Determination of [Formula: see text] using MLC‐collimated rectangular fields for absolute dosimetry of the CyberKnife

Traditional CyberKnife (CK) calibration uses TG‐51, which requires [Formula: see text] to be defined using the standard reference condition of 100 cm SSD in a [Formula: see text] field. Since the CK is calibrated using a 6 cm fixed‐aperture collimating cone at 80 cm SAD, the BJR‐25 method is commonly used to relate circular‐field PDDs to square‐field PDDs for [Formula: see text] determination. Using the InCise MLC system, the CK is able to deliver rectangular fields, allowing a more direct measurement of [Formula: see text] using conventional reference conditions. We define the PDD correction factor ([Formula: see text]) as the ratio of [Formula: see text] measured using CK reference conditions to that measured using standard TG‐51 reference conditions. Using four ionization chambers (A1SL, CC08, CC13, and A19), [Formula: see text] is measured using a 6 cm fixed cone at 80 cm SSD and at 100 cm SSD using an effective [Formula: see text] MLC‐collimated field. These values are used to calculate [Formula: see text] , while the latter is used to directly calculate a [Formula: see text] value. This direct [Formula: see text] value is then compared to values determined using the BJR‐25 method. Using the MLC system, this study demonstrates conversion between the [Formula: see text] measured using CyberKnife reference conditions and TG‐51 reference conditions. These values provide the means for derivation of a [Formula: see text] curve as a function of direct measurements of [Formula: see text] using a 6 cm fixed‐aperture collimating cone at 80 cm SSD. PACS number: 87.55.Qr