This is the first prospective, observational study assessing prostatic dimensions in castrated adult canines, with ultrasonography. The results of the present study provide references of prostatic dimensions, which show a clear positive correlation with the dog’s body weight. Both the measurements of prostatic length and depth, in the longitudinal plane, showed a significant increase with increasing body weight. In current clinical practice, during the ultrasonographic abdominal examination, the prostate is assessed based on subjective characteristics, such as shape, size and homogeneity [15, 16]. Therefore, evaluation of the prostate is highly operator dependent and interpretation of possible abnormalities can thus vary between operators. This weight-based prostatic size reference will aid both radiology specialists and general practitioners with a comprehensive, objective parameter and can function as an add-on to the overall assessment of the prostate gland of castrated dogs. However, care must be taken to assess not only size, but also factors as homogeneity, contour, periprostatic fat and regional lymph nodes. The present study focused merely on measurements in the sagittal plane, as earlier studies showed this to be the most reproducible measurement , and not on volumetric measurements or measurements in additional planes, which may be more sensitive for the detection of prostatomegaly.
In the current study, a linear correlation between body weight and prostatic dimensions was illustrated. From the available data set, formulas were derived in order to calculate a prostatic length and depth reference based on the dogs’ body weight. Three weight-based categories were chosen, in order to represent the vast variety of different sizes within this species, while also keeping the reference a practical tool for daily use.
The results of the current study are in line with a recent study comparing volumetric assessment of the prostate gland in 57 intact and 37 castrated canines, as measured using the slice addition technique in CT . A study regarding linear prostatic dimensions of 62 castrated dogs, as measured on computed tomographic images, revealed a positive association between body weight and prostate dimensions, also in line with our current findings . The work of Atalan et al.  failed to illustrate a relationship between prostatic size and weight of the dog in castrated animals, which was attributed to the very small sample size of castrated dogs (17 dogs were present in the castrated dogs group) in that study.
However, another recent study conducted by Spada et al.  provided noteworthy, contradictory results. This study primarily addressed contrast-enhanced ultrasonographic findings in normal prostates of castrated canines, and found no correlation between prostate volume and body weight. In this study the prostate volume was calculated based on three linear dimensions of the prostate gland as a continuous variable. A possible explanation for these contradictory results could be attributed to the multiple variables (prostatic length, height and width) contributing to the prostatic volume which is then compared to body weight in the study by Spada et al. , whereas in the current study (as well as in the mentioned studies by Haverkamp et al.  and Delaude et al. ), the different linear dimensions are compared separately with the dogs’ body weight. Instead of a single measurement, with a single possibility for measurement error, each of the multiple measurements poses a risk for an additional measurement error. This mechanism can potentially increase the possibility to obscure a possible true relation between prostatic dimensions and body weight. Also, the effect of body condition score was not evaluated in either the current study or the referenced studies, but could be of influence on the measurements. Dogs of the same body size but with different body condition scores could be classified in different weight categories, thus influencing the calculated references. Future anatomical studies, preferably with larger sample sizes, are warranted to investigate these contradictory imaging findings. To address differences in body condition score, references based on parameters other than weight could be of value (i.e. aorta diameter, vertebral length etc.).
In our current study, significant differences in prostatic dimensions were found between weight categories. However, a few outliers were reported as well. The two most notable outliers were found in the large dog group and consisted of the prostatic length measuring 42 and 16.7 mm respectively. The first dog was one of the largest dogs in our study population, with a body weight of 39.6 kg. The dog with a prostatic length well below the large dog groups’ median prostatic length (16.7 mm) was in fact relatively small compared to the other dogs in the group and only marginally fell into the large dog group with a body weight of 26.7 kg. We would suggest to relate these abnormal prostatic lengths to the relative position of these individuals within the large dog group. As per inclusion criteria, the prostates of both individuals were interpreted to be normal, based on the assessment by the radiologist or radiology resident performing the ultrasonographic examination, in accordance with the current standard of practice. Due to the relative invasiveness and moderate technical difficulty, cytological samples of the prostates of the included individuals were not included in the study design. Thus, disease cannot be completely excluded as a cause of the relatively large prostatic length in the mentioned individual, as cytology and/or histology is considered the gold standard in diagnosing canine prostatic disease. A too small prostatic length is not expected as indicator of prostatic disease and therefore the second individual, with a relatively small prostatic length compared to the group mean value, would be highly unlikely to represent pathology. Furthermore, prostatic depth dimensions were within the reported reference for their weight category for both individuals, supporting lack of pathology as a cause of the abnormal prostatic length. In clinical practice, these outliers, and thus overlap between size ranges of normal vs. abnormal prostates, should be kept in mind, in order to prevent over-interpretation of prostatic disease. Comparison of the different prostatic dimensions with the subjects age revealed no relationship between these parameters, which was in line with what was expected based on both previous studies and clinical findings in daily practice .
Several limitations to the current study design exist. First of all, as mentioned, there was no cytological or histological confirmation that the prostates under investigation indeed were normal. Due to the study population and study design this was not feasible, and the level of invasiveness required for obtaining cytological or histopathological specimens would be unethical. Furthermore, even with the availability of cytological or histopathological biopsies, the possibility remains that a lesion is missed (i.e. false negative), especially when encountering focal pathologies (as apposed to generalized prostatic disease). The possibility of prostatic pathology within our study population cannot be completely excluded, however due to the study inclusion criteria, the authors deem this to be unlikely.
In the current study design, data collection and measurements were carried at a single point in time by a single observer. This eliminates possible interobserver variability as a cause for spread in measurements. Future study designs could further explore the interobserver variability by introducing multiple observers, and different levels of experience between observers, in order to better reflect the clinical setting. Intraobserver variability could also be assessed in future studies.
Suboptimal imaging of the complete prostate gland was one of the exclusion criteria in the current study, given that improper or uncertain identification of organ margins could result in improper measurements and thus negatively influence the validity of the presented measurements. However, in clinical practice we do encounter situations where organ boundary identification can be problematic (e.g. obese patients, prostates with an intrapelvic position). Distinguishing the prostate gland from periprostatic fat and the urethra can be challenging, especially for less experienced ultrasonographers . Techniques have been described to better visualize the prostate gland, e.g. transrectal ultrasonographic examination or infusion of the urinary bladder with saline, which can surpass some of the described inherent patient factors . In the current study these techniques were not applied, as it would increase the invasiveness of the procedure. In clinical practice however, these techniques could be used when justified by the clinical presentation of the patient at the discretion of the clinician.
When exploring possible future aims of research, thought should be given to the specific ultrasonographic findings in canine prostatic neoplasia in castrated males, compared to normal prostates. Exploring these differences can aid further into prioritizing differentials for ultrasonographic findings, and can thus support the clinician into putting resources to appropriate follow-up diagnostics. For the present study this was not feasible, as the number of neutered male patients presenting to our clinic within the study timeframe, with confirmed prostatic neoplasia, was too small to make valid comparisons.