Volume 33, Issue 6, June 2006
Index of content:
- Therapy Symposium: Valencia A
Symposium in Memoriam of Robert Loevinger: How Accurately Can We Measure Dose Clinically
33(2006); http://dx.doi.org/10.1118/1.2241883View Description Hide Description
How perfectly appropriate is the Symposium: “How accurately can we measure dose clinically?” to memorialize Bob Loevinger. The clear threads in Bob's career and seminal work was his quest for improved accuracy in dosimetry measurements, his organization of calibration networks to transfer traceable measurement standards to the clinical physicist, and his development of protocols and formalisms to implement accurate measurements of dose at the clinical level. In his professional career of some 55 years, his contributions were many and important. Quiet and unassuming, Bob was the supreme dosimetrist: he designed and constructed extrapolation chambers for beta dosimetry in his early career, and then the Wide‐Angle‐Free‐Air‐Chamber (WAFAC) for prostate‐seed dosimetry in his late career; he developed the MIRD schema with Mones Berman for dose calculations in nuclear medicine, and led the development of the TG 21 protocol for the determination of absorbed dose from high‐energy photon and electron beams; at the IAEA in the 1960s he was instrumental in the creation of their Dosimetry Laboratory and the IAEA/WHO Secondary Standards Dosimetry Laboratory Network that serve the mostly developing member states, and initiated their postal dosimetry service to clinics in the member states; then at the NBS in the 1970s he worked with the AAPM (he was a charter member) to create the network of Accredited CalibrationDosimetry Laboratories to better disseminate USradiation measurement standards to the clinical community in North America. Much of his work was recognized during his lifetime by numerous awards given him by many organizations, but I think he would be most honored by this Symposium, which gathers like‐minded scientists to discuss issues on which he spent his career.
33(2006); http://dx.doi.org/10.1118/1.2241884View Description Hide Description
Typical procedures for clinical dosimetry are well established for measurements in charged particle equilibrium situations. These situations only form a subset of the measurement challenges of a clinical physicist. Dose measurements in regions of charged particle disequilibrium require special considerations before the measurement can be assumed accurate. These disequilibrium conditions occur in association with measurements performed for standard clinical treatment techniques as well as for special treatment techniques. In the symposium we identified four major areas that complicate accurate clinical dosimetry: (1) the photon build‐up region, (2) narrow photon beams, (3) measurements in heterogeneous phantoms and (4) modulated fields and dynamic measurements.
In the present lecture we describe the principles of accurate dose measurements in general in equilibrium and non‐equilibrium situations. Principles of detector response, energy dependence and characteristics of practical detectors are described with emphasis on the understanding of detector behaviour in non‐equilibrium conditions. By way of introduction, practical examples will be provided of non‐equilibrium measurements in the build‐up region, narrow beams, heterogeneous phantoms and IMRT beams. The lecture will provide general guidelines about detector suitability for commercially available devices for the discussed areas of application. Finally, uncertainties of the procedures in a clinical context will be discussed.
1. To understand the fundamental complications of non‐equilibrium measurements.
2. To understand the impact of detector properties on measurement accuracy in non‐equilibrium situations.
3. To place measurementuncertainties in non‐equilibrium conditions in a clinical perspective.
33(2006); http://dx.doi.org/10.1118/1.2241885View Description Hide Description
Dosimetric measurements for stereotactic radiosurgery or radiotherapy fields typically include determination of relative output factors, tissue maximum ratios and off‐axis beam profiles. Accurate measurements of each of these quantities are made more difficult when the measured field sizes are either comparable to the detector size, or less than the distance required for lateral electronic equilibrium. These difficulties are present when measuring either single, static‐field cone or micro‐MLC linac‐based delivery systems, or simultaneous multiple‐field Gamma Knife delivery systems.
Several detector characteristics should be considered when choosing a measurement system. The detector size in the radial direction will determine the extent of the beam profile integrated in the reading. Typically solid‐state detectors or film (radiographic or radiochromic) have been used to reduce this volume‐averaging effect. Several other detection systems have also been used, including pin‐point ionization chambers,thermoluminescencedosimetry, and diamond and scintillation detectors. Techniques exist to correct for finite detector volume, including the extrapolation of measured results to zero‐volume, and deconvolution of measured data with the detector response function.
The detector energy response also should be considered due to the variation in the number of low energy scattered photons with field size and depth. Additionally, for field sizes below lateral electronic equilibrium, variations in the stopping power ratios affect the conversion of ionization to dose, particularly for higher‐energy measurements. Both of these effects increase with beam energy and depth.
This lecture will provide an overview of the measurement techniques used to determine dosimetric quantities for stereotactic fields. Recommendations for measurement systems will be made, and limitations to each system will be discussed. Estimations of error will be provided, both for individual measurements and in the overall planned dose. When available, comparisons will be made with results predicted by theoretical Monte Carlo calculations.
1. Understand the basic principles and practical aspects of clinical dosimetry for stereotactic radiosurgery.
2. Understand the limitations and applicability of various detector types and sizes for small field measurements.
3. Understand current techniques available to correct data measurements in small fields.
33(2006); http://dx.doi.org/10.1118/1.2241886View Description Hide Description
Accurate measurements in the dose build‐up region for high energy photon beams are not easily obtained. While dosimetry in situations where electronic equilibrium exists is well understood, there is in general no consensus on the most accurate method for measuring doses in the build‐up region. In the past, data acquired with an extrapolation chamber were regarded as the benchmark by which the data from other more commonly used dosimeters are evaluated. As extrapolation chambers are clinically impractical devices there is a need to study the behaviour of commercially available and clinically suitable detector systems for accurate dosimetry in the build‐up region.
This lecture will provide an overview on the characteristics of the dose build‐up region measurements for photon beams, the suitability of several clinical dosimeters for build‐up dose measurement and limitations in measurement accuracy.
1. Understanding the reasons why doe measurements in the build‐up region are challenging.
2. Grasp issues of the suitability of several dosimeters for measurements in the dose build‐up region.
3. Discussion of ongoing research on dosimetry in the dose build‐up region.
4. Discussion of the associated uncertainties and their clinical relevance.
33(2006); http://dx.doi.org/10.1118/1.2241887View Description Hide Description
The human body consists of tissue types that have radiological properties that are different from water. These include, for example, lung, bone, and oral cavities. Presence of such tissue types and cavities in the treatment fields of high‐energy photon beams creates potential dosimetric problems. For inhomogeneities with density less than that of water electronic disequilibrium situations can be severe. Lateral electronic disequilibrium is present for small field sizes (i.e., 5 cm × 5 cm) and high energy beams (i.e., 15 MV) in a low density inhomogeneity such as lung. Perturbations at air‐tissue interfaces are complex to measure or calculate due to lack of electron equilibrium. The trend of data published in the literature show that for low density media i) dose generally increases beyond the depth of dose maximum; ii) build‐up and build‐down regions exist within tissue near the low density media‐tissue interfaces; the severity of these effects increase with increasing energy and decreasing field size; iii) the penumbra increases with energy in the region of low density region. For high density media the dose is found to decrease beyond the depth of dose maximum.
The dosimetric effects of these heterogeneous tissues become even more complicated for IMRT beams. This is because i) small radiation beams are inherently difficult to measure; ii) standard ion chambers have dimensions that are large compared to the beam and do not have the spatial resolutions that are needed to resolve the narrow central region of uniform dose and the sharp dose gradient regions of the penumbra and iii) the chambers may have a dose rate dependence.
The dosimetric impact of the presence of heterogeneous tissues in megavoltage photon beams have been addressed with varying degrees of success by various investigators i) experimentally by the use of specially designed ionization chambers, film, TLD and other dosimeters in specially constructed phantoms, ii) theoretically by the development of various dose calculation algorithms and iii) by the use of Monte Carlo simulation.
Develop an appreciation of
1. the challenges involved in accurate dosemeasurements in heterogeneous phantoms
2. the clinical implementation of the results of such measurements, various dose calculation algorithms and/or Monte Carlo simulations to various disease sites that involve the presence of various types of heterogeneous tissues.
33(2006); http://dx.doi.org/10.1118/1.2241888View Description Hide Description
The use of conformal radiotherapy, especially with the IMRT technique, is a major departure from the way radiotherapy is currently delivered. Although the use of multileaf collimator(MLC) provides the possibility of achieving better dose distributions conformed to tumor targets, it also increases the treatment complexity. The sequences of leaf movement and their associated effects on the dose delivered to the patient may vary significantly depending on the accelerator and the MLC design. Accurate measurement of IMRTdose distributions in a clinical setting is not an easy task since many factors may affect the measurement results. In this presentation we will review various factors affecting IMRTdose distributions including the variation of the accelerator head scatter component in the MLC‐collimated beam, the amount of photon leakage through the leaves, and the scatter from the leaf ends, the “tongue and groove” effect, and the effect of back‐scattered photons from the moving jaws and MLC leaves on the monitor chamber signal. We will describe the use of different detectors commonly used for absolute and relative dose determination with both static and dynamic beam delivery. We will discuss the effects of electron disequilibrium, detector perturbation and patient heterogeneous anatomy on the measurement accuracy.
Below are the educational objectives of this presentation:
1. Review dosimeters for absolute and relative IMRTdosemeasurements.
2. Discuss the process for dose determination under electron disequilibrium conditions.
3. Describe the use of Monte Carlo simulations to derive detector correction factors.
4. Discuss major factors affecting IMRTdosemeasurement accuracy.
33(2006); http://dx.doi.org/10.1118/1.2241890View Description Hide Description
Radiation therapy is rapidly moving into the direction of image guidance, functional target volumes, hypofractionation, dose escalation and 4D deliveries. In this era of joint imaging‐therapy developments, accurate dosimetry techniques albeit often overlooked, increasingly require dealings with complex charged particle disequilibrium measurements and their interpretation. In this symposium, clinical dosemeasurements have been reviewed with emphasis on accuracy in these charged particle disequilibrium conditions. After a brief introduction and review of fundamentals of dosimetry, clinical accuracy requirements, four distinct areas of dosimetry in charged particle disequilibrium and their clinical relevance have been discussed. Guided by the main conclusions of the contributions in the four areas, moderated discussions will be conducted around clinical relevance of dosimetric accuracy in on non‐equilibrium conditions, detector suitability, technical aspects and other areas of application.