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Impact of Decaying Dose-rate in Gamma knife Radiosurgery

To the Editor,

The recent publication by Niranjan et al1 in the Journal of Radiosurgery and SBRT, Volume 1, Issue 4 represents an attempt to address a long standing conundrum in Leksell Gamma Knife (LGK) stereotactic radiosurgery (SRS) as to whether the activity of the Cobalt-60 (60Co) sources, in isolation, affect the biological effectiveness of this method of treatment. This publication, in common with others in this field, expresses the activity of the 60Co sources in terms of the calibration dose-rate of the equipment, representing in effect the dose-rate as measured in the Elekta ABS Calibration Phantom using a fixed sized collimator: a standard output measurement in conventional radiotherapy terminology. In the published study this dose-rate ranged from 0.77 – 2.936 Gy/min. However, as alluded to in the discussion of their manuscript these calibration dose-rates do not represent the actual doserates in the target volume of individual patients. Even when using a single iso-center the dose-rate in the target volume will be modulated by the selected collimator, if different from that used for calibration, and by the individual treatment volume geometry of the patients. This disparity between fixed calibration dose-rate and varying dose-rates in individually treated patients has been discussed elsewhere2 for cases of trigeminal neuralgia. This may seem like stating the obvious but it must be recognized that it is dose-rate in the target volume of an individual patient that influences the biological effectiveness of a specified dose in the target of that individual patient and not the calibration dose-rate factor per se. Thus caution is needed if conclusions are reached, based on the subdivision of patients based on the calibration dose-rate factor 3 and not the dose-rate in the individual patient. It is also pointed out in the discussion of the publication by Niranjan et al1 that as the number of iso-centers used in a treatment are increased, the treatment times are also generally increased because the tissue dose-rates are lower, when the dose is divided between more iso-centers, and because of the increasing number of time gaps between iso-centers. The latter applies, in particular, to machine models prior to Perfexion®.With increasing total exposure times, more time is available for the repair of sublethal radiation damage both over the periods of actual exposure exposure (beam on time) and during the gaps (beam off times) between each of the iso-center exposures. In the gaps no additional damage is obviously induced but some repair does occur. The time-related dose-rates and dose profile changes for a patient treated for a Vestibular Schwannoma, over the course of treatment for an individual voxel receiving the prescription dose of 14 Gy on the 50% iso-surface, are illustrated in Figure 1. The different isocenter doses delivered to this specific voxel were 0.015 – 3.847 Gy given at dose-rates of between 0.0031 – 1.043 Gy/min. This to be compared with a calibration dose-rate of 2.963 Gy/min on the date this patient was treated. The dose-rates and dose profiles to other voxels on that same 50% physical iso-surface differed and thus for practical and reporting considerations this illustration would imply that the use of the phantom based single calibration doserate may, at best, be misleading…

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