Case 5 -
Spindle Cell Tumor of Prostate
Lawrence D. True
University of Washington Medical Center
Click on each slide thumbnail image for an enlarged view
This 65 year old male presented with a bulky pelvic tumor. Nine years prior to admission a routine
physical exam revealed an asymmetrically enlarged prostate. Prostate biopsies showed a "prostate
adenocarcinoma, poorly differentiated." The tumor was treated with 6,800 Gy of radiation to the pelvis.
The patient remained asymptomatic for 9 years, at which time he developed bladder outlet obstruction. A
biopsy revealed tumor, which was treated by radical cystoprostatectomy. Based on evidence of residual
tumor, additional radiation therapy, followed by en-bloc resection of residual tumor and rectum, was
undertaken. The slides are from this latter specimen.
Case 5 - Figure 1 - The lesion cells have a predominantly spindle shape and haphazard growth pattern. Markedly atypical cells are present in this relatively hypercellular field.
Case 5 - Figure 2 - Spindle shaped lesion cells exhibit only moderate atypia and cellularity in this field.
Case 5 - Figure 3 - Lesion cells are admixed with an osteoid stroma in this field.
Case 5 - Figure 4 - Lesion cells form palisading aggregates of Verocay bodies in this field.
Histology of the case
In the section is a multifocally necrotic tumor that has regions of histologically intact,
cytologically malignant spindle cells. The architecture of these spindle cells is undifferentiated.
Specifically, there is no formation of fascicles or of specific stromal elements and no epithelioid
features. The main differential diagnostic considerations are sarcoma of the prostate, sarcomatoid
prostate carcinoma, and pseudotumor that exhibits marked atypia.
Immunostains of the spindle cells in this and additional sections were positive only for vimentin.
No immunoreactivity for antibodies to keratin, smooth muscle actin, desmin, S100 protein, alk, HMB45, or
PSA was exhibited by the spindle cell population. However, keratin immunostains of other sections
revealed rare, PSA-positive nests and cribriform aggregates of epithelial cells that exhibited cytologic
atypia, characterized by mildly enlarged nuclei and somewhat prominent nucleoli. Based upon these
findings and the history of a previously diagnosed prostate carcinoma, a diagnosis of sarcomatoid
carcinoma of the prostate was made.
History (M181): 27 year-old male who presented with hematuria. Radiological exam showed a single,
large, midline, multicystic pelvic mass that was situated between and did not appear to involve the
prostate/bladder neck region or the rectum. Cystoscopy and colonoscopy failed to reveal a mucosal
lesion. An exploratory laparotomy showed a localized mass, which was biopsied. After treatment with
chemotherapy, the tumor was resected as a radical cystoprostatectomy/tumor mass resection.
Lesions to discuss
The differential pathologic diagnosis of the more common spindle cell tumors of the prostate are:
- Post-operative spindle cell nodule
- Inflammatory pseudotumor
- Stromal "tumors"
- Stromal hyperplasia/nodule/leiomyoma
- STUMP (stromal proliferation of uncertain malignant
- Sarcomatoid carcinoma
- Sarcomatoid prostate adenocarcinoma
- Sarcomatoid transitional cell carcinoma
- Epithelial-stromal tumors
- Phyllodes tumor
A number of terms apply to what is apparently the same benign biologic process, which consists of a
proliferation of predominantly fibroblastic cells and small numbers of admixed smooth muscle cells and
vessels. These terms include "Inflammatory pseudotumor", "Pseudosarcomatous" (fibromyxoid)
pseudotumor", and "Post-operative spindle cell nodule".
Post-operative spindle cell nodule presents as a mass at the site of a
previous surgical procedure, most frequently a bladder neck mass after a TURP or TURB. Typically, the
procedure was done within the 3 months prior to appearance of the mass. Occurring at any age, this rare
lesion is characterized by a proliferation of spindle cells that focally may form fascicles and/or
storiform aggregates. Often they have a high mitotic rate. Atypia and hypercellularity may be marked.
Immunohistochemically the only antigen diffusely expressed is vimentin. A minority of lesional cells
express desmin and smooth muscle actin; in up to 40% of cases keratin-immunoreactive cells are found. In
some post-operative spindle cell nodules cells are cytologically so atypical that malignancy should be
considered. You can avoid the pitfall of diagnosing sarcomatoid carcinoma or sarcoma by remembering that
a nodule that appears within 3 months of a local surgical procedure is most likely a benign, reactive
process. Localized excision is curative.
Inflammatory pseudotumor is, basically, indistinguishable from the
post-operative spindle cell nodule with respect to histology and benign outcome. Without a history of
prior local trauma/surgery, these are idiopathic masses that are self-limited and without malignant
potential. The range of cellularity, inflammation, and edematous/myxoid stroma exhibited by these
lesions has provided the basis for different terms
This lesion affects both prostate and bladder
. Our challenge is to distinguish a malignant stromal tumor from an inflammatory pseudotumor that
arises without a history of prior surgery and is composed of an atypical, hypercellular stroma. An
immunostain for smooth muscle actin that demonstrates no more than rare to infrequent smooth muscle cells
argues strongly against a leiomyosarcomas, which is the most frequent prostate sarcoma in adults.
Stromal Tumors of the prostate are predominantly of smooth muscle
Stromal hyperplasia/nodule/leiomyoma. The earliest stage of nodular
hyperplasia of the prostate is thought to be microscopic, periurethral nodules of stromal cells composed
of fibroblasts and smooth muscle cells and free of glands. Since the distinction between a stromal
cell-only nodule of a prostate with BPH and leiomyoma appears to be arbitrary, we can question whether a
large smooth muscle cell nodule warrants being termed "leiomyoma". These lesions are similar in a number
of ways to uterine smooth muscle tumors. Histologically atypical and "bizarre" stromal tumors of the
prostate have been reported . However, to my knowledge, the biology of these is no different than
that of stromal tumors free of atypia and/or of smaller size, specifically, they are benign. Distinction
of a stromal nodule/leiomyoma from leiomyosarcoma is based upon features used as criteria in many organs
– small size, circumscription and localization to the organ, lack of necrosis, absence of cell/nuclear
pleomorphism and low mitotic activity.
Atypical stromal tumors . Atypical spindle cell tumors (recently termed
STUMPs, for, "stromal proliferations of uncertain malignant potential") exhibit a wide range of
histological appearances, ranging from atypical stromal cells interposed between histologically benign
prostate glands to phyllodes tumors in which both epithelial and mesenchymal components are proliferative
(see below for more discussion of phyllodes tumors)
[4-6]. There is evidence, mostly anecdotal, that
these tumors represent a biologic continuum, and, consequently, that there may be progression between
stages characterized by overgrowth of the stromal component. Antigens variably expressed by STUMPs
include vimentin, smooth muscle actin, desmin, CD34 and estrogen and progesterone receptor .
Miscellaneous benign prostate spindle cell tumors have been reported, e.g.
nerve sheath tumor and solitary fibrous tumor. We have had the experience of a GIST (GI stromal tumor)
being regarded clinically to be of prostate origin. When resected, this was found to be a rectal GIST
that abutted the prostate gland .
Sarcomas of the prostate are rare, representing < 0.1% of primary
prostate neoplasms . The most frequent types are leiomyosarcomas (in adults in whom the median age is
60, with a range of 20 to 80 years) or rhabdomyosarcomas (in children in whom the median age is 4, with a
range from newborn to 6 years of age, though cases have been reported in adults). Other histologic types
of sarcomas have been reported, including angiosarcoma, osteosarcoma, chondrosarcoma, malignant
peripheral nerve sheath tumor, synovial sarcoma, and malignant mesenchymoma.
Leiomyosarcomas are typically hypercellular, composed of cytologically
high-grade spindle cells that tend to form fascicles, and are often necrotic. Other histologic patterns
include pleomorphic and epithelioid. Based on the Broders' grading system (a scale of 1 to 4) the
majority (>67%) are high grade. Mitoses and necrosis are typically frequent. A definitive diagnosis
can be made by demonstrating diffuse smooth muscle actin immunoreactivity. Leiomyosarcoma can usually be
distinguished from sarcomatoid carcinoma by absence of an epithelial tumor cell population and by
demonstrating widespread expression of smooth muscle-actin (66% of cases), desmin (20% of cases), and
vimentin (virtually all cases). Keratin expression does not rule out leiomyosarcoma since 25% of
leiomyosarcomas express keratin .
The median survival after diagnosis is 2 to 4 years . The
only known curative therapy is radical surgery. No systemic therapy has produced a tumor response.
Rhabdomyosarcoma of the prostate present at a median age of 4 to 5 (range:
newborn to 19 years of age, though cases have been reported in adults). Histologically the majority are
embryonal type. Expression of nuclear factors myogenin and myo-D and of cytoplasmic myoglobin confirm a
diagnosis of rhabdomyosarcoma. Multimodal therapy (tumor resection, chemotherapy, radiation therapy) has
resulted in cures of many patients with bladder preservation; 85% of patients survive at least 5 years.
Prognostic parameters of rhabdomyosarcoma include tumor stage, histologic type, and patient age .
However, that said, virtually all patients with metastatic disease have died of tumor.
Sarcomatoid carcinoma (or, "carcinosarcoma")
As with carcinomas of other organs, i.e. breast, lung, pancreas, and urinary bladder, adenocarcinomas
of the prostate rarely have a malignant spindle cell component. This may be the predominant component.
These tumors are extremely rare in the prostate; fewer than 50 cases have been published. Patients tend
to be older (median age 70; range 50 to 90 y.o.) than the median age of patients with pure prostate
adenocarcinoma. About half of the cases are associated with a previously diagnosed, pure acinar prostate
adenocarcinoma, which is often of high grade. In these patients there is a correlation, albeit weak,
with prior radiation or hormonal therapy that is followed by recurrence of tumor as sarcomatoid carcinoma
an average of 3 years after initial diagnosis. The other cases co-occur with an acinar prostate
Histology: The proportion of the tumor that is sarcomatoid is highly variable. Where the dominant
element is sarcomatoid, extensive sampling to find histologic evidence of epithelial differentiation may
be necessary. Immunostaining sections for epithelial cell antigens (for transitional/urothelial cell
carcinoma, though I am not aware of only sarcomatoid transitional/urothelial cell carcinomas that
clinically presented as a prostate tumor: low- and high-MW keratin, keratin 7/20 and p63; for prostate
adenocarcinoma, low-MW keratin or PSA) can either highlight rare epithelial tumor cell nests, or can
provide indirect evidence of epithelial differentiation in a tumor that is virtually exclusively composed
of spindle cells. When the only evidence of epithelial differentiation is the expression of keratin by
tumor cells, the caveat that sarcomas have been reported to, infrequently, express keratin should be
recollected. The sarcomatoid component may be composed of heterologous elements. In decreasing
frequency of occurrence these are osteosarcoma (40% of cases), chondrosarcoma (20%), leiomyosarcoma (5%),
rhabdomyosarcoma (5%), and angiosarcoma (1%). The presence of these elements is of no apparent clinical
significance. Regarding immunophenotype the sarcomatoid component expresses, in decreasing frequency,
vimentin (100%), smooth muscle actin (45%), prostatic acid phosphatase (35%), desmin (5%), keratin (5%),
and PSA (2%). A case in which the sarcomatoid element expressed progesterone receptor has been reported
Although the epithelial element in most cases is adenocarcinoma, there are case publications
that report a squamous and urothelial epithelial component .
In general, carcinomas that exhibit extensive sarcomatous differentiation are associated with a
particularly rapid course. The median survival in one series of patients with sarcomatoid carcinomas was
7 months (range: 1 to 48 mos.) following the appearance of sarcomatoid foci, and 36 mos. (range: 8 to
108 mos.) following the diagnosis of adenocarcinoma. The SEER (an NCI program: Surveillance,
Epidemiology and End Results) data provides further documentation of the malignant nature of sarcomatoid
prostate carcinomas. The histologic elements of metastases are, with equal frequency, pure epithelial,
pure sarcomatoid, and sarcomatoid carcinoma. These tumors appear to be non-responsive to systemic
therapy, including androgen blockade and chemotherapy.
With respect to etiology, the suggestion has been made that radiation, either external beam or
brachytherapy, confers an increased risk of prostate carcinosarcoma and/or prostate sarcoma . Given
the relatively short duration after radiation (< 5 years after radiotherapy) and rarity of
carcinosarcoma, and the even greater rarity of prostate sarcoma, this etiology seems unlikely, unless
there are yet to be identified host-specific predisposing factors . Regarding molecular
pathogenesis, little is known. That 2 patients exhibited increasing p53 provides a basis for arguing
that "the progressive accumulation of p53 suggests increasing clonal dominance of dedifferentiated tumor
cells with p53 mutations" .
A final point concerns semantics. "Sarcomatoid carcinoma", which has been strictly defined as a
spindle cell carcinoma, has the same clinical behavior as "carcinosarcoma", which has been strictly
defined as a malignant tumor composed of both carcinoma and sarcoma histologic elements. In the latest
WHO Classification of Tumors of the Prostate (2004), the authors state that "given their otherwise
similar clinico-pathologic features and identically poor prognosis, these two lesions are best considered
as one entity." Finally, the term "collision tumor"  has sometimes been applied to sarcomatoid
carcinomas of different organs. Since it is now clear that the sarcomatoid component appears to be a
histologically metaplastic manifestation of the respective carcinoma, and not a de novo tumor, the term
collision tumor is best reserved for co-occurent tumors of adjacent, but different organs, such as
concurrent rectal and prostate adenocarcinomas.
A number of reviews of sarcomatoid prostate adenocarcinoma have been published
discussions are also available in the recently published WHO monograph Tumours of the Urinary System and
Male Genital Organs .
Epithelial-stromal tumors of prostate
Primary epithelial-stromal tumors of the prostate are also extremely rare, consisting for the most
part of case reports. These tumors occur in adults (median age of 49 years; range 22 to 78 years of
age). Although a variety of different names have been used for these tumors, "phyllodes tumor" and
"fibroadenoma" seem the most appropriate since they have gross and microscopic similarities to the
corresponding tumors of the breast:
These tumors are composed of cytologically benign epithelial cells and a stroma that varies widely in
cellularity in different cases. Similar to phyllodes tumor of the breast, the histology of the stromal
cells contributes to predicting tumor behavior. A proliferative, cellular stroma leading to stromal
overgrowth and cellular atypia with increased (>2 to 9 per 10 hpf) mitotic rate predicts likely
recurrence of tumor with a significant probability of death due to the neoplasm. These tumors (termed
"malignant phyllodes tumor" or "cystosarcoma) are lethal in 75 % of patients due to either bulky local
tumor or metastases. Conversely, in the absence of these features, a diagnosis of "low-grade phyllodes
tumor (or, cystadenoma)" is appropriate, with the caveat that the course of the tumor in individual cases
is unpredictable. The risk of local recurrence of incompletely excised tumor is high. 80% of tumors that
were surgically treated by transurethral excision recurred locally. The risk of local invasion and of
distant metastases can be predicted base on the histologic features as discussed above. Responsiveness
to either hormonal therapy or to chemotherapy is unknown. These tumors are apparently insensitive to
radiation therapy .
- When large, they may be cystic; the cystic spaces are lined by the
same epithelium that lines the glands .
- The size of the lesion ranges from incidental, microscopic,
proliferative changes to bulky tumors.
- The stromal component has malignant potential .
Several intriguing observations can be made about sarcomatoid carcinomas:
Cases such as the present one raise yet-to-be answered questions: Why is a sarcomatoid element so
rare? What induces the sarcomatoid phenotype? Why does the sarcomatoid phenotype behave in a
particularly malignant manner? Basic science findings based on intermediate filament cell biology
provide intriguing clues to some answers. There are > 65 intermediate filaments (originally defined
as long, 10 nm diameter cytoplasmic filaments that are distinct from muscle filaments). The broad
classes of intermediate filaments include keratins (types I and II), vimentin, desmin, neurofilament,
glial fibrillary acidic protein, (nuclear) lamins, etc. The functional complexity of these proteins is
exemplified by such functions as cell and nuclear structure/scaffolding, intracellular transport, cell
signaling, and cell motility . Known for years is the loss (in general} of keratin immunoreactivity
and gain of vimentin expression by the spindle cell element of sarcomatoid carcinomas. Functional
features of these respective intermediate filaments that provide an explanation for the more aggressive
course of carcinomas with a sarcomatoid component are the following:
- The sarcomatoid
element virtually never has functional phenotypic features of the specific epithelium and, when free of
heterologous elements (such as skeletal muscle, bone, cartilage), are phenotypically
- Sarcomatoid carcinomas exhibit a
more malignant course than the carcinoma of the respective organ, growing more rapidly and disseminating
as metastases earlier in the course of disease.
- Sarcomatoid carcinomas are resistant to systemic therapy.
Although the reason for why some carcinomas undergo this switch in intermediate filament type is
unknown and whether the functions reported above for vimentin and some keratins are relevant to the
behavior of sarcomatoid carcinomas is unproven, characterizing molecular events begins to provide us with
specific tools to better understand the complex biology of tumors, and, in our present case, of such
phenotypically unusual tumors as sarcomatoid carcinomas.
- Cells that
express vimentin as their predominant intermediate filament are more rapidly motile than comparable cells
expressing keratin as the predominant intermediate filament .
- Transition of epithelial cells to a mesenchymal phenotype has been correlated
with resistance to EGFR inhibitor Erlotinib (shown in non-small cell lung carcinoma] .
Summary points (sarcomatoid prostate carcinoma)
- If it look like
sarcoma, rule out carcinoma and post-op spindle cell nodule
- Look for epithelial nests to confirm the diagnosis of sarcomatoid
- Immunostains may be less helpful than
expected in confirming the diagnosis since stromal cells and some smooth muscle cell tumors can
(infrequently and, then, only focally) express keratin
- Sarcomatoid prostate carcinoma is rapidly progressive and resistant to
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