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ZONAL ANATOMY OF THE PROSTATE GLAND
Gafurov B.K., Mirzarakhimov Kh.A., Sabirov A.A., Kupaysinov B.B.*
*Lecturer at the Department of General Surgery of the Chirchik branch of the Tashkent Medical
Academy
Annotation:
The article provides a historical background, presenting the approaches of
morphologists and radiologists to the issue of the lobar structure of the prostate gland. Given
the confusion regarding the term “central zone”, it is proposed to avoid its use in the
conclusions of magnetic resonance and ultrasound examination. It is recommended to use
the division of the gland into 39 sectors only when describing an MRI study of the prostate
performed in accordance with the recommendations of the PI-RADSv2 system.
Keywords:
prostate gland, zonal anatomy, magnetic resonance imaging, ultrasound
examination.
Introduction
When analyzing the conclusions formed based on the results of MRI studies of the male
pelvic organs, which are posted in the Unified Radiological Information System, it was
noted that the authors use an extremely wide range of terms characterizing the location of
the detected changes in the prostate gland. The gland is divided into lobes, zones, parts,
segments, sectors, regions, floors, thirds, ends, etc. This situation is largely determined by
the historically established interest of researchers to leave their contribution to history by
proposing a new classification of the gland structure. This led to significant confusion in the
terms found in the conclusions, and as a result - to an incorrect interpretation of the data of
the studies conducted.
History of the issue
There are many classifications of the lobar structure of the prostate gland. Urologists
traditionally distinguish 3 lobes - 2 lateral and a middle [1]. However, there were also other
classifications: division of the prostate gland into 5 lobes - anterior, posterior, middle and 2
lateral [2], into 6 lobes - 2 posterior, 2 internal and 2 lateral [3]. Currently, the most common
is the zonal anatomy of the prostate gland, developed by JE McNeal in 1981 [4]. He
identified 4 glandular zones (central, peripheral, 2 transitional) and 4 fibromuscular layers
(anterior fibromuscular stroma, preprostatic sphincter, longitudinal smooth muscle fibers of
the urethra and postprostatic sphincter). A schematic representation of the location of the
gland zones is shown in Fig. 1 and 2.
Based on the characteristics of the prostate radiographic picture, the classification proposed
by MD Rifkin et al. in 1990 [5] is of particular interest. According to it, the prostate gland is
divided into 3 parts. The anterior part corresponds to the fibromuscular stroma according to
the classification
JE McNeal. This fibrous zone is practically not visualized by TRUS and, since it does not
contain glandular tissue, does not play a role in the development of hyperplasia or prostate
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cancer. The “outer gland” or the external part of the gland is a combination of peripheral and
central zones. The “inner gland” or the internal part of the gland includes transition zones,
the zone of periurethral glands and the zone of muscle layers of the internal sphincter of the
urethra. It would seem that this classification most logically defines the structure of the
prostate gland used to analyze echographic and MR images (Fig. 3), but it has not managed
to gain wide distribution in practical work. One of the reasons is the inconvenience of the
terms themselves - “inner” and “outergland” and the difficulty of translating them into
Russian. We proposed to translate the terms “inner” and “outergland” as the internal and
external parts of the gland (see above), which could lead to greater uniformity in the
conclusions [6].
As for MRI diagnostics of prostate cancer, an International Working Group was formed in
2007 to standardize it. The result of its work was the PI-RADS (Prostate Imaging and
Reporting and Data System), which was published as recommendations of the European
Society of Urogenital Radiology (ESUR) in 2012 [7]. This system recommended using a
scheme for dividing the prostate gland into a number of segments. However, in 2015, the
second version of this system was published [8, 9]. This version proposes to describe the
localization of the suspected tumor based on the zonal anatomy according to McNeal and
dividing the gland into thirds: base, middle third, apex. Accordingly, the location of the
detected changes is tied to a scheme of 39 sectors: 36 sectors representing the prostate gland
itself, as well as the seminal vesicles and the membranous part of the urethra.
A
4
3
1
2
Fig. 1.
Schematic representation of the
zonal anatomy of the prostate gland
(according to JE McNeal, 1981). a – in
the sagittal plane; b – in the frontal
(coronal) plane. 1 – central zone; 2 –
peripheral zone; 3 – transition zones; 4
– anterior fibromuscular zone.
b
1
4
3
2
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A
b
V
G
A
b
Fig. 2.
Schematic representation of the zonal anatomy of the prostate gland (according to JE
McNeal, 1981) in the coronal plane, the zones indicated by arrows are sequentially
“removed”. a – all zones of the gland are presented (arrow – anterior fibromuscular stroma);
b – arrow – peripheral zone; c – arrow – central zone; d – arrow – transition zones.
Fig. 3.
Division of the prostate into parts. a – schematic image; b – T2-weighted MR image,
axial plane; c – transverse TRUS image. Arrow – external part of the gland; thin arrow –
internal part of the gland.
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SP
SP
Ur
A
PrK
b
MP
O
K
PrK
Fig. 4.
Zonal differentiation of the prostate on
normal T2-weighted MR images. a – axial plane; b – sagittal plane. Transitional zones are
outlined by a dotted line. UB – urinary bladder; RB – rectum; LB – pubic bone
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Fig. 5.
T2-weighted MR image of the prostatein the coronal plane. The central zone has
the form of a cone directed from the base of the gland to the seminal tubercle (highlighted
by the dotted line). SP – seminal vesicles; Ur – membranous section of the urethra.
Discussion
The main problem, in our opinion, is the misunderstanding of the term “cent”“central zone of
the gland”. Many reports of practicing ultrasound and MR diagnostics specialists, moreover,
in scientific publications describe changes in the central zone of the prostate gland, implying
their localization in the transitional zones. The reason for this error is that the transitional
zones, which have a reduced MR signal intensity on T2WI, are located on axial sections as
if in the center of the prostate gland and are surrounded by a peripheral zone with a high
signal intensity on T2WI (Fig. 4). The true central zone (according to the McNeal
classification) can be identified almost only in young patients with no pronounced benign
hyperplasia when scanning in the coronal plane. According to the PI-RADSv2 system, it is
“the tissues of the gland surrounding the ejaculatory ducts from behind and above, from the
base of the gland to the seminal tubercle; it has the shape of an inverted cone, with the base
directed toward the base of the gland; contains more stroma than glandular tissue” (Fig. 5).
At the same time, the same PI-RADSv2 system postulates that cancer affecting the central
zone is a tumor spread from the peripheral zone. Thus, in practical work, examining patients
mainly of middle and older age groups, in the presence of benign prostatic hyperplasia, we
are unable to differentiate the “true” central zone of the prostate gland. However, if such a
study is carried out in accordance with the PI-RADSv2 recommendations, then this system
itself offers a solution to this terminological dispute.
MP
MP
PrK
PrK
Fig. 6.
Combined MR-TRUS image obtained using Fusion technology. The tumor lesion in
the peripheral zone of the right lobe at the level of the middle third of the gland is outlined
in oval. a – T2-weighted MR image in oblique axial plane. There is a clear differentiation of
the prostate gland into peripheral and transitional zones; b – transrectal ultrasound scanning
of the prostate in the transverse plane. The peripheral and transitional zones have similar
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echogenicity and structure. The border between them is a faint hypoechoic band. UB –
urinary bladder; RC – rectum.
By using the proposed 39 sectors to localize pathological foci, we stop using the usual terms
based on zonal anatomy, and, finally, we can agree on the uniformity of indications of the
topic of detected changes in the prostate gland.
However, it should be taken into account that a significant part of MR examinations of the
pelvis in men is aimed at detecting tumors in the bladder or rectum. The protocols for such
studies, naturally, do not meet the requirements of the PI-RADSv2 system and we have no
right to use the recommended division of the gland into 39 sectors. The same situation
occurs with ultrasound examinations; it is not possible to divide the prostate gland into 39
segments due to the peculiarities of the echographic image. Moreover, in transrectal
ultrasound images, only the boundaries of the transition zones can be seen in the structure of
the prostate gland, although not as clearly as in MRI (Fig. 6). It is almost impossible to
differentiate the “true” central zone from the peripheral one, since both, as a rule,
demonstrate a similar level of echogenicity. Therefore, there is a need to have a unified
approach to topical diagnostics in the prostate gland, different from the PI-RADSv2
recommendations.
Conclusion
When describing images of the prostate gland, we propose the following approach:
•
when describing an MRI examination of the prostate performed in accordance with the
PI-RADSv2 system, be guided by the localization of pathological foci in 39 sectors;
•
When describing MRI studies that do not meet the requirements of the PI-RADSv2
system (for example, pelvic examinations for neoplasms of the bladder or rectum), as well
as TRUS, we propose using the following system for localizing pathological changes:
–
base, middle third and apex of the gland;
–
right and left lobes of the gland;
–
peripheral and transitional zones;
•
To avoid confusion regarding the term “central zone”, we propose to avoid it altogether
in the conclusions of both magnetic resonance and ultrasound examinations.
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