D. Rickey

A bench-top megavoltage fan-beam CT using CdWO4-photodiode detectors. I. System description and detector characterization

We describe the components of a bench-top megavoltage computed tomography (MVCT) scanner that uses an 80-element detector array consisting of CdWO4 scintillators coupled to photodiodes. Each CdWO4 crystal is 2.75 x 8 x 10 mm3. The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient (R2) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by this detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detectors large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation <0.5%); however, any smaller permanent damage could not be assessed.

An investigation of gantry angle data accuracy for cine-mode EPID images acquired during arc IMRT

EPID images acquired in cine mode during arc therapy have inaccurate gantry angles recorded in their image headers. In this work, methods were developed to assess the accuracy of the gantry potentiometer for linear accelerators. As well, assessments of the accuracy of other, more accessible, sources of gantry angle information (i.e., treatment log files, analysis of EPID image headers) were investigated. The methods used in this study are generally applicable to any linear accelerator unit, and have been demonstrated here with Clinac/Trilogy systems. Gantry angle data were simultaneously acquired using three methods: i) a direct gantry potentiometer measurement, ii) an incremental rotary encoder, and iii) a custom‐made radiographic gantry‐angle phantom which produced unique wire intersections as a function of gantry angle. All methods were compared to gantry angle data from the EPID image header and the linac MLC DynaLog file. The encoder and gantry‐angle phantom were used to validate the accuracy of the linacs potentiometer. The EPID image header gantry angles and the DynaLog file gantry angles were compared to the potentiometer. The encoder and gantry‐angle phantom mean angle differences with the potentiometer were 0.13∘±0.14∘ and 0.10∘±0.30∘, respectively. The EPID image header angles analyzed in this study were within ±1∘ of the potentiometer angles only 35% of the time. In some cases, EPID image header gantry angles disagreed by as much as 3° with the potentiometer. A time delay in frame acquisition was determined using the continuous acquisition mode of the EPID. After correcting for this time delay, 75% of the header angles, on average, were within ±1∘ of the true gantry angle, compared to an average of only 35% without the correction. Applying a boxcar smoothing filter to the corrected gantry angles further improved the accuracy of the header‐derived gantry angles to within ±1∘ for almost all images (99.4%). An angle accuracy of 0.11∘±0.04∘ was determined using a point‐by‐point comparison of the gantry angle data in the MLC DynaLog file and the potentiometer data. These simple correction methods can be easily applied to individual treatment EPID images in order to more accurately define the gantry angle. PACS numbers: 87.53.Kn, 87.55.T‐, 87.56.bd, 87.59.‐e

A quality assurance tool for high-dose-rate brachytherapy.

PURPOSE The purpose of this work was to develop a quality assurance (QA) tool for high-dose-rate (HDR) brachytherapy that would quickly and easily verify both source positioning (dwell positions) and durations (dwell times). METHODS The authors constructed a QA tool that combined radiochromic film to verify position with four photodiode detectors to verify dwell times. To characterize the temporal accuracy of the tool, a function generator powered four red light-emitting diodes that were optically coupled to the four photodiode detectors. The QA tool was used to verify the dwell positions and times of a commercial brachytherapy afterloader. Measurements of dwell time were independently verified by a one-dimensional optical camera that acquired 1000 lines/s. RESULTS The temporal accuracy of the QA tool was found to be about 1 ms. For visual assessment, the source position could be located within about 0.5 mm. Evaluating the accuracy and precision of an HDR brachytherapy afterloader, the authors found that the bias in dwell time can exceed 60 ms and the dwell time associated with the first dwell position had an unexpectedly large standard deviation of 30 ms. They found that the source locations were much easier to locate on the film if a plastic catheter was used instead of a metal treatment tube. Scanning the films enabled the dwell positions to be determined within about 0.2 mm. CONCLUSIONS For pretreatment QA, the authors found that this tool allowed verification of dwell positions and dwell times in about 6 min.

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‐E‐T‐210: Precise Gantry Angle Determination for EPID Images during Rotational IMRT

Purpose: Utilization of an aSi EPID to develop an in vivo patient dose verification system for rotational IMRT (rIMRT) delivery requires accurate knowledge of gantry angle as a function of time. Currently the accuracy of the gantry angle stamp in the header of the EPIDimage is limited to approximately +/−3 degrees. This work investigates several unique methods for a more accurate determination of the gantry angle during rIMRT. Methods: Gantry angles were determined using: (1) an incremental rotary encoder attached to the rotational axis of the gantry, (2) a direct analogue‐to‐digital measurement of the gantry potentiometer, and (3) through EPIDimage analyses of an in‐house phantom (manufactured at sub‐millimeter precision). The phantom consists of a cylindrical acrylic frame with one wire wrapped helically around its surface and one straight wire traversing its central axis. This design creates EPIDimages with unique and identifiable wire intersection points as a function of gantry orientation. Analysis of the treatment console log files was compared to the above methods. Results: The gantry potentiometer is considered the most accurate gantry angle but is unavailable during treatment. The ClinacLog produced discrepancies of up to ±2 degrees, the DynaLog up to ±1 degrees, and the encoder up to ±0.5 degrees with respect to the potentiometer. Preliminary analysis comparing our phantom‐determined gantry angles with the encoder gantry angles showed agreement within ±0.5 degrees of each other for 85% of the data and differed at most by 1.3 degrees from each other. Conclusions: We have developed several techniques to determine gantry angle as a function of time during rIMRT. We have shown a strong agreement in gantry determination by our phantom and encoder. This investigation of gantry angle is critical to develop an accurate in vivo patient dose verification system for rIMRT delivery.

Size and positioning reproducibility of an 192Ir brachytherapy stepping source.

In this paper we describe techniques for measuring the dimensions and position reproducibility of an 192Ir brachytherapy stepping source. Measurements were carried out using a 0.25x10x152 mm3 collimator placed in front of a detector of our own design. The brachytherapy source was translated past the collimator in 0.025 mm increments using a stepper motor. The source was found to be 3.58 mm long and 0.69 mm wide, which is in good agreement with the manufacturers values of 3.5x0.6 mm2. The source position was reproducible to within 0.12 mm.

Implant delivering hydroxychloroquine attenuates vaginal T lymphocyte activation and inflammation

&NA; Evidence suggests that women who are naturally resistant to HIV infection exhibit low baseline immune activation at the female genital tract (FGT). This “immune quiescent” state is associated with lower expression of T‐cell activation markers, reduced levels of gene transcription and pro‐inflammatory cytokine or chemokine production involved in HIV infection while maintaining an intact immune response against pathogens. Therefore, if this unique immune quiescent state can be pharmacologically induced locally, it will provide an excellent women‐oriented strategy against HIV infection To our knowledge, this is the first research article evaluating in vivo, an innovative trackable implant that can provide controlled delivery of hydroxychloroquine (HCQ) to successfully attenuate vaginal T lymphocyte activation and inflammation in a rabbit model as a potential strategy to induce an “immune quiescent” state within the FGT for the prevention of HIV infection. This biocompatible implant can deliver HCQ above therapeutic concentrations in a controlled manner, reduce submucosal immune cell recruitment, improve mucosal epithelium integrity, decrease protein and gene expression of T‐cell activation markers, and attenuate the induction of key pro‐inflammatory mediators. Our results suggest that microbicides designed to maintain a low level of immune activation at the FGT may offer a promising new strategy for reducing HIV infection.

Poster — Thur Eve — 51: An Investigation of Geometry Issues for EPID Dosimetry during Rotational IMRT

INTRODUCTION: Amorphous‐silicon electronic portal imaging devices(EPIDs) have been established as useful tools for dosimetry. To accurately reconstruct the patient dose delivered during rotational IMRT, one must acquire time‐resolved EPIDimages as a function of gantry‐angle. Dose reconstruction accuracy is directly impacted by the accuracy of the geometry of the imaging system, including the gantry‐angle readout (i.e. source geometry) and the EPID support‐arm sag (i.e. imager geometry). This work investigates these two factors. METHODS: The EPID support‐arm sag was investigated through measurements performed on Varian E‐arm and R‐arm models at two institutes and employing two different analysis methods. One method imaged an isocentric ball‐bearing whose position was tracked over all gantry‐angles. The second method involved analysing field edges to obtain the field centre location of all images. Gantry‐angle accuracy was examined by comparing the gantry‐angle indicated at the treatment console readout to the gantry‐angle written to the EPID DICOM header. We developed a method of measuring gantry‐angle directly from the gantry‐angle potentiometer. RESULTS: The E‐arm showed maximum displacement of roughly 0.6mm (cross‐plane) and 0.8mm (in‐plane). R‐arm results were significantly worse, estimated at 8.5mm (cross‐plane) and 5.0mm (in‐plane). Gantry‐angle analysis demonstrated approximately 2 degrees of uncertainty in the gantry‐angle contained in the EPIDimage. A direct measurement of the gantry angle potentiometer was demonstrated. CONCLUSIONS: Two main factors affecting patient dose reconstruction using EPIDdosimetry have been investigated. EPID support‐arm sag can be measured (and corrected). Near real‐time gantry‐angle measurement can be performed through directly monitoring the potentiometer signal.

The axolotl as an animal model for the comparison of 3-D ultrasound with plain film radiography

We assessed the usefulness of an animal model, the axolotl (Ambystoma mexicanum), in comparing 3-D ultrasound (3-D US) and plain film radiographs. Hindlimbs were amputated from 5 animals, at either the zeugopodial or stylopodial level, and each regenerating limb was imaged 16 times with 3-D US and 14 times with plain film X ray over 315 days. US images were acquired with a Siemens Sonoline Versa Pro and a 10-MHz linear array transducer. For 3-D US images, the probe was translated in a motor-driven linear stage while images were digitized. The regenerating tibia and fibula bones were detected on 3-D US an average of 37 days earlier than on plain film radiography, and regenerating phalangeal bones were detected on 3-D US an average of 18 days earlier. After 120 days, both imaging modalities consistently showed the bones. The average bone growth rates for the tibia and fibula were 0.019 +/- 0.001 mm/day and 0.019 +/- 0.001 mm/day, respectively.

Low-cost optical scanner and 3-dimensional printing technology to create lead shielding for radiation therapy of facial skin cancer: First clinical case series

Purpose Three-dimensional printing has been implemented at our institution to create customized treatment accessories, including lead shields used during radiation therapy for facial skin cancer. To effectively use 3-dimensional printing, the topography of the patient must first be acquired. We evaluated a low-cost, structured-light, 3-dimensional, optical scanner to assess the clinical viability of this technology. Methods and materials For ease of use, the scanner was mounted to a simple gantry that guided its motion and maintained an optimum distance between the scanner and the object. To characterize the spatial accuracy of the scanner, we used a geometric phantom and an anthropomorphic head phantom. The geometric phantom was machined from plastic and included hemispherical and tetrahedral protrusions that were roughly the dimensions of an average forehead and nose, respectively. Polygon meshes acquired by the optical scanner were compared with meshes generated from high-resolution computed tomography images. Most optical scans contained minor artifacts. Using an algorithm that calculated the distances between the 2 meshes, we found that most of the optical scanner measurements agreed with those from the computed tomography scanner within approximately 1 mm for the geometric phantom and approximately 2 mm for the head phantom. We used this optical scanner along with 3-dimensional printer technology to create custom lead shields for 10 patients receiving orthovoltage treatments of nonmelanoma skin cancers of the face. Patient, tumor, and treatment data were documented. Results Lead shields created using this approach were accurate, fitting the contours of each patients face. This process added to patient convenience and addressed potential claustrophobia and medical inability to lie supine. Conclusions The scanner was found to be clinically acceptable, and we suggest that the use of an optical scanner and 3-dimensional printer technology become the new standard of care to generate lead shielding for orthovoltage radiation therapy of nonmelanoma facial skin cancer.