Characterisation of out-of-field dose at shallow depths for external beam radiotherapy: implications for eye lens dose.

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Re-evaluation of the eye lens radio-sensitivity by the ICRP in 2011 resulted in a significant reduction of the threshold for lens opacities from 8Gy to 0.5Gy. This has led to an increase in concern for eye lens doses from treatment sites further from the eye than previously considered. The aim of this study was to examine the out-of-field dose far from the field edge and develop an effective method to accurately characterise the constituent components of this dose at varying depths. Dose profile scans using a 0.6cm3 cylindrical ionisation chamber in a motorised water tank were compared with previous studies and displayed good agreement. At points more than 20cm from the field edge patient scatter becomes insignificant, and the dose is dominated by head leakage and collimator scatter. Point depth-dose measurements made with a Roos parallel plate chamber in solid water at distances of 52cm and 76cm from central axis showed that the highest dose is at the surface. Since the sensitive region of the eye can be as shallow as 3mm, in vivo measurements carried out with a detector with buildup more than 3mm water equivalent thickness may be underestimating the dose to the lens. It is therefore recommended that for in vivo measurements for the eye lens further than 20cm from the field edge the detector should have only 3mm build-up material over the effective point of measurement.