Film Dosimetry System Research Articles

Correcting scan-to-scan response variability for a radiochromic film-based reference dosimetry system.

In radiochromic film dosimetry systems, measurements are usually obtained from film images acquired on a CCD-based flatbed scanner. The authors investigated factors affecting scan-to-scan response variability leading to increased dose measurement uncertainty. The authors used flatbed document scanners to repetitively scan EBT3 radiochromic films exposed to doses 0-1000 cGy, together with three neutral density filters and three blue optical filters. Scanning was performed under two conditions: scanner lid closed and scanner lid opened/closed between scans. The authors also placed a scanner in a cold room at 9 °C and later in a room at 22 °C and scanned EBT3 films to explore temperature effects. Finally, the authors investigated the effect of altering the distance between the film and the scanner's light source. Using a measurement protocol to isolate the contribution of the CCD and electronic circuitry of the scanners, the authors found that the standard deviation of response measurements for the EBT3 film model was about 0.17% for one scanner and 0.09% for the second. When the lid of the first scanner was opened and closed between scans, the average scan-to-scan difference of responses increased from 0.12% to 0.27%. Increasing the sample temperature during scanning changed the RGB response values by about -0.17, -0.14, and -0.05%/°C, respectively. Reducing the film-to-light source distance increased the RBG response values about 1.1, 1.3, and 1.4%/mm, respectively. The authors observed that films and film samples were often not flat with some areas up to 8 mm away from the scanner's glass window. In the absence of measures to deal with the response irregularities, each factor the authors investigated could lead to dose uncertainty >2%. Those factors related to the film-to-light source distance could be particularly impactful since the authors observed many instances where the curl of film samples had the potential to cause dose uncertainty in excess of 5%. Two expedients will eliminate the uncertainties: a transparent sheet (preferably glass) placed over the scanned film keeps the film-to-light source distance constant, and an EBT3 reference film included in all scans provides correction factors for measured response values.

SU‐E‐T‐462: Impact of the Radiochromic Film Energy Response On Dose Measurements of Low Energy Electronic Brachytherapy Sources

Purpose:We investigated the effect of the EBT3 GafChromicTM film model absorbed dose energy response when used for percent depth dose (PDD) measurements in low‐energy photon beams.Methods:We measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared them to PDD measurements acquired with a PTW‐TN34013 parallel‐plate ionization chamber. For the x‐ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Pieces of EBT3 film were irradiated in air and calibration curves were created in terms of air‐kerma in air ((Kair)air) for different beam qualities. Pieces of EBT3 film were positioned at distances of 2–6 cm from the Xoft source in a water phantom using a custom‐made holder, and irradiated at the same time. As scatter is incorporated in the measured film signal in water, measured (Kair)wat was subsequently converted into absorbed dose to water by the ratio of mass energy absorption coefficients following the AAPM TG‐61 dosimetry protocol.Results:Our results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 3%. However, if the calibration curve was established at MV beam quality, the error in absorbed dose could be as large as 15%. We observed agreement within 1% between PDD measurements using EBT3 film model (using a calibration curve obtained at 80 kVp, HVL=2.18 mm Al, Eeff=29.5 keV) and the parallel‐plate ionization chamber.Conclusion:Accurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at corresponding low energies, as large absorbed dose errors are expected for calibrations performed at MV beam qualities.

Radiochromic film-based quality assurance for CT-based high-dose-rate brachytherapy.

In the past, film dosimetry was developed into a powerful tool for external beam radiotherapy treatment verification and quality assurance. The objective of this work was the development and clinical testing of the EBT3 model GafChromic film based brachytherapy quality assurance (QA) system. Retrospective dosimetry study was performed to test a patient-specific QA system for preoperative endorectal brachytherapy that uses a radiochromic film dosimetry system. A dedicated phantom for brachytherapy applicator used for rectal cancer treatment was fabricated enabling us to compare calculated-to-measured dose distributions. Starting from the same criteria used for external beam intensity-modulated radiation therapy QA (3%, 3 mm), passing criteria for high- and low-dose gradient regions were subsequently determined. Finally, we investigated the QA system's sensitivity to controlled source positional errors on selected patient plans. In low-dose gradient regions, measured dose distributions with criteria of 3%, 3 mm barely passed the test, as they showed 95% passing pixels. However, in the high-dose gradient region, a more stringent condition could be established. Both criteria of 2%, 3 mm and 3%, 2 mm with gamma function calculated using normalization to the same absolute dose value in both measured and calculated dose distributions, and matrix sizes rescaled to match each other showed more than 95% of pixels passing, on average, for 15 patient plans analyzed. Although the necessity of the patient-specific brachytherapy QA needs yet to be justified, we described a radiochromic film dosimetry-based QA system that can be a part of the brachytherapy commissioning process, as well as yearly QA program.

Development of a PET cyclotron based irradiation setup for proton radiobiology

An out-of-yoke irradiation setup using the proton beam from a cyclotron that ordinary produces radioisotopes for positron emission tomography (PET) has been developed, characterized, calibrated and validated. The current from a 20 μm thick aluminum transmission foil is readout by home-made transimpedance electronics, providing online dose information. The main monitoring variables, delivered in real-time, include beam current, integrated charge and dose rate. Hence the dose and integrated current delivered at a given instant to an experimental setup can be computer-controlled with a shutter. In this work, we report on experimental results and Geant4 simulations of a setup which exploits for the first time the 18 MeV proton beam from a PET cyclotron to irradiate a selected region of a target using the developed irradiation system. By using this system, we are able to deliver a homogeneous beam on targets with 18 mm diameter, allowing to achieve the controlled irradiation of cell cultures located in biological multi-well dishes of 16 mm diameter. We found that the magnetic field applied inside the cyclotron plays a major role for achieving the referred to homogeneity. The quasi-Gaussian curve obtained by scanning the magnet current and measuring the corresponding dose rate must be measured before any irradiation procedure, with the shutter closed. At the optimum magnet current, which corresponds to the center of the Gaussian, a homogenous dose is observed over the whole target area. Making use of a rotating disk with a slit of 0.5 mm at a radius of 150 mm, we could measure dose rates on target ranging from 500 mGy/s down to 5 mGy/s. For validating the developed irradiation setup, several Gafchromic® EBT2 films were exposed to different values of dose. The absolute dose in the irradiated films were assessed in the 2D film dosimetry system of the Department of Radiotherapy of Coimbra University Hospital Center with a precision better than 2%. In the future, we plan to irradiate small animals, cell cultures, or other materials or samples.

A universal dose-response curve for radiochromic films.

This paper presents a model for dose-response curves of radiochromic films. It is based on a modified version of single-hit model to take into account the growth experienced by lithium salt of pentacosa-10,12-diynoic acid polymers after irradiation. Polymer growth in radiochromic films is a critical phenomenon that can be properly described by means of percolation theory to provide an appropriate distribution function for polymer sizes. Resulting functional form is a power function featuring a critical exponent and two adjustable parameters. Moreover, these parameters act as scaling factors setting a natural scale for sensitometric curves where the dependence on channel sensitivity is removed. A unique reduced response curve is then obtained from all the color channels describing film behavior independently of film dosimetry system. Resulting functional form has been successfully tested in several sensitometric curves from different Gafchromic EBT models, providing excellent agreement with experimental data in a wide dose range up to about 40 Gy and low dose uncertainty. The model presented in this paper describes accurately the sensitometric curves of radiochromic films in wide dose ranges covering all typical ranges used in external radiotherapy. Resulting dose uncertainty is low enough to render a reasonably good performance in clinical applications. Due to cross-correlation, only one of the adjustable parameters is totally independent and characterizes film batches.

Radiochromic film based dosimetry of image-guidance procedures on different radiotherapy modalities.

In this work we compare doses from imaging procedures performed on today's state‐of‐the‐art integrated imaging systems using a reference radiochromic film dosimetry system. Skin dose and dose profile measurements from different imaging systems were performed using radiochromic films at different anatomical sites on a humanoid RANDO phantom. EBT3 film was used to measure imaging doses from a TomoTherapy MVCT system, while XRQA2 film was used for dose measurements from kilovoltage imaging systems (CBCT on 21eX and TrueBeam Varian linear accelerators and CyberKnife stereoscopic orthogonal imagers). Maximum measured imaging doses in cGy at head, thorax, and pelvis regions were respectively 0.50, 1.01, and 4.91 for CBCT on 21eX, 0.38, 0.84, and 3.15 for CBCT on TrueBeam, 4.33, 3.86, and 6.50 for CyberKnife imagers, and 3.84, 1.90, and 2.09 for TomoTherapy MVCT. In addition, we have shown how an improved calibration system of XRQA2 film can achieve dose uncertainty level of better than 2% for doses above 0.25 cGy. In addition to simulation‐based studies in literature, this study provides the radiation oncology team with data necessary to aid in their decision about imaging frequency for image‐guided radiation therapy protocols.PACS number: 87.53.Bn, 87.55.Qr, 87.56.Fc

Characterization of calibration curves and energy dependence GafChromic™ XR-QA2 model based radiochromic film dosimetry system.

The authors investigated the energy response of XR-QA2 GafChromic™ film over a broad energy range used in diagnostic radiology examinations. The authors also made an assessment of the most suitable functions for both reference and relative dose measurements. Pieces of XR-QA2 film were irradiated to nine different values of air kerma in air, following reference calibration of a number of beam qualities ranging in HVLs from 0.16 to 8.25 mm Al, which corresponds to effective energy range from 12.7 keV to 56.3 keV. For each beam quality, the authors tested three functional forms (rational, linear exponential, and power) to assess the most suitable function by fitting the delivered air kerma in air as a function of film response in terms of reflectance change. The authors also introduced and tested a new parameter χ = netΔR·e(m netΔR) that linearizes the inherently nonlinear response of the film. The authors have found that in the energy range investigated, the response of the XR-QA2 based radiochromic film dosimetry system ranges from 0.222 to 0.420 in terms of netΔR at K(air)(air) = 8 cGy. For beam qualities commonly used in CT scanners (4.03-8.25 mm Al), the variation in film response (netΔR at K(air)(air) = 8 cGy) amounts to ± 5%, while variation in K(air)(air) amounts to ± 14%. Results of our investigation revealed that the use of XR-QA2 GafChromic™ film is accompanied by a rather pronounced energy dependent response for beam qualities used for x-ray based diagnostic imaging purposes. The authors also found that the most appropriate function for the reference radiochromic film dosimetry would be the power function, while for the relative dosimetry one may use the exponential response function that can be easily linearized.

EP-1466: Energy response of XR-QA2 based radiochromic film dosimetry system

EP-1466: Energy response of XR-QA2 based radiochromic film dosimetry system

Pre-Treatment Dose Verification of Imrt Using Gafchromic Ebt3 Film and 2DArray

Purpose: The present work deals with pre-treatment dosimetric verification of 10 prostate intensity-modulated radiotherapy (IMRT) plans obtained by Gafchromic EBT3 film and 2D-Array seven29 to establish quality control protocol for the delivery of prostate IMRT in our clinic. Methods: 10 prostate patients were irradiated according to IMRT techniques with 6 MV photon beams produced by a Varian DHX linear accelerator. Dose plans were computed using Eclipse 8.9 (version8.9, Varian, Palo Alto, CA, United States) treatment planning system (TPS). Pre-treatment dosimetric verification was carried out on field-perfield and total IMRT plan basis measuring 2D dose distributions in RW3 solid water phantom (PTW-Freiburg, Germany). For 10 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original and 0 degrees gantry, collimator and table angles. Thus IMRT quality control (QC) plans were generated. Dose distribution measurements were measured with Gafchromic EBT3 film and 2D-Array seven29 (PTW-Freiburg, Germany) two dimensional ionization chamber system. Verification measurements of total IMRT plans and each individual beams were compared with expected dose maps, and differences were evaluated by Verisoft program (PTW-Freiburg, Germany). To provide comparisons of multidimensional dose distributions, dose comparison tools such as gamma dose distribution, distance- to- agreement (DTA) and dose difference (DD) have been developed. The gamma dose distribution tool was used in our study. Three different gamma criteria of dose difference (DD) and distance to agreement (DTA) (3%/3 mm, 4%/4 mm and 5%/5 mm DD / DTA) are selected. These criteria are evaluated while suppressing the dose of 5% from dose distribution. Criterion validity accepted as section with gamma value less than or equal to 1 (γ ≤ 1) to be 90%. Results: In the comparison of dose distributions obtained from TPS and the results of PTW 2D-Array seven29 and Gafchromic EBT3 film dosimetry systems, it was showed that the compatibility of both methods were above 90% with respect to 3% DD and 3mm DTA criteria. PTW 2D-Array seven29 results compared to the results of the film was closer to the TPS data and this difference was statistically meaningful (p 0.05). Conclusions: The measurements performed with PTW 2D-Array seven29 were closer to the TPS data than film measurements and it takes less time during clinical use, so it could be preferred for routine use. The present results suggest the gamma criteria of 3%/3 mm as the most suitable criteria for prostate IMRT quality assurance.

Delta three-dimensional semiconductor array verification for intensity modulated planning of helical tomotherapy

Objective To investigate the feasibility of dose verification of intensity modulated planning of helical tomotherapy (HT) using three-dimensional semiconductor array (Delta4) and find a more time-consuming and accurate method to validate the delivery dose.Methods Delta4 detector array was used to verify the HT plan dose distribution of 10 patients.The precise setup of detector array was guided and registered by MVCT imaging.After the implementation of delivery,the measured dose distributions on the corresponding planes were compared with those calculated by the treatment planning system (TPS).According to the different acceptance criteria combination of dose difference or distance to agreement at different threshold level including 3％/2 mm,3％/3 mm,3％/4 mm,3％/5 mm at TH5,TH10,TH20,TH30,TH70,TH90,the γ analysis method was utilized to evaluate the dose verification.Results The dose distribution measured by the Delta4 was well consistent with that calculated by the TPS.The mean γpassing rates were all above 94.89％.Conclusions Delta4 detector array can get satisfying results when used in HT plan dose verification and is a viable tool to replace traditional film dosimetry system in clinical quality assurance work. Key words: Dose verification; Three-dimensional detector array; Helical tomotherapy

SU-D-103-05: Radiochromic Film Based System for CTDI Measurements

Purpose: We demonstrate a new method for computed tomography dose index (CTDI) measurements performed on CT scanners using XR‐QA model radiochromic film dosimetry system. Method was tested by comparing measured to CT scanner listed CTDI values for fifteen different CT scanners at various locations. Methods: Reference dosimetry system was calibrated in terms of air kerma in air. Dose profiles were measured with XR‐QA2 radiochromic film strips, sandwiched between acrylic rods cut in half and placed within CTDI phantoms. Film strips were scanned before and after irradiation with Epson Perfection V500 scanner in reflective mode. Reflectance change was measured using red color channel from TIFF images of scanned films and it was converted into air kerma in air using calibration curves. Measured air kerma in air was subsequently converted into absorbed dose to water following AAPM TG‐61 protocol. We also investigated the impact of scan length by scanning CTDI phantom 5, 10, and 15 cm longitudinally. Results: Average doses along profiles from each film piece were used to calculate weighted CTDIvol. Average difference for the first 10 scanners between calculated and listed CTDIvol was 21.8% for Head and 10.4% for Body protocol. Our study also showed that as the scanning length was increasing, the difference between measured and tabulated CTDIvol decreased. For the last 5 CT scanners, the scanning length of 5‐10 cm was changed to 15 cm for both Head and Body scanning protocols, resulting in average difference between measured and tabulated CTDIvol values of 3.8% for Head and 9.4% for Body scanning protocol. Conclusion: In contrast to ionization chamber measurement, the proposed method requires only a single exposure, does not need stem effect correction, and provides an acceptable accuracy when compared to CT scanner listed CTDIvol values. It also measures dose profiles allowing measurements of DLP and peak doses.

Comment on “Linearization of dose‐response curve of the radiochromic film dosimetry system” [Med. Phys. 39(8), 4850–4857 (2012)

Comment on “Linearization of dose‐response curve of the radiochromic film dosimetry system” [Med. Phys. 39(8), 4850–4857 (2012)

Response to "Comments on 'Linearization of dose-response curve of the radiochromic film dosimetry system'" [Med. Phys. 39(8), 4850-4857 (2012)

Response to "Comments on 'Linearization of dose-response curve of the radiochromic film dosimetry system'" [Med. Phys. 39(8), 4850-4857 (2012)

Evaluation of Gafchromic® EBT3 films characteristics in therapy photon, electron and proton beams

PurposeTo evaluate the uncertainties and characteristics of radiochromic film-based dosimetry system using the EBT3 model Gafchromic® film in therapy photon, electron and proton beams. Material and methodsEBT3 films were read using an EPSON Expression 10000XL/PRO scanner. They were irradiated in five beams, an Elekta SL25 6 MV and 18 MV photon beam, an IBA 100 MeV 5 × 5 cm2 proton beam delivered by pencil-beam scanning, a 60 MeV fixed proton beam and an Elekta SL25 6 MeV electron beam. Reference dosimetry was performed using a FC65-G chamber (Elekta beam), a PPC05 (IBA beam) and both Markus 1916 and PPC40 Roos ion-chambers (60 MeV proton beam). Calibration curves of the radiochromic film dosimetry system were acquired and compared within a dose range of 0.4–10 Gy. An uncertainty budget was estimated on films irradiated by Elekta SL25 by measuring intra-film and inter-film reproducibility and uniformity; scanner uniformity and reproducibility; room light and film reading delay influences. ResultsThe global uncertainty on acquired optical densities was within 0.55% and could be reduced to 0.1% by placing films consistently at the center of the scanner. For all beam types, the calibration curves are within uncertainties of measured dose and optical densities. The total uncertainties on calibration curve due to film reading and fitting were within 1.5% for photon and proton beams. For electrons, the uncertainty was within 2% for dose superior to 0.8 Gy. ConclusionsThe low combined uncertainty observed and low beam and energy-dependence make EBT3 suitable for dosimetry in various applications.

Linearization of dose-response curve of the radiochromic film dosimetry system.

Despite numerous advantages of radiochromic film dosimeter (high spatial resolution, near tissue equivalence, low energy dependence) to measure a relative dose distribution with film, one needs to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose values into relative doses. In this work, we present result of our efforts to obtain a functional form that would linearize the inherently nonlinear dose-response curve of the radiochromic film dosimetry system. Functional form [ζ = (-1)[middle dot]netOD((2∕3))∕ln(netOD)] was derived from calibration curves of various previously established radiochromic film dosimetry systems. In order to test the invariance of the proposed functional form with respect to the film model used we tested it with three different GAFCHROMIC™ film models (EBT, EBT2, and EBT3) irradiated to various doses and scanned on a same scanner. For one of the film models (EBT2), we tested the invariance of the functional form to the scanner model used by scanning irradiated film pieces with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, verification tests have been performed in clinical applications: percent depth dose measurements, IMRT quality assurance (QA), and brachytherapy QA. Obtained R(2) values indicate that the choice of the functional form of the new argument appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMIC™ EBT3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt-60 data. It was also found that criteria of 3%∕3 mm for an IMRT QA plan and 3%∕2 mm for a brachytherapy QA plan are passing 95% gamma function points. In this paper, we demonstrate the use of functional argument to linearize the inherently nonlinear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing calibration curve.

TH-E-BRB-02: Linearization of Dose Response Curve for the Radiochromic Film Dosimetry System

Purpose: In order to measure relative dose distribution with radiochromic film, one need to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose into relative dose. We present here result of our efforts to establish a functional form that converts the inherently non‐linear dose response curve of the radiochromic film dosimetry system into liner one. Methods: In order to test the invariance of the proposed functional form with respect to the film or scanner model used, we have tested it on film pieces from three different GAFCHROMICTM film models (EBT, EBT‐2, and EBT‐3) irradiated to various doses and scanned on a single scanner. for one of the film models we have also tested it by scanning with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, we performed a measurement of percent depth dose (PDD) curves in Cobalt‐60 beam. Results: Obtained R2 values indicate that the choice of the functional form of the new argument (ζ) appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMICTM EBT‐3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt‐60 data. Conclusions: We demonstrate the use of a unique functional argument to linearize the inherently non‐linear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing an absolute calibration curve. One author is from GAFCHROMICTM films manufacturer

Evaluation of a Gafchromic EBT2 film dosimetry system for radiotherapy quality assurance

This study examines the dosimetric accuracy of Gafchromic EBT2 model radiochromic film for use in radiotherapy quality assurance. In this study, film was scanned using an Epson Perfection V700 flatbed scanner in transmission mode at 75 DPI with the subsequent analysis performed using the red and blue colour channels and ImageJ software. Results of this study suggest that the conversion of film optical density to measured dose should, at present, utilise red channel data only, without application of a blue channel correction to the data. For the batch of film examined here, film uniformity and reproducibility appear to have improved compared with published results using older batches. The orientation of the film on the scanner and the side of the film facing the light source were found to have substantial effects on results. Based on the results of this study, it is possible to recommend the use of EBT2 film in routine quality assurance testing for radiotherapy, in situations where a dose uncertainty of up to 2.8% is acceptable.

Verification of patient‐specific dose distributions in proton therapy using a commercial two‐dimensional ion chamber array

The purpose of this study was to determine whether a two-dimensional (2D) ion chamber array detector quickly and accurately measures patient-specific dose distributions in treatment with passively scattered and spot scanning proton beams. The 2D ion chamber array detector MatriXX was used to measure the dose distributions in plastic water phantom from passively scattered and spot scanning proton beam fields planned for patient treatment. Planar dose distributions were measured using MatriXX, and the distributions were compared to those calculated using a treatment-planning system. The dose distributions generated by the treatment-planning system and a film dosimetry system were similarly compared. For passively scattered proton beams, the gamma index for the dose-distribution comparison for treatment fields for three patients with prostate cancer and for one patient with lung cancer was less than 1.0 for 99% and 100% of pixels for a 3% dose tolerance and 3 mm distance-to-dose agreement, respectively. For spot scanning beams, the mean (+/- standard deviation) percentages of pixels with gamma indices meeting the passing criteria were 97.1% +/- 1.4% and 98.8% +/- 1.4% for MatriXX and film dosimetry, respectively, for 20 fields used to treat patients with prostate cancer. Unlike film dosimetry, MatriXX provides not only 2D dose-distribution information but also absolute dosimetry in fractions of minutes with acceptable accuracy. The results of this study indicate that MatriXX can be used to verify patient-field specific dose distributions in proton therapy.

Evaluation of Dose Volume Histograms for Rectal and Prostate Cancer Patients Undergoing CBCT Based IGRT

Advances in radiotherapy treatment planning led to improved local tumor control and better healthy tissues sparing. Without image guidance at the time of therapy (IGRT), changes in position, shape and intra-fractional motions may prevent desired dose from being precisely delivered to the planning target. The rapid fall-off in dose outside the target in highly conformal radiotherapy techniques impose demand for accurate target localization at the time of the treatment to avoid underdosing of disease. Daily kV cone-beam computed tomography (CBCT) acquisition can provide accurate 3D volumetric patient's anatomy prior to every treatment fraction and it is suitable for adaptive corrections of treatment positioning variations. This study retrospectively assesses dose volume histograms (DVHs) with and without IGRT employing CBCT. We also report on additional radiation skin dose to patients during CBCT. Pre-treatment 3D imaging of 20 patients in treatment position was acquired using Pelvis CBCT protocol on on-board imaging (OBI) system (v. 1.4 Varian Medical Systems, Palo Alto) on a daily basis and twice a week for prostate and rectum cancer patients respectively. The obtained reconstructed CBCT images were co-registered with planning CT data using bony landmarks for rectal patients and prostate soft tissue visibility and prostate contours for prostate cancer patients. Obtained shifts were used in the Eclipse Treatment planning system to mimic the non-IGRT guided treatment. We calculated the dose distribution for original and simulated plan and obtained DVHs for targets and organs at risk were compared. Prior to CBCT scan, film pieces were taped on patient skin. Films were scanned before and after irradiation with the Epson Expression 10000XL document scanner in reflective mode. Response of the reference film dosimetry system was calibrated in terms of air kerma in air. Our preliminary results show small difference in PTV coverage for rectum patients with better sparing of the bladder in high dose region. For prostate patients, DVH of simulated plan shows 5% worse PTV coverage when compared to the original plan while the bladder sparing increases significantly. Measured anterior dose during CBCT amounts to 1.60 cGy in larger and up to 3.17 cGy in smaller patients, while the lateral dose ranges from 1.63 cGy in larger and up to 2.68 cGy in smaller patients. Our preliminary data indicate better sparing of bladder in high dose regions, and possible margin reduction for PTV definition if daily CBCT was used. This could lead to better tumor control and decrease NTCP, but the benefits must be weighted against the dose delivered to patients during CBCT.

Characterization and dosimetry of a practical X-ray alternative to self-shielded gamma irradiators

The Insect Pest Control Laboratory of the Joint FAO/IAEA 1 1 Food and Agriculture Organization/International Atomic Energy Agency. Division of Nuclear Techniques in Food and Agriculture recently purchased an X-ray irradiator as part of their programme to develop the sterile insect technique (SIT). It is a self-contained type with a maximum X-ray beam energy of 150 keV using a newly developed 4 π X-ray tube to provide a very uniform dose to the product. This paper describes the results of our characterization study, which includes determination of dose rate in the centre of a canister as well as establishing absorbed dose distribution in the canister. The irradiation geometry consists of five canisters rotating around an X-ray tube—the volume of each canister being 3.5 l. The dose rate at the maximum allowed power of the tube (about 6.75 kW) in the centre of a canister filled with insects (or a simulated product) is about 14 Gy min −1. The dose uniformity ratio is about 1.3. The dose rate was measured using a Farmer type 0.18-cm 3 ionization chamber calibrated at the relevant low photon energies. Routine absorbed dose measurement and absorbed dose mapping can be performed using a Gafchromic® film dosimetry system. The radiation response of Gafchromic film is almost independent of X-ray energy in the range 100–150 keV, but is very sensitive to the surrounding material with which it is in immediate contact. It is important, therefore, to ensure that all absorbed dose measurements are performed under identical conditions to those used for the calibration of the dosimetry system. Our study indicates that this X-ray irradiator provides a practical alternative to self-shielded gamma irradiators for SIT programmes.