Analysis of various dosimetric parameters using multiple detectors in the cyberknife® robotic radiosurgery system

Accurate dosimetry in CyberKnife® is challenging because of the unavailability of suitable detectors to satisfy all the criteria of small-field dosimetry. In this work, eight different small-field detectors from PTW and IBA Dosimetry were used to determine the dosimetric parameters for twelve fixed collimators in the CyberKnife® radiosurgery system. The scope of this work was to assist medical physicists in detector selection in small-field dosimetry.

Dosimetric parameters such as the surface dose (Ds), dose buildup (DB), percentage dose at 100 mm (D100), percentage dose at 200 mm (D200), depth of dose maximum (Dmax), and total scatter factor (Scp) were compared and analyzed from the acquired Percentage Depth Dose(PDD).

Large variations in Ds were observed with different detectors for smaller collimator sizes. On analyzing the dose buildup, considerable differences were observed with all detectors from the surface to  6 mm depth for the smallest cone of 5 mm diameter. The D100 and D200 values obtained using ion chambers were higher than those using diodes, likely due to the volume averaging effect. The depth of dose maximum was found to increase with increase in the field size for most of the detectors. Considerable variation in Scp was noticed with all detectors in smaller field ranges.

For small fields, the selection of detector is crucial, and awareness of the advantages, disadvantages and limitations of the detectors used is crucial. As in absolute dosimetry, the relative dosimetry in small fields is challenging and requires further studies and recommendations.