error of the "set-up" is very low and again virtually identical for both systems based on the actual measurements provided by Accuray, the manufacturer.
So, given the above, why would a hospital purchase it? The "cyberknife" does allow treatment of lung, prostate and liver tumors and thus provides the opportunity for the billing for extracranial sites. The radiosurgical linear accelerator is a dedicated device--only brain tumors are treated.
Let me propose two important questions to ask in the consideration of any treatment device or program:
1.) What is the number of patients who have been treated thus far, the duration of follow up, and the findings of the clinical as well as radiographic follow up.
2.) What is the total radiation dose and number of fractions to achieve the total dose. The corollary question is to describe the basis for the selection of dose and the predicted therapeutic gain (tumor killing vs. preservation of normal cranial nerve(s).
I hope this is helpful to you. I am pleased to continue the discussion with anyone who may have any questions in this regard. Thanks.
Jeffery A. Williams, M.D.
Director, Stereotactic Radiosurgery
Department of Neurosurgery
The Johns Hopkins Hospital
Harvey 811
600 North Wolfe Street
Baltimore, MD 21287-8811
Phone and Voice Mail: 410-614-2886
Fax: 410-614-2982
email: jw@jhu.edu
web: http://www.med.jhu.edu/radiosurgery
No radiosurgical device can "see" the tumor prior to treatment. To verify positioning prior to treatment, both the cyberknife and
the Hopkins FSR measure the location and position of the skull (and hence the contained AN) with reference to the treatment device via the bony landmarks of the skull.
Neither system visualizes (or can visualize) the tumor--this can only be done by MRI or CT that is acquired during the "simulation." These images are then provided to the treatment planning software system. The position of the patient's head (skull and hence AN) for treatment is defined during the treatment planning.
To visualize the skull and to ascertain correct positioning, the cyberknife uses fluoroscopy. The Hopkins FSR uses the actual radiographs (x-rays) taken by the treatment machine while the patient is in actual position for treatment, and takes the images immediately prior to treatment. The cyberknife uses an imaging system that is different from the machine that delivers the radiation. The Hopkins FSR uses the same machine that will treat the patient (and in the same geometric position as for the treatment) to take the images for verification. The Hopkins FSR system for verification is more labor intensive, but this method is intrinsically more accurate based upon the method for acquisition of the images.
To summarize, neither device "sees" the tumor. Both devices do use the bony landmarks of the skull to verify the correct positioning immediately prior to treatment. The key determinants of success are total dose, fractionation, and experience. I hope this is helpful.
Jeffery A. Williams, M.D.
Director, Stereotactic Radiosurgery
Department of Neurosurgery
The Johns Hopkins Hospital
Harvey 811
600 North Wolfe Street
Baltimore, MD 21287-8811
Phone and Voice Mail: 410-614-2886
Fax: 410-614-2982
email: jw@jhu.edu
web: http://www.med.jhu.edu/radiosurgery