Biomedical subjects

Indra J Das

Publications and source records attributed to Indra J Das.

17 recordsLinked to original sources

PubMed2006

Choice of beam energy and dosimetric implications for radiation treatment in a subpopulation of women with large breasts in the United States and Japan.

Radiation complications are often related to the dose inhomogeneity (hot spot) in breast tissue treated with conservative therapy, especially for large patients. The effect of photon energy on radiation dose distribution is analyzed to provide guidelines for the selection of beam energy when tangential fields and limited slices are used to treat women with large breasts. Forty-eight patients with chest wall separation > 22 cm were selected for dosimetric analysis. We compared the maximum dose in the central axis (CAX) plane (2D) using 6-, 10-, and 18-MV photon beams in all patients and 3D data set for 16 patients. Correlation between hot spot dose (HSD), separation, breast cup size, breast volume, and body weight was derived with beam energy. Among the 48 patients in this study, HSD > 10% in the CAX plane was noted in 98%, 46%, and 4% of the population when 2D dosimetry was performed; however, with 3D study, it was in 50%, 19%, and 6% of the patients with 6-MV, 10-MV and 18-MV beams, respectively. The chest wall separation, body weight, and breast volume were correlated with the HSD in both the 2D and 3D plans. Patient's bra size was not correlated with the hot spot. The chest wall separation was found to be the most important parameter to correlate with hot spot in tangential breast treatment. Simple guidelines are provided for dose uniformity in breast with respect to chest wall separation, body weight, bra size, and breast volume with tangential field irradiations.

Body Weight↗

PubMed2005

AAPM Task Group 103 report on peer review in clinical radiation oncology physics.

This report provides guidelines for a peer review process between two clinical radiation oncology physicists. While the Task Group's work was primarily focused on ensuring timely and productive independent reviews for physicists in solo practice, these guidelines may also be appropriate for physicists in a group setting, particularly when dispersed over multiple separate clinic locations. To ensure that such reviews enable a collegial exchange of professional ideas and productive critique of the entire clinical physics program, the reviews should not be used as an employee evaluation instrument by the employer. Such use is neither intended nor supported by this Task Group. Detailed guidelines are presented on the minimum content of such reviews, as well as a recommended format for reporting the findings of a review. In consideration of the full schedules faced by most clinical physicists, the process outlined herein was designed to be completed in one working day.

Health Physics↗

PubMed2005

Comparison of inhomogeneity correction algorithms in small photon fields.

Algorithms such as convolution superposition, Batho, and equivalent pathlength which were originally developed and validated for conventional treatments under conditions of electronic equilibrium using relatively large fields greater than 5 x 5 cm2 are routinely employed for inhomogeneity corrections. Modern day treatments using intensity modulated radiation therapy employ small beamlets characterized by the resolution of the multileaf collimator. These beamlets, in general, do not provide electronic equilibrium even in a homogeneous medium, and these effects are exaggerated in media with inhomogenieties. Monte Carlo simulations are becoming a tool of choice in understanding the dosimetry of small photon fields as they encounter low density media. In this study, depth dose data from the Monte Carlo simulations are compared to the results of the convolution superposition, Batho, and equivalent pathlength algorithms. The central axis dose within the low-density inhomogeneity as calculated by Monte Carlo simulation and convolution superposition decreases for small field sizes whereas it increases using the Batho and equivalent pathlength algorithms. The dose perturbation factor (DPF) is defined as the ratio of dose to a point within the inhomogeneity to the same point in a homogeneous phantom. The dose correction factor is defined as the ratio of dose calculated by an algorithm at a point to the Monte Carlo derived dose at the same point, respectively. DPF is noted to be significant for small fields and low density for all algorithms. Comparisons of the algorithms with Monte Carlo simulations is reflected in the DCF, which is close to 1.0 for the convolution-superposition algorithm. The Batho and equivalent pathlength algorithms differ significantly from Monte Carlo simulation for most field sizes and densities. Convolution superposition shows better agreement with Monte Carlo data versus the Batho or equivalent pathlength corrections. As the field size increases the DCF's for all algorithms converge toward 1.0. The largest differences in DCF are at the interface where changes in electron transport are greatest. For a 6 MV photon beam, electronic equilibrium is restored at field sizes above 3 cm diameter and all of the algorithms predict dose in and beyond the inhomogeneous region equally well. For accurate dosimetry of small fields within and near inhomogeneities, however, simple algorithms such as Batho and equivalent pathlength should be avoided.

Algorithms↗

PubMed2005

Dose perturbations due to contrast medium and air in mammosite treatment: an experimental and Monte Carlo study.

In the management of early breast cancer, a partial breast irradiation technique called MammoSite (Proxima Therapeutic Inc., Alpharetta, GA) has been advocated in recent years. In MammoSite, a balloon implanted at the surgical cavity during tumor excision is filled with a radio-opaque solution, and radiation is delivered via a high dose rate brachytherapy source situated at the center of the balloon. Frequently air may be introduced during placement of the balloon and/or injection of the contrast solution into the balloon. The purpose of this work is to quantify as well as to understand dose perturbations due to the presence of a high-Z contrast medium and/or an air bubble with measurements and Monte Carlo calculations. In addition, the measured dose distribution is compared with that obtained from a commercial treatment planning system (Nucletron PLATO system). For a balloon diameter of 42 mm, the dose variation as a function of distance from the balloon surface is measured for various concentrations of a radio-opaque solution (in the range 5%-25% by volume) with a small volume parallel plate ion chamber and a micro-diode detector placed perpendicular to the balloon axis. Monte Carlo simulations are performed to provide a basic understanding of the interaction mechanism and the magnitude of dose perturbation at the interface near balloon surface. Our results show that the radio-opaque concentration produces dose perturbation up to 6%. The dose perturbation occurs mostly within the distances <1 mm from the balloon surface. The Plato system that does not include heterogeneity correction may be sufficient for dose planning at distances > or = 10 mm from the balloon surface for the iodine concentrations used in the MammoSite procedures. The dose enhancement effect near the balloon surface (<1 mm) due to the higher iodine concentration is not correctly predicted by the Plato system. The dose near the balloon surface may be increased by 0.5% per cm3 of air. Monte Carlo simulation suggests that the interface effect (enhanced dose near surface) is primarily due to Compton electrons of short range (<0.5 mm). For more accurate dosimetry in MammoSite delivery, the dose perturbation due to the presence of a radio-opaque contrast medium and air bubbles should be considered in a brachytherapy planning system.

Artifacts↗

PubMed2004

Proposed guidelines for image-based intracavitary brachytherapy for cervical carcinoma: report from Image-Guided Brachytherapy Working Group.

PURPOSE: To present issues to be considered in, and make proposals for, image-based brachytherapy for cervical cancer. METHODS AND MATERIALS: The Image-Guided Brachytherapy Working Group, consisting of representatives from the Gynecology Oncology Group (GOG), Radiologic Physics Center (RPC), American Brachytherapy Society (ABS), American College of Radiology (ACR), American College of Radiology Imaging Network (ACRIN), American Association of Physicists in Medicine (AAPM), Radiation Therapy Oncology Group (RTOG), and American Society for Therapeutic Radiology and Oncology (ASTRO), proposed guidelines for image-based brachytherapy for cervical cancer. This report was based on their aggregate clinical experience and a review of the literature. It reflects only the personal opinions of the authors and is not meant to be an endorsement from any of the above organizations. RESULTS: The Group recommended T(2)-weighted MRI using a pelvic surface coil with MRI-compatible brachytherapy applicators in place for image-based intracavitary brachytherapy for cervical cancer. Imaging must be performed with the patient in the treatment position, with all other treatment conditions duplicated as closely as possible. Future use of positron emission tomography or positron emission tomography/CT may obviate the need for special applicators. The group proposed the following terminology for image-based brachytherapy. The GTV((I)) is defined as the gross tumor volume as defined through imaging, GTV is defined as the GTV((I)) plus any clinically visualized or palpable tumor extensions, and GTV + cx is defined as the GTV plus the entire cervix. The dose-volume histograms (DVH) of the GTV, GTV((I)), GTV + cx should be performed, and the dose to 100%, 95%, or 90% of the GTV (D(100), D(95), and D(90), respectively) and the percentage of the GTV covered by Point A dose (V(100)) should be reported. Similarly, the DVH of the bladder and rectum wall should be performed, and the maximal dose at any point within the bladder and rectal wall should be reported, along with the maximal dose to a contiguous 1, 2, and 5 cm(3) volume of the bladder and rectum, respectively. In addition, the dose at the International Commission on Radiation Units and Measurements reference point for the bladder and rectum should be reported. The Group thought that the current dose prescription method in use for cervical cancer brachytherapy (i.e., to prescribe to Point A in most institutions) should not be changed as yet, because image-based dosimetry is not ready for routine practice. The Group proposes that for research purposes, individual centers and cooperative groups (e.g., GOG, RTOG, ACRIN) collect image-based dosimetry information and perform DVHs and correlate these data with the clinical outcome to determine which of the above parameters are relevant. The Group encourages external funding for image-based dosimetry and recommends that brachytherapy manufacturers develop image-compatible applicators. CONCLUSION: Although current institutional brachytherapy prescription for cervical cancer should continue, image-based data collection and analysis are needed to optimize cervical cancer brachytherapy. Proposals are made for research in image-based brachytherapy for cervical cancer.

Brachytherapy↗

PubMed2004

Thermal and temporal response of ionization chambers in radiation dosimetry.

The temporal and thermal response of various commonly used ion chambers (NEL, Exradin, PTW) with different wall and electrode materials is studied in a water phantom. Measurements were taken in heating water bath having an automatic temperature control mechanism. All chambers were submersed at 3 cm depth in water phantom and connected to separate electrometers for simultaneous readings for a given dose from a 6 MV beam. The temporal response was studied at approximately +/- 10 degrees C from room temperature, i.e., 10 degrees C and 30 degrees C. Temporal results show that all chambers reach quick equilibrium response within < 2 minutes of submersion in water. The steady-state thermal response was dependent upon the water temperature and wall and electrode compositions. The temperature and pressure corrected response of the NEL chamber was least affected by the changes in water temperature, where as the Exradin chamber has positive and PTW has negative slope with water temperature. The response varied within +/-1.5% between 10 degrees C-50 degrees C temperature and is mainly dependent on volume changes rather than the humidity. A correction factor based on thermal coefficients is derived for each chamber. It is concluded that ion chamber correction factors can be divided into first order; temperature and pressure connection, second order; volume correction for thermal expansion, and third order; humidity correction. To eliminate dosimetric error, the temporal and thermal response should be known for a chamber or water phantom temperature should be maintained close to room temperature.

Calibration↗

PubMed2004

Procedures for establishing and maintaining consistent air-kerma strength standards for low-energy, photon-emitting brachytherapy sources: recommendations of the Calibration Laboratory Accreditation Subcommittee of the American Association of Physicists in Medicine.

Low dose rate brachytherapy is being used extensively for the treatment of prostate cancer. As of September 2003, there are a total of thirteen 125I and seven 103Pd sources that have calibrations from the National Institute of Standards and Technology (NIST) and the Accredited Dosimetry Calibration Laboratories (ADCLs) of the American Association of Physicists in Medicine (AAPM). The dosimetry standards for these sources are traceable to the NIST wide-angle free-air chamber. Procedures have been developed by the AAPM Calibration Laboratory Accreditation Subcommittee to standardize quality assurance and calibration, and to maintain the dosimetric traceability of these sources to ensure accurate clinical dosimetry. A description of these procedures is provided to the clinical users for traceability purposes as well as to provide guidance to the manufacturers of brachytherapy sources and ADCLs with regard to these procedures.

Air↗

PubMed2004

Transmission and dose perturbations with high-Z materials in clinical electron beams.

High density and atomic number (Z) materials used in various prostheses, eye shielding, and beam modifiers produce dose enhancements on the backscatter side in electron beams and is well documented. However, on the transmission side the dose perturbation is given very little clinical importance, which is investigated in this study. A simple and accurate method for dose perturbation at metallic interfaces with soft tissues and transmission through these materials is required for all clinical electron beams. Measurements were taken with thin-window parallel plate ion chambers for various high-Z materials (Al, Ti, Cu, and Pb) on a Varian and a Siemens accelerator in the energy range of 5-20 MeV. The dose enhancement on both sides of the metallic sheet is due to increased electron fluence that is dependent on the beam energy and Z. On the transmission side, the magnitude of dose enhancement depends on the thickness of the high-Z material. With increasing thickness, dose perturbation reduces to the electron transmission. The thickness of material to reduce 100% (range of dose perturbation), 50% and 10% transmission is linear with the beam energy. The slope (mm/MeV) of the transmission curve varies exponentially with Z. A nonlinear regression expression (t=E[alpha+beta exp(-0.1Z)]) is derived to calculate the thickness at a given transmission, namely 100%, 50%, and 10% for electron energy, E, which is simple, accurate and well suited for a quick estimation in clinical use. Caution should be given to clinicians for the selection of thickness of high-Z materials when used to shield critical structures as small thickness increases dose significantly at interfaces.

Electrons↗

PubMed2003

Beam characteristics of upper and lower physical wedge systems of Varian accelerators.

The beam characteristics of a dual physical wedge system, upper and lower, for Varian accelerators are studied over the energy range 6-18 MV. Wedge factors for both systems are measured in a water phantom as a function of field size, depth and source-to-wedge (SWD) distance. Our results indicate that apart from their physical differences, dosimetrically, the two wedge systems have <2% difference in central axis percentage depth dose beyond the build-up region. The lower wedge central axis percentage depth dose is consistently lower than that of the corresponding upper wedge, with the effect more pronounced for large field sizes. The wedge profiles are identical within 2% for all field sizes, depths and energies. The wedge factors for both wedge systems are also within 2% for all field sizes and depths for both 6 and 15 MV photons and slightly higher for the 18 MV beam and 45 degrees-60 degrees wedge angle. The wedge factor variation with SWD reveals an interesting fact that thinner wedges (15 degrees and 30 degrees) result in a higher surface dose in the central axis region than thicker wedges. As the SWD increases beyond 80 cm, the reverse is true, i.e. thicker wedges produce higher surface dose than thinner wedges. It is also verified that the wedge factor at any depth and for any field size can be calculated from the wedged and open field central axis percentage depth dose, and the wedge factor at dmax, resulting in nearly 44% reduction in water phantom scanning and 80% reduction in point measurements during commissioning.

Particle Accelerators↗

PubMed2003

Modification of Gill-Thomas-Cosman frame for extracranial head-and-neck stereotactic radiotherapy.

PURPOSE: A modification of a commercially available, noninvasive, relocatable, stereotactic Gill-Thomas-Cosman (GTC) head frame is presented for treatment of extracranial lesions of the head and neck, base of the skull, and inferior nasopharyngeal region. METHODS AND MATERIALS: Skull-based and nasopharyngeal lesions cannot be treated with the GTC frame because it obstructs the beam path. To treat those lesions, the GTC frame was modified without compromising the integrity, flexibility, or use of the treatment software. The modification uses a set of aluminum extension rods of variable lengths and bevels to support a modified dental plate. The extension rods allow the dental tray and attached GTC frame to be lowered so that the more inferior regions may be treated. Ten patients underwent CT with the modified frame and CT localizer. For some patients, MRI was acquired without the frame. Image fusion of MRI and CT scans was used to delineate the target volume, and planning was done with the existing software for proper treatment. RESULTS: The modification of the GTC frame has been successful in imaging, planning, and extending the treatment domain for the base of the skull, nasopharyngeal regions, and other superior lesions of the head and neck. The reproducibility of the modified frame and the patient localization helmet technique was identical to that of the unmodified frame. CONCLUSION: The modification of the GTC frame is simple and accurate. It provides flexibility in treating an extended range of the base of the skull, nasopharyngeal region, and other superior lesions of the head and neck that otherwise could not be treated with the GTC frame.

Equipment Design↗

PubMed2003

Impact of target volume coverage with Radiation Therapy Oncology Group (RTOG) 98-05 guidelines for transrectal ultrasound guided permanent Iodine-125 prostate implants.

PURPOSE: Despite the wide use of permanent prostate implants for the treatment of early stage prostate cancer, there is no consensus for optimal pre-implant planning guidelines that results in maximal post-implant target coverage. The purpose of this study was to compare post-implant target volume coverage and dosimetry between patients treated before and after Radiation Therapy Oncology Group (RTOG) 98-05 guidelines were adopted using several dosimetric endpoints. MATERIALS AND METHODS: Ten consecutively treated patients before the adoption of the RTOG 98-05 planning guidelines were compared with ten consecutively treated patients after implementation of the guidelines. Pre-implant planning for patients treated pre-RTOG was based on the clinical target volume (CTV) defined by the pre-implant TRUS definition of the prostate. The CTV was expanded in each dimension according to RTOG 98-05 and defined as the planning target volume. The evaluation target volume was defined as the post-implant computed tomography definition of the prostate based on RTOG 98-05 protocol recommendations. Implant quality indicators included V(100), V(90), V(100), and Coverage Index (CI). RESULTS: The pre-RTOG median V(100), V(90), D(90), and CI values were 82.8, 88.9%, 126.5 Gy, and 17.1, respectively. The median post-RTOG V(100), V(90), D(90), and CI values were 96.0, 97.8%, 169.2 Gy, and 4.0, respectively. These differences were all statistically significant. CONCLUSIONS: Implementation of the RTOG 98-05 implant planning guidelines has increased coverage of the prostate by the prescription isodose lines compared with our previous technique, as indicated by post-implant dosimetry indices such as V(100), V(90), D(90). The CI was also improved significantly with the protocol guidelines. Our data confirms the validity of the RTOG 98-05 implant guidelines for pre-implant planning as it relates to enlargement of the CTV to ensure adequate margin between the CTV and the prescription isodose lines.

Adult↗

PubMed2003

A Monte Carlo study of IMRT beamlets in inhomogeneous media.

Intensity Modulated Radiation Therapy (IMRT) has trended toward smaller multiple radiation fields thereby increasing the resolution of the intensity map. Vendors have introduced multileaf collimators with beam apertures of 0.5 cm and less. The beam characteristics of these smaller fields have not been adequately assessed, especially in the presence of inhomogeneities. Most dosimetric devices have significant limitations due to finite size, dose rate, and energy dependence. We studied the effect of inhomogeneities on small beamlets. The 6, 15, and 24 MV beams were modeled using the EGSnrc Monte Carlo code. Point source beams of circular field sizes 0.5, 1.0, 3.0, 5.0, and 10 cm were simulated in a water phantom at 100 SSD. A 3 cm inhomogeneity of lung tissue was incorporated between 3 and 6 cm in the phantom. The depth dose curves and profiles were compared by beam size and density of the inhomogeneity. The Monte Carlo simulations show that for small fields a marked dose decrease in the presence of low-density media due to the lack of lateral electronic equilibrium is observed. As the density and field size increase, the dose reduction is less pronounced and for the 10 cm field there is an increased dose as expected due to lack of attenuation. This data suggests that current TPS may dramatically over- or underestimate the dose in inhomogeneous media for small field sizes that are used for IMRT.

Anisotropy↗

PubMed2003

Dosimetric considerations for patients with HIP prostheses undergoing pelvic irradiation. Report of the AAPM Radiation Therapy Committee Task Group 63.

This document is the report of a task group of the Radiation Therapy Committee of the AAPM and has been prepared primarily to advise hospital physicists involved in external beam treatment of patients with pelvic malignancies who have high atomic number (Z) hip prostheses. The purpose of the report is to make the radiation oncology community aware of the problems arising from the presence of these devices in the radiation beam, to quantify the dose perturbations they cause, and, finally, to provide recommendations for treatment planning and delivery. Some of the data and recommendations are also applicable to patients having implanted high-Z prosthetic devices such as pins, humeral head replacements. The scientific understanding and methodology of clinical dosimetry for these situations is still incomplete. This report is intended to reflect the current state of scientific understanding and technical methodology in clinical dosimetry for radiation oncology patients with high-Z hip prostheses.

Advisory Committees↗

PubMed2003

Quality assurance for computed-tomography simulators and the computed-tomography-simulation process: report of the AAPM Radiation Therapy Committee Task Group No. 66.

This document presents recommendations of the American Association of Physicists in Medicine (AAPM) for quality assurance of computed-tomography- (CT) simulators and CT-simulation process. This report was prepared by Task Group No. 66 of the AAPM Radiation Therapy Committee. It was approved by the Radiation Therapy Committee and by the AAPM Science Council.

Computer Simulation↗

PubMed2003

Lateral loss and dose discrepancies of multileaf collimator segments in intensity modulated radiation therapy.

In the step-and-shoot technique delivery of intensity modulated radiation therapy (IMRT), each static field consists of a number of beamlets, some of which may be very small. In this study, we measured the dose characteristics for a range of field sizes: 2 x 2 to 12 x 10 cm2 for 6 and 15 MV x rays. For a given field length, a number of treatment fields are set up by sequentially increasing the field width using a multi leaf collimator. A set of fields is delivered with the accelerator operated in the IMRT mode. Using an ion chamber, the output factors at 1 cm and 3 cm laterally from a field edge are measured at different depths in a solid water phantom. Our results show that with insufficient lateral distance in at least one direction, the absorbed dose never reaches the equilibrium values, and can be significantly lower for very small field sizes. For example, the output factor of the 2 x 2 cm2 field relative to 10 x 10 cm2 at d(max0 is 0.832 and 0.790 for 6 MV and 15 MV x rays, respectively. Multiple output factor curves are obtained for different field lengths and different buildup conditions. Thus under nonequilibrium conditions, output factors are critically dependent on the field size and the conventional method of determining the equivalent square does not apply. Comparison of output factors acquired in the commissioning of the accelerator with those measured in the present study under conditions of nonequilibrium shows large discrepancies between the two sets of measurements. Thus monitor units generated by a treatment planning system using beam data commissioned with symmetric fields may be underestimated by > 5%, depending on the size and shape of the segments. To facilitate manual MU calculation as an independent check in step-and-shoot IMRT, the concept of effective equivalent square (EES) is introduced. Using EES, output factors can be calculated using existing beam data for fields with asymmetric collimator settings and under conditions of lateral disequilibrium.

Equipment Failure Analysis↗

PubMed2002

A dosimetric comparison of various multileaf collimators.

The dosimetric characteristics of three multileaf collimator (MLC) systems (Elekta, Siemens and Varian) having 10 mm leaf width are compared. A 6 MV photon beam was used from each unit for measurements. Film dosimetry was performed for the measurements and the analysis techniques were exactly duplicated in each system. Two of the collimators have rounded leaf ends (Elekta and Varian) and the third (Siemens) has a flat end that follows beam divergence. A scanning densitometer (Wellhöfer with 0.45 mm spot and 0.5 mm step size) was used for film analysis. The dosimetric characteristics studied include: penumbra width (80-20%) as a function of position of the leaf end in the field, inter- and intra-leaf radiation leakage, dose distribution of the tongue and groove, and isodose curves for stepped leaves forming 45 degrees angle beam edge. Results show that MLC designs with divergent and non-divergent leaves produce penumbra (80-20%) widths that are within 2.0 mm of each other. However, the distance of the collimator from the x-ray target plays an important role, and the smallest penumbra width was noted for the Varian MLC despite its rounded leaf-end design. Compared to the other systems, this collimator is positioned about 15 cm closer to the patient which affects the skin dose. The MLC with flat leaf end, although closer to the target, showed slightly poorer penumbra width. Inter-leaf leakage through the leaves is 1.3% for two of the collimators (Elekta and Varian) with the backup jaws and is nearly 1% for the third system (Siemens). The Siemens MLC produces reduced tongue-and-groove effect compared to the other two collimators (Elekta and Varian). The isodose undulation for a stepped edge is found to be significant for the collimator closest to the patient (Varian) and does not depend on the leaf-end shape. There is no perfect MLC system that can be recommended, rather each one has unique advantages and disadvantages that should be weighed with comfort, ease and cost effectiveness for clinical use.

Radiometry↗

PubMed2002

Suppression of dark current radiation in step-and-shoot intensity modulated radiation therapy by the initial pulse-forming network.

The effect of the initial pulse forming network (IPFN) on the suppression of dark current is investigated for a Siemens Primus accelerator. The dark current produces a spurious radiation, which is referred to as dark current radiation (DCR) in this study. In the step-and-shoot delivery of an intensity modulated radiation therapy (IMRT), the DCR could be of some concern for whole body dose along with leakage radiation through collimator jaws or multileaf collimator. By adjusting the IPFN-to-PFN ratio to >0.8, the DCR can be measured with an ion chamber during the "PAUSE" state of the accelerator in the IMRT mode. For 15 MV x rays, the magnitude of the DCR is approximately equal to 0.7% of the dose at dmax for a 10 x 10 cm2 field. The DCR has a similar central axis depth dose as a 15 MV beam as determined from a water phantom scan. When the IPFN-to-PFN ratio is lowered to <0.8, no DCR is detected. For low energy x rays (6 MV), no DCR is detected regardless of the IPFN-to-PFN ratio. Although the DCR is studied only for the Siemens Primus model accelerator, the same precaution applies to other models of modern accelerators from other vendors. Due to the large number of field segments used in a step-and-shoot IMRT, it is imperative therefore, that dark current evaluation be part of machine commissioning and annual calibration for high-energy photon beams. Should DCR be detected, the medical physicist should work with a service engineer to rectify the problem. In view of DCR and whole body dose, low-energy photon beams are advisable for IMRT.

Calibration↗