Volume 35, Issue 12, December 2008
- medical physics letters
- vision 20/20
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- ultrasound physics
- anatomy and physiology
- radiation biology
- anniversary papers
- books and publications
- note to readers of medical physics
- ultrasound physics
Index of content:
- MEDICAL PHYSICS LETTERS
35(2008); http://dx.doi.org/10.1118/1.3014197View Description Hide Description
Frequent and repeated imaging procedures such as those performed in image-guidedradiotherapy(IGRT) programs may add significant dose to radiosensitive organs of radiotherapy patients. It has been shown that kV-CBCT results in doses to bone that are up to a factor of 3–4 higher than those in surrounding soft tissue.Imaging guidance procedures are necessary due to their potential benefits, but the additional incremental dose per treatment fraction may exceed an individual organ tolerance. Hence it is important to manage and account for this additional dose from imaging for radiotherapy patients. Currently available model-based dose calculation methods in radiation treatment planning (RTP) systems are not suitable for low-energy x rays, and new and fast calculation algorithms are needed for a RTP system for kilovoltage dose computations. This study presents a new dose calculation algorithm, referred to as the medium-dependent-correction (MDC) algorithm, for accurate patient dose calculation resulting from kilovoltage x rays. The accuracy of the new algorithm is validated against Monte Carlo calculations. The new algorithm overcomes the deficiency of existing density correction based algorithms in dose calculations for inhomogeneous media, especially for CT-based human volumetric images used in radiotherapytreatment planning.
35(2008); http://dx.doi.org/10.1118/1.3026593View Description Hide Description
Current methods for organ and effective dose estimations in pediatric CT are largely patient generic. Physical phantoms and computer models have only been developed for standard/limited patient sizes at discrete ages (e.g., 0, 1, 5, 10, old) and do not reflect the variability of patient anatomy and body habitus within the same size/age group. In this investigation, full-body computer models of seven pediatric patients in the same size/protocol group (weight: ) were created based on the patients’ actual multi-detector array CT (MDCT) data. Organs and structures in the scan coverage were individually segmented. Other organs and structures were created by morphing existing adult models (developed from visible human data) to match the framework defined by the segmented organs, referencing the organ volume and anthropometry data in ICRP Publication 89. Organ and effective dose of these patients from a chest MDCT scan protocol (64 slice LightSpeed VCT scanner,, 70 or , gantry rotation period, pitch of 1.375, beam collimation, and small body scan field-of-view) was calculated using a Monte Carlo program previously developed and validated to simulate radiation transport in the same CT system. The seven patients had normalized effective dose of (coefficient of variation: 10.8%). Normalized lungdose and heartdose were and , respectively. Organdose variations across the patients were generally small for large organs in the scan coverage , but large for small organs in the scan coverage (9%–18%) and for partially or indirectly exposed organs (11%–77%). Normalized effective dose correlated weakly with body weight (correlation coefficient: ). Normalized lungdose and heartdose correlated strongly with mid-chest equivalent diameter (lung:, heart:); these strong correlation relationships can be used to estimate patient-specific organdose for any other patient in the same size/protocol group who undergoes the chest scan. In summary, this work reported the first assessment of dose variations across pediatric CT patients in the same size/protocol group due to the variability of patient anatomy and body habitus and provided a previously unavailable method for patient-specific organdose estimation, which will help in assessing patient risk and optimizing dose reduction strategies, including the development of scan protocols.
- VISION 20/20
35(2008); http://dx.doi.org/10.1118/1.3002305View Description Hide Description
Intensity modulated radiation therapy(IMRT) is an advanced form of external beam radiation therapy.IMRT offers an additional dimension of freedom as compared with field shaping in three-dimensional conformal radiation therapy because the radiation intensities within a radiation field can be varied according to the preferences of locations within a given beam direction from which the radiation is directed to the tumor. This added freedom allows the treatment planning system to better shape the radiationdoses to conform to the target volume while sparing surrounding normal structures. The resulting dosimetric advantage has shown to translate into clinical advantages of improving local and regional tumor control. It also offers a valuable mechanism for dose escalation to tumors while simultaneously reducing radiation toxicities to the surrounding normal tissue and sensitive structures. In less than a decade, IMRT has become common practice in radiationoncology. Looking forward, the authors wonder if IMRT has matured to such a point that the room for further improvement has diminished and so it is pertinent to ask what the future will hold for IMRT. This article attempts to look from the perspective of the current state of the technology to predict the immediate trends and the future directions. This article will (1) review the clinical experience of IMRT; (2) review what we learned in IMRT planning; (3) review different treatment delivery techniques; and finally, (4) predict the areas of advancements in the years to come.
35(2008); http://dx.doi.org/10.1118/1.3002307View Description Hide Description
Currently, there is an increasing interest in heavy ion radiotherapy (RT) and a number of new facilities are being installed in Europe and Japan. This development is accompanied by intensive technical, physical, and clinical research. The authors identify six research fields where progress is likely and propose a thesis on the expected achievements for each of the fields: (1) Synchrotrons with active energy variation and three-dimensional beam scanning will be the standard in ion beam RT. (2) Common standards for precise measurement, prescription, and reporting of dose will be available. (3) Intensity-modulated particle therapy will be state-of-the-art. (4) Time-adaptive treatments of moving targets will be feasible. (5) Therapeutic effectiveness of heavy ions will be known for the most important indications while cost effectiveness will remain to be shown. (6) The potential of high-linear energy transfer radiation will be known. The rationale for each of these theses is described.
35(2008); http://dx.doi.org/10.1118/1.3020593View Description Hide Description
The current climate of rapid technological evolution is reflected in newer and better methods to modulate and direct radiation beams for cancer therapy. This Vision paper focuses on part of this evolution, locating and targeting moving tumors. The two processes are somewhat independent and in principle different implementations of the locating and targeting processes can be interchanged. Advanced localization and targeting methods have an impact on treatment planning and also present new challenges for quality assurance (QA), that of verifying real-time delivery. Some methods to locate and target moving tumors with radiation beams are currently FDA approved for clinical use—and this availability and implementation will increase with time. Extensions of current capabilities will be the integration of higher order dimensionality, such as rotation and deformation in addition to translation, into the estimate of the patient pose and real-time reoptimization and adaption of delivery to the dynamically changing anatomy of cancer patients.
35(2008); http://dx.doi.org/10.1118/1.3013698View Description Hide Description
Commercially available high-resolution three-dimensional optical imaging modalities—including confocal microscopy, two-photon microscopy, and optical coherence tomography—have fundamentally impacted biomedicine. Unfortunately, such tools cannot penetrate biological tissue deeper than the optical transport mean free path ( in the skin). Photoacoustictomography, which combines strong optical contrast and high ultrasonic resolution in a single modality, has broken through this fundamental depth limitation and achieved superdepth high-resolution optical imaging. In parallel, radio frequency-or microwave-induced thermoacoustic tomography is being actively developed to combine radio frequency or microwave contrast with ultrasonic resolution. In this Vision article, the prospects of photoacoustictomography are envisaged in the following aspects: (1) photoacoustic microscopy of optical absorption emerging as a mainstream technology, (2) melanoma detection using photoacoustic microscopy, (3) photoacoustic endoscopy, (4) simultaneous functional and molecular photoacoustictomography, (5) photoacoustictomography of gene expression, (6) Doppler photoacoustictomography for flow measurement, (7) photoacoustictomography of metabolic rate of oxygen, (8) photoacoustic mapping of sentinel lymph nodes, (9) multiscale photoacoustic imagingin vivo with common signal origins, (10) simultaneous photoacoustic and thermoacoustic tomography of the breast, (11) photoacoustic and thermoacoustic tomography of the brain, and (12) low-background thermoacoustic molecular imaging.
- RADIATION THERAPY PHYSICS
35(2008); http://dx.doi.org/10.1118/1.3002312View Description Hide Description
This work summarizes Monte Carlo results in order to evaluate the potential of using HDR sources in accelerated partial breast irradiation (APBI) with the MammoSite® applicator. Simulations have been performed using the MCNP5 Monte Carlo Code, in simple geometries comprised of two concentric spheres; the internal consisting of selected concentrations, C, of a radiographic contrast solution in water (Omnipaque 300™) to simulate the MammoSite balloon and the external consisting of water to simulate surrounding tissue. The magnitude of the perturbation of delivered dose due to the radiographic contrast medium used in the MammoSite® applicator is calculated. At the very close vicinity of the balloon surface, a dose build-up region is observed, which leads to a dose overestimation by the treatment planning system (TPS) which depends on Omnipaque™ 300 solution concentration (and is in order of 2.3%, 3.0%, and 4.5%, respectively, at 1 mm away from the balloon - water interface, for C=10%, 15%, and 20%). However, dose overestimation by the TPS is minimal for points lying at the prescription distance ( cm) or beyond, for all simulated concentrations and radii of MammoSite® balloon. An analytical estimation of the integral dose outside the CTV in the simple geometries simulated shows that dose to the breast for MammoSite® applications is expected to be comparable using HDR and sources, and higher than that for . The higher enegies of sources result to approximately twice radiation protection requirements as compared to sources. However, they allow for more accurate dosimetry calculation with currently used treatment planning algorithms for sources, compared to .
A simplified method of four-dimensional dose accumulation using the mean patient density representation35(2008); http://dx.doi.org/10.1118/1.3002304View Description Hide Description
The purpose of this work was to demonstrate, both in phantom and patient, the feasibility of using an average 4DCT image set (AVG-CT) for 4D cumulative dose estimation. A series of 4DCT numerical phantoms and corresponding AVG-CTs were generated. For full 4D dose summation, static dose was calculated on each phase and cumulative dose was determined by combining each phase’s static dose distribution with known tumor displacement. The AVG-CT cumulative dose was calculated similarly, although the same AVG-CT static dose distribution was used for all phases (i.e., tumor displacements). Four lungcancer cases were also evaluated for stereotactic body radiotherapy and conformal treatments; however, deformable image registration of the 4DCTs was used to generate the displacement vector fields (DVFs) describing patient-specific motion. Dose discrepancy between full 4D summation and AVG-CT approach was calculated and compared. For all phantoms, AVG-CT approximation yielded slightly higher cumulative doses compared to full 4D summation, with dose discrepancy increasing with increased tumor excursion. In vivo, using the AVG-CT coupled with deformable registration yielded clinically insignificant differences for all GTV parameters including the minimum, mean, maximum, dose to 99% of target, and dose to 1% of target. Furthermore, analysis of the spinal cord, esophagus, and heart revealed negligible differences in major dosimetric indices and dose coverage between the two dose calculation techniques. Simplifying 4D dose accumulation via the AVG-CT, while fully accounting for tumor deformation due to respiratory motion, has been validated, thereby, introducing the potential to streamline the use of 4D dose calculations in clinical practice, particularly for adaptive planning purposes.
Monte Carlo evaluation of a treatment planning system for helical tomotherapy in an anthropomorphic heterogeneous phantom and for clinical treatment plans35(2008); http://dx.doi.org/10.1118/1.3002316View Description Hide Description
Helical tomotherapy is an increasingly common form of intensity modulated radiation therapy that allows for image guided adaptive radiotherapy. Its treatment planning system (TPS) uses a convolution superposition algorithm for dose distribution calculations. The accuracy of this algorithm in the presence of heterogeneities was evaluated against Monte Carlo(MC) calculations and measurements. This work performed BEAMnrc-and DOSXYZnrc-based MCdose calculations of tomotherapy deliveries to a CIRS anthropomorphic heterogeneous phantom with typical clinical inverse planning and delivery settings. Point measurements with A1SL ion chambers and relative measurements with Kodak EDR2 film were carried out in the phantom. The experimental results were used to evaluate both the TPS and MCdose calculations. Furthermore, the dose distribution for a clinical head-and-neck cancer plan was calculated on the TPS and MC systems. The results support this MC system as a viable option for the accurate simulation of the tomotherapy process in the presence of heterogeneities where direct measurement may not be practical. Ion chamber measurements in the CIRS phantom suggested the TPS has an average relative difference of 2.3%, with the largest difference being in one of the organs at risk. The MC system accurately predicted the dose to these measurement points within statistical uncertainty. The film measurements in the CIRS phantom demonstrated 90.7% (of pixels) agreed with the MC system using a acceptance criteria, where only 50.3% agreed with the TPS. In the clinical head-and-neck cancer plan evaluation where MC served as a reference against which to compare the TPS result, an average of 92.7% of the voxels within volumes of interest passed a criteria. The PTV54 region showed the worst agreement with 85.4% of the volume passing the criteria. In general, the criterion was found to be a challenge for the TPS in the presence of lung inhomogeneity.
35(2008); http://dx.doi.org/10.1118/1.3005480View Description Hide Description
The meaningful sharing and combining of clinical results from different centers in the world performing boronneutron capture therapy (BNCT) requires improved precision in dose specification between programs. To this end absorbed dose normalizations were performed for the European clinical centers at the Joint Research Centre of the European Commission, Petten (The Netherlands), Nuclear Research Institute, Rez (Czech Republic), VTT, Espoo (Finland), and Studsvik, Nyköping (Sweden). Each European group prepared a treatment plan calculation that was benchmarked against Massachusetts Institute of Technology (MIT) dosimetry performed in a large, water-filled phantom to uniformly evaluate dose specifications with an estimated precision of . These normalizations were compared with those derived from an earlier exchange between Brookhaven National Laboratory (BNL) and MIT in the USA. Neglecting the uncertainties related to biological weighting factors, large variations between calculated and measured dose are apparent that depend upon the uptake in tissue. Assuming a boron concentration of in normal tissue, differences in the evaluated maximum dose to brain for the same nominal specification of at the different facilities range between 7.6 and in the trials using boronophenylalanine (BPA) as the boron delivery compound and between 8.9 and in the two boron sulfhydryl (BSH) studies. Most notably, the value for the same specified dose of determined at the different participating centers using BPA is significantly higher than at BNL by 32% (MIT), 43% (VTT), 49% (JRC), and 74% (Studsvik). Conversion of dose specification is now possible between all active participants and should be incorporated into future multi-center patient analyses.
Technical note: Heterogeneity dose calculation accuracy in IMRT: Study of five commercial treatment planning systems using an anthropomorphic thorax phantom35(2008); http://dx.doi.org/10.1118/1.3006353View Description Hide Description
The purpose of this study was to determine the accuracy of five commonly used intensity-modulated radiation therapy(IMRT) treatment planning systems (TPSs), 3 using convolution superposition algorithms or the analytical anisotropic algorithm (CSA/AAAs) and 2 using pencil beam algorithms (PBAs), in calculating the absorbed dose within a low-density, heterogeneous region when compared with measurements made in an anthropomorphic thorax phantom. The dose predicted in the target center met the test criteria (5% of the dose normalization point or 3 mm distance to agreement) for all TPSs tested; however, at the tumor-lung interface and at the peripheral lung in the vicinity of the tumor, the CSA/AAAs performed better than the PBAs (85% and 50%, respectively, of pixels meeting the 5%/3-mm test criteria), and thus should be used to determine dose in heterogeneous regions.
35(2008); http://dx.doi.org/10.1118/1.3002417View Description Hide Description
A model based on the linear quadratic model that has been corrected for repopulation, sublethal cell damage repair, and RBE effect has been used to determine the prescription dose for prostate permanent brachytherapy using seeds loaded with a mixture of and or a mixture of and . The prescription dose was determined by comparing the tumor cell survival fractions between the considered biradionuclide seed implant and one monoradionuclide seed implant chosen from , , and . Prostate edema is included in the model. The influence of the value of the radiobiological parameters and RBE were also investigated. Two mixtures of radionuclides were considered: and , where the subscripts indicate the fractions of total initial internal activity in the biradionuclide seed. These fractions were selected in order to obtain a dose distribution that lies between that of and . As expected, the computed prescription dose values are dependent on the model parameters (edema half-life and magnitude, radiobiogical parameters, and RBE). The radionuclide used as a benchmark also has a strong impact on the derived prescribed dose. The large uncertainties in the radiobiological parameters and RBE values produce big errors in the computed prescribed dose. Averaged over the range of all the parameters and depending on the radionuclide used as a benchmark (in subscript), the derived prescription dose for the first mixture (PdI) would be: and ; and and for the PdCs mixture. The uncertainties could be reduced if the radiobiological parameters and RBE value were known more accurately. However, as edema characteristics are patient dependent and can be obtained only after the treatment, an unpredictable error is unavoidable.
35(2008); http://dx.doi.org/10.1118/1.3002313View Description Hide Description
Real-time tracking can provide high accuracy localization for a moving target and minimize the effect of motion. Simultaneous kV-MV imaging has been proposed as a real-time tracking technique by utilizing the existing kV on-board imager (OBI) and the MV electronic portal device (EPID) mounted on the linear accelerator. The orthogonal pair of kV-MV images acquired simultaneously can provide 3-D localization in real-time. However, the kV and MV beams cross shooting the target interfere with each other with beam scattering, which affects the quality of images. The success of this modality heavily relies on the image quality, especially the visibility of the target, which was investigated in this study. The kV and MV images were acquired for a gold implant marker that was used as a surrogate of the target and placed in an IMRT thorax phantom, a dynamic phantom, and a pelvis phantom to test the image quality in different situations. Contrast-to-noise ration (CNR) was used to quantitatively describe the visibility of the target in the image.CNR can be obtained by statistical calculation from image processing and physics analysis with ion chamber measurement. The difference is described by contrast detection efficiency (CDE). By comparing the ratio of CNR with and without the MV beam on, the MV beam scatter was found to have dramatically reduced the target visibility in the kV images, which was supported by an independent physics analysis that treats beam scatter as a noise. In contrast, the kV scatter effect on the MV images was minor . The effect of tumor motion was visible but tolerable for the target tracking purpose. CNR varied with different tumor sites and was lower for the pelvis than the thorax. Different kV imaging parameters such as kVp, mAs, and exposure time ms were tested for different cases. Considering a threshold of 1.0 CNR as a measure for the target visibility, a range of CNR from 1.3 to 4.2 was reached with appropriate tuning of those imaging parameters. This study has shown that CNR is a key parameter that can be used for assessing the visibility of the target in digital imaging and the quality of kV/MV images. It has also been shown that reasonable target visibility can be obtained using simultaneous kV-MV imaging for real-time target tracking.
35(2008); http://dx.doi.org/10.1118/1.3002412View Description Hide Description
A Monte Carlo study of dosimetry for eye plaque brachytherapy is performed. BrachyDose, an EGSnrc user code which makes use of Yegin’s multi-geometry package, is used to fully model (model 6711) and (model 200) brachytherapy seeds and the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). Three-dimensional dose distributions in the eye region are obtained. In general, dose to water is scored; however, the implications of replacing water with eye tissues are explored. The effect of the gold alloy (Modulay) backing is investigated and the dose is found to be sensitive to the elemental composition of the backing. The presence of the silicone polymer (Silastic) seed carrier results in substantial dose decreases relative to water, particularly for . For a plaque with a Modulay backing and Silastic insert, fully loaded with 24 seeds, the dose decrease relative to water is of the order of 14% for and 20% for at a distance of from the inner sclera along the plaque’s central axis. For the configurations of seeds used in COMS plaques, interseed attenuation is a small effect within the eye region. The introduction of an air interface results in a dose reduction in its vicinity which depends on the plaque’s position within the eye and the radionuclide. Introducing bone in the eye’s vicinity also causes dose reductions. The dose distributions in the eye for the two different radionuclides are compared and, for the same prescription dose, generally offers a lower dose to critical normal structures. BrachyDose is sufficiently fast to allow full Monte Carlodose calculations for routine clinical treatment planning.
Clinical application of radiophotoluminescent glass dosimeter for dose verification of prostate HDR procedure35(2008); http://dx.doi.org/10.1118/1.3005478View Description Hide Description
High dose rate brachytherapy (HDR-BT) is one of the many modalities for prostate cancer treatment. Due to the nature of HDR-BT, in vivodosimetry is feasible and can be used to verify consistent dose delivery. In order to validate a dose verification system for HDR-BT prostate cancer treatment, a radiophotoluminescent glass dosimeter (RPLGD) was used and the measurements were compared with those from a thermoluminescent dosimeter. The RPLGD shows many advantages in HDR-BT dose measurement, such as repeatability, stability, and small effective size. These advantages make the RPLGD a superior option for use as a dosimeter in HDR-BT. The results described here show that the difference between the measured dose and the treatment planned dose is less than 5%. A Monte Carlo simulation for the dose was performed using Monte Carlo -particle to investigate position error. This study concludes that the RPLGD is a promising and reliable dosimeter for HDR-BT in vivodosimetry with clinically acceptable accuracy.
35(2008); http://dx.doi.org/10.1118/1.3013568View Description Hide Description
The purpose of this study was to evaluate the quality and accuracy of cone beam computed tomography(CBCT) gated by active breathing control (ABC), which may be useful for image guidance in the presence of respiration. Comparisons were made between conventional ABC-CBCT (stop and go), fast ABC-CBCT (a method to speed up the acquisition by slowing the gantry instead of stopping during free breathing), and free breathing respiration correlated CBCT.Image quality was assessed in phantom. Accuracy of reconstructed voxel intensity, uniformity, and root mean square error were evaluated. Registration accuracy (bony and soft tissue) was quantified with both an anthropomorphic and a quality assurance phantom. Gantry angle accuracy was measured with respect to gantry speed modulation. Conventional ABC-CBCT scan time ranged from . Fast ABC-CBCT scan time ranged from , and respiratory correlated CBCT scans took to complete. Voxel intensity value for ABC gated scans was accurate relative to a normal clinical scan with all projections. Uniformity and root mean square error performance degraded as the number of projections used in the reconstruction of the fast ABC-CBCT scans decreased (shortest breath hold, longest free breathing segment). Registration accuracy for small, large, and rotational corrections was within and 1°. Gantry angle accuracy was within 1° for all scans. For high-contrast targets, performance for image-guidance purposes was similar for fast and conventional ABC-CBCT scans and respiration correlated CBCT.
35(2008); http://dx.doi.org/10.1118/1.2990779View Description Hide Description
Total body irradiation (TBI) is used as a preconditioning regimen prior to bone marrow transplant for treatment of hematologic malignancies. During TBI, large volumes of normal tissue are irradiated, and this can lead to toxicities, most significantly in the lungs. Intensity modulated total marrow irradiation (IMTMI) may be able to reduce these toxicities by directly targeting the bone marrow while minimizing the dose to critical structures. The goal of this study was to assess the feasibility of IMTMI by following the planning and delivery process for a Rando phantom. A three isocenter technique was used to provide a full body plan for treatment on a linear accelerator. Thermoluminescent detectors(TLDs) were placed at 22 positions throughout the phantom to compare the delivered doses to the planned doses. Individual intensity modulated radiation therapy verification plans were delivered to a solid water phantom for the three isocenters, and doses measured from an ion chamber and film were compared to the planned doses. The treatment plan indicated that target coverage was achieved with this IMTMI technique, and that the doses to critical structures were reduced by 29%–65% compared to conventional TBI. TLD readings demonstrated accurate dose delivery, with an average difference of 3.5% from the calculated dose. Ion chamber readings for the verification plans were all within 3% of the expected dose, and film measurements showed accurate dose distributions. Results from this study suggest that IMTMI using the three isocenter technique can be accurately delivered and may result in substantial dose reductions to critical structures.
A Comparison of daily megavoltage CT and ultrasound image guided radiation therapy for prostate cancer35(2008); http://dx.doi.org/10.1118/1.3013550View Description Hide Description
In order to quantify the differences between ultrasound-imaging and megavoltage-CT (MVCT) daily prostate localization in prostate-cancer radiotherapy and their dosimetric impacts, daily shifts were analyzed for a total of 140 prostate cancer patients; 106 positioned using ultrasound-based imaging [B-mode Acquisition and Targeting (BAT)], and 34 using the MVCT from a TomoTherapy Hi-Art unit. The shifts indicated by the two systems were compared statistically along the right/left (R/L), superior/inferior (S/I), and anterior/posterior (A/P) directions. The systematic and random variations among the daily alignments were calculated. Margins to account for these shifts were estimated. The mean shifts and standard deviations along the R/L, S/I, and A/P directions were , , and for BAT localizations and , , and for MVCT localizations, respectively. The systematic and random variations in daily shifts based on MVCT were generally smaller than those based on BAT, especially along the A/P direction. A t-test showed this difference to be statistically significant. The planning target volume margins in the A/P direction estimated to account for daily variations were 8.81 and based on MVCT and BAT data, respectively. There was no statistically significant difference in the daily prostate movement pattern between the first few fractions and the remaining fractions. Dosimetric comparison of MVCT and BAT prostate alignments was performed for seven fractions from a patient. The degradation from the plan caused by the MVCT alignment is trivial, while that by BAT is substantial. The MVCT technique results in smaller variations in daily shifts than ultrasound imaging, indicating that MVCT is more reliable and precise for prostate localization. Ultrasound-based localization may overestimate the daily prostate motion, particularly in the A/P direction, negatively impacting prostate dose coverage and rectal sparing.