This research investigate the effect of radiation depth, field size , wedge and beam energy on peripheral dose distribution and discuss the constitution of peripheral dose. We also discussed the feasibility of using diode system to detect peripheral dose and measure breast,thyroid and lens dose using CRIS phantom.
Gopiraj Annamalai et al found that: as depth increases, the peripheral dose increase. At setup of field size 20cm*20cm,distance from edge is 1cm at 1.5cm, 5cm and 10cm depth, the peripheral dose is: 7.8%, 10% and 16%. They show different results from us, which are 13% consistent at different depths. The reason of this difference is unsure but it may cause by the precision of position and the choice of measurement detector in spite of field size changes in MLC for different measurement. These reasons may cause differences especially in measurement near the field edge. R. Balasubramanian found that: PD distribution distance at 5cm – 20cm from field edge in different depth is approximately equal shows similar result with us.
Our results show that as field size increases, phantom scatter factor increase and PD increases accordingly. It shows similar result with most research. This result, however, did not take into account that MLC can form same field size as collimator. Robin L. Stern found that: In same field size condition, field formed by MLC can reduce peripheral dose by 6% to 50%.
Effects of physical wedge and virtual wedge to peripheral dose are complex. For physical wedges, there are four kinds of effects: 1. When physical wedge added, beam quality changes. 2. Physical wedge blocks some of the scattering radiation from collimator. 3. Radiation beam’s angle changes when enter the phantom. 4. It takes more MU to deliver same amount of radiation dose. Scrimger et al. and Svensson et al. also found the increases in peripheral dose when adding physical wedges.
Virtual wedges changes dose distribution by moving jaw, when jaw moving, it blocks parts of scattering radiation generated from collimator and the volume of phantom irradiated are decrease as jaw moving. Accordingly, phantom scatter factor decrease and then the peripheral dose decreases.
Radiation beam will generate Compton effects with phantom, and with beam energy increases, backscatter electron will have more tendency to scatter in front direction. In this case, lower the peripheral dose in more distance area from field edge. And also, in the same dose condition, higher energy radiation beam requires less MU than that of lower energy radiation beam which cause lower the collimator scatter factor in high energy radiation beam. These two reason mentioned above caused higher energy radiation beam have less peripheral dose than lower energy radiation beam. R. Balasubramanian et al. found that: in 15MV and 6MV beam energy, for peripheral dose at 5cm distance from field edge, the peripheral dose is 3.42% and 3.07%, which have similar result with us.
Gopiraj Annamalai et al.’s research about Primus linear accelerator found that: at 10cm-20cm distance from radiation field edge, phantom scatter is dominant and at 30cm distance, phantom scatter dose is approximately equal to leakage dose. In our research, ‘PDscatter’ dominant in near field edge area while ‘PDleakage’ dominant remoteness area. The junction point of these two dominances is at 7cm approximately where the ‘PDleakage’ and ‘PDscatter’ are equal and ‘PDleakage’ dominant in more distance area over 7cm. Steffen Lissner et al.’s research about Tomotherapy accelerator found that: because of its unique principle of operation and shielding design, leakage dose’s percentage at 30cm distance is less than 40%. In other words, phantom scatter peripheral dose dominant in any position in Tomotherapy accelerator.
For measurement of peripheral dose, choice of measurement instrument is always a controversial topic. For now, most researchers select TLD, Tilo Wiezorek and E. D’Agostino et al. uses TLD to measure peripheral dose in intensity modulated treatment. TLD have advantages that it has no dose rate response and have large range of measurement, relatively small volume and can repeatedly use makes it widely used for peripheral dose measurement. TLD is hard to operate and cannot read dose timely, however, limit its usage. This research considering using Diode ionization chamber to measure peripheral dose as it’s easy to operate, strong timeliness and have high sensitivity response. In this research, when measuring 6MV radiation beam, Diode and CC13 ionization chamber has good conformity, for 18MV radiation beam, however, it does not show good enough conformity in account of high energy radiation beam has a complex energy spectrum and for 18MV diode detector has a narrow range.
Benedick A Fraass et al. found that adding lead shielding outside radiation field can reduce region of interest radiation dose. This research has mentioned that lead shielding can reduce breast, thyroid and lens’ radiation dose. It shows a little difference than Ming X Jia et al.’s results.The reason of the differences maybe the size difference of radiation target, the distance differences of organs of interest and radiation target.