S Kim

Radiation dose from cone beam CT in a pediatric phantom: risk estimation of cancer incidence.

OBJECTIVE The objective of our study was to measure absorbed doses and calculate effective dose (ED) from cone beam CT (CBCT) with metal oxide semiconductor field effect transistor (MOSFET) detectors in an anthropomorphic phantom and to estimate the risk of cancer incidence for CBCT. MATERIALS AND METHODS Abdominal CBCT was performed in an anthropomorphic phantom of a 5-year-old child using the On-Board Imager with arbitrarily designated standard-dose (125 kVp, 80 mA, 25 milliseconds) and low-dose (125 kVp, 40 mA, 10 milliseconds) modes. The full-fan mode was used, and 20 MOSFET dosimeters were used to measure the absorbed doses in various organs. We calculated the ED, the lifetime attributable risk (LAR) for cancer incidence, and relative risk (RR) of cancer induction from a single scan for both standard- and low-dose modes in 5-year-old children. RESULTS The highest absorbed doses were found in the skin, ascending colon, and stomach. The mean ED was 37.8+/-0.7 (SD) mSv for the standard-dose mode and 8.1+/-0.2 mSv for the low-dose mode. The LAR of cancer incidence ranged from 23 to 144 cases per 100,000 exposed persons for the standard-dose mode and from five to 31 cases per 100,000 exposed persons for the low-dose mode. The RR of cancer incidence ranged from 1.003 to 1.054 for the standard-dose mode and from 1.001 to 1.012 for the low-dose mode. CONCLUSION The ED from pediatric CBCT using the standard-dose mode was considerably higher than that of MDCT, whereas the ED for CBCT using the low-dose mode was comparable to that of abdominal MDCT. For abdominal CBCT in the pediatric phantom, the highest LARs were for colon and bladder cancers and the highest RRs were for stomach and liver cancers.

Computed tomography dose index and dose length product for cone‐beam CT: Monte Carlo simulations of a commercial system

Dosimetry in kilovoltage cone beam computed tomography (CBCT) is a challenge due to the limitation of physical measurements. To address this, we used a Monte Carlo (MC) method to estimate the CT dose index (CTDI) and the dose length product (DLP) for a commercial CBCT system. As Dixon and Boone (1) showed that CTDI concept can be applicable to both CBCT and conventional CT, we evaluated weighted CT dose index (CTDIw) and DLP for a commercial CBCT system. Two extended CT phantoms were created in our BEAMnrc/EGSnrc MC system. Before the simulations, the beam collimation of a Varian On‐Board Imager (OBI) system was measured with radiochromic films (model: XR‐QA). The MC model of the OBI X‐ray tube, validated in a previous study, was used to acquire the phase space files of the full‐fan and half‐fan cone beams. Then, DOSXYZnrc user code simulated a total of 20 CBCT scans for the nominal beam widths from 1 cm to 10 cm. After the simulations, CBCT dose profiles at center and peripheral locations were extracted and integrated (dose profile integral, DPI) to calculate the CTDI per each beam width. The weighted cone‐beam CTDI (CTDIw,l) was calculated from DPI values and mean CTDIw,l(CTDIw,l)¯ and DLP were derived. We also evaluated the differences of CTDIw values between MC simulations and point dose measurements using standard CT phantoms. In results, it was found that CTDIw,600¯ was 8.74±0.01 cGy for head and CTDIw,900¯ was 4.26±0.01 cGy for body scan. The DLP was found to be proportional to the beam collimation. We also found that the point dose measurements with standard CT phantoms can estimate the CTDI within 3% difference compared to the full integrated CTDI from the MC method. This study showed the usability of CTDI as a dose index and DLP as a total dose descriptor in CBCT scans. PACS number: 87.57.uq

Comparison of radiation doses between cone beam CT and multi detector CT: TLD measurements

In recent years, wide beam computed tomography (CT) technique has become standard in radiation oncology whereas there is little information about radiation dose assessments for the technique. A point dose measurement method was employed to assess the radiation doses of cone beams CT (CBCT) and multi detectors CT (MDCT). The radiation doses of both modalities were measured using thermoluminescence dosemeters (TLDs) in head and body CT phantoms. Four TLD chips were placed at the centre and each peripheral channel to measure the doses. From the measurements, the weighted CT dose index for CBCT (CTDI(w)(CBCT)) and volume CT dose index for MDCT (CTDI(vol)(MDCT)) were derived. In the results, the CTDI(w)(CBCT) was 89.7 +/- 4.0 mGy and the CTDI(vol)(MDCT) was 137.0 +/- 7.4 mGy for the head scan. For the body scan, they were 37.9 +/- 1.4 and 74.3 +/- 5.3 mGy, respectively. In conclusion, CTDI(w)(CBCT) for the head scan was 35% lower than CTDI(vol)(MDCT), and CTDI(w)(CBCT) for the body scan was also 49% lower than CTDI(vol)(MDCT).

Bismuth shielding in CT: support for use in children

Since 1993, the number of CT examination has been dramatically increasing by approximately 10% per year [1]. For all ages, up to 62 million CT examinations were performed in the United States during 2006 and the radiation dose from CT accounts for nearly half of the total ionizing radiation dose from medical imaging. Among the total CT examinations performed from 2004 to 2006, approximately 10% of the procedures were in children [1]. Children have a higher risk of cancer from radiation than adults for several well-recognized reasons [2], especially longer life expectancy. Additionally, radiation to organs such as breast, eye lens and thyroid are of concern during CT. For these reasons, strategies to optimize radiation dose to these radiosensitive organs are critical in this population. Recognized radiation dose reduction methods for CT include reducing tube current time product (mAs), using tube current modulation, reducing peak-voltage (kVp), using relatively higher pitches, and limiting both scan regions and multiphase examinations. In addition, shielding of radiosensitive organs in the region of scanning (in-plane shielding), generally with 4-ply (adult or older children) or 2-ply (young children) bismuth material, has been advocated. A bismuth shield primarily functions to remove the lower-energy photons that deposit energy causing ionizations in tissue; these low-energy photons do not contribute to image formation but do contribute to dose. However, the use of bismuth shielding in children depends on several considerations: examination type, patient’s body habitus, position and number of bismuth layers of the shield, and cost-effectiveness. These are in addition to data regarding dose reduction versus image quality (Table 1). In pediatric phantoms and patients, Mukundan et al. [3] reported a 42% lens dose reduction when using a 2-ply bismuth shield for axial brain and helical craniofacial pediatric CT protocols. Fricke et al. [4] found that bismuth breast shields for pediatric CT reduce dose by 29% in anthropomorphic phantoms, with no significant reduction in image quality as measured by image noise. Coursey et al. [5] assessed the dose reduction of the automatic tube current modulation with bismuth shields and reported a dose reduction of 52% when the shield was employed after the acquisition of scout view using the automatic tube current modulation in a chest MDCT protocol. Perisinakis et al. [14] also investigated the orbital bismuth shielding technique in children and found the average eye dose reduction of 38% and 33% for CT scans of the orbit and whole head in their Monte Carlo simulations as well as 34% and 20% for the entire and partial eye globe scans in pediatric patients, respectively. All the above studies state that the use of the bismuth shield did not significantly increase the image noise clinically at the region of interest. However, the use or routine use of these shields has been called into question, particularly by Geleijns et al. [12], Kalra et al. [9] and Vollmar and Kalender [13]. Geleijns et al. and Vollmar et al. [13] reported that the dose reduction can be better achieved with tube current reduction alone with relatively less impact on image quality [12]. Their findings were supported by Kalra et al. [9], indicating that the reduction in dose using bismuth shields was less than expected based on measured image noise. These three groups of investigators concluded that use of reduction in or modulation of tube S. Kim Medical Physics Graduate Program, DukeUniversityMedical Center, Durham, NC, USA

Clinical commissioning and use of the Novalis Tx linear accelerator for SRS and SBRT

The purpose of this study was to perform comprehensive measurements and testing of a Novalis Tx linear accelerator, and to develop technical guidelines for commissioning from the time of acceptance testing to the first clinical treatment. The Novalis Tx (NTX) linear accelerator is equipped with, among other features, a high‐definition MLC (HD120 MLC) with 2.5 mm central leaves, a 6D robotic couch, an optical guidance positioning system, as well as X‐ray‐based image guidance tools to provide high accuracy radiation delivery for stereotactic radiosurgery and stereotactic body radiation therapy procedures. We have performed extensive tests for each of the components, and analyzed the clinical data collected in our clinic. We present technical guidelines in this report focusing on methods for: (1) efficient and accurate beam data collection for commissioning treatment planning systems, including small field output measurements conducted using a wide range of detectors; (2) commissioning tests for the HD120 MLC; (3) data collection for the baseline characteristics of the on‐board imager (OBI) and ExacTrac X‐ray (ETX) image guidance systems in conjunction with the 6D robotic couch; and (4) end‐to‐end testing of the entire clinical process. Established from our clinical experience thus far, recommendations are provided for accurate and efficient use of the OBI and ETX localization systems for intra‐ and extracranial treatment sites. Four results are presented. (1) Basic beam data measurements: Our measurements confirmed the necessity of using small detectors for small fields. Total scatter factors varied significantly (30% to approximately 62%) for small field measurements among detectors. Unshielded stereotactic field diode (SFD) overestimated dose by ~ 2% for large field sizes. Ion chambers with active diameters of 6 mm suffered from significant volume averaging. The sharpest profile penumbra was observed for the SFD because of its small active diameter (0.6 mm). (2) MLC commissioning: Winston Lutz test, light/radiation field congruence, and Picket Fence tests were performed and were within criteria established by the relevant task group reports. The measured mean MLC transmission and dynamic leaf gap of 6 MV SRS beam were 1.17% and 0.36 mm, respectively. (3) Baseline characteristics of OBI and ETX: The isocenter localization errors in the left/right, posterior/anterior, and superior/inferior directions were, respectively, −0.2±0.2 mm, −0.8±0.2 mm, and −0.8±0.4 mm for ETX, and 0.5±0.7 mm, 0.6±0.5 mm, and 0.0±0.5 mm for OBI cone‐beam computed tomography. The registration angular discrepancy was 0.1±0.2°, and the maximum robotic couch error was 0.2°. (4) End‐to‐end tests: The measured isocenter dose differences from the planned values were 0.8% and 0.4%, measured respectively by an ion chamber and film. The gamma pass rate, measured by EBT2 film, was 95% (3% DD and 1 mm DTA). Through a systematic series of quantitative commissioning experiments and end‐to‐end tests and our initial clinical experience, described in this report, we demonstrate that the NTX is a robust system, with the image guidance and MLC requirements to treat a wide variety of sites — in particular for highly accurate delivery of SRS and SBRT‐based treatments. PACS numbers: 87.55.Qr, 87.53.Ly, 87.59.‐e

Application of MOSFET Detectors for Dosimetry in Small Animal Radiography Using Short Exposure Times

Abstract Lin, M. D., Toncheva, G., Nguyen, G., Kim, S., Anderson-Evans, C., Johnson, G. A. and Yoshizumi, T. T. Application of MOSFET Detectors for Dosimetry in Small Animal Radiography Using Short Exposure Times. Radiat. Res. 170, 260– 263 (2008). Digital subtraction angiography (DSA) X-ray imaging for small animals can be used for functional phenotyping given its ability to capture rapid physiological changes at high spatial and temporal resolution. The higher temporal and spatial requirements for small-animal imaging drive the need for short, high-flux X-ray pulses. However, high doses of ionizing radiation can affect the physiology. The purpose of this study was to verify and apply metal oxide semiconductor field effect transistor (MOSFET) technology to dosimetry for small-animal diagnostic imaging. A tungsten anode X-ray source was used to expose a tissue-equivalent mouse phantom. Dose measurements were made on the phantom surface and interior. The MOSFETs were verified with thermoluminescence dosimeters (TLDs). Bland-Altman analysis showed that the MOSFET results agreed with the TLD results (bias, 0.0625). Using typical small animal DSA scan parameters, the dose ranged from 0.7 to 2.2 cGy. Application of the MOSFETs in the small animal environment provided two main benefits: (1) the availability of results in near real-time instead of the hours needed for TLD processes and (2) the ability to support multiple exposures with different X-ray techniques (various of kVp, mA and ms) using the same MOSFET. This MOSFET technology has proven to be a fast, reliable small animal dosimetry method for DSA imaging and is a good system for dose monitoring for serial and gene expression studies.

Kerma Area Product Method for Effective Dose Estimation During Lumbar Epidural Steroid Injection Procedures: Phantom Study

OBJECTIVE The purpose of this study was to derive from the kerma area product the dose conversion coefficient for estimating the effective dose for lumbar epidural steroid injection procedures. MATERIALS AND METHODS A mobile fluoroscopy system was used for fluoroscopic imaging guidance of lumbar epidural steroid injection procedures. For acquisition of organ dose measurements, 20 diagnostic metal oxide semiconductor field effect transistor detectors were placed at each organ in an anthropomorphic phantom of a man, and these detectors were attached to four mobile metal oxide semiconductor field effect transistor wireless bias supplies to obtain the organ dose readings. The kerma area product was recorded from the system console and independently validated with an ion chamber and therapeutic x-ray film. Fluoroscopy was performed on the phantom for 10 minutes for acquisition of the dose rate for each organ, and the average clinical procedure time was multiplied by each organ dose rate for acquisition of individual organ doses. The effective dose was computed by summing the product of each organ dose and the corresponding tissue weighting factor from International Commission on Radiologic Protection publication 60. RESULTS The effective dose was computed as 0.93 mSv for an average lumbar epidural steroid injection procedure (fluoroscopic time, 40.7 seconds). The corresponding kerma area product was 2.80 Gy.cm(2). The dose conversion coefficient was derived as 0.33 mSv/(Gy.cm(2)). CONCLUSION The effective dose for lumbar epidural steroid injection can be easily estimated by multiplying the derived dose conversion coefficient by the console-displayed kerma area product.

Estimation of absorbed doses from paediatric cone-beam CT scans: MOSFET measurements and Monte Carlo simulations

The purpose of this study was to establish a dose estimation tool with Monte Carlo (MC) simulations. A 5-y-old paediatric anthropomorphic phantom was computed tomography (CT) scanned to create a voxelised phantom and used as an input for the abdominal cone-beam CT in a BEAMnrc/EGSnrc MC system. An X-ray tube model of the Varian On-Board Imager((R)) was built in the MC system. To validate the model, the absorbed doses at each organ location for standard-dose and low-dose modes were measured in the physical phantom with MOSFET detectors; effective doses were also calculated. In the results, the MC simulations were comparable to the MOSFET measurements. This voxelised phantom approach could produce a more accurate dose estimation than the stylised phantom method. This model can be easily applied to multi-detector CT dosimetry.

Dosimetric characterisation of bismuth shields in CT: measurements and Monte Carlo simulations

Although bismuth shields are frequently used in radiology to reduce radiation dose, its mechanism has not been fully investigated. Dosimetric characteristics of bismuth shields in computed tomography (CT) were assessed with ion chamber and Monte Carlo (MC) simulations. Primary attenuation and backscatter effects of paediatric (2-ply) and adult (4-ply) bismuth shields were measured. Simulated CT beams were used for ion chamber measurements. Radiation doses were measured free-in-air and in the tissue-equivalent slabs. MC simulations for the same settings were also performed. Mean dose reductions from primary attenuation were 23% (2-ply) and 40% (4-ply). The dose increase from backscatter was 2% for both shields. MC simulations for primary beam dose reduction were 20% (2-ply) and 38% (4-ply); the backscatter dose increase was around 6% for both shields. In summary, primary attenuation is the major factor that introduces the dose reduction in bismuth and the dose increase from backscatter is negligible.

Transient Activation of Hedgehog Pathway Rescued Irradiation-Induced Hyposalivation by Preserving Salivary Stem/Progenitor Cells and Parasympathetic Innervation

Purpose: To examine the effects and mechanisms of transient activation of the Hedgehog pathway on rescuing radiotherapy-induced hyposalivation in survivors of head and neck cancer. Experimental Design: Mouse salivary glands and cultured human salivary epithelial cells were irradiated by a single 15-Gy dose. The Hedgehog pathway was transiently activated in mouse salivary glands, by briefly overexpressing the Sonic hedgehog (Shh) transgene or administrating smoothened agonist, and in human salivary epithelial cells, by infecting with adenovirus encoding Gli1. The activity of Hedgehog signaling was examined by the expression of the Ptch1-lacZ reporter and endogenous Hedgehog target genes. The salivary flow rate was measured following pilocarpine stimulation. Salivary stem/progenitor cells (SSPC), parasympathetic innervation, and expression of related genes were examined by flow cytometry, salisphere assay, immunohistochemistry, quantitative reverse transcription PCR, Western blotting, and ELISA. Results: Irradiation does not activate Hedgehog signaling in mouse salivary glands. Transient Shh overexpression activated the Hedgehog pathway in ductal epithelia and, after irradiation, rescued salivary function in male mice, which is related with preservation of functional SSPCs and parasympathetic innervation. The preservation of SSPCs was likely mediated by the rescue of signaling activities of the Bmi1 and Chrm1–HB-EGF pathways. The preservation of parasympathetic innervation was associated with the rescue of the expression of neurotrophic factors such as Bdnf and Nrtn. The expression of genes related with maintenance of SSPCs and parasympathetic innervation in female salivary glands and cultured human salivary epithelial cells was similarly affected by irradiation and transient Hedgehog activation. Conclusions: These findings suggest that transient activation of the Hedgehog pathway has the potential to restore salivary gland function after irradiation-induced dysfunction. Clin Cancer Res; 20(1); 140–50. ©2013 AACR.