**6. High-Z elements and cancer radiotherapy**

The augmentation of dose due to secondary photoelectron emission from high-Z elements is not a new concept; it is just that it has been ignored for the purposes of radioprotection. The idea of employing high-Z elements and their photoelectron emission to augment radiotherapy doses was advanced by Matsudeira et al. who measured the radio-enhancing effect of iodine on cell cultures [28]. Nath et al. incorporated iodine into cellular DNA with iododeoxyuridine in vitro and found a radiation enhancement of about 3-fold [29]. Herold et al. injected gold particles directly into a tumour followed by irradiation and found that the excised cells had reduced plating efficiency [30]. Mello et al. found that direct tumour injection with iodine contrast medium followed by 100 kVp X-rays completely suppressed growth of 80% of tumours in mice [31]. Norman et al. modified a CT scanner to deliver X-rays to spontaneous brain tumours in dogs after iodine injection and found a 53% longer survival [32]. Synchrotron radiation was used in combination with the tumour injected drug cisplatin to treat brain tumours in rats [33]. The issue of the mechanism of cisplatin is revisited below.

The photoelectron enhancement by high-Z nanoparticles was exploited in cancer radiotherapy by Hainfeld et al. who attempted to increase the dose delivered to tumours by injecting 1.9 nm gold nanoparticles into mice [34]. The authors also made the method the subject of a patent.
