Skin in vivo Dosimetry in Radiotherapy

Document Type: Conference Proceedings

Authors

1 Student Research Committee, School of Medicine, Kermanshah University of Medical Sciences, Kermanshah, Iran

2 Associate Professor,Medical Physics and Medical Engineering Department, Faculty of Medicine, Kermanshah University of Medical Sciences, Kermanshah, Iran

Abstract

Introduction: Due to the prevalence of skin problems in patients after radiotherapy, skin dose measuring is importance.
Content: Skin in vivo dosimetry means measuring the patient's (or phantom) skin dose during radiotherapy.
According to the ICRP 59, the dose at the depth of 0.07 mm is known as a skin dose. The most radiosensitive epidermis cells are located at this depth.
Several studies have assessed the skin diseases in different areas of the body caused by radiotherapy. The most common problems are skin dermatitis and alopecia depending on the exposure conditions and physiological features of the patient`s body occur several days to several weeks after treatment.
Skin dose in radiotherapy, arising from the primary photon beam, backscatter radiation from more depths, scattering from other equipments in the path of radiation, treatment room and also head leakage. The main challenge of the skin dosimetry is the lack of particle equilibrium at 0.07 mm depth, as well as dosimetry in the build-up region with a high-dose gradient. On the other hand, studies have shown that treatment planning system (TPS), especially in Intensity-modulated radiotherapy (IMRT) and tomotherapy, overestimates the skin dose. Skin dosimetry is more important in modern radiotherapy techniques (such as IMRT) because these treatments use more tangential beams than 3D-CRT, which increase the dose and cause skin problems.
Various dosimeters, such as TLDs, Films, Diodes, and MOSFETs, are used for skin dosimetry, each with advantages and disadvantages. TLDs are small in size, but require long-term pre- and post-processing and are incapable to real-time display of dosimetric information. According to tissue equivalency, the Gafchromic EBT films show acceptable dose accuracy. These films are able to display a 2D dose distribution, but these dosimeters cannot display dosimetric information in real-time. Diodes and MOSFETs, due to their small size, can be appropriate choices for skin dosimetry and provide high spatial resolution.
The newest device proposed for the skin dosimetry is MOSkin, which is based on the MOSFET structure. In addition to its small sensitive volume, this dosimeter acts as a real-time dosimeter. The reproducibility and linearity of the MOSkin response have been approved at the water equivalent depth of 0.07 mm and within the range of 50-300 cGy. This limited dynamic range doesn’t allow this dosimeter to be made in the 2D array. The most important problem with the MOSkin is requirement of a wire to external voltage supply. Presenting a wireless version will be an important step in the field of skin dosimetry in radiotherapy.
Results: Advantages of MOSkin dosimetry outperformed its disadvantages for skin
dosimetry in radiotherapy.
Conclusion: The MOSkin is an appropriate choice for skin in vivo
dosimetry because it has small sensitive volume and acts as a real-time dosimeter.

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