Faiz M. Khan

Clinical electron-beam dosimetry: report of AAPM Radiation Therapy Committee Task Group No. 25.

DISCLAIMER: This publication is based on sources and information believed to be reliable, but the AAPM and the editors disclaim any warranty or liability based on or relating to the contents of this publication. The AAPM does not endorse any products, manufacturers, or suppliers. Nothing in this publication should be interpreted as implying such endorsement.

Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.

Film dosimetry of megavoltage photon beams: a practical method of isodensity-to-isodose curve conversion

The central problems of photon beam film dosimetry are the dependence of film response upon photon energy, processing conditions, and film plane orientation. We have overcome these problems by accurately fitting the depth-dependent sensitometric curve of Kodak XV-2 film (exposed parallel to beam axis) to the equation OD(D,d) = ODs(1 - exp [- alpha 0[1 + beta(d-dm)]D]) where OD(D,d) is the optical density for dose D at depth d. ODs, alpha 0, and beta are constants characteristic of the film and beam energy but are independent of field size. Only central axis depth dose data for a single field are required to determine their values. A computer program based upon this equation has been written which successfully generates single field isodose curves from film data for a variety of field sizes (including wedged fields) with an accuracy of +/- 3%. Data are presented for 60Co, and 4 and 10 MV x rays.

Revision of tissue‐maximum ratio and scatter‐maximum ratio concepts for cobalt 60 and higher energy x‐ray beams

The concept of tissue-maximum ratios (TMR) as a basis of dose computations has posed problems when applied to a whole range of clinically used megavoltage beams. Another problem is the definition of scatter-maximum ratio (SMR) which assumes a value of zero at the depth of maximum dose. This paper describes new methods of measuring collimator and phantom scatter correction factors. The definitions of TMR and SMR are modified in order to compute phantom scatter at any depth, including depth of maximum dose. The revised concept is basic and general enough that it can be applied to x-ray beams of any energy fields of any shape and isocentric as well as non-isocentric modes of treatment.

Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Radiobiological basis of total body irradiation with different dose rate and fractionation: Repair capacity of hemopoietic cells

Abstract Total body irradiation (TBI) followed by bone marrow transplantation is being used in the treatment of malignant or non-malignant hemopoietic disorders. It has been believed that the ability of hemopoietic cells to repair sublethal radiation damage is negligible. Therefore, several school of investigators suggested that TBI in a single exposure at extremely low dose rate (5 rad/min) over several hours, or in several fractions in 2–3 days, should yield a higher therapeutic gain, as compared with a single exposure at a high dose rate (26 rad/min). We reviewed the existing data in the literature, in particular, the response of hemopoietic cells to fractionated doses of irradiation and found that the repair capacity of both malignant and non-malignant hemopoietic cells might be greather than has been thought. It is concluded that we should not underestimate the ability of hemopoietic cells to repair sublethal radiation damage in using TBI.

Second neoplasms following megavoltage radiation in a pediatric population

Previous reports of radiation‐related neoplasia have relied primarily upon patients treated by orthovoltage to low doses for benign disease. This survey is believed to be the first to assess the incidence of second neoplasms following megavoltage therapy. The source was the records of all long‐term pediatric survivors (88 patients) who were treated with megavoltage radiation (cobalt 60) at the University of Minnesota. There was an average follow‐up period of 14 years during which 7 second neoplasms were discovered (8%). Five were not associated with prior radiation. Both radiation‐related neoplasms were associated with low doses and one was without significant morbidity. Two of the seven neoplasms were malignant; one was not associated with radiation while the other was associated with prolonged chemotherapy and low dose radiation (1%). The only fatal second neoplasm was not associated with radiation but developed 5 years after prolonged chlorambucil treatment. This review reveals the tendency of childhood cancer victims to develop other neoplasms regardless of radiation. The finding of neoplasia induction only at low radiation doses supports the Gray hypothesis of decreased tumor induction at high doses through increased cell killing.

Dosimetry of asymmetric x ray collimators

We have studied the dosimetry of an independent jaw system (provided with the Varian Clinac 2,500) using ionometric measurements performed both in air and in a water phantom. Our study shows that the effect of the independent jaw on the dose distribution is similar to that of secondary blocking except for changes produced in the collimator scatter. A system of dose calculation was developed which takes into account the changes in the collimator scatter as well as in the isodose distribution. A method is described to correctly generate isodose curves for fields shaped by an independent jaw using a modified AECL TP11 treatment planning system. The primary modification in the program consists of correcting the zero-area tissue-maximum ratios for the off-axis variation in beam quality.

The polarity effect for commercially available plane-parallel ionization chambers

The polarity effect was investigated for three different commercially available plane-parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30-329 chamber manufactured by PTW, Frieburg, and the Capintec PS-033 thin-window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6-, 10-, 18-, and 24-MV x-ray beams, and 9- and 22-MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane-parallel ionization chambers that we studied, we found a polarity effect of only 1%-2% for electron beams at the depth of dmax. At depths greater than dmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high-energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.

A report of the work party: comparison of total body irradiation techniques for bone marrow transplantation

The report presents a survey of total body irradiation techniques for bone marrow transplantation in nine institutions in North America and England. The survey compares their nominal dose, dose rate, point of dose prescription, type of machine used, patients position during treatment, and use of compensators. This experience has emphasized the need for a system of uniform dose reporting and for uniform dose prescription in total body irradiation.