VALIDATION OF AN INDEXED RADIOTHERAPY HEAD POSITIONING DEVICE FOR USE IN DOGS AND CATS

Setup variability affects the appropriate delivery of radiation and informs the setup margin required to treat radiation patients. Twenty‐four veterinary patients with head and neck cancers were enrolled in this prospective, cross‐sectional study to determine the accuracy of an indexed board immobilization device for positioning. Couch position values were defined at the first treatment based on setup films. At subsequent treatments, patients were moved to the previously defined couch location, orthogonal films were acquired, table position was modified, and displacement was recorded. The mean systematic displacement, random displacement, overall displacement, and mean displacement values of the three‐dimensional (3D) vector were calculated. Three hundred thirty‐two pairs of orthogonal setup films were analyzed for displacement in cranial–caudal, lateral, and dorsal–ventral directions. The mean systematic displacements were 0.5, 0.8, and 0.5 mm, respectively. The mean random displacements were 1.0, 1.1, and 0.7 mm, respectively. The overall displacements were 1.1, 1.4, and 0.9 mm, respectively. The mean 3D vector value was 1.6 mm with a standard deviation of 1.2 mm. Ninety‐five percent of the vectors were <3.6 mm. These values were compared to data obtained with a previously used immobilization device. A t‐test was used to compare the two devices. The 3D vector, random displacement in all directions, and overall displacement in the cranial–caudal and dorsal–ventral directions were significantly smaller than displacements with the previous device. The precision and accuracy of the indexed board device was superior to the historical head and neck device.     

COMPARISON OF THE ACCURACY OF POSITIONING DEVICES FOR RADIATION THERAPY OF CANINE AND FELINE HEAD TUMORS

The purpose of this retrospective study was to evaluate the repositioning accuracy of different positioning devices in order to determine their applicability for potential use in conformal radiation therapy for animals. Forty-four animals with spontaneous tumors of the head were included. The animals were divided into 3 groups determined according to the positioning device used. Group 1 animals were positioned using a thermoplastic mask. Group 2 animals were positioned using a head holder. Group 3 animals were positioned using the head holder and an inflatable pillow. The time of presentation determined which position device was used. Port films of the 44 patients were reviewed retrospectively, and the repositioning precision was recorded by measurements in three orthogonal planes. Groups 2 and 3 had significantly better repositioning accuracy (P < or = 0.05) compared to Group 1. The position variation was not significantly different (P < or = 0.05) between Groups 2 and 3 in the lateral and longitudinal direction. Group 3 had a median reposition variation of 0.5 to 1.0 mm, with a standard deviation of 1.0 to 1.5 mm.     

ASSESSMENT OF A RADIOTHERAPY PATIENT IMMOBILIZATION DEVICE USING SINGLE PLANE PORT RADIOGRAPHS AND A REMOTE COMPUTED TOMOGRAPHY SCANNER

Radiation treatment requires a precise procedure for interfraction repositioning of the patient. The purpose of this study was to determine the accuracy of our fixation device in treatment position and to evaluate the setup accuracy with two different methods. The positioning data of 19 canine patients with tumors in the head region (oral, nasal, cerebral) treated with photon or proton irradiation were included in this study. The patients were immobilized by means of an individualized fixation device. Focus was set upon interfraction displacement with systematic and random components. In one method, treatment position was evaluated using single plane port radiographs and megavoltage x-rays. In the other method, two orthogonal CT-topograms were acquired to evaluate the precision of positioning of the patient in the immobilization device. Systematic and random displacements were calculated and presented as mean values with corresponding 95% confidence intervals. In spite of a difference between both methods, the positioning seemed to be accurate within the expected range. It seems that a safety margin of 3.7 mm would be enough for both methods to take into account systematic and random position variability in the fixation device, thereby preventing geometric inaccuracies of treatment delivery. The reported immobilization protocol provides accurate patient immobilization for photon and conformal proton radiation therapy.     

The Elekta Fraxion™ system is not suitable for maxillary fixation in canine conformal radiation therapy techniques

In this prospective, exploratory study, we evaluated the positioning accuracy in a group of 15 dogs undergoing fractionated stereotactic radiotherapy for tumors affecting the head, using a modified human maxillary fixation device (Elekta Fraxion? system). Positioning was assessed using on‐board volumetric imaging, with a six‐degrees‐of‐freedom image registration technique. Prior to treatment delivery, CBCT images were obtained and patient alignment was corrected, in both translational and rotational planes, using a six‐degrees‐of‐freedom robotic patient positioning system (HexaPOD Evo RT System). The maximum angular inter‐fraction motions observed were 6.1° (yaw), 10.9° (pitch), and 4.5° (roll). The mean systematic translational errors were 4.7, 2.6, and 2.3 mm, mean random translational errors were 3.0, 2.2, and 2.5 mm, and mean overall translational errors were 2.4, 0.7, and 2.3 mm in the cranial‐caudal, lateral, and dorsal‐ventral directions, respectively. The mean systematic rotational errors were 1.17°, 0.77°, and 1.43°, the mean rotational random errors were 1.65°, 1.46°, and 1.34° and the mean overall rotational errors were 0.56°, 0.22°, and 0.29° in the yaw, pitch, and roll directions, respectively. The mean error of the three‐dimensional vector was 6.9 mm with a standard deviation of 3.8 mm. Ninety‐five percent of the three‐dimensional vectors were <14.8 mm. This study demonstrates that this maxillary fixation device relies on six‐degrees‐of‐freedom registration and an ability to apply corrections using a six‐degrees‐of‐freedom couch for accurate patient positioning and tumor targeting. Its use in conformal radiation therapy in dogs is not recommended.     

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.     

EVALUATION OF A HEAD-REPOSITIONER AND Z-PLATE SYSTEM FOR IMPROVED ACCURACY OF DOSE DELIVERY

Radiation therapy requires accurate dose delivery to targets often identifiable only on computed tomography (CT) images. Translation between the isocenter localized on CT and laser setup for radiation treatment, and interfractional head repositioning are frequent sources of positioning error. The objective was to design a simple, accurate apparatus to eliminate these sources of error. System accuracy was confirmed with phantom and in vivo measurements. A head repositioner that fixates the maxilla via dental mold with fiducial marker Z-plates attached was fabricated to facilitate the connection between the isocenter on CT and laser treatment setup. A phantom study targeting steel balls randomly located within the head repositioner was performed. The center of each ball was marked on a transverse CT slice on which six points of the Z-plate were also visible. Based on the relative position of the six Z-plate points and the ball center, the laser setup position on each Z-plate and a top plate was calculated. Based on these setup marks, orthogonal port films, directed toward each target, were evaluated for accuracy without regard to visual setup. A similar procedure was followed to confirm accuracy of in vivo treatment setups in four dogs using implanted gold seeds. Sequential port films of three dogs were made to confirm interfractional accuracy. Phantom and in vivo measurements confirmed accuracy of 2 mm between isocenter on CT and the center of the treatment dose distribution. Port films confirmed similar accuracy for interfractional treatments. The system reliably connects CT target localization to accurate initial and interfractional radiation treatment setup.     

ACCURACY OF POSITIONING THE CERVICAL SPINE FOR RADIATION THERAPY AND THE RELATIONSHIP TO GTV, CTV AND PTV

The purpose of the study was to evaluate the accuracy and precision of a rigid positioning device for repositioning the cervical spine accurately and precisely during conformal radiation therapy of dogs. Fifteen purpose bred research dogs in a radiation therapy study were included. The dogs were positioned using a head holder and a deflatable pillow attached to the treatment table. Port films were reviewed retrospectively, and repositioning precision was recorded by measurements in three orthogonal planes of the head, 2nd cervical vertebra and 1st thoracic spinous process. Mean treatment position was compared to the planning position for a measurement of systematic set-up error. Mean interfraction position variation of the 2nd cervical vertebra was 0.2, 0.1 and 0.2 cm for the ventrodorsal, caudocranial and laterolateral directions respectively, and the average systematic set up error was 0.2, 0.1 and 0.2 cm for the ventrodorsal, caudocranial and laterolateral directions respectively. Knowledge of the magnitude of reposition errors should be included when determining the margins around the tumor.     

COMPARISON OF INTERFRACTIONAL VARIATION IN CANINE HEAD POSITION USING PALPATION AND A HEAD‐REPOSITIONING DEVICE

Radiation treatment planning is performed on images that do not take variation in patient position into account. To compensate for expected variations in position of the patient, a three‐dimensional expansion of the clinical target volume, or set‐up margin, is added. Variations in patient position can be decreased through use of an immobilization device, allowing selection of a smaller set‐up margin. The objective of this prospective study was comparison of interfractional variation in patient position between set‐ups of the canine head region using palpation of bony landmarks and set‐ups using a head‐repositioning device. Fiducial markers were attached to the skull bones of three research dogs, and the dogs were positioned as for a typical radiation treatment of the head region using both set‐up methods. A kilovoltage on‐board imager was used to acquire orthogonal images and the difference between the x‐, y‐, and z‐axis coordinates of each fiducial marker relative to the initial reference isocenter was measured. The difference in patient position for each axis coordinate was significantly lower for set‐ups using the head‐repositioning device than for set‐ups using bony landmarks (P<0.05). Ninety‐five percent of the absolute values of the displacement vector differences were <4.62 mm for set‐up using bony landmarks, and <1.93 mm for set‐up using the head‐repositioning device. A minimum set‐up margin of 5–6 mm is recommended when patient set‐up is based on bony landmarks and of 2–3 mm when the head‐repositioning device is used.     

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.     

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.     

Influence of limb positioning and measurement method on the magnitude of the tibial plateau angle

OBJECTIVE: To evaluate the effect of limb positioning and measurement technique on the magnitude of the radiographically determined tibial plateau angle (R-TPA). STUDY DESIGN: In vitro study, R-TPA was determined by 6 blinded observers and image measurement software. ANIMALS: Five canine cadaver hind limbs. METHODS: The legs were positioned on a custom-made positioning device simulating a radiographic tabletop technique in lateral recumbency. True lateral positioning was defined by superimposition of femoral and tibial condyles on the radiographic projection. Radiographs were taken while the specimens were relocated in a proximal, distal, caudal, and cranial direction with respect to the radiographic beam. For each specimen, 25 different radiographic views were obtained and 6 blinded observers determined the radiographic TPA using 2 different methods. The conventional method used precise anatomic landmarks to determine the tibial plateau. To simulate osteoarthritic changes complicating identification of these landmarks, the tangential method estimated the tibial plateau as the tangent to the central portion of the tibial plateau. After periarticular soft tissue dissection the anatomic tibial plateau angle (A-TPA) was determined. The A-TPA and the R-TPA were compared. RESULTS: The R-TPA significantly decreased as limb position with respect to the X-ray beam changed from cranial proximal to caudal distal. The maximal mean radiographic R-TPA difference was 3.6 degrees with the first and 5.7 degrees with the second method. Regardless of the method used there was no significant difference between A-TPA and R-TPA in the true lateral position. In the peripheral positions, however, significant differences between anatomic and radiographic TPA were seen. CONCLUSIONS: Limb positioning influenced the radiographic appearance of the tibial plateau and the magnitude of the measured TPA. Cranial and proximal positioning of the limb relative to the X-ray beam leads to overestimation whereas caudal and distal positioning leads to underestimation of the TPA. CLINICAL RELEVANCE: True lateral positioning of the tibia defined by superimposition of the femoral and tibial condyles should be used for accurate TPA determination before tibial plateau leveling osteotomy.     

A NOVEL,REMOVABLE, CERROBEND,BEAM‐BLOCKING DEVICE FOR RADIATION THERAPY OF THE CANINE HEAD AND NECK: PILOT STUDY

Radiation therapy of the head and neck can result in mucositis and other acute affects in the oral cavity. This prospective pilot study evaluated a novel, intraoral, beam‐blocking device for use during imaging and therapeutic procedures. The beam‐blocking device was made from a metal alloy inserted into a coated frozen dessert mold (Popsicle® Mold, Cost Plus World Market, Oakland, CA). The device was designed so that it could be inserted into an outer shell, which in turn allowed it to be placed or removed depending on the need due to beam configuration. A Farmer type ionization chamber and virtual water phantom were used to assess effects of field size on transmission. Six large breed cadaver dogs, donated by the owner after death, were recruited for the study. Delivered dose at the dorsal and ventral surfaces of the device, with and without the alloy block in place, were measured using radiochromic film. It was determined that transmission was field size dependent with larger field sizes leading to decreased attenuation of the beam, likely secondary to scatter. The mean and median transmission on the ventral surface without the beam‐blocking device was 0.94 [range 0.94–0.96]. The mean and median transmission with the beam‐blocking device was 0.52 [range 0.50–0.57]. The mean and median increase in dose due to backscatter on the dorsal surface of the beam‐blocking device was 0.04 [range 0.02–0.04]. Findings indicated that this novel device can help attenuate radiation dose ventral to the block in dogs, with minimal backscatter.     

VALIDATION OF NEW BITE BLOCK‐TYPE HEAD‐IMMOBILIZATION DEVICES FOR RADIOTHERAPY IN DOGS

An ideal head‐immobilization method provides a high level of accuracy and reproducibility in the immobilization. Various head‐immobilization methods for radiotherapy have been published and are excellent in terms of accuracy; however, these methods are complicated to use, and labor intensive. The present study describes two new bite block‐type head‐immobilization devices designed for higher stability and lower vertical variation. The device designed in our previous study (the bite block‐type head‐immobilization device; Device A) was modified by making a groove on the top the horizontal plate (Device B) for a stable ventral‐dorsal position, or beneath the horizontal plate (Device C) for a stable dorsal‐ventral position. The three devices were objectively compared with respect to setup time, and accuracy of the computed tomography scan images by two authors independently. Five male healthy beagles were used in this study. For each device, the setup time and the variation in the coordinates were measured five times for each dog. The mean setup times for Devices A, B, and C were 3.3, 1.5, and 2.4 min, respectively, showing the groove modifications were able to reduce the setup time (in device B, by at least 50%). Moreover, three‐dimensional analysis of the computed tomography images revealed that the measurement variability of Device A (1.6 ± 1.0 mm) was significantly higher than that of Device C (0.7 ± 0.4 mm; P < 0.001). Collectively, our results show that use of a bite block‐type head‐immobilization device with a groove improves the setup time and head‐immobilization accuracy.     

ULTRASONOGRAPHY OF THE NORMAL FELINE PANCREAS AND ASSOCIATED ANATOMIC LANDMARKS: A PROSPECTIVE STUDY OF 20 CATS

The sonographic appearance of the feline pancreas and associated anatomic landmarks including the pancreatic duct, duodenum, duodenal papilla, portal vein, and gastric lymph node were evaluated in 20 healthy, awake cats. The pancreas appeared nearly isoechoic to surrounding mesenteric tissues, isoechoic to slightly hyperechoic to adjacent liver lobes, and hypoechoic to the spleen. The mean thickness measurements for the right pancreatic lobe, body, and left pancreatic lobe were 4.5 mm (range 2.8-5.9), 6.6 mm (range 4.7-9.5), and 5.4 mm (range 3.4-9.0), respectively. The pancreatic duct was consistently visualized in the left pancreatic lobe and had a mean thickness of 0.8 mm (range 0.5-1.3). It could be differentiated from the pancreatic vessel, by its central location, and the duct's lack of Doppler flow signal. The duodenum was used as a landmark to identify the right lobe of the pancreas. The mean duodenal wall thickness measurement was 2.8 mm (range 2.1-3.8) in sagittal section, and 3.0 mm (range 2.2-4.4) in transverse section. The duodenal papilla was identified in 4 of 20 cats. It ranged in size from 2.9 to 5.5 mm in width, and had a maximum height of 4.0 mm in transverse section. The portal vein was used as a consistent anatomic landmark for identification of the left lobe and body of the pancreas. The mean diameter of the portal vein at the level where the pancreatic body joins the left pancreatic lobe was 4.3 mm (range 2.7-5.9) when viewed in sagittal section, and 4.5 mm (range 3.6-6.1) in transverse section. The gastric lymph node was identified cranial and ventromedial to the pyloroduodenal angle in 6 of 20 cats. It had an asymmetrical shape with a larger caudal pole in five of the six cats. The largest dimensions of the gastric lymph node were 10 mm in length, and 6 mm in width for the larger caudal pole, and 5.1 mm in width for the smaller cranial pole.     

Chronic ocular lesions associated with bi-directional microbeam radiation therapy in an experimental rat study for therapy of C6 and F98 gliomas

Objectives  To describe the chronic ocular lesions associated with microbeam radiation therapy (MRT) in an experimental rat study.
Procedures  MRT was administered bi-directionally with a skin entry dose of 350 Gy. During laterally directed irradiation, the beam entered the head on the right with the center of the beam array 3 mm posterior to the center of the right eye. During irradiation in anterior-posterior direction, the right eye was almost completely in the path of the beam array. Twelve months after MRT ophthalmic examinations were completed on 37 treated (MRT+) and 16 control (MRT–) rats. Electroretinography (ERG) was completed in two MRT+ and one MRT– rat. Histopathology was performed on eyes of 16 MRT+ and 9 MRT– rats, and retinal and choroidal thicknesses were measured.
Results  Biomicroscopic and indirect ophthalmoscopic examinations revealed fundus pallor and retinal vascular attenuation in 33 of 37 right and 2 of 37 left eyes of MRT+ rats. Cataracts were present in the right eyes of 12 of 37 MRT+ rats. ERG amplitudes were reduced in the eyes of MRT+ rats. Light microscopy revealed retinal lesions ranging in severity with loss of outer to inner retinal cell layers, in 16 of 16 right and of 8 of 16 left eyes MRT+ rats. The mean right retinal thickness of MRT+ rats was reduced.
Conclusions  Eyes within the field treated with MRT at a dose of 350 Gy develop retinal degeneration and occasionally, cataract.     

ULTRASONOGRAPHY OF THE LIVER, SPLEEN, AND URINARY TRACT OF THE CHEETAH (ACINONYX JUBATUS)

Diseases of the abdomen of the cheetah (Acinonyx jubatus) include those affecting the liver, spleen, and urinary tract. The most common diseases of captive-bred cheetah are gastritis, gastric ulceration, glomerulosclerosis, and hepatic veno-occlusive disease, and are the most frequent causes of mortality in these animals. The purpose of this study was to describe the ultrasonographic anatomy of the normal liver, spleen, kidney, and urinary bladder of the anesthetized captive-bred cheetah. Twenty-one cheetahs were examined. Eight of the 21 animals had subclinical evidence of either gastritis or chronic renal disease. The ultrasonographic appearances of the liver, gall bladder, common bile duct, and spleen were evaluated and various measurements made. Statistical analyses of the measurements were performed on all the healthy and subclinically ill animals taking sex, age, mass, and anesthetic protocol into account. There were no significant differences in any parameters between the healthy and subclinically ill animals (P > 0.25) and data were combined for statistical analyses. The mean mass was 41.1kg ( +/- 8.8) and the mean age was 5.0 years (+/- 2.2). The mean thickness of the liver medial to the gall bladder was 67.0 mm (+/- 14.8) and the liver was within the left costal arch in 75% of animals, extended caudal to the right costal arch in 50% of animals for an average of 30 mm, and extended caudal to the sternum in 63% of animals for an average of 32.5 mm. The maximum mean hepatic vein diameter at the entrance to the caudal vena cava was 8.6 +/- 2.8 mm; the mean diameters of the portal vein at the hilus and that of the caudal vena cava as it entered the liver were 7.5 +/- 1.6 and 9.9 +/- 4.1 mm, respectively. The mean diameter of the caudal vena cava was significantly affected by the type of anesthetic used (P < 0.10). The mass of the animals was significant in explaining the variance in maximum portal vein diameters (P < 0.10). The mean maximum velocity of the hepatic vein flow at the entrance to the caudal vena cava was 25.3 +/- 2.8 cm/s (n=4), the hilar portal vein was 11.7 +/- 3.3 cm/s (n=7), and the caudal vena cava was 33.8 +/- 19.8 cm/s (n=5). The mean maximum gall bladder length and width, and the mean common bile duct diameters were 44.6 mm (+/- 10.4), 23.3 mm (+/- 5.0), and 8.1 mm (+/- 2.4), respectively. Age was significant in explaining the variance in gall bladder lengths (P<0.10). Urinary tract ultrasonography was performed only in animals that had normal urea and creatinine levels (n=13). Renal cortico-medullary distinction was present in all kidneys and a cortico-medullary rim sign was seen in 21 of 26 kidneys. Mean kidney length, height, and width was 63.9 +/- 5.7, 38.1 +/- 5.2, and 42.1 +/- 5 mm, respectively. The average resistivity index was 0.58 (n=5). Mean urinary bladder length, height, and width were 57.0, 19.2, and 34.9 mm, respectively.     

Factors affecting width of the canine femorotibial joint space in nonweight-bearing radiographs.

Caudocranial stifle radiographs with variations in positioning were made in two greyhound cadavers. Radiographs were repeated after each of three interventions: cranial cruciate desmotomy; release of the caudal horn of the medial meniscus; complete medial meniscectomy. The joint space on medial and lateral aspects of the joint was measured by a observer who was unaware of positioning or intervention. One dog had significantly wider joint space than the other (1.0 vs. 1.5mm). The lateral aspect of the joint space was wider than the medial aspect (1.7 vs. 0.7 mm). Medial rotation of the stifle resulted in an increase of 0.4 mm in width of the lateral joint space, whereas lateral rotation of the stifle reduced the lateral joint space by an average of 0.4 mm. Decentering the X-ray beam had no significant effect on joint space width. Tension increased the width of the medial joint space by an average of 1.2 mm and the lateral aspect by an average of 1.3 mm. Cranial cruciate desmotomy resulted in an average 0.3 mm increase in width of the joint space, and medial meniscectomy with an average 0.2 mm reduction of the joint space. Although the femorotibial joint space was affected by iatrogenic stifle injuries and by medial or lateral rotation, these changes were less than the differences between the two dogs. Hence it seems unlikely that the small changes in joint space width associated with cruciate ligament desmotomy and medial meniscectomy will be detectable in clinical practice.     

The Intercondylar Fossa of the Normal Canine Stifle: An Anatomic and Radiographic Study

The intercondylar fossa (ICF) in dogs consists of a cranial outlet, intercondylar shelf, caudal arch, caudal outlet, a medial wall, and a lateral wall. The normal cranial outlet is bell-shape and, in mixed-breed dogs (mean body weight 19.2 kg, N = 21), measured 5.8 mm cranially, 8.1 mm centrally, and 10.3 mm caudally. The ICF is oriented 12° from the dorsal plane of the femoral diaphysis and obliqued 7°, proximolateral to distomedial, in the sagittal plane. To adjust for dog size, a fossa width index (FWI) was calculated by dividing the cranial outlet width by the distance between epicondyles. The normal FWI as determined in this study was 0.18 cranially, 0.25 centrally, and 0.32 caudally. The fossa height index was 0.31. Contact between the ICF and the cranial cruciate ligament began at about 115° of extension. The contact area moved cranially in the intercondylar fossa as the stifle was extended. Evaluation of the ICF can be performed radiographically but positioning is critical.     

RADIATION THERAPY COMMUNICATION: NASAL PASSAGE AND PARANASAL SINUS LYMPHOMA IN A PONY

An aged pony with extensive paranasal sinus and nasal passage B-cell lymphoma was treated with palliative radiation therapy. Sixteen gray were administered in two fractions, 7 days apart. A lateral field was used for the first fraction and a dorsal field for the second. Because of tumor being present in the left frontal sinus, gross tumor was knowingly excluded from the treated volume in the lateral field. The tumor regressed within 2 months and the pony remained free of clinical disease for 2.5 years. Acute, temporary blindness developed shortly after the second radiation fraction, but a direct causal relationship with the radiation therapy was not confirmed. The only radiation side effect was leukotrichia. Palliative treatment was successful in improving and prolonging the quality of life. These results suggest that localized equine B-cell lymphoma is radiosensitive, and that palliative radiation therapy is a reasonable consideration for large tumors, even when tumor volume prevents all gross tumor from being irradiated.     

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.