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 …

Physical Aspects of Quality Assurance in Radiation Therapy

Literature was reviewed to assess the physical aspects governing the present and emerging technologies used in intraoperative radiation therapy (IORT). Three major technologies were identified: treatment with electrons, treatment with external generators of kV X-rays and electronic brachytherapy. Although also used in IORT, literature on brachytherapy with radioactive sources is not systematically reviewed since an extensive own body of specialized literature and reviews exists in this field. A comparison with radioactive sources is made in the use of balloon catheters for partial breast irradiation where these are applied in almost an identical applicator technique as used with kV X-ray sources. The physical constraints of adaption of the dose distribution to the extended target in breast IORT are compared. Concerning further physical issues, the literature on radiation protection, commissioning, calibration, quality assurance (QA) and in-vivo dosimetry of the three technologies was reviewed. Several issues were found in the calibration and the use of dosimetry detectors and phantoms for low energy X-rays which require further investigation. The uncertainties in the different steps of dose determination were estimated, leading to an estimated total uncertainty of around 10-15% for IORT procedures. The dose inhomogeneity caused by the prescription of electrons at 90% and by the steep dose gradient of kV X-rays causes additional deviations from prescription dose which must be considered in the assessment of dose response in IORT.

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Present state and issues in IORT Physics

A multichannel OSL fiber optic dosimeter based on optically stimulated luminescence (OSL) of alumina is proposed for online in vivo dosimetry (IVD) in radiation therapy (RT). Two types of dosimetric-grade Al2O3:C crystals are compared and show different behavior according to manufacturing process. Metrological validations have been performed with a Saturne 43 LINAC in reference conditions at CEA LIST LNHB (French Ionizing Radiation Reference Laboratory). The dose response of OSL integrals under photon beam irradiation (6, 12, and 20 MV) show sublinearity behavior modeled by second-order equations and exhibit a small energy dependence (between 0.7% and 1.4%), explained by a modified intermediate cavity model adapted to a LINAC photon spectrum. Preclinical tests at Institut Gustave Roussy (IGR) prove that a proper design for a PMMA build-up cap leads to a low dependence vs photon beam orientation (± 1.5% and ± 0.9%) and vs field size in view of surface measurements.

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Multichannel Dosimeter and $\alpha$ -Al $_{2}$ O $_{3}$ :C Optically Stimulated Luminescence (OSL) Fiber Sensors for Use in Radiation Therapy—Evaluation With Photon Beams

Ventilation and perfusion SPECT images during tidal breathing were studied in 15 cases of lung cancer using 81mKr gas and 99mTc-microspheres. Furthermore, functional images of V/Q ratio and Q/V ratio were prepared, and their clinical significance is descussed with reference to general lung function. There was a decreas in %VC and %FEV 1.0in 7 of 15 cases, and an increase of AaDo2 in the blood gas analysis in 12 of 15 cases. Both planar and SPECT images showed ventilation and perfusion abnormalities in all 15 cases. Of these, 12 patients showed matched ventilation and perfusion defects, 2 patients a dead-space effect and 1 patient a shunt effect. In comparing planar and SPECT images, depiction of ventilation and perfusion impairments were equally clear in 11 cases, but in 3, showing a lobar or segmental defect with a shunt effect, the SPECT images were superior. In a patient with markedly impaired function of the affected lung, the remaining function could not be depicted by SPECT. From the above, it seems that better information can be obtained for understanding the ventilation and perfusion states of lung cancer by adding the SPECT images to the planar image.

Assessment of SPECT ventilation-perfusion imaging in patients with lung cancer

F. H. Attix and W. C. Roesch (Eds) New York: Academic Press. 1967 Pp. xviii + 462. Price £8 In the last 10 years since Hine and Brownell edited the first edition of Radiation Dosimetry there has been a tremendous increase in the applications of dosimetry and a corresponding growth in fundamental studies and instrumentation. Although 'Hine and Brownell' is still the standard book heavily used by most radiation workers it is now out of date, and will no doubt be most eagerly replaced by the new edition.

B M Wheatley

Papers

Radiation dosimetry, 2nd edition, Vol. II, Instrumentation