Dose indicator for CyberKnife image-guided radiation therapy

The purpose of this study was to calculate dose distributions from CyberKnife image-guided radiation therapy (IGRT) for brain, H&N, lung, and pelvis treatment regions and use them to extract the corresponding effective dose and estimate-related risk.

We developed a CyberKnife IGRT kV beam model in a standard treatment planning system and validated it against measurements in heterogeneous phantoms. Five brain, five head and neck, five thorax, and 10 (five male and five female) pelvis patient computed tomographies (CTs) were contoured. The dose distribution resulting from different CyberKnife IGRT protocols was calculated. From them, the effective dose was calculated according to ICRP publication Nr 103, using the average dose to contoured organs. The corresponding risk factors were calculated. Entrance surface dose (ESD) was also calculated and compared with existing data.

The maximum effective dose produced by CyberKnife IGRT protocols was 0.8 mSv (brain), 1.9 mSv (H&N), 20.2 (pelvis), and 42.4 mSv (thorax) per fraction for a risk estimate of 0.004% (brain), 0.01% (H&N), 0.1% (pelvis), and 0.2% (thorax). Calculated ESD were compatible with existing data.

Dose calculation models for CyberKnife IGRT kV beams were implemented in a clinical treatment planning system and validated in water and heterogeneous phantoms. We determined the effective dose and the related risk estimate resulting from CyberKnife IGRT protocols for brain, head and neck, thorax, and pelvis cases. The effective doses calculated for CyberKnife IGRT protocols were similar to those obtained for cone beam CT protocols on conventional C-arm linear accelerators, except for extreme irradiation conditions for thorax cases (140 kV X-ray tube tension).