![]() ![]() Traditionally films were used to verify that the light and radiation fields agreed with each other and with the indicated jaw settings. A case in point is the American Association of Physicist in Medicine (AAPM) task group report no 40 (TG 40)- and most recently report no 142 (TG 142) of the same organization. No wonder that most recommendations for quality assurance of megavoltage radiation therapy equipment require checks of the degree of congruence of the light and radiation fields. It is therefore necessary that the light field agrees (is congruent) with the radiation field. Traditional radiation therapy clinical practice involves aligning the treatment unit light field with the skin marks on the patient as the final confirmation that the patient is correctly positioned with respect to the radiation beam. Depending on the manufacturer, this light source may be positioned at the location of the x-rays target by a rotating carousel or a sliding drawer assembly, or it may be positioned to one side of the collimator axis of rotation with the light reflected from a mirror. The radiation field on most megavoltage radiation therapy units are indicated by a light field projected through the collimator by a light source mounted inside the collimator. The QA tool can be used with either an EPID or CR to provide a simple and rapid method to verify light and radiation field congruence. This will satisfy the American Association of Physicists in Medicine task group report number 142 recommendation of 2 mm tolerance. ![]() The light and radiation congruency could be detected within 1 mm. Resultsīoth the single and double exposure techniques were evaluated, with double exposure technique providing a better visualization of the light-radiation field markers. We tested this QA tool using Varian PortalVision and Elekta iViewGT EPID systems and Kodak CR system. ![]() MethodĪ simple quality assurance tool has been designed for rapid and simple test of the light field and radiation field using electronic portal images device (EPID) or computed radiography (CR). Hence it is imperative that the light field is congruent with the radiation field. The light field is traditionally used for patient alignment. The radiation field on most megavoltage radiation therapy units are shown by a light field projected through the collimator by a light source mounted inside the collimator. ![]()
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