Use of CT and MR imaging in radiation therapy planning of imaging-diagnosed canine intracranial meningioma achieves better tumor coverage than CT alone.

The aim of this retrospective, secondary analysis study was to quantify the dosimetric impact of the lack of interobserver agreement on gross tumor volume (GTV) delineation for canine meningioma. This study used a previously reported population of 13 dogs with GTVs contoured on CT alone and on registered CT-MR by 18 radiation oncologists. The "true" GTV was generated for each dog using a simultaneous truth and performance-level estimation algorithm, and "true" brain was defined as the whole brain minus true GTV. Treatment plans were generated for each dog and observer combination, using criteria applied to the observer's GTV and brain contours. Plans were then categorized as a pass (met all planning criteria for true GTV and true brain) or fail. A mixed-effects linear regression was performed to examine differences in metrics between CT and CT-MR plans and mixed-effects logistic regression was performed to examine differences in percentages of pass/fail between CT and CT-MRI plans. The mean percent coverage of true GTV by prescribed dose was higher for CT-MR plans than for CT plans (mean difference 5.9%; 95% CI, 3.7-8.0; P < 0.001). There was no difference in the mean volume of true brain receiving ≥24 Gy and in maximum true brain dose between CT plans and CT-MR plans (P ≥ 0.198). CT-MR plans were significantly more likely to pass the criteria for true GTV and true brain than CT plans (OR 1.75; 95% CI, 1.02-3.01; P = 0.044). This study demonstrated significant dosimetric impact when GTV contouring was performed on CT alone compared with CT-MR.

Blood and Hand Surface Lead in Veterinary Workers Using Lead Shielding During Diagnostic Radiography.

OBJECTIVES: The objectives are to compare lead blood concentrations in veterinary workers using lead shielding with concentrations in a control population, to measure hand surface lead before and after use of shielding, and to compare hand surface lead with and without the use of disposable gloves worn under hand shielding. METHODS: Blood and hand wipe samples were analyzed for lead using inductively coupled plasma mass spectrometry. RESULTS: There was no difference in blood lead between exposed and control groups. After lead glove use, 69% (18/26) of hand surface lead samples from workers not using disposable gloves were greater than 500 µg, 42% (11/26) were greater than 1000 µg, and 12% (3/26) were greater than 2000 µg. CONCLUSIONS: If lead shielding use is unavoidable, disposable gloves should be worn, and skin should be decontaminated after use.

Contouring in the optic plane improves the accuracy of computed tomography-based segmentation of the optic pathway.

Canine optic pathway structures are often contoured on CT images, despite the difficulty of visualizing the optic pathway with CT using standard planes. The purpose of this prospective, analytical, diagnostic accuracy study was to examine the accuracy of optic pathway contouring by veterinary radiation oncologists (ROs) before and after training on optic plane contouring. Optic pathway contours used as the gold standard for comparison were created based on expert consensus from registered CT and MRI for eight dogs. Twenty-one ROs contoured the optic pathway on CT using their preferred method, and again following atlas and video training demonstrating contouring on the optic plane. The Dice similarity coefficient (DSC) was used to assess contour accuracy. A multilevel mixed model with random effects to account for repeated measures was used to examine DSC differences. The median DSC (5th and 95th percentile) before and after training was 0.31 (0.06, 0.48) and 0.41 (0.18, 0.53), respectively. The mean DSC was significantly higher after training compared with before training (mean difference = 0.10; 95% CI, 0.08-0.12; P < 0.001) across all observers and patients. DSC values were comparable to those reported (0.4-0.5) for segmentation of the optic chiasm and nerves in human patients. Contour accuracy improved after training but remained low, potentially due to the small optic pathway volumes. When registered CT-MRI images are not available, our study supports routine addition of an optic plane with specific window settings to improve segmentation accuracy in mesaticephalic dogs ≥11 kg.

Providing comparison normal examples alongside pathologic thoracic radiographic cases can improve veterinary students' ability to identify abnormal findings or diagnose disease.

Learning by comparison is a frequently employed education strategy used across many disciplines and levels. Interpreting radiographs requires both skills of perception and pattern recognition, which makes comparison techniques particularly useful in this field. In this randomized, prospective, parallel-group study, students enrolled in second and third-year radiology veterinary courses were given a case-based thoracic radiographic interpretation assignment. A cohort of the participants was given cases with side-by-side comparison normal images while the other cohort only had access to the cases. Twelve cases in total were presented to the students, with 10 cases depicting examples of common thoracic pathologies, while 2 cases were examples of normal. Radiographs of both feline and canine species were represented. Correctness of response to multiple choice questions was tracked, as was year and group (group 1: non compare, Control; group 2: compare, Intervention). Students assigned to group 1 had a lower percentage of correct answers than students assigned to group 2 (45% Control vs. 52% Intervention; P = 0.01). This indicates that side-by-side comparison to a normal example is helpful in identifying disease. No statistical significance was noted for the correctness of responses according to the year of training (P = 0.90). The overall poor performance on the assignment, regardless of group or year, shows that students in the early years of undergraduate veterinary radiology training struggle with the interpretation of common pathologies, likely a result of a lack of exposure to a multitude of cases and normal variants.

X-ray tube operators can be exposed to equal or higher scattered radiation doses to the hand as cassette holders during diagnostic radiographic procedures of the equine vertebral column and limbs.

OBJECTIVE: The objectives of this study were to investigate scattered radiation doses to the hands of equine workers holding the cassette and the x-ray tube by hand, for both limb and vertebral column studies, and to compare the scattered radiation attenuation of lead with radiation protection lead-free gloves. Radiation doses to the hands of the cassette holder in the primary beam were also investigated. SAMPLE: A whole-body horse cadaver. PROCEDURES: A portable x-ray unit was used to simulate 6 radiographic study types in the horse cadaver. Doses were measured with no shielding and, for cassette holders, with the ion chamber enclosed in a lead glove and a lead-free glove. Thirty exposures were performed for each study view and condition (n = 1,920). RESULTS: Mean scattered doses to x-ray unit operators were higher than doses to cassette holders for ungula (hoof), thoracic vertebrae, and lumbar vertebrae studies, whereas doses to cassette holders were higher than doses to x-ray tube operators for studies of the metacarpophalangeal joint (fetlock) and tarsus (hock). Doses did not differ for the stifle joint. Mean percentage decrease in scattered radiation dose was 99.58% with lead gloves and 98.9% with lead-free gloves. CLINICAL RELEVANCE: X-ray tube operators can be exposed to equal or higher scattered radiation doses to the hand as cassette holders. Lead-free hand shielding should only be considered as an alternative to lead gloves if their lighter weight increases frequency of use by workers.

Computed tomography features of presumed normal mandibular and medial retropharyngeal lymph nodes in dogs.

Computed tomographic (CT) features of lymph nodes are used for diagnosis, staging, and response evaluation in veterinary patients. The objectives of this study were to describe maximum short- and long-axis transverse diameters (SATD and LATD, respectively), volume, and attenuation of presumed normal canine mandibular lymph nodes (MLNs) and medial retropharyngeal lymph nodes (MRLNs), to investigate the effect of variables on these features, and to describe inter-observer reliability of transverse diameter and attenuation measurement. Computed tomography studies of 161 dogs were retrospectively reviewed. Median values for SATD were 5.2 and 5.4 mm, median values for volume were 0.26 and 0.54 cm3, and median values for attenuation were 32.3 and 31.8 Hounsfield units (HU) for MLN and MRLN, respectively. Median LATD was 11 mm for MLNs. Height of the first cervical vertebra (HCV1) and weight were positively associated with transverse diameters of both MLNs and MRLNs (P < 0.001). Grade of dental disease was negatively associated with transverse diameters of both MLNs (P = 0.001) and MRLNs (P < 0.03). The intraclass correlation coefficient (ICC) was 0.94 [95% confidence interval (CI): 0.94 to 0.95] for transverse diameter measurement. This study provides descriptive data on the CT characteristics of presumed normal MLNs and MRLNs in dogs and describes a highly reliable method of measuring transverse diameter.

Les caractéristiques tomodensitométriques (CT) des ganglions lymphatiques sont utilisées pour le diagnostic, la stadification et l'évaluation de la réponse chez les patients vétérinaires. Les objectifs de cette étude étaient de décrire les diamètres transversaux maximaux à axe court et long (SATD et LATD, respectivement), le volume et l'atténuation des ganglions lymphatiques mandibulaires canins (MLN) et des ganglions lymphatiques rétropharyngés médians (MRLN) présumés normaux, afin d'étudier l'effet de variables sur ces caractéristiques, et pour décrire la fiabilité inter-observateur de la mesure du diamètre transversal et de l'atténuation. Des études de tomodensitométrie de 161 chiens ont été examinées rétrospectivement. Les valeurs médianes pour la SATD étaient de 5,2 et 5,4 mm, les valeurs médianes pour le volume étaient de 0,26 et 0,54 cm3, et les valeurs médianes pour l'atténuation étaient de 32,3 et 31,8 unités Hounsfield (HU) pour MLN et MRLN, respectivement. Le LATD médian était de 11 mm pour les MLN. La hauteur de la première vertèbre cervicale (VHC1) et le poids étaient positivement associés aux diamètres transversaux des MLN et des MRLN (P < 0,001). Le grade de maladie dentaire était négativement associé aux diamètres transversaux des MLN (P = 0,001) et des MRLN (P < 0,03). Le coefficient de corrélation intraclasse (ICC) était de 0,94 [intervalle de confiance (IC) à 95 % : 0,94 à 0,95] pour la mesure du diamètre transversal. Cette étude fournit des données descriptives sur les caractéristiques CT des MLN et MRLN présumés normaux chez le chien et décrit une méthode très fiable de mesure du diamètre transversal.(Traduit par Docteur Serge Messier).

Inter- and intraobserver agreement for CT measurement of mandibular and medial retropharyngeal lymph nodes is excellent in dogs with histologically confirmed oral melanoma.

Mandibular and medial retropharyngeal lymph nodes are routinely evaluated with CT when staging dogs with oral melanomas. While size alone is considered inadequate for detecting nodal metastasis, it is critical in evaluating treatment response, as clinical decisions are based on changes in size. It is common for different radiologists to measure the size of pre- and posttreatment lymph nodes in the same patient. The objective of this retrospective, observer agreement study was to evaluate the inter- and intraobserver agreement in measuring canine mandibular and medial retropharyngeal lymph nodes by a diverse population of veterinary radiologists and trainees. Fourteen dogs with documented oral melanoma and head CT studies identified from records of a single institution were included in this study. North American veterinary radiologists and trainees were recruited to measure the mandibular and medial retropharyngeal lymph nodes; in triplicate. Prior to performing the study measurements, participants completed a training tool demonstrating the lymph node measurements. Overall, interobserver intraclass correlation coefficient (ICC) was 0.961 (95% confidence interval [CI]: 0.946, 0.972) and intraobserver ICC was 0.977 (95% CI: 0.968, 0.983), indicating excellent agreement (ICC > 0.9 considered excellent). Similar findings were noted following sub-analysis for most variables (experience, size, laterality, axis of measurement). These results suggest that follow-up measurement of the long and short axis of the mandibular lymph nodes and short axis of the medial retropharyngeal lymph nodes in the transverse plane, performed by different veterinary radiologists using the same method of measure, should have minimal impact on clinical decision making.

Workers should take steps to mitigate surface lead exposure when using lead-containing personal protective equipment.

Veterinary workers use lead shielding for protection against harm from ionizing radiation during diagnostic imaging. Surface lead on shielding has been reported as a potential exposure hazard. The purpose of this prospective, analytical, descriptive study was to measure surface lead on lead-containing shielding in the field service and small animal radiology areas of a veterinary teaching hospital, and to examine potential risk factors for surface lead. Surface lead on a convenience sample of 54 lead shielding items was measured in a commercial laboratory using inductively coupled plasma mass spectrometry. The median and range of surface lead for aprons, thyroid collars, and gloves were 3.6 µg/dm2 (range, 1.3-22.4 µg/dm2 ), 23.1 µg/dm2 (range, 2.6-116 µg/dm2 ), and 134 µg/dm2 (range, 1.5-155 µg/dm2 ), respectively. In the final multivariable analysis, the mean surface lead on thyroid collars and gloves was higher than on aprons (relative differences 4.8, 95% confidence interval [CI] 2.2, 10.5, P < 0.001; and 9.5, 95% CI 4.4, 20.6, P < 0.001, respectively). The mean surface lead on shielding in the worst condition was higher than on shielding in the intermediate and best conditions (P < 0.001). Lead shielding provides effective protection against the harmful effects of ionizing radiation, and shielding should always be worn during radiation exposure. Based on our findings and the common use of lead shielding in veterinary workplaces, we recommend that employers inform workers of this hazard and provide training on steps to mitigate exposure, including the use of disposable gloves when wearing lead shielding and handwashing after use.

Survey of self-reported radiation safety practices among North American veterinary technicians involved in equine radiography using portable x-ray equipment.

OBJECTIVES: To describe self-reported radiation safety practices by equine veterinary technicians in North America and identify factors associated with these practices. SAMPLE: 154 equine technicians. PROCEDURES: An electronic questionnaire regarding radiation safety practices during the use of portable x-ray equipment was sent to 884 members of the American Association of Equine Veterinary Technicians and Assistants. Data were summarized, and various factors were evaluated for associations with reported safety practices. RESULTS: 221 of 884 (25.0%) questionnaires were completed, including 154 by equine technicians who had been involved in equine radiography as x-ray tube operators, cassette holders, or both in the previous year. Lead apron use was suboptimal, reported as "always" for 80.0% (104/130) of tube operators and 83.1% (123/148) of cassette holders. Approximately 20% of participants never wore thyroid shields, and approximately 90% never wore lead eyeglasses. Almost 50% of participants did not have lead eyeglasses available. Although > 55% of participants always held the x-ray equipment by hand, 58.4% (73/125) of tube operators and 25.0% (35/140) of cassette holders never wore gloves. Cassette holders wore lead gloves and personal radiation dose-monitoring devices significantly more frequently than did tube operators. CONCLUSIONS AND CLINICAL RELEVANCE: Compliance of North American equine technicians with radiation safety recommendations by the National Council on Radiation Protection and Measurements was suboptimal. Improvements in radiation safety training and education, strengthening the connection between academic institutions and private practices, and greater availability and requirement of personal protective equipment use by senior clinicians and employers might aid in improving safety practices.

Self-reported radiation safety behaviors among veterinary specialists and residents performing fluoroscopic procedures on small animals.

OBJECTIVE: To describe the radiation safety behaviors of veterinary specialists performing small animal fluoroscopic procedures and examine potential risk factors for these behaviors, including knowledge of radiation risk and training regarding machine operating parameters. SAMPLE: 197 veterinary specialists and residents in training. PROCEDURES: An electronic questionnaire was distributed to members of the American Colleges of Veterinary Internal Medicine (subspecialties of cardiology and small animal internal medicine), Veterinary Radiology, and Veterinary Surgery. RESULTS: The overall survey response rate was 6% (240/4,274 email recipients). Of the 240 respondents, 197 (82%) had operated an x-ray unit for a small animal fluoroscopic procedure in the preceding year and fully completed the questionnaire. More than 95% of respondents believed that radiation causes cancer, yet approximately 60% of respondents never wore hand or eye protection during fluoroscopic procedures, and 28% never adjusted the fluoroscopy machine operating parameters for the purpose of reducing their radiation dose. The most common reasons for not wearing eye shielding included no requirement to wear eyeglasses, poor fit, discomfort, and interference of eyeglasses with task performance. Respondents who had received training regarding machine operating parameters adjusted those parameters to reduce their radiation dose during procedures significantly more frequently than did respondents who had not received training. CONCLUSIONS AND CLINICAL RELEVANCE: On the basis of the self-reported suboptimal radiation safety practices among veterinary fluoroscopy users, we recommend formal incorporation of radiation safety education into residency training programs. All fluoroscopy machine operators should be trained regarding the machine operating parameters that can be adjusted to reduce occupational radiation exposure.

Contrast enhanced computed tomography assessment of superficial inguinal lymph node metastasis in canine mammary gland tumors.

Mammary gland neoplasms are predominant in dogs. However, sentinel lymph node (SLN) status assessment criteria have not been established for these cases. In this retrospective, secondary analysis, diagnostic case control study, CT images of 65 superficial inguinal SLNs were obtained before and 1, 3, 5, and 10 min after intravenous administration of contrast agent (iopamidol 370 mgI/mL). The presence and degree of postcontrast enhancement were assessed, by means of the median absolute density value and the maximum absolute density value at any time point in the center and in the periphery of each SLN measured in Hounsfield units (HU), before and after contrast agent administration. These values were compared with histopathological findings postsurgical excision. Receiver operating characteristic analysis was conducted. The absolute density values ranged widely at each time point and within each group of nodes (negative, positive, control group). At all time points, the median density value in the center and in the periphery was significantly higher in metastatic than in non-metastatic SLNs (P ≤ .014). Among the parameters tested, the median absolute density value measured in the periphery of the SLN 3 min after injection showed the highest sensitivity, specificity, and accuracy (AUC) (87.5%, 82.1%, and 92.1% respectively), with a cutoff value of 50.9 HU. The maximum absolute density value at any time point in the center and periphery of the SLNs was also significantly higher in metastatic SLNs compared to non-metastatic (P ≤ .001). With a cutoff value of 59.5 HU, the maximum absolute density value in the periphery of the SLN displayed high sensitivity and specificity (87.5% and 89.3%, respectively). The results of this study support the hypothesis that contrast enhanced CT imaging may aid in the assessment of SLN metastasis in dogs with mammary gland neoplasms.

Radiation safety practices among Canadian equine veterinary workers during diagnostic procedures with portable X-ray equipment.

The objective of this study was to report observed radiation safety practices among equine workers using portable X-ray units at 3 veterinary practices in Canada. Practice 1 was comprised of equine field service workers from an academic institution, whereas Practices 2 and 3 were comprised of workers from private equine referral practices. Out of 2969 exposure-worker observations, protective aprons were worn for > 99% (2968/2969), thyroid shields for 87% (2584/2969), and eyeglasses for 10% (310/2969). The cassette was held by hand for 97% (1078/1114) of observations, and the X-ray unit was held by hand for 100% (1293/1293) of observations. Gloves were used for 54% (606/1114) and < 1% (5/1293) of observations involving cassette holders and X-ray tube operators, respectively. Use of gloves was more frequent among technologists than DVMs (P = 0.004), and among workers in Practices 1 and 3 than in Practice 2 (P < 0.001). In conclusion, given that workers commonly hold cassettes and X-ray tubes by hand, improvement in shielding use is needed. Consideration should be given to specific behavior recommendations in radiation regulations.

Pratiques de radioprotection chez les travailleurs vétérinaires équins canadiens pendant les procédures de diagnostic avec un équipement de radiographie portable. L'objectif de la présente étude était de rapporter les pratiques de radioprotection observées chez les travailleurs équins utilisant des appareils de radiographie portatifs dans trois pratiques vétérinaires au Canada. La pratique 1 était composée de travailleurs équins sur le terrain d'une institution universitaire, tandis que les pratiques 2 et 3 étaient composées de travailleurs sur le terrain issus de pratiques de référence équines privées. Sur 2969 observations de travailleurs exposés, des tabliers de protection ont été portés pour > 99 % (2968/2969), des écrans thyroïdiens pour 87 % (2584/2969) et des lunettes pour 10 % (310/2969). La cassette a été tenue à la main pour 97 % (1078/1114) des observations et l'unité de radiographie a été tenue à la main pour 100 % (1293/1293) des observations. Des gants ont été utilisés pour 54 % (606/1114) et < 1 % (5/1293) des observations impliquant des porteurs de cassette et des opérateurs de tubes à rayons X, respectivement. L'utilisation de gants était plus fréquente chez les technologues que chez les vétérinaires (P = 0,004) et chez les travailleurs des Pratiques 1 et 3 que ceux de la Pratique 2 (P < 0,001). En conclusion, étant donné que les travailleurs tiennent généralement des cassettes et des tubes à rayons X à la main, une amélioration de l'utilisation du blindage est nécessaire. Il faudrait tenir compte des recommandations de comportement spécifiques dans les réglementations relatives aux rayonnements.(Traduit par Dr Serge Messier).

Comparing Two Resources Used to Teach Pulmonary Patterns for a Flipped Veterinary Radiology Course.

The flipped classroom has been gaining momentum within medical education circles. Pre-class assignments are an important component of this pedagogical approach. In this study, a section of the introductory course to veterinary medical imaging was taught using a flipped classroom, and the effectiveness of two different pre-classroom assignments was evaluated. The pre-classroom assignments consisted of either short videos or readings. Both had similar content, which included basic information about pulmonary patterns of disease on chest radiographs. Learning outcomes were assessed by in-classroom and final examination questions. Student learning self-assessments and student satisfaction were also evaluated via an online survey. Students in the video group answered more of the in-classroom questions correctly (71% video vs. 63% reading group; p = .01) and had higher scores on the final examination (83% video vs. 75% reading group; p = .02). There was also a higher student satisfaction with the videos versus the reading assignment. However, we found no significant difference in the student self-assessments of learning or participation in class. An additional finding of this study related to the ongoing difficulties students were having with the learning objectives, including differentiating a pathological process from a normal, or normal variant, recognizing the different pulmonary patterns, and developing a differential diagnoses list, despite the pre-classroom assignments and large group learning sessions. This speaks to the difficulty in developing confidence in pulmonary pattern recognition on chest radiographs, a skill that requires considerable training and time investment.

A 7-min video training intervention improves worker short-term radiation safety behavior during small animal diagnostic radiography.

Although manual restraint for small animal diagnostic radiography is common, worker protection is often not optimized, particularly for hands and eyes. Radiation safety training videos generally include hours of material on content other than personal protective equipment (PPE), have limited content, if any, on reducing dose to the lens of the eye, and are presented at the level of veterinary professionals. The objectives of this prospective, observational study were to develop a short, open access video training intervention at the layperson level, focused on proper use of PPE, and to test the effectiveness of the training video in changing behavior of workers. The use of PPE, optimal head position, and body position relative to the source of scattered radiation were compared before and after the video training was completed by workers. Results of unconditional and multivariable analyses were similar. In final multivariable analysis, workers wore gloves correctly more frequently (odds ratio [OR] = 2.09; 95% confidence interval [CI], 1.68-2.59; P < .001) and wore eyewear more frequently (OR = 1.85; 95% CI, 1.23-2.78; P = .003) after completing the training intervention. Workers also had an optimal head position more frequently (OR = 1.27; 95% CI, 1.03-1.58; P = .03) and stood straight or leaned back more frequently (OR = 1.85; 95% CI, 1.48-2.23; P < .001) after completing the training. The video training developed in this study is an effective tool that can be incorporated into a radiation protection program to improve worker radiation safety behaviors during manual restraint for small animal diagnostic radiography.

Use of MRI increases interobserver agreement on gross tumor volume for imaging-diagnosed canine intracranial meningioma.

There is a lack of information regarding interobserver agreement on canine meningioma gross tumor volume (GTV) delineation, and on the impact of MRI on this agreement. The objectives of this retrospective, secondary analysis, observer agreement study were to describe agreement between veterinary radiation oncologists on GTV for canine intracranial meningioma, and to compare interobserver agreement between delineation based on CT alone and delineation based on fused CT-MRI. Eighteen radiation oncologists delineated GTV for 13 dogs with an imaging diagnosis of meningioma on pre- and postcontrast CT, pre- and postcontrast T1-weighted magnetic resonance, and T2-weighted magnetic resonance images. Dice similarity coefficient (DSC), concordance index (CI), and center of volume (COV) were used to quantify interobserver agreement. Multilevel mixed models were used to examine the difference in volume, DSC, CI and COV 3D distance between CT and CT-MR imaging. The mean volume for GTV contours delineated using fused CT-MRI was larger than when CT alone was used for delineation (mean difference CT-MR - CT = 0.89 cm3, 95% CI 0.66 to 1.12, P < .001). Interobserver agreement on GTV was improved when MRI was used; the mean DSC and CI were higher, and the mean COV 3D distance was lower, when fused CT-MRI was used than when CT alone was used (P < .001 for all differences). Based on our results, fused CT-MRI is recommended for radiation therapy planning of canine intracranial meningioma.

Setup error with and without image guidance using two canine intracranial positioning systems for radiation therapy.

Daily image guidance reduces inter-fractional variation in patient position for intracranial radiation therapy. However, the ability to detect and correct positioning errors is limited below a certain level. Because of these limitations, the accuracy achieved with a positioning system prior to image guidance may affect the error remaining after image guidance (the residual setup error). The objective of this study was to compare the setup accuracy achieved before and after megavoltage (MV) and cone-beam computed tomography (CBCT) guidance between two intracranial positioning systems. Equipment included a four degrees-of-freedom couch capable of 1 mm translational moves. Six dog cadavers were positioned 24 times as for clinical treatment in a head re-positioner (HPS), and the coordinates of five fiducial markers were measured before and after image-guided correction. The values obtained for the HPS were compared with those previously reported for the standard positioning system (SPS) used at this facility. The mean three-dimensional distance vector (3DDV) was lower for the HPS than for the SPS when no image guidance was used (P = .019). The mean 3DDV after MV guidance was lower for the HPS than for the SPS (P = .027), but not different after CBCT guidance (P = .231). The 95th percentiles of the 3DDV after MV and CBCT guidance were 2.1 and 2.9 mm, respectively, for the HPS, and 2.8 and 3.6 mm for the SPS. The setup error after MV guidance was lower for the positioning system that achieved a more accurate patient position before image guidance.

Residual setup error in the canine intracranial region after megavoltage, kilovoltage, or cone-beam computed tomographic image guidance for radiation therapy.

Sources of residual setup error after image guidance include image localization accuracy, errors associated with image registration, and inability of some treatment couches to correct submillimeter translational errors and/or pitch and roll errors. The purpose of this experimental study was to measure setup error after image-guided correction of the canine intracranial region, using a four degrees-of-freedom couch capable of 1 mm translational moves. Six cadaver dogs were positioned 45 times as for clinical treatment using a vacuum deformable body cushion, a customizable head cushion, a thermoplastic mask and an indexed maxillary plate with a dental mould. The location of five fiducial markers in the skull bones was compared between the reference position and after megavoltage (MV), kilovoltage (kV) and cone-beam computed tomography (CBCT)-guided correction using orthogonal kV images. The mean three-dimensional distance vectors (3DDV) after MV, kV and CBCT-guided correction were 1.7, 1.5 and 2.2 mm, respectively. All values were significantly different (P < .01). The 95th percentiles of the 3DDV after online MV, kV and CBCT-guided correction were 2.8, 2.6 and 3.6 mm, respectively. Residual setup error in the clinical scenario examined was on the order of millimetres and should be considered when choosing PTV margins for image-guided radiation therapy of the canine intracranial region.