Question

Where is the build up region? and How can the build up region affect patient treatments?

please explain clearly.

Answer 1

Ans) This region is called the build-up region and is responsible for the skin sparing effect. The dose at dmax is defined as 100% and then the radiation dose decreases as the depth increases, the energy being absorbed within the tissue.

- At the physical phantom depths, the surface and buildup region doses found to be the highest with MOSFET. When the WET values of dosimeters are considered and the same water equivalent depths are taken into account, MOSFET results are slightly lower for the first 5 mm. The EBT3 film shows closer agreement to the Markus chamber at these depths as a consequence of WET of dosimeters. The dose differences between dosimeters are greater with 6 MV at all field sizes when the results for the surface dose measurements at 0.07 mm depth were compared. MOSFET gives much higher results compared to EBT3 and Markus at oblique measurements as a result of its WET value. This physical property of MOSFET causes a special consideration for in vivo usage.

- The EBT3 film has advantages like its dosimetric properties of homogenous material and tissue equivalence, but it is not a practical dosimeter for in vivo usage. The EBT3 film requires a waiting time before scanning and a calibration curve to acquire the absolute doses. The MOSFET dosimeter is easy to use and gives immediate results. The MOSFET dosimeter gives a higher response at surface measurement but the overestimation of detectors can be measured with another reference dosimetry system. When a factor of overestimation is found, the MOSFET dosimeters can be used for in vivo surface dosimetry.

- There are some publications about the surface dose measurements which take part in literature, but in these publications the surface doses were investigated at 0 mm physical depth. It is necessary to know the effective depth of the dosimeters. In this study, surface and buildup region doses were measured and calculated by considering the WET of the dosimeters.

- The surface and buildup region doses should be evaluated considering the depth. The effective measurement depths of dosimeters vary; thus, the results of the measurements could be different. This issue can lead to mistakes at surface and buildup dosimetry and must be taken into account.