Physical Aspects of Quality Assurance in Radiation Therapy

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of the publisher. 48 50 51 52 53 54-4-CHAPTER 1 INTRODUCTION Quality assurance in radiation therapy includes those procedures that ensure a consistent and safe fulfillment of the dose prescription to the target volume, with minimal dose to norma1 tissues and minimal exposure to personnel. A comprehensive quality assurance program is necessary because of the importance of accuracy in dose delivery in radiation therapy. The dare-response curve in radiation therapy is quite steep in certain cases, and there is evidence that a 7-10% change in the dose to the target volume may result in a significant change in tuner control probability [53]. Similarly, such a dose change may also result in a sharp change in the incidence and severity of radiation induced morbidity. Surveying the evidence on effective and excessive dose levels, Herring and Compton [38] concluded that the therapeutic system should be capable of delivering a dose to the tumor volume within 5% of the dose prescribed. Report 24 from the International Commission on Radiation Units and Measurements [53] lists several studier in support of this conclusion. Surveys have indicated that errors occur with some finite frequency even in institutions which are regularly reviewed by physicists from the Radiological Physics Center [31] and the Centers for Radiological Physics [117]. A nationwide survey of Co-60 teletherapy units was conducted by the National Center for Devices and Radiological Health (NCDRH), which was fomerly the bureau of Radiological Earth and the National Bureau of Standards [128}. The survey included 75% of the Co-60 units (751) in use and showed that doses delivered by 17% of these units differed from the requested doses by at least 5% and that 4% of the delivered doses differed by 10% or more. Sources of error in radiation therapy include tumor localization, lack of patient immobilization, field placement, human errors in calibration, calculation, daily patient setup, and equipment-related problems. Many of these equipment and calculational errors can be minimized through a program of periodic checks. The discussion and accompanying flow chart of a systems approach to radiation therapy given in ICRU Report No.24 [53] and in the CR0S Blue Book [106] points out that the planning and delivery of a course …