Helical tomotherapy setup variations in canine nasal tumor patients immobilized with a bite block

The purpose of our study was to compare setup variation in four degrees of freedom (vertical, longitudinal, lateral, and roll) between canine nasal tumor patients immobilized with a mattress and bite block, versus a mattress alone. Our secondary aim was to define a clinical target volume (CTV) to planning target volume (PTV) expansion margin based on our mean systematic error values associated with nasal tumor patients immobilized by a mattress and bite block. We evaluated six parameters for setup corrections: systematic error, random error, patient-patient variation in systematic errors, the magnitude of patient-specific random errors (root mean square [RMS]), distance error, and the variation of setup corrections from zero shift. The variations in all parameters were statistically smaller in the group immobilized by a mattress and bite block. The mean setup corrections in the mattress and bite block group ranged from 0.91 mm to 1.59 mm for the translational errors and 0.5°. Although most veterinary radiation facilities do not have access to Image-guided radiotherapy (IGRT), we identified a need for more rigid fixation, established the value of adding IGRT to veterinary radiation therapy, and define the CTV-PTV setup error margin for canine nasal tumor patients immobilized in a mattress and bite block.     

Treatment outcomes and target delineation utilizing CT and MRI in 13 dogs treated with a uniform stereotactic radiation therapy protocol (16 Gy single fraction) for pituitary masses: (2014‐2017)

Canine pituitary tumours are increasingly treated with stereotactic radiotherapy (SRT). Here, we report clinical outcomes in dogs treated with single‐fraction SRT; we also explore technical aspects of SRT treatment planning. A single‐institution retrospective study was performed, including any dog with a pituitary mass (PM) that was treated using a standardized single‐fraction (16 Gy) SRT protocol between 2014 and 2017. Via medical records review, 13 cases were identified. Nine dogs neurologically improved after SRT. Four dogs experienced MRI‐documented tumour volume reduction. Nine dogs experienced neurologic decline in 1.5 to 18 months after SRT and were euthanized. The median overall survival time was 357 days, with 15% alive 18 months after SRT. To better understand whether SRT target delineation is predictably altered by use of magnetic resonance imaging (MRI) in addition to computed tomography (CT), two radiation oncologists (RO) retrospectively re‐evaluated all imaging studies used for SRT planning in these 13 cases. Gross tumour volume (GTV) was contoured on co‐registered CT and MRIs for each case. In seven cases, CT alone was deemed inadequate for GTV contouring by at least one RO. T1 post‐contrast MRI was considered the ideal image for GTV contouring in 11 cases. Contouring on MRI yielded larger GTV than CT for 11 cases. Inter‐observer variability existed in each case and was greater for MRI. In summary, use of co‐registered CT and MRI images is generally considered advantageous for PM delineation when using SRT. Notably, survival times reported herein are shorter than what has previously been reported for PM treated with finely fractionated full‐course RT protocols.     

COMPARISON OF ISODOSE DISTRIBUTIONS IN CANINE BRAIN IN HETEROGENEITY-CORRECTED VERSUS UNCORRECTED TREATMENT PLANS USING 6 MV PHOTONS

Magnetic resonance (MR) images may be useful for radiation planning due to greater contrast resolution. One disadvantage of MR images for radiation planning is the inability to incorporate electron density information into the dose calculation algorithm. To assess the magnitude of this problem, we evaluated radiation dose distribution in canine brain by comparing computed tomography (CT)-based radiotherapy plans with and without electron density correction. Computerized radiotherapy plans were generated for 13 dogs with brain tumors using 6 MV photons. A tissue-contouring program was used to outline the gross tumor volume (GTV) and the planning target volume (PTV) for each patient. Two treatment plans were generated for each dog. First, the plan was optimized without heterogeneity correction. Then the heterogeneity correction was implemented without changing any other plan parameters. Isodose distributions and dose volume histograms (DVHs) were used to compare the two plans. The D95 (dose delivered to 95% of the volume) within the PTV was calculated for each treatment plan and differences in the D95s were compared. The mean D95s without and with heterogeneity correction were 49.1 +/- 0.7 and 48.9 +/- 1.0Gy, respectively. The absolute mean percent dose difference without and with heterogeneity correction was 1.0 - 0.9% (-1.3-3.2%) and was not considered to be clinically significant. We found no clinically significant difference between CT-based radiotherapy plans without and with heterogeneity correction for brain tumors in small animals, which supports the use of MR-based treatment planning for radiotherapy of small animal brain tumors.     

REPEATABILITY OF A PLANNING TARGET VOLUME EXPANSION PROTOCOL FOR RADIATION THERAPY OF REGIONAL LYMPH NODES IN CANINE AND FELINE PATIENTS WITH HEAD TUMORS

For canine and feline patients with head tumors, simultaneous irradiation of the primary tumor and mandibular and retropharyngeal lymph nodes (LNs) is often indicated. The purpose of this study was to assess the repeatability of a planning target volume (PTV) expansion protocol for these LNs. Two CT image sets from 44 dogs and 37 cats that underwent radiation therapy for head tumors were compared to determine LN repositioning accuracy and precision; planning‐CT (for radiation therapy planning) and cone‐beam CT (at the time of actual treatment sessions). Eleven percent of dogs and 65% of cats received treatment to their LNs. In dogs, the mandibular LNs were positioned more caudally (P = 0.0002) and the right mandibular and right retropharyngeal LNs were positioned more to the left side of the patient (P = 0.00015 and P = 0.003, respectively). In cats, left mandibular LN was positioned higher (toward roof) than the planning‐CT (P = 0.028). In conclusion, when the patient immobilization devices and bony anatomy matching are used to align the primary head target and these LNs are treated simultaneously, an asymmetrical PTV expansion that ranges 4–9 mm (dogs) and 2–4 mm (cats), depending on the directions of couch movement, should be used to include the LNs within the PTV at least 95% of the time.     

Changes in target volume during irradiation of canine intranasal tumors can significantly impact radiation dosimetry

Nasal tumor size can change during radiation therapy (RT). The amount of peritumoral fluid (eg, mucohemorrhagic effusions) can also fluctuate. How often this occurs and the magnitude of change are unknown. Likewise, there are no data which describe dosimetric effects of these changing volumes during a course of RT in veterinary medicine. This study addresses that gap in knowledge. Using pet dogs with nasal tumors, three CT image sets were created. Different Hounsfield units were applied to the gross tumor volume (GTV) of each image set: unchanged, –1000 (AIR), –1000 (to the portion of the GTV that actually underwent volume reduction during clinical RT; REAL). Two plans were created: 18‐fraction three‐dimensional conformal RT (3DCRT) and three‐fraction intensity‐modulated stereotactic RT (IM‐SRT). For nearby normal tissues and GTV, near‐maximum doses (D_2% and D_5%) and volumes receiving clinically significant doses were recorded. To verify “AIR” results, thermoluminescent dosimeters recorded dose in cadavers that were irradiated using both 3DCRT and IM‐SRT plans. “AIR” scenario had ≤1.5 Gray (Gy) increases in D_2% and ≤3.2 cc increases of volume. “REAL” scenario had ≤0.97 Gy increases in D_5% and ≤0.55 cc increases of volume at clinically relevant doses. Both were statistical significant. Results suggest that near‐complete resolution of GTV warrants plan revision.     

A COMPARISON OF NORMAL TISSUE COMPLICATION PROBABILITY OF BRAIN FOR PROTON AND PHOTON THERAPY OF CANINE NASAL TUMORS

This study compared the calculated normal tissue complication probability of brain in dogs with a nasal tumor, which had both photon and proton treatment planning. Nine dogs diagnosed with a variety of histologies, but all with large, caudally located nasal tumors were studied. Three-dimensional (3-D) photon dose distribution, and a proton dose distribution was calculated for each dog. To calculate the normal tissue complication probability (NTCP) for brain, the partial brain volume irradiated with the prescribed dose was determined, then a mathematic model relating complications to partial volume and radiation dose was used. The NTCP was always smaller for proton plans as compared to photon plans, indicating conformation of the dose to the target allows a higher dose to be given. If a 5% NTCP were accepted, the mean applicable dose for this group of dogs was 50.2 Gy for photons, but 58.3 Gy for protons. Not all dogs would benefit the same from proton irradiation. If a large partial brain volume has to be irradiated, the advantage becomes minimal. There is also a minimal advantage if the planning target volume (PTV) includes a small, superficial brain volume. However, for a complex PTV shape the degree of conformation is clearly superior for protons and results in smaller calculated NTCPs.     

INVITED REVIEW—IMAGE REGISTRATION IN VETERINARY RADIATION ONCOLOGY: INDICATIONS,IMPLICATIONS, AND FUTURE ADVANCES

The field of veterinary radiation therapy (RT) has gained substantial momentum in recent decades with significant advances in conformal treatment planning, image‐guided radiation therapy (IGRT), and intensity‐modulated (IMRT) techniques. At the root of these advancements lie improvements in tumor imaging, image alignment (registration), target volume delineation, and identification of critical structures. Image registration has been widely used to combine information from multimodality images such as computerized tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) to improve the accuracy of radiation delivery and reliably identify tumor‐bearing areas. Many different techniques have been applied in image registration. This review provides an overview of medical image registration in RT and its applications in veterinary oncology. A summary of the most commonly used approaches in human and veterinary medicine is presented along with their current use in IGRT and adaptive radiation therapy (ART). It is important to realize that registration does not guarantee that target volumes, such as the gross tumor volume (GTV), are correctly identified on the image being registered, as limitations unique to registration algorithms exist. Research involving novel registration frameworks for automatic segmentation of tumor volumes is ongoing and comparative oncology programs offer a unique opportunity to test the efficacy of proposed algorithms.     

Interobserver reliability of computed tomographic contouring of canine tonsils in radiation therapy treatment planning

In radiation therapy (RT) treatment planning for canine head and neck cancer, the tonsils may be included as part of the treated volume. Delineation of tonsils on computed tomography (CT) scans is difficult. Error or uncertainty in the volume and location of contoured structures may result in treatment failure. The purpose of this prospective, observer agreement study was to assess the interobserver agreement of tonsillar contouring by two groups of trained observers. Thirty dogs undergoing pre‐ and post‐contrast CT studies of the head were included. After the pre‐ and postcontrast CT scans, the tonsils were identified via direct visualization, barium paste was applied bilaterally to the visible tonsils, and a third CT scan was acquired. Data from each of the three CT scans were registered in an RT treatment planning system. Two groups of observers (one veterinary radiologist and one veterinary radiation oncologist in each group) contoured bilateral tonsils by consensus, obtaining three sets of contours. Tonsil volume and location data were obtained from both groups. The contour volumes and locations were compared between groups using mixed (fixed and random effect) linear models. There was no significant difference between each group's contours in terms of three‐dimensional coordinates. However there was a significant difference between each group's contours in terms of the tonsillar volume (P < 0.0001). Pre‐ and postcontrast CT can be used to identify the location of canine tonsils with reasonable agreement between trained observers. Discrepancy in tonsillar volume between groups of trained observers may affect RT treatment outcome.     

A comparison of two dose calculation algorithms—anisotropic analytical algorithm and Acuros XB—for radiation therapy planning of canine intranasal tumors

Although anisotropic analytical algorithm (AAA) and Acuros XB (AXB) are both radiation dose calculation algorithms that take into account the heterogeneity within the radiation field, Acuros XB is inherently more accurate. The purpose of this retrospective method comparison study was to compare them and evaluate the dose discrepancy within the planning target volume (PTV). Radiation therapy (RT) plans of 11 dogs with intranasal tumors treated by radiation therapy at the University of Georgia were evaluated. All dogs were planned for intensity‐modulated radiation therapy using nine coplanar X‐ray beams that were equally spaced, then dose calculated with anisotropic analytical algorithm. The same plan with the same monitor units was then recalculated using Acuros XB for comparisons. Each dog's planning target volume was separated into air, bone, and tissue and evaluated. The mean dose to the planning target volume estimated by Acuros XB was 1.3% lower. It was 1.4% higher for air, 3.7% lower for bone, and 0.9% lower for tissue. The volume of planning target volume covered by the prescribed dose decreased by 21% when Acuros XB was used due to increased dose heterogeneity within the planning target volume. Anisotropic analytical algorithm relatively underestimates the dose heterogeneity and relatively overestimates the dose to the bone and tissue within the planning target volume for the radiation therapy planning of canine intranasal tumors. This can be clinically significant especially if the tumor cells are present within the bone, because it may result in relative underdosing of the tumor.     

USE OF CONE‐BEAM COMPUTED TOMOGRAPHY TO CHARACTERIZE DAILY URINARY BLADDER VARIATIONS DURING FRACTIONATED RADIOTHERAPY FOR CANINE BLADDER CANCER

Urinary bladder cancer is difficult to treat accurately with fractionated radiation therapy (RT) due to daily positional changes of the bladder and surrounding soft‐tissue structures. We quantified the daily motion experienced by the canine bladder with patients in dorsal vs. sternal vs. lateral recumbency. We also described the dose distribution for three different planning target volume expansions (5, 10, and 15 mm) for each of the three positions to ensure adequate bladder dose and minimize irradiation of nearby healthy tissues. Analysis was based on data from retrospective daily cone‐beam computed tomography (CT) (CBCT) images obtained for positioning of canine patients undergoing routine RT. Organs of interest were contoured on each CBCT data set and the images, along with the contours, were registered to the original planning CT. All measurements were made relative to the planning CT and dosimetric data for the organs of interest was determined using a dose volume histogram generated from sample parallel‐opposed beam configuration. There was a wide range in bladder position throughout treatment. The least amount of bladder variation and the lowest rectal dose was with dogs in lateral recumbency. It was also determined that a margin of 10 mm would allow for sufficient dose to be delivered to the bladder while minimizing rectal dose.     

Using biologically based objectives to optimize boost intensity‐modulated radiation therapy planning for brainstem tumors in dogs

Irradiated brain tumors commonly progress at the primary site, generating interest in focal dose escalation. The aim of this retrospective observational study was to use biological optimization objectives for a modeling exercise with simultaneously‐integrated boost IMRT (SIB‐IMRT) to generate a dose‐escalated protocol with acceptable late radiation toxicity risk estimate and improve tumor control for brainstem tumors in dogs safely. We re‐planned 20 dog brainstem tumor datasets with SIB‐IMRT, prescribing 20 × 2.81 Gy to the gross tumor volume (GTV) and 20 × 2.5 Gy to the planning target volume. During the optimization process, we used biologically equivalent generalized equivalent uniform doses (gEUD) as planning aids. These were derived from human data, calculated to adhere to normal tissue complication probability (NTCP) ≤5%, and converted to the herein used fractionation schedule. We extracted the absolute organ at risk dose‐volume histograms to calculate NTCP of each individual plan. For planning optimization, gEUD_{(a = 4)} = 39.8 Gy for brain and gEUD_{(a = 6.3)} = 43.8 Gy for brainstem were applied. Mean brain NTCP was low with 0.43% (SD ±0.49%, range 0.01‐2.04%); mean brainstem NTCP was higher with 7.18% (SD ±4.29%, range 2.87‐20.72%). Nevertheless, NTCP of < 10% in brainstem was achievable in 80% (16/20) of dogs. Spearman's correlation between relative GTV and NTCP was high (ρ = 0.798, P < .001), emphasizing increased risk with relative size even with subvolume‐boost. Including biologically based gEUD values into optimization allowed estimating NTCP during the planning process. In conclusion, gEUD‐based SIB‐IMRT planning resulted in dose‐escalated treatment plans with acceptable risk estimate of NTCP < 10% in the majority of dogs with brainstem tumors. Risk was correlated with relative tumor size.     

Three‐dimensional conformal versus non‐graphic radiation treatment planning for apocrine gland adenocarcinoma of the anal sac in 18 dogs (2002–2007)

Differences in dose homogeneity and irradiated volumes of target and surrounding normal tissues between 3D conformal radiation treatment planning and simulated non‐graphic manual treatment planning were evaluated in 18 dogs with apocrine gland adenocarcinoma of the anal sac. Overall, 3D conformal treatment planning resulted in more homogenous dose distribution to target tissues with lower hot spots and dose ranges. Dose homogeneity and guarantee of not under‐dosing target tissues with 3D conformal planning came at the cost, however, of delivering greater mean doses of radiation and of irradiating greater volumes of surrounding normal tissue structures.     

DOSIMETRIC IMPACT OF DAILY SETUP VARIATIONS DURING TREATMENT OF CANINE NASAL TUMORS USING INTENSITY-MODULATED RADIATION THERAPY

Intensity-modulated radiation therapy (IMRT) can be employed to yield precise dose distributions that tightly conform to targets and reduce high doses to normal structures by generating steep dose gradients. Because of these sharp gradients, daily setup variations may have an adverse effect on clinical outcome such that an adjacent normal structure may be overdosed and/or the target may be underdosed. This study provides a detailed analysis of the impact of daily setup variations on optimized IMRT canine nasal tumor treatment plans when variations are not accounted for due to the lack of image guidance. Setup histories of ten patients with nasal tumors previously treated using helical tomotherapy were replanned retrospectively to study the impact of daily setup variations on IMRT dose distributions. Daily setup shifts were applied to IMRT plans on a fraction-by-fraction basis. Using mattress immobilization and laser alignment, mean setup error magnitude in any single dimension was at least 2.5 mm (0–10.0 mm). With inclusions of all three translational coordinates, mean composite offset vector was 5.9±3.3 mm. Due to variations, a loss of equivalent uniform dose for target volumes of up to 5.6% was noted which corresponded to a potential loss in tumor control probability of 39.5%. Overdosing of eyes and brain was noted by increases in mean normalized total dose and highest normalized dose given to 2% of the volume. Findings suggest that successful implementation of canine nasal IMRT requires daily image guidance to ensure accurate delivery of precise IMRT distributions when non-rigid immobilization techniques are utilized. Unrecognized geographical misses may result in tumor recurrence and/or radiation toxicities to the eyes and brain.     

DOSIMETRIC CONSEQUENCES OF USING CONTRAST‐ENHANCED COMPUTED TOMOGRAPHIC IMAGES FOR INTENSITY‐MODULATED STEREOTACTIC BODY RADIOTHERAPY PLANNING

Potential benefits of planning radiation therapy on a contrast‐enhanced computed tomography scan (ceCT) should be weighed against the possibility that this practice may be associated with an inadvertent risk of overdosing nearby normal tissues. This study investigated the influence of ceCT on intensity‐modulated stereotactic body radiotherapy (IM‐SBRT) planning. Dogs with head and neck, pelvic, or appendicular tumors were included in this retrospective cross‐sectional study. All IM‐SBRT plans were constructed on a pre‐ or ceCT. Contours for tumor and organs at risk (OAR) were manually constructed and copied onto both CT's; IM‐SBRT plans were calculated on each CT in a manner that resulted in equal radiation fluence. The maximum and mean doses for OAR, and minimum, maximum, and mean doses for targets were compared. Data were collected from 40 dogs per anatomic site (head and neck, pelvis, and limbs). The average dose difference between minimum, maximum, and mean doses as calculated on pre‐ and ceCT plans for the gross tumor volume was less than 1% for all anatomic sites. Similarly, the differences between mean and maximum doses for OAR were less than 1%. The difference in dose distribution between plans made on CTs with and without contrast enhancement was tolerable at all treatment sites. Therefore, although caution would be recommended when planning IM‐SBRT for tumors near “reservoirs” for contrast media (such as the heart and urinary bladder), findings supported the use of ceCT with this dose calculation algorithm for both target delineation and IM‐SBRT treatment planning.     

ASSESSMENT OF A RADIOTHERAPY PATIENT CRANIAL IMMOBILIZATION DEVICE USING DAILY ON-BOARD KILOVOLTAGE IMAGING

The purpose of this study was to utilize state-of-the-art on-board digital kilovoltage (kV) imaging to determine the systematic and random set-up errors of an immobilization device designed for canine and feline cranial radiotherapy treatments. The immobilization device is comprised of a custom made support bridge, bite block, vacuum-based foam mold and a modified thermoplastic mask attached to a commercially available head rest designed for human radiotherapy treatments. The immobilization device was indexed to a Varian exact couch-top designed for image guided radiation therapy (IGRT). Daily orthogonal kV images were compared to Eclipse treatment planning digitally reconstructed radiographs (DRRs). The orthogonal kV images and DRRs were directly compared online utilizing the Varian on-board imaging (OBI) system with set-up corrections immediately and remotely transferred to the treatment couch prior to treatment delivery. Off-line review of 124 patient treatments indicates systematic errors consisting of +0.18 mm vertical, +0.39 mm longitudinal and −0.08 mm lateral. The random errors corresponding to 2 standard deviations (95% CI) consist of 4.02 mm vertical, 2.97 mm longitudinal and 2.53 mm lateral and represent conservative CTV to PTV margins if kV OBI is not available. Use of daily kV OBI along with the cranial immobilization device permits reduction of the CTV to PTV margins to approximately 2.0 mm.     

PNEUMOPERICARDIOGRAPHY IN THE NORMAL DOG

To define the normal radiographic anatomy of the canine heart and pericardial space as outlined by air, pneumopericardiography was performed in ten normal, anesthetized dogs using a percutaneously introduced pericardial catheter. Room air was injected to produce pneumopericardiums without causing cardiac tamponade, and radiographs were obtained using a vertical beam with the dogs positioned in right lateral (RLAT), left lateral(LLAT), dorsal (VD), and ventral (DV) recumbency. Selective and nonselective angiocardiography was used to confirm the identity of the outlined structures. The RLAT and LLAT positions provided more information than the DV or VD positions. Pericardial air consistently outlined a distinct interventricular sulcus and the recesses around the aorta and pulmonary artery. The right auricle, outlined along the cranial heart border ventral to the ascending aorta in both RLAT and LLAT positions, varied considerably in size. The RLAT position best outlined structures to the left of midline, including the left auricle, interventricular sulcus, outflow region of the right ventricle, and the origin of the pulmonary artery. The LLAT position best demonstrated structures to the right of midline, including the right atrium, proximal part of the cranial and caudal vena cavae, and ascending aorta. The considerable range of normal variation between dogs in this study must be considered in the interpretation of clinical pneumopericardiograms.     

PROOF OF PRINCIPLE OF OCULAR SPARING IN DOGS WITH SINONASAL TUMORS TREATED WITH INTENSITY‐MODULATED RADIATION THERAPY

Intensity‐modulated radiation therapy (IMRT) allows optimization of radiation dose delivery to complex tumor volumes with rapid dose drop‐off to surrounding normal tissues. A prospective study was performed to evaluate the concept of conformal avoidance using IMRT in canine sinonasal cancer. The potential of IMRT to improve clinical outcome with respect to acute and late ocular toxicity was evaluated. Thirty‐one dogs with sinonasal cancer were treated definitively with IMRT using helical tomotherapy and/or dynamic multileaf collimator (DMLC) delivery. Ocular toxicity was evaluated prospectively and compared with a comparable group of historical controls treated with conventional two‐dimensional radiotherapy (2D‐RT) techniques. Treatment plans were devised for each dog using helical tomotherapy and DMLC that achieved the target dose to the planning treatment volume and limited critical normal tissues to the prescribed dose–volume constraints. Overall acute and late toxicities were limited and minor, detectable by an experienced observer. This was in contrast to the profound ocular morbidity observed in the historical control group treated with 2D‐RT. Overall median survival for IMRT‐treated and 2D‐treated dogs was 420 and 411 days, respectively. Compared with conventional techniques, IMRT reduced dose delivered to eyes and resulted in bilateral ocular sparing in the dogs reported herein. These data provide proof‐of‐principle that conformal avoidance radiotherapy can be delivered through high conformity IMRT, resulting in decreased normal tissue toxicity as compared with historical controls treated with 2D‐RT.     

Malignant amelanotic melanoma behind the left eye in a female Crj:CD(SD)IGS rat: a case report

A tumor behind the left eye in a female Crj:CD(SD)IGS rat was investigated histopathologically, immunohistopathologically, and electron microscopically. The tumor invaded and destroyed orbital tissues and bones. It consisted of various tumor cells; namely, spindle-shaped, epithelioid, anaplastic melanoma cells, and had prominent eosinophilic cytoplasm and nuclei with a greater variation in size. Immunohistochemically, almost all of the tumor cells were positive for antimelanoma, PNL2 antibody. Ultrastructurally, the tumor cells were rich in small vesicles containing fine granules and filamentous structures. This is the first report describing an amelanotic melanoma in the head of an albino rat.     

COMPUTED TOMOGRAPHIC QUANTIFICATION OF CANINE ADRENAL GLAND VOLUME AND ATTENUATION

We conducted a retrospective study in presumed normal dogs to determine the adrenal gland attenuation and volume values. Multidetector computer tomography (MDCT 16) analysis of the gland was carried out in 48 adult dogs without evidence of adrenal gland disease that underwent CT examination for acute spinal injuries. The mean nonenhanced attenuation value +/- SD of the left adrenal gland was 36.0 +/- 5.3 HU (range: 22.0-42.0 HU). The mean nonenhanced attenuation value +/- SD of the right gland was 34.3 +/- 7.0 HU (range: 20.4-48.6HU). The mean enhanced attenuation value +/- SD were: left gland 101.5 +/- 10.6HU (range: 86.8-128.0 HU), and right gland 97.4 +/- 12.4 HU (range: 58.9-123.6 HU). The mean CT volume +/- SD were: left gland was 0.60 cm3 (range: 0.20-0.95; SD 0.17), and right gland (0.55cm3, range: 0.22-1.01; SD 0.19). Attenuation values and volume data were related to age, weight, and gender, using ANOVA. There was no statistically significant difference between the left and right side or in adrenal measurements, because of body weight class effects. The animal effect was the most important source of variation for all adrenal measurements. Based on our study, CT is an effective method for assessing adrenal characteristics in the dog. Normative CT data are provided to allow estimation of normal adrenal gland size and volume.