Investigation of dosimetric differences between the TMR 10 and convolution algorithm for Gamma Knife stereotactic radiosurgery

Abstract

Since its inception, doses applied using Gamma Knife Radiosurgery (GKR) have been calculated using a simple TMR algorithm, which assumes the patient's head is of even density, the same as water. This results in a significant approximation of the dose delivered by the Gamma Knife. We investigated how GKR dose calculations varied when using a new convolution algorithm clinically available for GKR planning that takes into account density variations in the head compared with the established calculation algorithm. Fifty‐five patients undergoing GKR and harboring 85 lesions were voluntarily and prospectively enrolled into the study. Their clinical treatment plans were created and delivered using TMR 10, but were then recalculated using the density correction algorithm. Dosimetric differences between the planning algorithms were noted. Beam on time (BOT), which is directly proportional to dose, was the main value investigated. Changes of mean and maximum dose to organs at risk (OAR) were also assessed. Phantom studies were performed to investigate the effect of frame and pin materials on dose calculation using the convolution algorithm. Convolution yielded a mean increase in BOT of 7.4% (3.6%–11.6%). However, approximately 1.5% of this amount was due to the head contour being derived from the CT scans, as opposed to measurements using the Skull Scaling Instrument with TMR. Dose to the cochlea calculated with the convolution algorithm was approximately 7% lower than with the TMR 10 algorithm. No significant difference in relative dose distribution was noted and CT artifact typically caused by the stereotactic frame, glue embolization material or different fixation pin materials did not systematically affect convolution isodoses. Nonetheless, substantial error was introduced to the convolution calculation in one target located exactly in the area of major CT artifact caused by a fixation pin. Inhomogeneity correction using the convolution algorithm results in a considerable, but consistent, dose shift compared to the TMR 10 algorithm traditionally used for GKR. A reduction of the prescription dose may be necessary to obtain the same clinical effect with the convolution algorithm. Head shape definition using CT outlining can reduce treatment uncertainty from head shape approximations.PACS number(s): 87.53.‐j; 87.55.D; 87.55.kd

Recommendations on the practice of calibration, dosimetry, and quality assurance for gamma stereotactic radiosurgery: Report of AAPM Task Group 178

“…Several reports have appeared in the literature comparing the two algorithms. [65][66][67][68][69][70][71][72][73] These reports indicate that using the convolution algorithm as opposed to TMR10 for the same treatment plan (i.e., for the same shot arrangement and prescription dose) tends to increase the total treatment time by between about 4% and 10%, depending on the tumor type and location, without significant change in the dose distribution. 73 Thus, slightly higher doses are effectively delivered using the convolution algorithm instead of the standard dose algorithm.…”

“…Although the convolution method is inherently more accurate than simpler ray‐tracing dose calculations, as of this writing, it has not been widely adopted in GSR. Several reports have appeared in the literature comparing the two algorithms 65–73 . These reports indicate that using the convolution algorithm as opposed to TMR10 for the same treatment plan (i.e., for the same shot arrangement and prescription dose) tends to increase the total treatment time by between about 4% and 10%, depending on the tumor type and location, without significant change in the dose distribution 73 .…”

Validation of PTV margin for Gamma Knife Icon frameless treatment using a PseudoPatient® Prime anthropomorphic phantom

“…The dose differences between the TMR 10 and convolution algorithms correlate well with other reported studies. Rojas‐Villabona et al reported that dose calculations generated by the convolution algorithm closely matched the measurement values with an average treatment time that was 5.9% longer than in the TMR 10 algorithm 24 . Choi et al reported that Monte Carlo dose calculations were lower than those calculated by TMR 10 by about 4% 25 …”

Dosimetric comparison of TMR10 and convolution dose calculation algorithms in GammaPlan treatment planning system

“…These algorithms have been compared in previous studies. [4][5][6][7][8] It was found that the convolution algorithm provided greater accuracy to films within a RANDO phantom to dose distributions than that with TMR10. 4 This conclusion was supported by Nakazawa et al, 5 who advocated the use of convolution algorithm to reduce dosimetric uncertainty.…”

“…Another study revealed the convolution algorithm reducing dose to cochlea by 7% when compared to TMR10. 7 Treatment times were studied in one paper revealing an 8•4% larger average treatment times for identical prescriptions and shot arrangements when the convolution dose calculation was used compared to the TMR10 algorithm. 8 When implementing new dose calculation algorithms, special considerations must be given to the dose prescription.…”