R Barnett

Dose assessment from an online kilovoltage imaging system in radiation therapy

We have investigated the dosimetric properties of a commercial kilovoltage cone beam computerised tomography (kV-CBCT) system. The kV-CBCT doses were measured in 16 and 32 cm diameter standard cylindrical Perspex computerised tomography (CT) and Rando anthropomorphic phantoms using 125 kVp and 1.0-2.0 mA s per projection. We also measured skin doses using thermoluminescence dosimeters placed on the skin surfaces of prostate cancer patients undergoing kV-kV image matching for daily set-up. The skin doses from kV-kV image matching of prostate cancer patients on the anterior and lateral skin surfaces ranged from 0.03 +/- 0.01 to 0.64 +/- 0.01 cGy depending on the beam filtration and technique factors employed. The mean doses on the Rando phantom ranged from 3.0 +/- 0.1 to 5.1 +/- 0.3 cGy for full-fan scans and from 3.8 +/- 0.1 to 6.6 +/- 0.2 cGy for half-fan scans using 125 kVp and 2 mA s per projection. The isocentre cone beam dose index (CBDI) in the 16 and 32 cm Perspex phantoms is 4.65 and 1.81 cGy, respectively (using a 0.6 cm(3) Capintec PR06C Farmer chamber) for full-fan scans, and the corresponding normalised CBDIs are 0.72 and 0.28 cGy/100 mA s, respectively. The mean weighted CBDIs are 4.93 and 2.14 cGy, and the normalised weighted CBDIs are 0.76 and 0.33 cGy/100 mA s for the 16 and 32 cm phantoms, respectively (full-fan scans). The normalised weighted CBDI for the half-fan scan is 0.41 cGy/100 mA s for the 32 cm diameter phantom. All measurements of the CBDI using the 0.6 cm(3) Farmer chamber are within 2-5% of measurements taken with the 100 mm CT chamber. The CBDI technique and definitions can be used to benchmark CBCT systems and to provide estimates of imaging doses to patients undergoing on-board imager (OBI)/CBCT image guided radiation therapy.

Design and evaluation of quantum dot sensors for making superficial x-ray energy radiation measurements.

The extraordinary physical properties of quantum dot (QD) materials such as high radiation sensitivity and good radiation resistivity indicate their potential for use in the fabrication of radiation sensors. This paper reports the design and fabrication of two kinds of radiation sensors based on ZnO and CdTe QDs. Both sensors are characterized using a Gulmay Medical D3000 DXR unit for superficial x-ray irradiation with source photon energies that range from 36.9 to 64.9 keV. The QD radiation sensors exhibit excellent linearity with respect to different photon energy doses, radiation source to device surface distances, and field sizes. The effects of the electrode separation and the area density of the QD layer are also investigated. All sensors characterized show an outstanding repeatability under photon irradiation, with a signal variation less than 1%.

Software for the estimation of organ equivalent and effective doses from diagnostic radiology procedures

Diagnostic radiological imaging such as conventional radiography, fluoroscopy and computed tomography (CT) examinations will continue to provide tremendous benefits in modern healthcare. The benefit derived by the patient should far outweigh the risk associated with a properly conducted imaging examination. Nonetheless, it is very important to be able to quantify the risk associated with any radiological examination of patients, and effective dose has been considered a useful indicator of patient exposure. Quantification of the risks associated with radiological imaging is very important as such information will be helpful to physicians and their patients for comparing risks from various imaging examinations and for making informed decisions whenever there is a need for any radiological imaging. The determination of equivalent and effective doses in diagnostic radiology is of interest as a basis for estimates of risk from medical exposures. In this paper we describe a simple computer program OrgDose, which calculates the doses to 27 organs in the body and then calculates the organ equivalent and effective doses and the risk from various procedures in the radiology department including conventional radiography, fluoroscopy and computed tomography examinations. The program will be a useful tool for the medical and paramedical personnel who are involved with assessing organ and effective doses and risks from diagnostic radiology procedures.

A carbon nanotube-based radiation sensor

Dosimetric measurements and monitoring play an essential role in radiotherapy. Because of their sensitivity and relatively flat energy response ionization chambers remain the most important dosimeters. However, ionization chambers usually have large physical dimensions and require high bias voltages to achieve acceptable ionization collection efficiency. Such disadvantages limit their applications for in vivo dose measurements. The availability of novel materials such as carbon nanotubes (CNTs) has created the potential to miniaturize traditional ionization chambers and lower the bias voltages. This paper describes a new CNT-based radiation sensor. In the first stage, characteristics of the sensor were examined with two stainless steel electrodes. The sensor displayed excellent linear responses to exposure and showed accurate responses to oblique incident beam measurements. These experimental results showed that the prototype sensor is suitable for studying the ionization collection efficiency of CNTs. In the second stage, square- and irregular-shaped CNTs electrodes were designed. Saturation characteristics of the sensor with the CNTs electrodes were measured. Experimental results and ongoing work are presented and discussed in this paper.

Margin selection to compensate for loss of target dose coverage due to target motion during external-beam radiation therapy of the lung.

The aim of this study is to provide guidelines for the selection of external‐beam radiation therapy target margins to compensate for target motion in the lung during treatment planning. A convolution model was employed to predict the effect of target motion on the delivered dose distribution. The accuracy of the model was confirmed with radiochromic film measurements in both static and dynamic phantom modes. 502 unique patient breathing traces were recorded and used to simulate the effect of target motion on a dose distribution. A 1D probability density function (PDF) representing the position of the target throughout the breathing cycle was generated from each breathing trace obtained during 4D CT. Changes in the target D95 (the minimum dose received by 95% of the treatment target) due to target motion were analyzed and shown to correlate with the standard deviation of the PDF. Furthermore, the amount of target D95 recovered per millimeter of increased field width was also shown to correlate with the standard deviation of the PDF. The sensitivity of changes in dose coverage with respect to target size was also determined. Margin selection recommendations that can be used to compensate for loss of target D95 were generated based on the simulation results. These results are discussed in the context of clinical plans. We conclude that, for PDF standard deviations less than 0.4 cm with target sizes greater than 5 cm, little or no additional margins are required. Targets which are smaller than 5 cm with PDF standard deviations larger than 0.4 cm are most susceptible to loss of coverage. The largest additional required margin in this study was determined to be 8 mm. PACS numbers: 87.53.Bn, 87.53.Kn, 87.55.D‐, 87.55.Gh

A Customized Radiation Sensor for Ionization Collection

The measurement of absorbed doses is fundamental to radiation biology and oncology. A customized parallel plate radiation sensor was designed and fabricated as a precursor to investigating novel materials, such as carbon nanotubes, as a substitute for conventional metallic conducting plates or active volume medium. This sensor contains two thick and large-area electrodes that provide the sensor with a good signal-to-noise ratio. The 6 MV and 15 MV photon beams produced by a Varian Clinac 21 EX medical linear accelerator were used in the experiments. The linear accelerator was calibrated such that 1 monitor unit (MU) produces 1 cGy of dose in water with depth of 5 cm for a calibration geometry of source-to-axis distance equal to 100 cm and 10times10 cm2 field size at the point of measurement. Ionization measurements were performed by varying the bias voltages, electrode separations, exposures, and angles of the incident beam to characterize the sensor. Signal saturation characteristics of the sensor with different electrode separations and exposures were investigated. This sensor displayed excellent linear response to exposure up to 600 MU. An analytical modeling using the pencil beam model and simulations based on device configuration were given to explain the results. In oblique incident beam experiments, the prototype sensor showed an accurate response compared to simulation results for a small field size of 1times1 cm2. The sensor was tested to be suitable in the study of ionization collection efficiencies for different materials

Sci‐AM1 Sat ‐ 06: Improved absorbed dose calculations incorporating internal organ motion

The goal of radiation therapy is to deliver a highly conformal dose to a prescribed target volume and to spare surrounding healthy tissue as much as possible. Present commercial dose planning systems assume that patients anatomy is static over the course of treatment. During treatmentdelivery, however, dosimetric uncertainties arising from patient repositioning and internal organ motion are unavoidable practically. The purpose of this study is to evaluate the effect of prostate motion on the physical dose distribution by PTV ¯ 7 model based on the Pinnacle treatment planning system. Prostate motion, within the PTV, was represented by a weighted average of seven individually shifted PTVs ( PTV ¯ 7 ). As already well known, internal organ motion always leads to blurred contour surfaces. The dose coverage of PTV and critical organs is less as indicated by dose at “edge” of contours and also by decreased DVH particularly at high dose region. The averaged decrease of TCP between the static planning and PTV ¯ 7 model is 2.9%, and the rectum is spared if motion is equally weighted and symmetric. The PTV ¯ 7 configurations yield a better estimate of the actual dose in the rectal wall with decreasing NTCP. The effects of different shifting weight to TCP and NTCP in L‐R, A‐P and S‐I directions were also quantitatively analyzed. The calculation of the cumulative dose incorporating internal organ motion plays an important role in pursuing adaptive radiation therapy and dose escalation for IMRT with the goal of decreasing the dosedelivered to the normal critical structures.

Sci‐AM2 Sat ‐ 06: IMRT prostate planning‐determination of the minimum MU/segment

For step and shoot IMRT, the combination of high dose rate, multiple beam segments and low dose per segment can lead to significant differences between the planned and delivered dose to the patient. This problem, known as an “overshoot” effect, is the result of current dose servo limitations and is demonstrated by over‐ and under‐dose in the first and the last segment respectively. Segment dose inaccuracy in the range of 10 to 60% of the both segments for 1 monitor unit (MU) per segment irradiated with dose rate (DR) of 100 to 600 MU/min was measured. The object of this study was to find a method for segment dose correction when small MU and high DR are used, and specify the prostate IMRT planning limits for MU/segment and the DR. The reported results were obtained using Pinnacle3‐V6 and Varian Clinac 2100 EX linear accelerator equipped with a 120‐leaf millennium MLC. The methodology for correcting dose employs small field segment dose ratio. The segment dose error after correction was measured to be less than 5 % for all dose rates. For prostate step and shoot IMRT a low limit of 1 MU per segment and DR with upper limit of 600 MU/min can be used. The results of this work relate to the agreement between planned and delivered doses of the prostate IMRT.

Design and evaluation of quantum dot sensors for making superficial x-ray energy radiation measurements

The extraordinary physical properties of quantum dot (QD) materials such as high radiation sensitivity and good radiation resistivity indicate their potential for use in the fabrication of radiation sensors. This paper reports the design and fabrication of two kinds of radiation sensors based on ZnO and CdTe QDs. Both sensors are characterized using a Gulmay Medical D3000 DXR unit for superficial x-ray irradiation with source photon energies that range from 36.9 to 64.9 keV. The QD radiation sensors exhibit excellent linearity with respect to different photon energy doses, radiation source to device surface distances, and field sizes. The effects of the electrode separation and the area density of the QD layer are also investigated. All sensors characterized show an outstanding repeatability under photon irradiation, with a signal variation less than 1%.

SU‐E‐T‐601: Patient Specific Margin Selection to Compensate for Intrafraction Motion during External Beam Radiation Therapy of the Lung

Purpose: Attaining a positive outcome from external beam radiation therapy (RT) is heavily dependent on delivering the prescribed radiationdose to the intended structures. RT of lungcancers is complicated by target motion caused by the patients natural breathing during treatmentdelivery, commonly known as ‘intrafraction motion’. This target motion must be managed to help ensure successful treatment. Methods: In order to accommodate target motion on a patient specific basis, we have employed a convolution model to predict the ‘blurred’ dose distribution delivered in the presence of known target motion. The convolution model requires two inputs: the planned ‘static’ dose distribution and a probability distribution function (PDF) describing the position of the target over the course of a breathing cycle. The model was experimentally verified by Gafchromic EBT2 film measurements. In a simulation study, 502 unique patient breathing traces were used to generate corresponding blurred dose distributions. Results: Analysis of the differences between the static and blurred dose distributions with respect to characteristics of the PDFs revealed strong trends between dose coverage metrics (percent mean dose difference and penumbral width) and features of the PDFs (amplitude, standard deviation, magnitude of maximum gradient and location of maximum gradient). In particular the magnitude of the maximum PDF gradient showed a clear inverse relationship with penumbral width, while the location of the maximum PDF gradient (measured relative to the PDFs geometric center) has a clear positive correlation with penumbral width. Conclusion: The convolution model has been verified for intrafraction target motion and analysis of changes present in the blurred dose distributions highlighted trends which can be used by physicians to guide target margin selection on a patient specific basis.