M Islam

Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy

Kilovoltage cone-beam computerized tomography (kV-CBCT) systems integrated into the gantry of linear accelerators can be used to acquire high-resolution volumetric images of the patient in the treatment position. Using on-line software and hardware, patient position can be determined accurately with a high degree of precision and, subsequently, set-up parameters can be adjusted to deliver the intended treatment. While the patient dose due to a single volumetric imaging acquisition is small compared to the therapy dose, repeated and daily image guidance procedures can lead to substantial dose to normal tissue. The dosimetric properties of a clinical CBCT system have been studied on an Elekta linear accelerator (Synergy RP, XVI system) and additional measurements performed on a laboratory system with identical geometry. Dose measurements were performed with an ion chamber and MOSFET detectors at the center, periphery, and surface of 30 and 16-cm-diam cylindrical shaped water phantoms, as a function of x-ray energy and longitudinal field-of-view (FOV) settings of 5,10,15, and 26 cm. The measurements were performed for full 360 degrees CBCT acquisition as well as for half-rotation scans for 120 kVp beams using the 30-cm-diam phantom. The dose at the center and surface of the body phantom were determined to be 1.6 and 2.3 cGy for a typical imaging protocol, using full rotation scan, with a technique setting of 120 kVp and 660 mAs. The results of our measurements have been presented in terms of a dose conversion factor fCBCT, expressed in cGy/R. These factors depend on beam quality and phantom size as well as on scan geometry and can be utilized to estimate dose for any arbitrary mAs setting and reference exposure rate of the x-ray tube at standard distance. The results demonstrate the opportunity to manipulate the scanning parameters to reduce the dose to the patient by employing lower energy (kVp) beams, smaller FOV, or by using half-rotation scan.

The stability of mechanical calibration for a kV cone beam computed tomography system integrated with linear accelerator

The geometric accuracy and precision of an image-guided treatment system were assessed. Image guidance is performed using an x-ray volume imaging (XVI) system integrated with a linear accelerator and treatment planning system. Using an amorphous silicon detector and x-ray tube, volumetric computed tomography images are reconstructed from kilovoltage radiographs by filtered backprojection. Image fusion and assessment of geometric targeting are supported by the treatment planning system. To assess the limiting accuracy and precision of image-guided treatment delivery, a rigid spherical target embedded in an opaque phantom was subjected to 21 treatment sessions over a three-month period. For each session, a volumetric data set was acquired and loaded directly into an active treatment planning session. Image fusion was used to ascertain the couch correction required to position the target at the prescribed iso-center. Corrections were validated independently using megavoltage electronic portal imaging to record the target position with respect to symmetric treatment beam apertures. An initial calibration cycle followed by repeated image-guidance sessions demonstrated the XVI system could be used to relocate an unambiguous object to within less than 1 mm of the prescribed location. Treatment could then proceed within the mechanical accuracy and precision of the delivery system. The calibration procedure maintained excellent spatial resolution and delivery precision over the duration of this study, while the linear accelerator was in routine clinical use. Based on these results, the mechanical accuracy and precision of the system are ideal for supporting high-precision localization and treatment of soft-tissue targets.

Performance characteristics of a microMOSFET as an in vivo dosimeter in radiation therapy

The commercially available microMOSFET dosimeter was characterized for its dosimetric properties in radiotherapy treatments. The MOSFET exhibited excellent correlation with the dose and was linear in the range of 5-500 cGy. No measurable effect in response was observed in the temperature range of 20-40 degrees C. No significant change in response was observed by changing the dose rate between 100 and 600 monitor units (MU) min(-1) or change in the dose per pulse. A 3% post-irradiation fading was observed within the first 5 h of exposure and thereafter it remained stable up to 60 h. A uniform energy response was observed in the therapy range between 4 MV and 18 MV. However, below 0.6 MeV (Cs-132), the MOSFET response increased with the decrease in energy. The MOSFET also had a uniform dose response in 6-20 MeV electron beams. The directional dependence of MOSFET was within +/-2% for all the energies studied. The inherent build-up of the MOSFET was evaluated dosimetrically and found to have varying water equivalent thickness, depending on the energy and the side of the beam entry. At depth, a single calibration factor obtained by averaging the MOSFET response over different field sizes, energies, orientation and depths reproduced the ion chamber measured dose to within 5%. The stereotactic and the penumbral measurements demonstrated that the MOSFET could be used in a high gradient field such as IMRT. The study showed that the microMOSFET dosimeter could be used as an in vivo dosimeter to verify the dose delivery to the patient to within +/-5%.

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

PURPOSE To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. METHODS The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. RESULTS For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. CONCLUSIONS The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

Evaluation of the effect of patient dose from cone beam computed tomography on prostate IMRT using Monte Carlo simulation

The aim of this study is to evaluate the impact of the patient dose due to the kilovoltage cone beam computed tomography (kV-CBCT) in a prostate intensity-modulated radiation therapy (IMRT). The dose distributions for the five prostate IMRTs were calculated using the Pinnacle treatment planning system. To calculate the patient dose from CBCT, phase-space beams of a CBCT head based on the ELEKTA x-ray volume imaging system were generated using the Monte Carlo BEAMnr code for 100, 120, 130, and 140 kVp energies. An in-house graphical user interface called DOSCTP (DOSXYZnrc-based) developed using MATLAB was used to calculate the dose distributions due to a 360 degrees photon arc from the CBCT beam with the same patient CT image sets as used in Pinnacle. The two calculated dose distributions were added together by setting the CBCT doses equal to 1%, 1.5%, 2%, and 2.5% of the prescription dose of the prostate IMRT. The prostate plan and the summed dose distributions were then processed in the CERR platform to determine the dose-volume histograms (DVHs) of the regions of interest. Moreover, dose profiles along the x- and y-axes crossing the isocenter with and without addition of the CBCT dose were determined. It was found that the added doses due to CBCT are most significant at the femur heads. Higher doses were found at the bones for a relatively low energy CBCT beam such as 100 kVp. Apart from the bones, the CBCT dose was observed to be most concentrated on the anterior and posterior side of the patient anatomy. Analysis of the DVHs for the prostate and other critical tissues showed that they vary only slightly with the added CBCT dose at different beam energies. On the other hand, the changes of the DVHs for the femur heads due to the CBCT dose and beam energy were more significant than those of rectal and bladder wall. By analyzing the vertical and horizontal dose profiles crossing the femur heads and isocenter, with and without the CBCT dose equal to 2% of the prescribed dose, it was found that there is about a 5% increase of dose at the femur head. Still, such an increase in the femur head dose is well below the dose limit of the bone in our IMRT plans. Therefore, under these dose fractionation conditions, it is concluded that, though CBCT causes a higher dose deposited at the bones, there may be no significant effect in the DVHs of critical tissues in the prostate IMRT.

Applying usability heuristics to radiotherapy systems

BACKGROUND AND PURPOSE Heuristic evaluations have been used to evaluate safety of medical devices by identifying and assessing usability issues. Since radiotherapy treatment delivery systems often consist of multiple complex user-interfaces, a heuristic evaluation was conducted to assess the potential safety issues of such a system. MATERIAL AND METHODS A heuristic evaluation was conducted to evaluate the treatment delivery system at Princess Margaret Hospital (Toronto, Canada). Two independent evaluators identified usability issues with the user-interfaces and rated the severity of each issue. RESULTS The evaluators identified 75 usability issues in total. Eighteen of them were rated as high severity, indicating the potential to have a major impact on patient safety. A majority of issues were found on the record and verify system, and many were associated with the patient setup process. While the hospital has processes in place to ensure patient safety, recommendations were developed to further mitigate the risks of potential consequences. CONCLUSIONS Heuristic evaluation is an efficient and inexpensive method that can be successfully applied to radiotherapy delivery systems to identify usability issues and improve patient safety. Although this study was conducted only at one site, the findings may have broad implications for the design of these systems.

The use of human factors methods to identify and mitigate safety issues in radiation therapy

BACKGROUND AND PURPOSE New radiation therapy technologies can enhance the quality of treatment and reduce error. However, the treatment process has become more complex, and radiation dose is not always delivered as intended. Using human factors methods, a radiotherapy treatment delivery process was evaluated, and a redesign was undertaken to determine the effect on system safety. MATERIAL AND METHODS An ethnographic field study and workflow analysis was conducted to identify human factors issues of the treatment delivery process. To address specific issues, components of the user interface were redesigned through a user-centered approach. Sixteen radiation therapy students were then used to experimentally evaluate the redesigned system through a usability test to determine the effectiveness in mitigating use errors. RESULTS According to findings from the usability test, the redesigned system successfully reduced the error rates of two common errors (p<.04 and p<.01). It also improved the mean task completion time by 5.5% (p<.02) and achieved a higher level of user satisfaction. CONCLUSIONS These findings demonstrated the importance and benefits of applying human factors methods in the design of radiation therapy systems. Many other opportunities still exist to improve patient safety in this area using human factors methods.

EDR2 film dosimetry for IMRT verification using low-energy photon filters

Recently the EDR2 (extended dose range) film has been introduced commercially for applications in radiation therapy dosimetry. In addition to characterizing the wide dynamic range, several authors have reported a reduced energy dependence of this film compared to that of X-Omatic Verification (XV) films for megavoltage photon beams. However, those investigations were performed under limited geometrical conditions. We have investigated the dosimetric performance of EDR2 film for the verification of IMRT fields at more clinically relevant conditions by comparing the film doses with the doses measured with an ion chamber and XV films. The effects of using a low energy scattered photon filter on EDR2 film dosimetry was also studied. In contrast to previous reports our results show that EDR2 film still exhibits considerable energy dependence (a maximum discrepancy of 9%, compared with an ion chamber) at clinically relevant conditions (10 cm depth for IMRT fields). However, by using the low-energy filters the discrepancy is reduced to within 3%. Therefore, EDR2 film, in combination with the filters, is found to be a promising two-dimensional dosimeter for verification of IMRT treatment fields.

A simplified shielding approach for limiting fetal dose during radiation therapy of pregnant patients

PURPOSE To investigate the effectiveness of a simple and practical shielding device to reduce fetal dose for a patient undergoing radiation therapy of nasopharyngeal carcinoma. METHODS AND MATERIALS Using 5-cm-thick lead bricks and a heavy-duty steel cart, a 50 x 50-cm portable shield was designed and fabricated to reduce fetal dose due to collimator scatter and head leakage radiation. With the gantry at 90 degrees /270 degrees the shield can be easily positioned between the machine head and the fetus to reduce peripheral dose. Dose measurements for 6-MV X-rays and 9-MeV electrons have been made, utilizing a Rando phantom, to quantify the effect of the shield. RESULTS Measurements show that the peripheral dose to the fetus can be reduced by 60% when the simple shielding device is used.

Assessment and management of radiotherapy beam intersections with the treatment couch

The impact of the treatment couch on a radiotherapy plan is rarely fully assessed during the treatment planning process. Incorporating a couch model into the treatment planning system (TPS) enables the planner to avoid or dosimetrically evaluate beam‐couch intersections. In this work, we demonstrate how existing TPS tools can be used to establish this capability and assess the accuracy and effectiveness of the system through dose measurements and planning studies. Such capabilities may be particularly relevant for the planning of arc therapies. Treatment couch top models were introduced into a TPS by fusing their CT image sets with the patient CT dataset. Regions of interest characterizing couch elements were then imported and assigned appropriate densities in the TPS. Measurements in phantom agreed with TPS calculations to within 2% dose and 1° gantry rotation. To clinically validate the model, a retrospective study was performed on patient plans that posed difficulties in beam‐couch intersection during setup. Beam‐couch intersection caused up to a 3% reduction in PTV coverage, defined by the 95% of the prescribed dose, and up to a 1% reduction in mean CTV coverage. Dose compensation strategies for IMRT treatments with beams passing through couch elements were investigated using a four‐field IMRT plan with three beams passing through couch elements. In this study, ignoring couch effects resulted in point dose reductions of 8±3%. A methodology for incorporating detailed couch characteristics into a TPS has been established and explored. The method can be used to predict beam‐couch intersections during planning, potentially eliminating the need for pretreatment appointments. Alternatively, if a beam‐couch intersection problem arises, the impact of the couch can be assessed on a case‐by‐case basis and a clinical decision made based on full dosimetric information. PACS numbers: 87.53.Bn;87.55.Gh;87.55.de;87.56.nk