Assessment of patient organ doses and effective doses using the VIP-Man adult male phantom for selected cone-beam CT imaging procedures during image guided radiation therapy

Development of scalable lymphatic system in the 4D XCAT phantom: Application to quantitative evaluation of lymphoma PET segmentations

BACKGROUND

Digital anthropomorphic phantoms, such as the 4D extended cardiac-torso (XCAT) phantom, are actively used to develop, optimize, and evaluate a variety of imaging applications, allowing for realistic patient modeling and knowledge of ground truth. The XCAT phantom defines the activity and attenuation for a simulated patient, which includes a complete set of organs, muscle, bone, and soft tissue, while also accounting for cardiac and respiratory motion. However, the XCAT phantom does not currently include the lymphatic system, critical for evaluating medical imaging tasks such as sentinel node detection, node density measurement, and radiation dosimetry.

PURPOSE

In this study, we aimed to develop a scalable lymphatic system in the XCAT phantom, to facilitate improved research of the lymphatic system in medical imaging. Using this scalable lymphatic system, we modeled the lymph node conglomerate pathology that is characteristically observed in primary mediastinal B-cell lymphoma (PMBCL). As an extended application, we evaluated positron emission tomography (PET) image quantification of metabolic tumor volume (MTV) and total lesion glycolysis (TLG) of these simulated lymphomas, though the phantoms may be applied to other imaging modalities and study design paradigms (e.g., image quality, detection).

METHODS

A template model for the lymphatic system was developed based on anatomical data from the Visible Human Project of the National Library of Medicine. The segmented nodes and vessels were fit with non-uniform rational basis spline surfaces, and multichannel large deformation diffeomorphic metric mapping was used to propagate the template to different XCAT anatomies. To model conglomerates observed in PMBCL, lymph nodes were enlarged, converged within the mediastinum, and tracer concentration was increased. We used the phantoms as inputs to a PET simulation tool, which generated images using ordered subsets expectation maximization reconstruction with 2-8 mm Gaussian filters. Fixed thresholding (FT) and gradient segmentation were used to determine MTV and TLG. Percent bias (%Bias) and coefficient of variation (COV) were computed as measures of accuracy and precision, respectively, for each MTV and TLG measurement.

RESULTS

Using the methodology described above, we introduced a scalable lymphatic system in the XCAT phantom, which allows for the radioactivity and attenuation ground truth to be generated in 116 ± 2.5 s using a 2.3 GHz processor. Within the Rhinoceros interface, lymph node anatomy and function were modified to create a cohort of 10 phantoms with lymph node conglomerates. Using the lymphoma phantoms to evaluate PET quantification of MTV, mean %Bias values were -9.3%, -41.3%, and 20.9%, while COV values were 4.08%, 7.6%, and 3.4% using 25% FT, 40% FT, and gradient segmentations, respectively. Comparatively for TLG, mean %Bias values were -27.4%, -45.8%, and -16.0%, while COV values were 1.9%, 5.7%, and 1.4%, for the 25% FT, 40% FT, and gradient segmentations, respectively.

CONCLUSIONS

In this work, we upgraded the XCAT phantom to include a lymphatic system, comprised of a network of 276 scalable lymph nodes and corresponding vessels. As an application, we created a cohort of phantoms with lymph node conglomerates to evaluate lymphoma quantification in PET imaging, which highlights an important application of this work.

Dose estimation for cone-beam computed tomography in image-guided radiation therapy using mesh-type reference computational phantoms and assuming head and neck cancer

This study aimed to estimate the additional dose the cone-beam computed tomography (CBCT) system integrated into the Varian TrueBeam linear accelerator delivers to a patient with head and neck cancer using mesh-type International Commission on Radiological Protection reference computational phantoms. In the first part, for use as a benchmark for the accuracy of the Monte Carlo geometry of CBCT, Particle and Heavy Ion Transport code System (PHITS) calculations were confirmed against measured lateral and depth dose profiles using a computed tomography dose profiler. After obtaining good agreement, organ dose calculations were performed by PHITS using mesh-type reference computational phantom (MRCP) and irradiating the neck region; the effective dose was calculated utilising absorbed organ doses and tissue weighting factors for male and female MRCP. Substantially, it has been found that the effective doses for male and female MRCP are 0.81 and 1.06 mSv, respectively. As this study aimed to assess the imaging dose from the CBCT system used in image-guided radiation therapy, it is required to take into account this dose in terms of both the target organ and surrounding tissues. Although the absorbed organ dose values and effective dose values obtained for both MRCP males and females were small, attention should be paid to the additional dose resulting from CBCT. This study can create awareness on the importance of doses arising from imaging techniques, especially CBCT.

Dose reduction potential of using gold fiducial markers for kilovoltage image-guided radiotherapy

This study aimed to evaluate the possibility of reducing the imaging dose for image-guided radiotherapy by using planar kilovoltage orthogonal imaging and fiducial markers (kV-FM). We tested kilovoltage planar images under clinical imaging conditions for the pelvis (75 kVp, 200 mA, 50 ms) at a decreasing tube current (from 200 to 10 mA). Imaging doses were measured with a semiconductor detector. The visibility of the kV-FM, aspects of image quality (spatial resolution, low contrast resolution), and the resultant image registration reproducibility were evaluated using various shapes (folded, linear, tadpole-like) of fiducial markers containing 0.5% iron [Gold Anchor™ (GA); Naslund Medical AB, Huddinge, Sweden]. The GA phantom was created by placing these variously shaped GAs in an agar phantom. The imaging doses with 200 and 10 mA were approximately 0.74 and 0.04 mGy and they were correlated to the tube current (R2  = 0.999). Regardless of the marker's shape, the GA phantom ensured visibility even when the tube current was reduced to the minimum value (10 mA). The low contrast resolution was gradually decreased at less than 50 mA, but the spatial resolution did not change. Although the auto-registration function could not be used, manual-registration could be achieved with an accuracy of within 1 mm, even when the imaging dose was reduced to 1/20 of the clinical imaging condition for the pelvis. When using the GA as the fiducial marker, the imaging dose could be reduced to 1/20 of that used clinically while maintaining the accuracy of manual-registration using the kV-FM for image-guided radiotherapy of the pelvis.

Registration accuracy with the low dose kilovoltage cone-beam CT: A phantom study

Objective:

The aim of this study was to investigate low-dose kilovoltage cone-beam CT (kV-CBCT) for image-guided radiotherapy, with a particular focus on the accuracy of image registration with low-dose protocols.

Methods:

Imaging doses were measured with a NOMEX semiconductor detector positioned at the front of head, thorax, and pelvis human body phantoms while kV-CBCT scans were acquired at different tube currents. Aspects of image quality (spatial resolution, noise, uniformity, contrast, geometric distortion, and Hounsfield unit sensitivity) and image registration accuracy using bone and soft tissue were evaluated.

Results:

With preset and the lowest tube currents, the imaging doses were 0.16 and 0.08 mGy, 5.29 and 2.80 mGy, and 18.23 and 2.69 mGy for head, thorax, and pelvis, respectively. Noise was the only quality aspect directly dependent on tube current, being increased by 1.5 times with a tube current half that of the preset in head and thorax, and by 2.2 times with a tube current 1/8 of the preset in the pelvis. Accurate auto-bone matching was performed within 1 mm at the lowest tube current. The auto-soft tissue matching could not be performed with the lowest tube current; however, manual-soft tissue matching could still be performed within 2 mm or less.

Conclusion:

Noise was the only image quality aspect dependent on the imaging dose. Auto-bone and manual-soft tissue matching could still be performed at the lowest imaging dose.

Advances in knowledge:

When optimizing kV-CBCT imaging dose, the impact on bone and soft tissue image registration accuracy should be evaluated.

Determination of the effective dose delivered by image guided radiotherapy in head & neck and breast treatments

PURPOSE

Image guided radiotherapy (IGRT) improves patient positioning for treatment delivery at the cost of an additional dose. This work aimed to calculate the effective dose (as an indicator of dose) for head & neck (H&N) and breast IGRT treatments by implementing dose calculation models to determine the dose distributions.

METHODS

The kV dose-models were created for the IGRT systems of Elekta Synergy (XVI) and Varian Clinac (OBI) linear accelerators within Philips Pinnacle TPS. Profiles and depth dose curves were measured in water. The models were validated in a CIRS thorax phantom. The IGRT dose distributions for five H&N and five breast patients were calculated. The effective dose was determined from the dose distributions following ICRP 103 recommendations. Moreover, time-saving approximations were studied in order to propose an alternative way of segmenting the tissues for a clinical implementation of the method.

RESULTS AND CONCLUSION

The effective dose specifically associated with IGRT varied from 1 to 10mSv depending on the protocol. The kV dose-model allowed us to calculate the dose distributions from IGRT for different configurations and patients, and to determine effective dose for IGRT protocols. The clinical implementation of the method was found to reduce time and to introduce a small enough increase of uncertainty in the results to be clinically usable.

A Monte Carlo study on quantifying the amount of dose reduction by shielding the superficial organs of an Iranian 11-year-old boy

A method for minimizing organ dose during computed tomography examinations is the use of shielding to protect superficial organs. There are some scientific reports that usage of shielding technique reduces the surface dose to patients with no appreciable loss in diagnostic quality. Therefore, in this Monte Carlo study based on the phantom of a 11-year-old Iranian boy, the effect of using an optimized shield on dose reduction to body organs was quantified. Based on the impact of shield on image quality, lead shields with thicknesses of 0.2 and 0.4 mm were considered for organs exposed directly and indirectly in the scan range, respectively. The results showed that there is 50%–62% reduction in amounts of dose for organs located fully or partly in the scan range at different tube voltages and modeling the true location of all organs in human anatomy, especially the ones located at the border of the scan, range affects the results up to 49%.

Evaluation of dose conversion coefficients for an eight-year-old Iranian male phantom undergoing computed tomography

In order to construct a library of Iranian pediatric voxel phantoms for radiological protection and dosimetry applications, an Iranian eight-year-old phantom was constructed from a series of CT images. Organ and effective dose conversion coefficients to this phantom were calculated for head, chest, abdominopelvis and chest-abdomen-pelvis scans at tube voltages of 80, 100 and 120 kVp. To validate the results, the organ and effective dose conversion coefficients obtained were compared with those of the University of Florida eight-year-old voxel female phantom as a function of examination type and anatomical scan area. For a detailed study, depth distributions of organs together with the thickness of surrounding tissues located in the beam path, which are shielding the internal organs, were determined for these two voxel phantoms. The relation between the anatomical differences and the level of delivered dose was investigated and the discrepancies among the results justified.

Accounting for patient size in the optimization of dose and image quality of pelvis cone beam CT protocols on the Varian OBI system

OBJECTIVE

The purpose of this study was to develop size-based radiotherapy kilovoltage cone beam CT (CBCT) protocols for the pelvis.

METHODS

Image noise was measured in an elliptical phantom of varying size for a range of exposure factors. Based on a previously defined "small pelvis" reference patient and CBCT protocol, appropriate exposure factors for small, medium, large and extra-large patients were derived which approximate the image noise behaviour observed on a Philips CT scanner (Philips Medical Systems, Best, Netherlands) with automatic exposure control (AEC). Selection criteria, based on maximum tube current-time product per rotation selected during the radiotherapy treatment planning scan, were derived based on an audit of patient size.

RESULTS

It has been demonstrated that 110 kVp yields acceptable image noise for reduced patient dose in pelvic CBCT scans of small, medium and large patients, when compared with manufacturer's default settings (125 kVp). Conversely, extra-large patients require increased exposure factors to give acceptable images. 57% of patients in the local population now receive much lower radiation doses, whereas 13% require higher doses (but now yield acceptable images).

CONCLUSION

The implementation of size-based exposure protocols has significantly reduced radiation dose to the majority of patients with no negative impact on image quality. Increased doses are required on the largest patients to give adequate image quality.

ADVANCES IN KNOWLEDGE

The development of size-based CBCT protocols that use the planning CT scan (with AEC) to determine which protocol is appropriate ensures adequate image quality whilst minimizing patient radiation dose.

Imaging dose from cone beam computed tomography in radiation therapy

Imaging dose in radiation therapy has traditionally been ignored due to its low magnitude and frequency in comparison to therapeutic dose used to treat patients. The advent of modern, volumetric, imaging modalities, often as an integral part of linear accelerators, has facilitated the implementation of image-guided radiation therapy (IGRT), which is often accomplished by daily imaging of patients. Daily imaging results in additional dose delivered to patient that warrants new attention be given to imaging dose. This review summarizes the imaging dose delivered to patients as the result of cone beam computed tomography (CBCT) imaging performed in radiation therapy using current methods and equipment. This review also summarizes methods to calculate the imaging dose, including the use of Monte Carlo (MC) and treatment planning systems (TPS). Peripheral dose from CBCT imaging, dose reduction methods, the use of effective dose in describing imaging dose, and the measurement of CT dose index (CTDI) in CBCT systems are also reviewed.

Validation of a technique for estimating organ doses for kilovoltage cone-beam CT of the prostate using the PCXMC 2.0 patient dose calculator

The use of cone beam CT in common radiotherapy treatments is increasing with the growth of image guided radiotherapy. Whilst the benefits that this technology offers are clear, such as improved patient positioning prior to treatment, it is always important to consider the implications of such intensive imaging regimes on the patient, especially when considering the fundamental radiation protection requirements for justification and optimisation.The purpose of this study was to develop a technique that uses readily available dose calculation software (PCXMC 2.0) to estimate the organ and effective doses that result from these types of examination in prostate treatments on the Varian OBI system. It has been shown that by separating these types of examinations into 28 different projections, with a range of x-ray beam qualities, it is possible to reproduce the complex geometry that is used on these imaging systems in PCXMC i.e. asymmetric radiation field with a half bowtie filter rotating 360° around the patient.This new technique has been validated with thermo-luminescent dosimeter measurements in the Rando anthropomorphic phantom, and has been shown to give excellent agreement with this established method (R(2) = 0.995). This technique will prove to be valuable to radiotherapy departments that are looking to optimise their CBCT imaging protocols as it allows a rapid evaluation of the impact of any changes on patient dose. It also serves to further highlight the levels of dose that these types of patient are subject to when having daily CBCT scans as part of the treatment, which further reinforces the need for optimisation of both patient dose and image quality on these systems.

Effects of shielding the radiosensitive superficial organs of ORNL pediatric phantoms on dose reduction in computed tomography

In computed tomography (CT), some superficial organs which have increased sensitivity to radiation, receive doses that are significant enough to be matter of concern. Therefore, in this study, the effects of using shields on the amount of dose reduction and image quality was investigated for pediatric imaging. Absorbed doses of breasts, eyes, thyroid and testes of a series of pediatric phantoms without and with different thickness of bismuth and lead were calculated by Monte Carlo simulation. Appropriate thicknesses of shields were chosen based on their weights, X-ray spectrum, and the amount of dose reduction. In addition, the effect of lead shield on image quality of a simple phantom was assessed quantitatively using region of interest (ROI) measurements. Considering the maximum reduction in absorbed doses and X-ray spectrum, using a lead shield with a maximum thickness of 0.4 mm would be appropriate for testes and thyroid and two other organs (which are exposed directly) should be protected with thinner shields. Moreover, the image quality assessment showed that lead was associated with significant increases in both noise and CT attenuation values, especially in the anterior of the phantom. Overall, the results suggested that shielding is a useful optimization tool in CT.

An exponential growth of computational phantom research in radiation protection, imaging, and radiotherapy: a review of the fifty-year history

Radiation dose calculation using models of the human anatomy has been a subject of great interest to radiation protection, medical imaging, and radiotherapy. However, early pioneers of this field did not foresee the exponential growth of research activity as observed today. This review article walks the reader through the history of the research and development in this field of study which started some 50 years ago. This review identifies a clear progression of computational phantom complexity which can be denoted by three distinct generations. The first generation of stylized phantoms, representing a grouping of less than dozen models, was initially developed in the 1960s at Oak Ridge National Laboratory to calculate internal doses from nuclear medicine procedures. Despite their anatomical simplicity, these computational phantoms were the best tools available at the time for internal/external dosimetry, image evaluation, and treatment dose evaluations. A second generation of a large number of voxelized phantoms arose rapidly in the late 1980s as a result of the increased availability of tomographic medical imaging and computers. Surprisingly, the last decade saw the emergence of the third generation of phantoms which are based on advanced geometries called boundary representation (BREP) in the form of Non-Uniform Rational B-Splines (NURBS) or polygonal meshes. This new class of phantoms now consists of over 287 models including those used for non-ionizing radiation applications. This review article aims to provide the reader with a general understanding of how the field of computational phantoms came about and the technical challenges it faced at different times. This goal is achieved by defining basic geometry modeling techniques and by analyzing selected phantoms in terms of geometrical features and dosimetric problems to be solved. The rich historical information is summarized in four tables that are aided by highlights in the text on how some of the most well-known phantoms were developed and used in practice. Some of the information covered in this review has not been previously reported, for example, the CAM and CAF phantoms developed in 1970s for space radiation applications. The author also clarifies confusion about 'population-average' prospective dosimetry needed for radiological protection under the current ICRP radiation protection system and 'individualized' retrospective dosimetry often performed for medical physics studies. To illustrate the impact of computational phantoms, a section of this article is devoted to examples from the author's own research group. Finally the author explains an unexpected finding during the course of preparing for this article that the phantoms from the past 50 years followed a pattern of exponential growth. The review ends on a brief discussion of future research needs (a supplementary file '3DPhantoms.pdf' to figure 15 is available for download that will allow a reader to interactively visualize the phantoms in 3D).

A dosimetry technique for measuring kilovoltage cone-beam CT dose on a linear accelerator using radiotherapy equipment

This work develops a technique for kilovoltage cone‐beam CT (CBCT) dosimetry that incorporates both point dose and integral dose in the form of dose length product, and uses readily available radiotherapy equipment. The dose from imaging protocols for a range of imaging parameters and treatment sites was evaluated. Conventional CT dosimetry using 100 mm long pencil chambers has been shown to be inadequate for the large fields in CBCT and has been replaced in this work by a combination of point dose and integral dose. Absolute dose measurements were made with a small volume ion chamber at the central slice of a radiotherapy phantom. Beam profiles were measured using a linear diode array large enough to capture the entire imaging field. These profiles were normalized to absolute dose to form dose line integrals, which were then weighted with radial depth to form the DLP_CBCT. This metric is analogous to the standard dose length product (DLP), but derived differently to suit the unique properties of CBCT. Imaging protocols for head and neck, chest, and prostate sites delivered absolute doses of 0.9, 2.2, and 2.9 cGy to the center of the phantom, and DLPCBCT of 28.2, 665.1, and 565.3 mGy.cm, respectively. Results are displayed as dose per 100 mAs and as a function of key imaging parameters such as kVp, mAs, and collimator selection in a summary table. DLPCBCT was found to correlate closely with the dimension of the imaging region and provided a good indication of integral dose. It is important to assess integral dose when determining radiation doses to patients using CBCT. By incorporating measured beam profiles and DLP, this technique provides a CBCT dosimetry in radiotherapy phantoms and allows the prediction of imaging dose for new CBCT protocols.

PACS number: 87.57.uq

Estimation of effective imaging dose for kilovoltage intratreatment monitoring of the prostate position during cancer radiotherapy

Kilovoltage intratreatment monitoring (KIM) is a novel real-time localization modality where the tumor position is continuously measured during intensity modulated radiation therapy (IMRT) or intensity modulated arc therapy (IMAT) by a kilovoltage (kV) x-ray imager. Adding kV imaging during therapy adds radiation dose. The additional effective dose is quantified for prostate radiotherapy and compared to dose from other localization modalities. The software PCXMC 2.0 was used to calculate the effective dose delivered to a phantom as a function of imager angle and field size for a Varian On-Board Imager. The average angular effective dose was calculated for a field size of 6 cm × 6 cm. The average angular effective dose was used in calculations for different treatment scenarios. Treatment scenarios considered were treatment type and fractionation. For all treatment scenarios, (i.e. conventionally fractionated and stereotactic body radiotherapy (SBRT), IMRT and IMAT), the total KIM dose at 1 Hz ranged from 2-10 mSv. This imaging dose is less than the Navotek radioactive implant dose (64 mSv) and a standard SBRT cone beam computed tomography pretreatment scan dose (22 mSv) over an entire treatment regime. KIM delivers an acceptably low effective dose for daily use as a real-time image-guidance method for prostate radiotherapy.

Reducing excess radiation from portal imaging of pediatric brain tumors

Previously we have shown that our routine portal imaging (PI) of the craniofacial region in pediatric brain tumor patients contributed an additional 2%‐3% of the prescribed dose and up to 200 cGy to the planning target volume (PTV) and nearby organs at risk (OARs). The purpose of this study is to quantify the reduction in dose to PTV and OARs from portal imaging (PI) of the craniofacial region of pediatric patients treated after the implementation of changes in our portal imaging practices. Twenty consecutive pediatric patients were retrospectively studied since the implementation of changes to our portal imaging procedure. Each received portal imaging of treatment fields and orthogonal setup fields to the craniofacial region. PI modifications included a reduction in the field size of setup orthogonal fields without loss of radiographic information needed for treatment verification. In addition, treatment fields were imaged using a single exposure, rather than double exposure. Dose‐volume histograms were generated to quantify the dose to the target and critical structures through PI acquisition. These results were compared with our previous cohort of 20 patients who were treated using the former portal imaging practices. The mean additional target dose from portal imaging following the new guidelines was 1.5% of the prescribed dose compared to 2.5% prior to the new portal image practices (p < 0.001). With the new portal imaging practices, the percentage decrease in portal imaging dose to the brainstem, optic structures, cochlea, hypothalamus, temporal lobes, thyroid, and eyes were 25%, 35%, 35%, 51%, 45%, 80%, and 55%, respectively. Reductions in portal imaging doses were significant in all OARs with exception of the brainstem, which showed a trend towards significance. Changes to portal imaging practices can reduce the radiation dose contribution from portal imaging to surrounding OARs by up to 80%. This may have implications on both late toxicity and second cancer development in pediatric brain tumors.

PACS number: 87

[Dose distribution from kV-cone beam computed tomography in image-guided radiotherapy]

Image-guided radiotherapy (IGRT) is an increasingly commonly adopted technique. As a result, however, total patient dose is increasing rapidly, especially when kV-cone beam computed tomography (CBCT) is applied. This study investigated the dosimetry of kV-CBCT using a Farmer ionization chamber with a (60)Co absorbed-dose calibration factor. The absorbed-dose measurements were performed using an I'mRT phantom (RW3, IBA) which is employed for dose verification of intensity-modulated radiotherapy (IMRT). The I'mRT phantom was used as a substitute for head and pelvis phantoms. The kV-CBCT absorbed dose was evaluated from a beam quality conversion factor of kV to (60)Co and the ionization ratio of the I'mRT phantom and water, calculated using the Monte Carlo method. The dose distribution in the I'mRT phantom was also measured using a radiophotoluminescent glass dosimeter (RGD). The absorbed doses for the pelvis phantom (full scan) ranged from 2.5-4 cGy for kV-CBCT and 4-8 cGy for MV-CBCT. TomoTherapy resulted in a lower dose of approximately 1.3 cGy due to fan-beam. For the head phantom (half scan), the doses ranged from 0.1-0.7 cGy for kV-CBCT and 3-5 cGy for MVCBCT. The results for RGD were similar to ion chamber measurements. It is necessary to decrease the absorbed dose of the organs at risk every time IGRT is applied.

Calculation of organ doses from breast cancer radiotherapy: a Monte Carlo study

The current study aimed to: a) utilize Monte Carlo simulation methods for the assessment of radiation doses imparted to all organs at risk to develop secondary radiation induced cancer, for patients undergoing radiotherapy for breast cancer; and b) evaluate the effect of breast size on dose to organs outside the irradiation field. A simulated linear accelerator model was generated. The in-field accuracy of the simulated photon beam properties was verified against percentage depth dose (PDD) and dose profile measurements on an actual water phantom. Off-axis dose calculations were verified with thermoluminescent dosimetry (TLD) measurements on a humanoid physical phantom. An anthropomorphic mathematical phantom was used to simulate breast cancer radiotherapy with medial and lateral fields. The effect of breast size on the calculated organ dose was investigated. Local differences between measured and calculated PDDs and dose profiles did not exceed 2% for the points at depths beyond the depth of maximum dose and the plateau region of the profile, respectively. For the penumbral regions of the dose profiles, the distance to agreement (DTA) did not exceed 2 mm. The mean difference between calculated out-of-field doses and TLD measurements was 11.4% ± 5.9%. The calculated doses to peripheral organs ranged from 2.32 cGy up to 161.41 cGy depending on breast size and thus the field dimensions applied, as well as the proximity of the organs to the primary beam. An increase to the therapeutic field area by 50% to account for the large breast led to a mean organ dose elevation by up to 85.2% for lateral exposure. The contralateral breast dose ranged between 1.4% and 1.6% of the prescribed dose to the tumor. Breast size affects dose deposition substantially.

Radiation dose and contralateral breast cancer risk associated with megavoltage cone-beam computed tomographic image verification in breast radiation therapy

PURPOSE

To measure and compare organ doses from a standard tangential breast radiation therapy treatment (50 Gy delivered in 25 fractions) and a megavoltage cone-beam computed tomography (MV-CBCT), taken for weekly image verification, and assess the risk of radiation-induced contralateral breast cancer.

METHODS AND MATERIALS

Organ doses were measured with thermoluminescent dosimeters placed strategically within a female anthropomorphic phantom. The risk of radiation-induced secondary cancer of the contralateral breast was estimated from these values using excess absolute risk and excess relative risk models.

RESULTS

The effective dose from a MV-CBCT (8-monitor units) was 35.9 ± 0.2 mSv. Weekly MV-CBCT imaging verification contributes 0.5% and 17% to the total ipsilateral and contralateral breast dose, respectively. For a woman irradiated at age 50 years, the 10-year postirradiation excess relative risk was estimated to be 0.8 and 0.9 for treatment alone and treatment plus weekly MV-CBCT imaging, respectively. The 10-year postirradiation excess absolute risk was estimated to be 4.7 and 5.6 per 10,000 women-years.

CONCLUSIONS

The increased dose and consequent radiation-induced second cancer risk as calculated by this study introduced by the imaging verification protocols utilizing MV-CBCT in breast radiation therapy must be weighed against the benefits of more accurate treatment. As additional image verification becomes more common, it is important that data be collected in regard to long-term malignancy risk.

Extension of RPI-adult male and female computational phantoms to obese patients and a Monte Carlo study of the effect on CT imaging dose

Although it is known that obesity has a profound effect on x-ray computed tomography (CT) image quality and patient organ dose, quantitative data describing this relationship are not currently available. This study examines the effect of obesity on the calculated radiation dose to organs and tissues from CT using newly developed phantoms representing overweight and obese patients. These phantoms were derived from the previously developed RPI-adult male and female computational phantoms. The result was a set of ten phantoms (five males, five females) with body mass indexes ranging from 23.5 (normal body weight) to 46.4 kg m(-2) (morbidly obese). The phantoms were modeled using triangular mesh geometry and include specified amounts of the subcutaneous adipose tissue and visceral adipose tissue. The mesh-based phantoms were then voxelized and defined in the Monte Carlo N-Particle Extended code to calculate organ doses from CT imaging. Chest-abdomen-pelvis scanning protocols for a GE LightSpeed 16 scanner operating at 120 and 140 kVp were considered. It was found that for the same scanner operating parameters, radiation doses to organs deep in the abdomen (e.g., colon) can be up to 59% smaller for obese individuals compared to those of normal body weight. This effect was found to be less significant for shallow organs. On the other hand, increasing the tube potential from 120 to 140 kVp for the same obese individual resulted in increased organ doses by as much as 56% for organs within the scan field (e.g., stomach) and 62% for those out of the scan field (e.g., thyroid), respectively. As higher tube currents are often used for larger patients to maintain image quality, it was of interest to quantify the associated effective dose. It was found from this study that when the mAs was doubled for the obese level-I, obese level-II and morbidly-obese phantoms, the effective dose relative to that of the normal weight phantom increased by 57%, 42% and 23%, respectively. This set of new obese phantoms can be used in the future to study the optimization of image quality and radiation dose for patients of different weight classifications. Our ultimate goal is to compile all the data derived from these phantoms into a comprehensive dosimetry database defined in the VirtualDose software.

Organ and effective doses from verification techniques in image-guided radiotherapy

The purpose of this work was an evaluation of organ doses and effective doses from three verification techniques in Image-Guided Radiotherapy: from kilovoltage (kV) cone beam computed tomography (CBCT) scans, from two orthogonal kV images and from two orthogonal megavoltage (MV) images for two different treatment sites: pelvis and head and neck (H&N). For comparison reasons, organ doses and effective doses from prostate and H&N radiotherapy were also evaluated. Measurements of organ doses were performed in a male anthropomorphic Rando phantom by means of thermoluminescent dosemeters. In this investigation, measured organ doses from one CBCT scan, from two MV images and from two kV images of pelvis represent typically 1-6, 1-10 and 0.05-1 %, respectively, of organ doses resulting from one fraction of prostate radiotherapy. The maximum effective doses from CBCT scans, kV images and MV images of pelvis are 5.6, 0.8 and 11.9 mSv, respectively.

Monte Carlo calculation of imaging doses from diagnostic multidetector CT and kilovoltage cone-beam CT as part of prostate cancer treatment plans

PURPOSE

To calculate imaging doses to the rectum, bladder, and femoral heads as part of a prostate cancer treatment plans, assuming an image guided radiation therapy (IGRT) procedure involving either the multidetector CT (MDCT) or kilovoltage cone-beam CT (kV CBCT).

METHODS

This study considered an IGRT treatment plan for a prostate carcinoma patient involving 50.4 Gy from 28 initial fractions and a boost of 28.8 Gy from 16 fractions. A total of 45 CT imaging procedures, each involving a MDCT or a kV CBCT scan procedure, were carefully modeled using the MCNPX code version 2.5.0. The MDCT scanner model is based on the GE LightSpeed 16-MDCT scanner and the kV CBCT scanner model is based on the Varian On-Board Imager using parameters reported by the CT manufacturers and literatures. A patient-specific treatment planning CT data set was used to construct the phantom for the dose calculation. The target, organs-at-risk (OARs), and background voxels in the CT data set were categorized into six tissue types according to CT numbers for Monte Carlo calculations.

RESULTS

For a total of 45 imaging procedures, it was found that the rectum received 78.4 and 76.7 cGy from MDCT and kV CBCT, respectively. The bladder received slightly greater doses of 82.4 and 77.9 cGy, while the femoral heads received much higher doses of 182.3 and 141.3 cGy from MDCT and kV CBCT, respectively. To investigate the impact of these imaging doses on treatment planning, OAR doses from MDCT or kV CBCT imaging procedures were added to the corresponding dose matrix reported by the original treatment plans to construct dose volume histograms. It was found that after the imaging dose is added, the rectum volumes irradiated to 75 and 70 Gy increased from 13.9% and 21.2%, respectively, in the original plan to 14.8% and 21.8%. The bladder volumes receiving 80 Gy increased to 4.6% from 4.1% in the original plan and the volume receiving 75 Gy increased to 7.9% from 7.5%. All values remained within the tolerance levels: V70<25%, V75 <15% for rectum and V75 < 25%, V80 < 15% for bladder. The irradiation of femoral heads was also acceptable with no volume receiving >45 Gy.

CONCLUSIONS

IGRT procedures can irradiate the OARs to an imaging dose level that is great enough to require careful evaluation and perhaps even adjustment of original treatment planning in order to still satisfy the dose constraints. This study only considered one patient CT because the CT x rays cover a relatively larger volume of the body and the dose distribution is considerably more uniform than those associated with the therapeutic beams. As a result, the dose to an organ from CT imaging doses does not vary much from one patient to the other for the same CT settings. One factor that would potentially affect such CT dose level is the size of the patient body. More studies are needed to develop accurate and convenient methods of accounting for the imaging doses as part of treatment planning.

Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition

PURPOSE

A common approach for dose assessment during cone beam computed tomography (CBCT) acquisition is to use thermoluminescent detectors for skin dose measurements (on patients or phantoms) or ionization chamber (in phantoms) for body dose measurements. However, the benefits of a daily CBCT image acquisition such as margin reduction in planning target volume and the image quality must be weighted against the extra dose received during CBCT acquisitions.

METHODS

The authors describe a two-dimensional reference dosimetry technique for measuring dose from CBCT scans using the on-board imaging system on a Varian Clinac-iX linear accelerator that employs the XR-QA radiochromic film model, specifically designed for dose measurements at low energy photons. The CBCT dose measurements were performed for three different body regions (head and neck, pelvis, and thorax) using humanoid Rando phantom.

RESULTS

The authors report on both surface dose and dose profiles measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Our measurements show that the surface doses per CBCT scan can range anywhere between 0.1 and 4.7 cGy, with the lowest surface dose observed in the head and neck region, while the highest surface dose was observed for the Pelvis spot light CBCT protocol in the pelvic region, on the posterior side of the Rando phantom. The authors also present results of the uncertainty analysis of our XR-QA radiochromic film dosimetry system.

CONCLUSIONS

Radiochromic film dosimetry protocol described in this work was used to perform dose measurements during CBCT acquisitions with the one-sigma dose measurement uncertainty of up to 3% for doses above 1 cGy. Our protocol is based on film exposure calibration in terms of "air kerma in air," which simplifies both the calibration procedure and reference dosimetry measurements. The results from a full Monte Carlo investigation of the dose conversion of measured XR-QA film dose at the surface into dose to water (or water kerma) at the surface of the phantom indicate that, for typical beam qualities used in CBCT, this conversion can be approximated by simple mass-energy absorption coefficient ratios water-to-air.

An organ and effective dose study of XVI and OBI cone-beam CT systems

The main purpose of this work was to quantify patient organ doses from the two kilovoltage cone beam computed tomography (CBCT) systems currently available on medical linear accelerators, namely the X‐ray Volumetric Imager (XVI, Elekta Oncology Systems) and the On‐Board Imager (OBI, Varian Medical Systems). Organ dose measurements were performed using a fiber‐optic coupled (FOC) dosimetry system along with an adult male anthropomorphic phantom for three different clinically relevant scan sites: head, chest, and pelvis. The FOC dosimeter was previously characterized at diagnostic energies by Hyer et al. [Med Phys 2009;36(5):1711–16] and a total uncertainty of approximately 4% was found for in‐phantom dose measurements. All scans were performed using current manufacturer‐installed clinical protocols and appropriate bow‐tie filters. A comparison of image quality between these manufacturer‐installed protocols was also performed using a Catphan 440 image quality phantom. Results indicated that for the XVI, the dose to the lens of the eye (1.07 mGy) was highest in a head scan, thyroid dose (19.24 mGy) was highest in a chest scan, and gonad dose (29 mGy) was highest in a pelvis scan. For the OBI, brain dose (3.01 mGy) was highest in a head scan, breast dose (5.34 mGy) was highest in a chest scan, and gonad dose (34.61 mGy) was highest in a pelvis scan. Image quality measurements demonstrated that the OBI provided superior image quality for all protocols, with both better spatial resolution and low‐contrast detectability. The measured organ doses were also used to calculate a reference male effective dose to allow further comparison of the two machines and imaging protocols. The head, chest, and pelvis scans yielded effective doses of 0.04, 7.15, and 3.73 mSv for the XVI, and 0.12, 1.82, and 4.34 mSv for the OBI, respectively.

PACS number: 87.57.uq

The development, validation and application of a multi-detector CT (MDCT) scanner model for assessing organ doses to the pregnant patient and the fetus using Monte Carlo simulations

The latest multiple-detector technologies have further increased the popularity of x-ray CT as a diagnostic imaging modality. There is a continuing need to assess the potential radiation risk associated with such rapidly evolving multi-detector CT (MDCT) modalities and scanning protocols. This need can be met by the use of CT source models that are integrated with patient computational phantoms for organ dose calculations. Based on this purpose, this work developed and validated an MDCT scanner using the Monte Carlo method, and meanwhile the pregnant patient phantoms were integrated into the MDCT scanner model for assessment of the dose to the fetus as well as doses to the organs or tissues of the pregnant patient phantom. A Monte Carlo code, MCNPX, was used to simulate the x-ray source including the energy spectrum, filter and scan trajectory. Detailed CT scanner components were specified using an iterative trial-and-error procedure for a GE LightSpeed CT scanner. The scanner model was validated by comparing simulated results against measured CTDI values and dose profiles reported in the literature. The source movement along the helical trajectory was simulated using the pitch of 0.9375 and 1.375, respectively. The validated scanner model was then integrated with phantoms of a pregnant patient in three different gestational periods to calculate organ doses. It was found that the dose to the fetus of the 3 month pregnant patient phantom was 0.13 mGy/100 mAs and 0.57 mGy/100 mAs from the chest and kidney scan, respectively. For the chest scan of the 6 month patient phantom and the 9 month patient phantom, the fetal doses were 0.21 mGy/100 mAs and 0.26 mGy/100 mAs, respectively. The paper also discusses how these fetal dose values can be used to evaluate imaging procedures and to assess risk using recommendations of the report from AAPM Task Group 36. This work demonstrates the ability of modeling and validating an MDCT scanner by the Monte Carlo method, as well as assessing fetal and organ doses by combining the MDCT scanner model and the pregnant patient phantom.