Page 653 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 29 Tumors of the Male Reproductive System 631

the cat is unclear because of the sparse literature available. Metas- and 16% were diagnosed with PCA. 144–146 One study of 177
tasis of a Sertoli cell tumor to the liver and spleen, and a teratoma dogs found that PCA was the most common disease in neutered
dogs whereas bacterial prostatitis and prostatic cysts were more
to the omentum have been reported; therefore staging is recom-
VetBooks.ir mended. 110,113 Optimal therapy other than orchiectomy is not common in intact male dogs. 145 Elderly dogs are more com-
monly diagnosed with PCA, with a median age at diagnosis of
known.
10 years. 135,146–148 The underlying etiology of canine prostatic
Comparative Aspects cancer is unknown; however, high-grade prostatic intraepithelial
neoplasia (PIN or HGPIN), which is believed to be a precursor
In the United States testicular cancer is the most common cancer of human prostate carcinoma, has been detected in both dogs
in men 15 to 44 years old, but is one of the most curable cancers without evidence of prostatic disease and in those with existing
with early diagnosis. 107–109 Studies have shown an increase in tes- PCA. 149–152 The occurrence of PIN in dogs with concurrent car-
ticular cancer over the past half-century, with significant shifts in cinoma varies from 7% to 72%, although in two large studies of
the age at presentation over time. 107,108,110 Causal factors such as dogs without histologic evidence of PCA, the occurrence was low
genetic predisposition, maternal estrogen exposure, occupational at 0% to 3%. 134,149–152
hazards, dietary factors, smoking habits, and birthplace have been Most tumors of the canine prostate are carcinomas, with most
evaluated but, to date, the most established risk factor remains being transitional cell carcinoma (TCC) or adenocarcinoma in
cryptorchidism. 23,107,111–120 Most cancers in men are germ cell origin. 153–156 Prostate tumors in the dog likely have a urothelial or
tumors and are broadly divided into pure seminomas and non- ductular origin rather than acinar because most canine tumors are
seminomas, with seminomas comprising 50% of tumors in this androgen independent. 153,154,157–160 Other types of carcinomas
group. 13,20,109 Seminomas are further classified into SE, atypical, include mixed carcinomas and squamous cell carcinomas (SCCs).
or SS subtypes; however, diagnostic and therapeutic management Classifying carcinomas on the basis of subtype is somewhat sub-
does not vary considerably between tumor types. 20,109 Pure semi- jective and there is not currently a standard for definitive diagnosis
nomas are more likely to behave in a clinically aggressive fash- of canine prostate tumors as there is with humans. 151,152,161–165
ion; therefore when both seminoma and nonseminoma features Fibrosarcoma, leiomyosarcoma, osteosarcoma, lymphoma, and
are present, therapeutic management follows the guidelines for hemangiosarcoma have also been reported to affect the pros-
nonseminoma. 109 Standard staging in human seminomas consists tate. 134,166–171 Benign tumors of the prostate are rare. 172 TCC of
of physical examination, radiographic studies, determination of the prostatic urethra will frequently invade the prostate, and it
serum markers, including alpha-fetoprotein, human chorionic may be difficult to distinguish primary PCA from secondary inva-
gonadotropin, and lactate dehydrogenase, and histopathologic sion of a urethral tumor.
assessment. SS is a rare variant of germ cell neoplasia that is most
commonly seen in older men and carries a low risk of metasta- Risk Factors
sis, suggesting similar behavior to most canine seminomas. 21,23
Treatment generally includes surgery for stage I seminomas, and/ Both intact and castrated dogs develop PCAs, although mul-
or RT and chemotherapy for individuals with higher stage dis- tiple studies have suggested there is an increased risk of PCA
ease. 106,121–123 Cisplatin-based chemotherapy protocols are gener- in castrated male dogs compared with intact male dogs, with
ally employed for patients with greater than stage I disease, with an odds ratio of approximately 2.3 to 4.3. 132,146,147,153,157
cure rates in the range of 70% to 80% despite high tumor bur- More aggressive tumors may develop in castrated males with
dens. 23,106,109,124 The dog has been proposed as a model for stud- a higher risk of metastasis. 147 The reason for this difference is
ies of the male reproductive system, and in particular for studies unclear, although it is possible that castrated dogs live longer
evaluating the development of testicular tumors. 18,21,22,33 Fur- than intact dogs and are thus predisposed to developing age-
ther work on the molecular pathogenesis, classification system, related cancers. 146,153 It is also possible that androgens provide
and behavior of canine tumors may yield more support for use of a protective effect on prostatic tissue, or that, after castra-
spontaneously occurring testicular tumors as good comparative tion, the relative estrogen effect aids in neoplastic transforma-
models for human disease. tion. 153,173–175 The lowest relative risk of neutered males in a
recent study was for PCA, which may occur relatively more
Canine Prostate Tumors frequently in intact male dogs. 134,151,153 Although several stud-
ies have suggested that androgens may not be required for
Incidence/Prevalence initiation or progression of adenocarcinoma of the canine pros-
tate, further controlled studies should be done to definitively
Prostatic tumors are relatively uncommon in dogs and have a determine the role of androgens or the effect of early or late
low prevalence at less than 1% (0.2%–0.6%). 132–136 In a col- castration. 153,154,157,159,160,176
lection of more than 17,000 confirmed neoplasms of the dog Breeds that may be at increased risk of developing PCA include
collected from veterinary schools in North America, only 11 the Bouvier des Flandres, Doberman pinscher, Shetland sheep-
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prostate carcinomas (PCAs) were identified (0.06%). A study dog, Scottish terrier, beagle, miniature poodle, German short-
evaluating lifetime occurrence of neoplasia in predominantly haired pointer, Airedale terrier, and Norwegian elkhound. 146,153
intact German shepherd dogs and Belgian Malinois working The Shetland sheepdog and Scottish terrier remained at increased
dogs showed that more than 30% developed at least one cancer; risk even when TCC was excluded in one study. 153 Mixed-breed
however only 2 of the 104 primary tumors were PCAs. Despite dogs have also been reported as at increased risk for PCA, regard-
17
this low incidence, the dog is one of the few domestic species to less of neutering status, suggesting that environmental influences
develop spontaneous prostate cancer, thus sparking interest in may play a significant role in tumor development. 153 The Ameri-
the dog as a model for prostate cancer in men. 135,137–143 In three can cocker spaniel, miniature poodle, and dachshund may be at
retrospective reviews of dogs with prostatic disease, between 7% decreased risk for developing PCA. 153