G Asuni

The influence of a novel transmission detector on 6 MV x-ray beam characteristics.

The purpose of this work was to investigate the influence of a new transmission detector on 6 MV x-ray beam properties. The device, COMPASS (IBA Dosimetry, Germany), contains 1600 plane parallel ionization chambers with a detector spacing of 6.5 mm and an active volume of 0.02 cm3. Surface dose measurements were carried out using a Markus chamber and radiochromic film for a range of field sizes and source-to-surface distances (SSDs). The surface dose and dose in the build-up region for COMPASS fields were compared to open fields. For moderately narrow beam geometric conditions, the increase in surface dose was small. For the largest field size investigated (20x20 cm2) at a 90 cm SSD, the surface dose with the detector was 34.9% versus 26.8% in the open field. However, the increase in surface dose in COMPASS fields was less than that observed with a standard block tray in the field (38.7% in the above example). It was found that beyond dmax, the difference in relative dose (profiles and PDDs) between open and COMPASS fields was insignificant. The mean transmission factor of the detector was 0.967 (standard deviation=0.002) measured over a range of field sizes from 3x3 to 20x20 cm2 at SSDs from 70 cm to 90 cm. In summary, the transmission detector was found to increase the relative dose in the buildup region but had a negligible effect on the beam parameters beyond dmax.

A Monte Carlo investigation of contaminant electrons due to a novel in vivo transmission detector.

A novel transmission detector (IBA Dosimetry, Germany) developed as an IMRT quality assurance tool, intended for in vivo patient dose measurements, is studied here. The goal of this investigation is to use Monte Carlo techniques to characterize treatment beam parameters in the presence of the detector and to compare to those of a plastic block tray (a frequently used clinical device). Particular attention is paid to the impact of the detector on electron contamination model parameters of two commercial dose calculation algorithms. The linac head together with the COMPASS transmission detector (TRD) was modeled using BEAMnrc code. To understand the effect of the TRD on treatment beams, the contaminant electron fluence, energy spectra, and angular distributions at different SSDs were analyzed for open and non-open (i.e. TRD and block tray) fields. Contaminant electrons in the BEAMnrc simulations were separated according to where they were created. Calculation of surface dose and the evaluation of contributions from contaminant electrons were performed using the DOSXYZnrc user code. The effect of the TRD on contaminant electrons model parameters in Eclipse AAA and Pinnacle(3) dose calculation algorithms was investigated. Comparisons of the fluence of contaminant electrons produced in the non-open fields versus open field show that electrons created in the non-open fields increase at shorter SSD, but most of the electrons at shorter SSD are of low energy with large angular spread. These electrons are out-scattered or absorbed in air and contribute less to surface dose at larger SSD. Calculated surface doses with the block tray are higher than those with the TRD. Contribution of contaminant electrons to dose in the buildup region increases with increasing field size. The additional contribution of electrons to surface dose increases with field size for TRD and block tray. The introduction of the TRD results in a 12% and 15% increase in the Gaussian widths used in the contaminant electron source model of the Eclipse AAA dose algorithm. The off-axis coefficient in the Pinnacle(3) dose calculation algorithm decreases in the presence of TRD compared to without the device. The electron model parameters were modified to reflect the increase in electron contamination with the TRD, a necessary step for accurate beam modeling when using the device.

Investigation of the spatial resolution of an online dose verification device

PURPOSE The aim of this work is to characterize a new online dose verification device, COMPASS transmission detector array (IBA Dosimetry, Schwarzenbruck, Germany). The array is composed of 1600 cylindrical ionization chambers of 3.8 mm diameter, separated by 6.5 mm center-to-center spacing, in a 40 × 40 arrangement. METHODS The line spread function (LSF) of a single ion chamber in the detector was measured with a narrow slit collimator for a 6 MV photon beam. The 0.25 × 10 mm(2) slit was formed by two machined lead blocks. The LSF was obtained by laterally translating the detector in 0.25 mm steps underneath the slit over a range of 24 mm and taking a measurement at each step. This measurement was validated with Monte Carlo simulation using BEAMnrc and DOSXYZnrc. The presampling modulation transfer function (MTF), the Fourier transform of the line spread function, was determined and compared to calculated (Monte Carlo and analytical) MTFs. Two head-and-neck intensity modulated radiation therapy (IMRT) fields were measured using the device and were used to validate the LSF measurement. These fields were simulated with the BEAMnrc Monte Carlo model, and the Monte Carlo generated incident fluence was convolved with the 2D detector response function (derived from the measured LSF) to obtain calculated dose. The measured and calculated dose distributions were then quantitatively compared using χ-comparison criteria of 3% dose difference and 3 mm distance-to-agreement for in-field points (defined as those above the 10% maximum dose threshold). RESULTS The full width at half-maximum (FWHM) of the measured detector response for a single chamber is 4.3 mm, which is comparable to the chamber diameter of 3.8 mm. The pre-sampling MTF was calculated, and the resolution of one chamber was estimated as 0.25 lp∕mm from the first zero crossing. For both examined IMRT fields, the χ-comparison between measured and calculated data show good agreement with 95.1% and 96.3% of in-field points below χ of 1.0 for fields 1 and 2, respectively (with an average χ of 0.29 for IMRT field 1 and 0.24 for IMRT field 2). CONCLUSIONS The LSF for a new novel online detector has been measured at 6 MV using a narrow slit technique, and this measurement has been validated by Monte Carlo simulation. The detector response function derived from line spread function has been applied to recover measured IMRT fields. The results have shown that the device measures IMRT fields accurately within acceptable tolerance.

SU-GG-T-150: Commissioning and Validation of a Novel Measurement-Based IMRT QA Method, Incorporating Dose Recalculation On Patient CT Data

Purpose: A novel measurement‐based IMRT QA method was tested which provides an accurate reconstruction of the 3D dose distribution in the patient model. This approach is a significant improvement over current QA methods since it allows direct and independent comparison of the doses calculated by the treatment planning system (TPS), including the 3D spatial dose distribution overlaid on CT data and contoured structures, as well as DVHs. Method and Materials: The challenging RPC Head and Neck phantom was used for initial evaluation. A 6 MV, 7 field, 79 segment, step and shoot plan was developed satisfying required dose metrics. A 2D‐array of dose chambers (MatriXX, IBA Dosimetry) was mounted on a linear accelerator. This device captured the delivered IMRT plan fluence in a pretreatment QA context. The measurement data were read directly by the control software (COMPASS, IBA Dosimetry), which also provides the ability to import patient plan data from the TPS. The COMPASS software also includes a dose calculation engine and head fluence model. Beam commissioning procedures analogous to those of a TPS were required. Reconstructed dose and DVHs were compared to those calculated by the TPS. Results: The beam model in the COMPASS software was able to predict percentage depth dose and X and Y profiles (Dmax, 5, 10, 20 cm depths) for MLC‐defined apertures ranging from 1×1–20×20 cm∧2 to within 1.5% (percentage depth‐dose), 2.0% (in‐field profiles), and 2.5% (out‐of‐field profiles). The reconstructed doses in the RPC Head & Neck phantom were within −3 to +4% of those in the treatment planning system. DVHs compared to within 1%. Conclusion: A novel measurement‐based IMRT QA method was tested. Reconstructed doses were overlaid on CT data and contoured structures, to enable a clinically relevant understanding of delivered under‐ or over‐doseages as compared to the TPS plan. Research partially sponsored by IBA Dosimetry.

A model-based 3D patient-specific pre-treatment QA method for VMAT using the EPID

This study reports the development and validation of a model-based, 3D patient dose reconstruction method for pre-treatment quality assurance using EPID images. The method is also investigated for sensitivity to potential MLC delivery errors. Each cine-mode EPID image acquired during plan delivery was processed using a previously developed back-projection dose reconstruction model providing a 3D dose estimate on the CT simulation data. Validation was carried out using 24 SBRT-VMAT patient plans by comparing: (1) ion chamber point dose measurements in a solid water phantom, (2) the treatment planning system (TPS) predicted 3D dose to the EPID reconstructed 3D dose in a solid water phantom, and (3) the TPS predicted 3D dose to the EPID and our forward predicted reconstructed 3D dose in the patient (CT data). AAA and AcurosXB were used for TPS predictions. Dose distributions were compared using 3%/3 mm (95% tolerance) and 2%/2 mm (90% tolerance) γ-tests in the planning target volume (PTV) and 20% dose volumes. The average percentage point dose differences between the ion chamber and the EPID, AcurosXB, and AAA were 0.73  ±  1.25%, 0.38  ±  0.96% and 1.06  ±  1.34% respectively. For the patient (CT) dose comparisons, seven (3%/3 mm) and nine (2%/2 mm) plans failed the EPID versus AAA. All plans passed the EPID versus Acuros XB and the EPID versus forward model γ-comparisons. Four types of MLC sensitive errors (opening, shifting, stuck, and retracting), of varying magnitude (0.2, 0.5, 1.0, 2.0 mm), were introduced into six different SBRT-VMAT plans. γ-comparisons of the erroneous EPID dose and original predicted dose were carried out using the same criteria as above. For all plans, the sensitivity testing using a 3%/3 mm γ-test in the PTV successfully determined MLC errors on the order of 1.0 mm, except for the single leaf retraction-type error. A 2%/2 mm criteria produced similar results with two more additional detected errors.

Reply to 'Comments on "The influence of a novel transmission detector on 6 MV x-ray beam characteristics"'

The purpose of this work was to investigate the influence of a new transmission detector on 6 MV x-ray beam properties. The device, COMPASS (IBA Dosimetry, Germany), contains 1600 plane parallel ionization chambers with a detector spacing of 6.5 mm and an active volume of 0.02 cm3. Butson et al (1996 Australas. Phys. Eng. Sci. Med 19 74–82) studied the causes for the surface dose and dose in the build-up region and concluded that the surface dose at 6 MV is mostly due to electron contamination. We used PTW Markus parallel plane chamber for measurements at the surface and in the build-up region and corrected the over-response using the Mellenberg method (Mellenberg 1990 Med. Phys. 17 1041–4) and we found that for moderately narrow beam geometric conditions, the increase in surface dose was small. For the largest field size investigated (20 × 20 cm2) at 90 cm SSD, the surface dose with the detector was 34.9% versus 26.8% in the open field. It was found that beyond dmax, the difference in relative dose (profiles and PDDs) between open and COMPASS fields was insignificant. In summary, the transmission detector was found to increase the relative dose in the buildup region, but had a negligible effect on the beam parameters beyond dmax.

Sci—Sat AM: Stereo — 01: 3D Pre-treatment Dose Verification for Stereotactic Body Radiation Therapy Patients

Radical treatment techniques such as stereotactic body radiation therapy (SBRT) are becoming popular and they involve delivery of large doses in fewer fractions. Due to this feature of SBRT, a high-resolution, pre-treatment dose verification method that makes use of a 3D patient representation would be appropriate. Such a technique will provide additional information about dose delivered to the target volume(s) and organs-at-risk (OARs) in the patient volume compared to 2D verification methods. In this work, we investigate an electronic portal imaging device (EPID) based pre-treatment QA method which provides an accurate reconstruction of the 3D-dose distribution in the patient model. Customized patient plans are delivered ‘in air’ and the portal images are collected using the EPID in cine mode. The images are then analysed to determine an estimate of the incident energy fluence. This is then passed to a collapsed-cone convolution dose algorithm which reconstructs a 3D patient dose estimate on the CT imaging dataset. To date, the method has been applied to 5 SBRT patient plans. Reconstructed doses were compared to those calculated by the TPS. Reconstructed mean doses were mostly within 3% of those in the TPS. DVHs of target volumes and OARs compared well. The Chi pass rates using 3%/3mm in the high dose region are greater than 97% in all cases. These initial results demonstrate clinical feasibility and utility of a robust, efficient, effective and convenient pre-treatment QA method using EPID. Research sponsored in part by Varian Medical Systems.

SU‐E‐T‐216: Investigation of the System Performance of An Online Dose Verification Device

Purpose: To measure the detector response function of an online dose verification device and to validate our measurement by applying the detector response to Monte Carlo (MC) derived incident fluence. Methods: A transmission detector array (IBA Dosimetry, Schwarzenbruck, Germany) was recently developed as an IMRTquality assurance tool. To understand the detector system performance for verification of IMRT fluence, the detector response function at 6MV of a single ion chamber in the detector was measured with a narrow slit collimator. The narrow slit was formed by two 24 × 72 × 152 mm3 lead blocks, which provided a slit of 0.25 × 10 mm2. The LSF of the detector was obtained by laterally translating it in 0.25 mm steps underneath the slit. Two head‐and‐neck IMRT fields were also measured using the device. The same fields were simulated in our BEAMnrc MC model, and the MC incident fluence was convolved with the 2D detector response to obtain calculated dose. The measured and calculated dose distributions were then quantitatively compared using chi comparison (3%/3mm) for in‐field points (defined as those above 10% maximum dose). Results: The FWHM of the measured detector response for a single chamber is 4.3 mm which is comparable to the chamber diameter of 3.8 mm. For both examined IMRT fields, the Chi comparison between measured and calculated data show good agreement with 100% of in‐field points below a chi of 1.0 (chi <0.6 for IMRT field 1 and <0.12 for IMRT field 2). Conclusions: The detector response function for a new novel online detector has been measured at 6 MV using a narrow slit technique. The good comparison shown between measured and calculated dose of the two IMRT fields is a validation of our response function measurement.

Poster — Thur Eve — 03: A Monte Carlo Investigation of the Effects of a Novel In Vivo Transmission Detector on a 6 MV Photon Beam

The effect of a transmission detector (TRD), a novel IMRTquality assurance tool to be used for in vivo dose measurements, was investigated. As a scattering material, the device will be a source of contaminantelectrons and could potentially affect prescribed dose to patients during treatment. The goals of this investigation are to characterize the effect of the TRD on the clinical photon beam including IMRT fields, using Monte Carlo simulation. The linear accelerator head and the TRD were modeled using BEAMnrc. Particles scored at 70 cm and 100cm SSD for different field sizes (i.e. 5×5 cm2, 10×10 cm2, and 20 × 20 cm2) with and without the TRD were separated according to where they were created in the linac head. In addition, two IMRT fields with and without the TRD were simulated and their respective absolute dose distributions were compared. Without the TRD, air is the major source of contaminantelectrons. When TRD is used, it absorbs nearly all the incident contaminantelectrons, while becoming the major source of contaminantelectrons. For both IMRT fields, the percentage dose differences of the calculated absolute doses with and without TRD at the isocenter were found to be within uncertainty of the measured transmission factor for the TRD. Although a major source of contaminantelectrons, the TRD does not introduce excessive electron contamination into a clinical 6 MV photon beam and is therefore suitable for in vivo dose measurements if fully commissioned as an IMRTquality assurance tool. Research partly sponsored by IBA Dosimetry.

Poster - Thur Eve - 04: The Line Spread Function Measurement of a Novel Transmission Detector

Measurement‐baseddose verification of planned intensity modulated radiotherapy(IMRT) fields requires a detector with good spatial resolution. A novel, transmission detector array has been recently developed by IBA Dosimetry (Germany) as an IMRTquality assurance tool. To characterize the detector system performance for verification of IMRT fluence, the line spread function (LSF) of a single ion chamber in the detector in a clinical 6 MV photon beam was measured with a narrow slit of width 0.25 mm. The LSF of the detector was obtained by laterally translating it in 0.5 mm steps underneath the slit, from the center to 16 mm distance from the center in the cross‐plane direction. The measured signal was corrected for background and leakage. The FWHM of the LSF for the single chamber was found to be 4.2 mm, which is comparable to the chamber size of 3.8 mm. The MTF of the detector system is a sinc function and with a first zero crossing of the MTF at spatial frequency of 0.25 lp/mm. This compares well to the value of 0.15 lp/mm found for the PTW (Germany) 2D‐ARRAY type 10024, also used for dose verification of IMRT fluence. We have successfully measured the LSF for a new novel transmission detector using a narrow slit method and a clinical 6MV photon beam. The transmission detector was found to have a better first zero crossing of the MTF compared to the 2D‐ARRAY type 10024. Research partly sponsored by IBA Dosimetry.