Measured Lens Doses Research Articles

Evaluation of Radiation Protection Methods for Assistant Staff during CT Imaging in High-energy Trauma: Lens Dosimetry with a Phantom Study.

Staff are exposed to radiation in the scanning room when assisting with CT scans of patients requiring ventilatory support during procedures. We measured lens doses using a phantom during a high-energy trauma protocol. Dosimetry showed that the unprotected lens received 2.02 mGy on the right and 1.91 mGy on the left, which are not negligible doses. Respective exposures to the right and left lens were 53.6% and 55.1% when wearing 0.07 mm Pb protective glasses with side covers; 53.7% and 64.2% when wearing 0.7 mm Pb glasses without side covers when facing away from the patient couch; and 92.1% and 91.2% using protective shielding in the gantry. Since the face direction may change during assistance with CT imaging, it is desirable that the protective glasses have a shape with a side cover. The protective shielding had a major radiation reduction effect, although it is expensive to acquire, install, and maintain.

Verification of eye lens dose in IMRT by MOSFET measurement

The eye lens is recognized as one of the most radiosensitive structures in the human body. The widespread use of intensity-modulated radiotherapy (IMRT) complicates dose verification and necessitates high standards of dose computation. The purpose of this work was to assess the computed dose accuracy of eye lens through measurements using a metal–oxide–semiconductor field-effect transistor (MOSFET) dosimetry system. Sixteen clinical IMRT plans of head and neck patients were copied to an anthropomorphic head phantom. Measurements were performed using the MOSFET dosimetry system based on the head phantom. Two MOSFET detectors were imbedded in the eyes of the head phantom as the left and the right lens, covered by approximately 5-mm-thick paraffin wax. The measurement results were compared with the calculated values with a dose grid size of 1 mm. Sixteen IMRT plans were delivered, and 32 measured lens doses were obtained for analysis. The MOSFET dosimetry system can be used to verify the lens dose, and our measurements showed that the treatment planning system used in our clinic can provide adequate dose assessment in eye lenses. The average discrepancy between measurement and calculation was 6.7 ± 3.4%, and the largest discrepancy was 14.3%, which met the acceptability criterion set by the American Association of Physicists in Medicine Task Group 53 for external beam calculation for multileaf collimator-shaped fields in buildup regions.

Occupational radiation dose to eyes from interventional radiology procedures in light of the new eye lens dose limit from the International Commission on Radiological Protection.

In 2011, the International Commission on Radiological Protection (ICRP) recommended a substantial reduction in the equivalent dose limit for the lens of the eye, in line with a reduced threshold of absorbed dose for radiation-induced cataracts. This is of particular relevance in interventional radiology (IR) where it is well established that staff doses can be significant, however, there is a lack of data on IR eye doses in terms of Hp(3). Hp(3) is the personal dose equivalent at a depth of 3 mm in soft tissue and is used for measuring lens dose. We aimed to obtain a reliable estimate of eye dose to IR operators. Lens doses were measured for four interventional radiologists over a 3-month period using dosemeters specifically designed to measure Hp(3). Based on their typical workloads, two of the four interventional radiologists would exceed the new ICRP dose limit with annual estimated doses of 31 and 45 mSv to their left eye. These results are for an "unprotected" eye, and for IR staff who routinely wear lead glasses, the dose beneath the glasses is likely to be significantly lower. Staff eye dose normalized to patient kerma-area product and eye dose per procedure have been included in the analysis. Eye doses to IR operators have been established using a dedicated Hp(3) dosemeter. Estimated annual doses have the potential to exceed the new ICRP limit. We have estimated lens dose to interventional radiologists in terms of Hp(3) for the first time in an Irish hospital setting.

SU‐E‐J‐39: Effect of Jaw Size in MVCT On Image Quality and Dose in Helical Tomotherapy

Purpose: The jaw size for the TomoTherapy Hi‐Art II (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye. Methods: MVCT image quality (image noise, uniformity, contrast linearity, high‐contrast resolution, and full width at half‐maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements. Results: The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high‐contrast resolution. However, the full‐width at half‐maximum reduced from approximately 6.7 at J4 to 4.1 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69∼2.32 cGy (peripheral: 0.83∼2.49 cGy) at J4 to 0.85∼2.81 cGy (peripheral: 1.05∼2.86 cGy) at J1. The measured lens dose increased from 0.92∼3.36 cGy at J4 to 1.06∼3.91 cGy at J1. Conclusion: The change in jaw size can be expected to accelerate treatment planning and improve image registration accuracy. An additional dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in imaging dose over that with J4 should be provided with J1. This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (Grant No. 2009‐00420) and program of Basic Atomic Energy Research Institute (BAERI) which is a part of the Nuclear R and D programs (No. 2012000486) funded by the MEST.

Effect of jaw size in megavoltage CT on image quality and dose

Recently, the jaw size for the TomoTherapy Hi-Art II(®) (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal (IEC-Y) resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye, which is a highly radiosensitive tissue. MVCT image quality (image noise, uniformity, contrast linearity, high-contrast resolution, and full width at half-maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements. The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high-contrast resolution. However, the full-width at half-maximum reduced from approximately 7.2 at J4 to 4.5 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69-2.32 cGy (peripheral: 0.83-2.49 cGy) at J4 to 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) at J1. The measured lens dose increased from 0.92-3.36 cGy at J4 to 1.06-3.91 cGy at J1. The change in jaw size improved longitudinal resolution. The MVCT imaging dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in lens dose over that with J4 should be provided with J1.

Changes in Dose-Area Product, Entrance Surface Dose, and Lens Dose to the Radiologist in a Vascular Interventional Laboratory when an Old X-ray System Is Exchanged with a New System

The aim of this study was to compare dose-area product (DAP), entrance surface dose (ESD), and lens dose to radiologists for an old and a new X-ray system in a vascular interventional laboratory. DAP, ESD, fluoroscopy time, number of images, and patient weight were recorded for patients undergoing the following four procedures: percutaneous transluminal angioplasty (PTA) and stenting (divided into two subgroups, lower extremities and pelvis), nephrostomy, and treatment for varicocele. Halfway through the registration period, the 9-year-old X-ray equipment was exchanged with a new system. Lens doses to the radiologist were measured. There was a reduction in DAP for all procedures: PTA lower extremities 31% (12-8Gycm(2)), PTA/stenting pelvis 67% (134-44Gycm(2)), nephrostomy 39% (7-4Gycm(2)), and varicocele 70% (37-11 Gycm(2)). The reduction in number of images was 17% (158-131), 23% (153-118), 68% (2-1), and 31% (50-35), explaining a part of the dose reduction. The reduction in ESD was 33, 60, 38, and 46%. The differences in measured lens doses indicate a dose reduction in three procedures (19-53%) and an increase in one (56%), but differences are not statistically significant. DAP and ESD from the X-ray system were reduced for all procedures. The reduction was greater in the more radiation-demanding procedures.

Lens dose in MLC-based IMRT treatments of the head and neck

Purpose The objectives of this work are: ( 1) to determine typical dose to the lens during step-and-shoot intensity modulated radiotherapy (IMRT) treatments of the head and neck, and ( 2) to report on the dose calculation accuracy of a commercial inverse planning system in predicting lens dose. Methods and materials The Corvus inverse treatment planning system (Nomos, Cranberry Township, PA) was used to plan IMRT treatments for patients with head-and-neck cancers in our clinic. Patients were treated on Varian C-series linacs (Varian, Palo Alto, CA) with 4-MV or 6-MV X-rays. A Rando phantom (Alderson Laboratories, Stamford, CT) was specially modified to accommodate 1 × 1 × 1 mm 3 thermoluminescent dosimeters at the position of the lens. The IMRT treatment plans were then delivered to the modified Rando phantom. The thermoluminescent dosimeter measurements were converted to dose and taken as an estimate of the lens dose. A total of 20 cases were used in this study (15 cases with 4 MV and 5 cases with 6 MV). Results Expressed as a percentage of the prescription dose, the mean dose to the left and right lens for all 4-MV cases was 9.1% (range, 2.0% to 61.3%). For the 6-MV cases, the mean dose to the left and right lens was 12.8% (range, 3.6% to 41.3%). For both the 4-MV and 6-MV cases, the case of maximum dose occurred when the IMRT treatment target included volumes superior to the level of the lens. The field size and number of monitor units did not correlate with the measured lens dose. The only factor of significance affecting lens dose was the inferior-to-superior distance of the target to the lens. For target–lens distance ≥6 mm, the maximum measured lens doses were 5.9% and 9.0% relative to the prescribed dose for the 4-MV and 6-MV beams, respectively. These data are similar to those observed in conventional head-and-neck treatments. For all cases, the difference between the dose measured and that predicted by Corvus was less than 2% and 4% of the dose prescribed to the gross tumor volume for the 4-MV and 6-MV cases, respectively. Conclusion In IMRT, factors such as leaf leakage and number of monitor units play a secondary role and are not more significant than what is observed in conventional head-and-neck treatment when the lens is shielded by the collimator jaws. The target–lens distance is the parameter that affects the lens dose most strongly. For cases where the tumor is at or above the level of the lens, the lens dose can amount to an appreciable fraction of the prescription dose. To keep the lens dose to a minimum, noncoplanar beams that enter or exit into the lens should not be used.

Minimizing and measuring lens dose when giving cranial irradiation.

Three different techniques of administering cranial irradiation were used to determine the dose to the lens as measured in the Rando phantom. The techniques employed were as follows: (1) the central axis of the radiation beam was placed at the thickest portion of the cranium; (2) the central axis of the radiation beam was placed at the lateral orbital rim (bony canthus); (3) the central axis of the radiation beam was placed at the thickest portion of the cranium but with the beam angled 5 degrees posteriorly away from the eye. Thermal luminescent dosimeters (TLD) were placed in a phantom, at a point determined from a life-sized anatomical section of the plane through the midsection of the eye, to be at the location of the posterior capsule of the lens. In addition, TLDs were placed on the outer surface of the phantom head, directly lateral to the location determined to be where the lens would lie. With equally weighted lateral opposed beams, delivering a midplane dose of 200 cGy, the TLDs at the point of the lens measured 21, 9.9 and 10.6% of the midplane doses from the three techniques respectively. TLDs placed directly lateral to the lens on the surface of the phantom head gave an approximation of the lens dose, particularly when techniques 2 and 3 were used. Isodose curve generated by a General Electric treatment planning computer gave lens doses similar to those of the phantom data for each of the three different radiotherapy techniques. Cranial irradiation should be carried out by either technique 2 or technique 3 to minimize radiation dose to the lens.

Minimizing and measuring lens dose when giving cranial irradiation

Minimizing and measuring lens dose when giving cranial irradiation