Measurement of off-centre ratios (OCRs) is a requirement for the commissioning of cyberknife. The fixed source to axis distance (SAD) technique is required for the measurement of OCRs which is time-consuming and tedious. The fixed source to surface distance (SSD) technique, on the other hand, is easy to set up and requires less time. The OCRs have been measured with SAD and SSD setup and compared to assess the difference between each other.

The research is carried out on an Accuracy cyberknife M6, installed at NORI Cancer Hospital Islamabad. The OCRs are measured with Sun Nuclear ‘EDGE’ diode detector on a Sun Nuclear SNC 3D dosimetry system. The OCRs were measured for 12 cones and at three depths. Each OCR measured with the SAD setup is compared with the corresponding OCR measured with SSD setup using % dose distance and distance to agreement (2%/0·2mm).

For the within-the-beam and out-of-the beam regions, both OCRs are matching with each other. The percentage difference is in the order of less than 1% while the distance-to-agreement results in 100% matching for all cones and all depths. For the penumbra region, the percentage difference is higher than the other two regions. The maximum percentage difference is 2·96%. Generally, the percentage difference is higher for small cones and for OCRs measured at larger depths.

The OCRs on a cyberknife system measured with a fixed SSD setup and fixed SAD setup coincide within an acceptable limit and can be measured with both setups with similar accuracy.