Interobserver Agreement in Histopathological Subtyping of Malignant Pleural Mesotheliomas
Mona MLIKA, Faouzi MEZNI
Department of Pathology, Abderrahman Mami Hospital, University of Medicine of Tunis, University of Tunis El Manar, Research Laboratory: LR18SP06, TUNIS, TUNISIA
Keywords: Interobserver agreement, Malignant pleural mesothelioma, Mesothelioma subtypes
Many recent studies are pointing out the heterogeneity between pathologists in the classification of malignant pleural mesotheliomas.
Besides, they reported the prognostic impact of classifying epithelioid mesotheliomas according to the predominant architectural features and
the nuclear grade. The authors assessed the interobserver and the intraobserver agreement of subtyping mesotheliomas between 2 pathologists
used to thoracic pathology.
Material and Method: The observers reviewed all the slides of the malignant pleural mesotheliomas diagnosed during the period ranging from
2004 to 2017. The Cohen Kappa was performed in order to evaluate the agreement between both observers into classifying mesotheliomas,
subtyping and grading epithelioid mesotheliomas. Two rounds of examination were planned with a delay period of one month. After the first
round, the reviewers discussed the different difficulties and challenges they faced. All the statistic tests were performed using the SPSS software
Results: After the first round, a fair agreement between both observers was reported. After the second round, an improvement of the concordance
rate with a good agreement in subtyping epithelioid mesotheliomas was noticed. Concerning the grading of mesotheliomas, the interobserver
agreement was poor even after the second round examination. The intraobserver reproducibility of epithelioid mesothelioma subtyping was fair
or moderate for both reviewers. The intraobserver agreement was poor concerning the grading of epithelioid mesothelioma.
Conclusion: Integrating subtyping and grading of epithelioid mesotheliomas into a new classification necessitates an important training of the
pathologists. The architectural features’ definitions have to be clarified in order to avoid using own subjective opinions and habits by pathologists.
Malignant pleural mesothelioma is a rare tumour with a poor
prognosis. The management of these tumours is based on
a multidisciplinary approach associating surgical resection,
radiation therapy and chemotherapy. The most relevant
prognostic factors consist in the stage and the histologic
subtype. The microscopic diagnosis is made in accordance
with the 2015 World Health Organization Classification 1
This classification made huge modifications in lung cancer
classification concerning the subtyping of adenocarcinomas
according to the predominant architecture. On the other
hand, it put emphasis on the heterogeneous architecture
of epithelioid pleural mesotheliomas. In fact, pleural
mesotheliomas are classified into diffuse or localized
tumours based on their distribution and into epithelioid,
sarcomatoid or biphasic tumours according to their
microscopic features. These tumours present different
prognoses. The best prognosis is attributed to epithelioid
tumours and the worse one to sarcomatoid tumours. As it was noticed in lung primary adenocarcinomas, the group
of epithelioid mesotheliomas seemed to be heterogeneous
in terms of prognoses, molecular features and microscopic
aspects. Many different architectural subtypes have been
identified into the WHO classification and reported by the
international mesothelioma interest group 2-5
subtypes consisted of solid, trabecular, acinar, tubulopapillary,
micropapillary, adenomatoid, deciduoid and
transitional. Every subtype is characterized by specific
diagnostic features, which were slightly described in the
WHO classification 1
. Many authors reported the different
prognostic impact of these subtypes pointing out the poor
prognosis of transitional epithelioid mesotheliomas that
was reported to be similar to sarcomatoid mesotheliomas
. In addition to these different subtypes, many authors
reported the prognostic impact of grading epithelioid
mesotheliomas. The grading system takes into account
nuclear grade, mitoses and necrosis 7
. It has been
reported to be a relevant prognostic feature by many authors. Recently, the international mesothelioma interest
group stipulated the necessity of reporting the subtype
of epithelioid mesotheliomas in addition to the grade 3,5
. This fact made us wonder about the reproducibility of
The authors aimed to assess the inter-observer and
intra-observer reproducibility of classifying pleural
mesotheliomas and subtyping epithelioid mesotheliomas
according to the predominant architecture and the grade.
The authors reported a descriptive, retrospective and
transversal study including pleural mesotheliomas
diagnosed in a single institution. The authors reviewed
all the slides of the malignant pleural mesotheliomas
diagnosed during the period ranging from 2004 and 2017.
They reviewed hematoxylin eosin stain slides and silanized
slides used for immunohistochemistry.
All the malignant pleural mesotheliomas diagnosed during
the period of the study were retrieved from the archives of
the Department of Pathology.
The specimen for which no paraffin blocks were available,
Other pleural tumours were excluded from this study.
All the slides were reviewed by 2 pathologists who were
used to thoracic pathology. The first pathologist had 25
years of experience and the second pathologist had 10 years
of experience in thoracic pathology.
The authors were given a sheet with different items to record
including the number of slides reviewed, the number of
samples, the mesothelioma subtypes including epithelioid,
sarcomatoid, biphasic and the architecture-based
subtyping in case of epithelioid mesothelioma including
trabecular, solid, micropapillary, tubulo-papillary, acinar,
adenomatoid, transitional, deciduoid or special variant
including pleomorphic cells or signet ring cells. The grade
was also recorded in epithelioid mesotheliomas. Two
rounds of evaluation, with an interval of one month, were
performed. After the first round, the two reviewers met for a clarification session to present the different difficulties
and challenges they faced.
Subtyping of pleural mesothelioma was made according
to the WHO classification 1. Epithelioid mesothelioma
was defined as a tumour made of polygonal or ovoid cells.
Sarcomatoid mesothelioma was characterized by elongated
and tapered mesothelial cells with various degrees of
atypia and mitoses. Biphasic tumours were defined by the
association of an epithelioid component to a sarcomatoid
one with a minimum proportion of 10 % for each
component. Concerning the epithelioid mesotheliomas,
the trabecular subtype consisted in small cells arranged into
thin cords or single files (Figure 1A). Nests of tumour cells
defined the solid subtype (Figure 1B). Papillary subtype was
characterized by papillary structures with a fibrovascular
core (Figure 1C). Micropapillary structures were digitiform
structures without a fibrovascular core. Acinar structures
were glandular structures (Figure 1D). Adenomatoid
structures were characterized by pseudoglandular
structures (Figure 1E). Deciduoid subtype was composed
by large cells with atypical nuclei, abundant cytoplasm with
pronounced eosinophilia, and glassy cytoplasm mimicking
deciduoid cells. The description of the transitional subtype
was not clear in the WHO classification, and that is why
the authors adopted the definition of Galateau Sallé, et al.
Transitional subtype was defined as sheets of plump cells
starting to lose their epithelioid morphology but not overtly
spindle shaped and lacking frank sarcomatous features 6.
Click Here to Zoom
|Figure 1: A) The trabecular subtype with small cells arranged into
thin cords or single files (H&E; x400). B) The solid subtype with
nests of tumour cells (H&E; x200). C) The papillary subtype with
papillary structures with a fibrovascular core. D) Acinar structures
with glandular structures (H&E; x200). E) The adenomatoid
structures with pseudoglandular structures (H&E; x200).
Grading was performed according to the IASLC proposition
criteria and was based on the nuclear grading, mitotic
index and necrosis 3. A two-tier system was established
with low-grade tumours consisting of grade I or II tumours
without necrosis and high-grade tumours consisting of
grade III tumours and grade II tumours with necrosis.
Nuclear grade was scored 1, 2 or 3 for respectively mild,
moderate or severe nuclear atypia. Mitotic count was
scored 1, 2 or 3 for tumours with respectively less than 1
mitosis per 2 mm2, 2 to 4 mitoses per 2 mm2 and more than
5 mitoses per 2 mm2. A sum of 2 or 3 was considered as a
grade I, a sum of 4 or 5 was considered as a grade II, and a
sum of 6 was consistent with grade III tumour. The reviewers
had no limit of time to review the cases (Figure 2A,B).
Click Here to Zoom
|Figure 2: A) Epithelioid mesothelioma characterized by a low nuclear grade with regular nuclei and rare mitoses (H&E; x400).
B) Epithelioid mesothelioma characterized by a high-grade nuclear grade with atypical nuclei and numerous mitoses (H&E; x400).
The Cohen Kappa was performed in order to evaluate
the agreement between both pathologists into classifying
mesotheliomas, subtyping epithelioid mesotheliomas and
reporting the grade in epithelioid mesothelioma.
Two rounds of examination were planned with a delay
period of one month. After the first round, the reviewers
discussed the different difficulties and challenges they faced.
Interobserver agreement and intraobserver agreement
were performed. The strength of agreement was considered
excellent for kappa >0.8, good for 0.61
All the blocks included were anonymized and no
information concerning the patients was used in this study.
A reference number was attributed to each case.
The diagnosis of malignant mesothelioma was made on
needle biopsies in 16 cases, thoracoscopic biopsy in 32 cases,
and surgical specimens in 2 cases. The latter consisted of
a bullectomy and an extra pleural pneumonectomy. Total
concordance between pathologists was observed in 10/16
needle biopsies, 21/32 thoracoscopic biopsies, and 1/2
surgical specimens. Both pathologists reviewed the same
number of slides.
Interobserver Reproducibility Concerning the
Subtyping of Mesotheliomas
This study included 50 malignant pleural mesotheliomas.
Both pathologists reviewed all cases with a mean of 7 blocks
(slides)/ case (min 1 block, max 21 blocks). The different
cases consisted of epithelioid mesotheliomas in 44 cases (88%), sarcomatoid mesothelioma in 4 cases (8%), and
biphasic mesothelioma in 2 cases (4%).
The concordance rate accounted for 32% (16/50) when
taking into account the subtyping and the grading. It
reached 44% (22/50) when taking into account only
the subtyping of mesotheliomas without the grading.
The pathologists were confused when differentiating
solid from deciduoid subtypes (4 cases), papillary from
trabecular subtypes (4 cases), acinar from trabecular
subtypes (4 cases), signet-ring cell from solid subtypes (2
cases), trabecular from micropapillary subtype (2 cases),
solid from trabecular subtype (2 cases), solid from acinar
subtype (2 cases), sarcomatoid from pleomorphic (2
cases), sarcomatoid from trabecular subtypes (2 cases), and
biphasic from epithelioid subtypes (4 cases) (Figures 3A,B).
The highest concordance rates were observed in the tubulopapillary,
trabecular, solid, biphasic, and sarcomatoid
subtypes with ratios reaching respectively 8/12, 14/18,
6/6, 2/2 and 4/6. The deciduoid and the acinar subtypes
presented the lowest concordance rates, reaching 0/2 and
2/4 respectively. The first reviewer recognized 8 trabecular
mesotheliomas, 6 acinar, 6 solid, 14 tubulo-papillary,
2 micropapillary, and 8 special variants consisting of 2
pleomorphic cases, 2 signet ring cells cases, 2 deciduoid
cases, and 6 sarcomatoid mesotheliomas. The second
reviewer identified 14 trabecular mesotheliomas that were
judged by the first reviewer as trabecular in 4 cases, acinar
in 4 cases, tubulo-papillary in 2 cases, micropapillary in
2 cases, and sarcomatoid in 2 cases. The second reviewer
recognized 16 solid mesotheliomas classified by the first
reviewer as trabecular in 2 cases, acinar in 2 cases, and
solid in 6 cases. The second reviewer identified also 6 biphasic mesotheliomas classified as trabecular in 2 cases,
papillary in 2 cases, and a special variant in 6 cases by the
first reviewer. The Cohen Kappa reached the value of 0.34
corresponding to a fair agreement.
Click Here to Zoom
|Figure 3: A) Case 15 was considered as a trabecular subtype by observer 1 and acinar subtype by observer 2 (H&E; x200). B) Case 35 was
considered as biphasic mesothelioma by observer 2 and tubulopapillary mesothelioma by observer 1 (H&E; x200).
After the second session, the inter-observer agreement was
good with a weighted Kappa value of 0.62. The highest
concordance rates were observed for solid, acinar, and
sarcomatoid subtypes reaching 100%. The concordance
rate accounted for 50% for papillary subtype.
Interobserver Reproducibility Concerning Nuclear
Concerning the nuclear grading of the epithelioid
mesotheliomas, the first reviewer considered 16 high-grade
and 28 low-grade tumours. The second reviewer considered
22 low-grade and 22 high-grade tumours. Among the 32
high-grade tumours recorded by the second reviewer,
16 cases were also considered as high-grade by the first
reviewer. Among the 12 low-grade tumours recorded by
the second reviewer, 10 cases were also recorded as lowgrade
by the first one. The concordance between the judges
accounted for 59% (26/44). The agreement between the
reviewers was fair (Cohen Kappa=0.28). After the second
round, the agreement of nuclear grade was poor with a
weighted Kappa accounting for 0.
Intraobserver Reproducibility Concerning the
Subtyping of Mesotheliomas and the Grading of
Between both sessions of examination, the intraobserver
agreement of mesothelioma subtyping was fair (Kappa=
0.27) and moderate (Kappa=0.44) for the first and the
second reviewer respectively. For the first reviewer, the
worse concordance rates were recorded for solid, trabecular,
acinar, the special variants, and micro-papillary subtypes.
For the second reviewer, the worse concordance rates were
recorded for the trabecular and biphasic subtypes.
Concerning the grading, the intraobserver agreement was
very poor (Kappa=0) and poor (Kappa=0.2) for reviewer 1
and 2 respectively.
The different values of the weighted kappa are represented
in Table I.
Click Here to Zoom
|Table I: The distribution of kappa scores for subtyping mesotheliomas and assessing nuclear grade.
In this retrospective and descriptive study, the authors
assessed the reproducibility of subtyping the pleural
mesotheliomas after two-round sessions. They included
the subtype and grade of epithelioid mesotheliomas.
The second round was performed after a clarification
session between the 2 reviewers who were used to
practicing Thoracic Pathology. During the first round,
they reported a fair agreement between both observers
concerning mesothelioma subtyping and the grading.
After the second round, they noticed an improvement of
the concordance rate with a good agreement in subtyping
epithelioid mesotheliomas. Concerning the grading, the
interobserver agreement was poor even after the second
round examination. The intraobserver agreement of
epithelioid mesothelioma subtyping was fair or moderate
for both reviewers. The intra-observer agreement was poor
concerning the grading. This kind of study seems necessary
in order to assess the validity of new criteria that can be
integrated in these tumours’ classification. The 2015 World
Health Organization Classification of lung cancer resulted
from numerous studies that reported the heterogeneity
of the adenocarcinomas and the reproducibility of a
classification based on the major architectural subtypes
. In this classification, the heterogeneity of the
epithelioid mesotheliomas was pointed out without a real
recommendation to subtype these tumours according to the
predominant architectural feature. The prognostic impact of
subtyping epithelioid mesotheliomas was reported by many
. Rosen L, et al. reported a better prognosis
of mesotheliomas with trabecular or tubulo-papillary
patterns in comparison to those with other patterns 7
In a study including 108 pleural mesotheliomas, Brcic L,
et al. reported a good agreement with a Kappa coefficient
reaching 0.72 9. In 2018, the same authors assessed the
interobserver and intraobserver agreement between the
main types of mesotheliomas and the subtypes of epithelioid
mesothelioma in a study about 200 patients. In opposition
to their first manuscript, they reported a fair interobserver
agreement, which was substantially improved after a
clarification session between the observers. In this study, the
clarification between both reviewers induced an improved
agreement between pathologists. The clarification had no
effect on the reproducibility of the grading of epithelioid
mesotheliomas. In this study, the clarification between
both observers made the first observer realize the accurate
definition of the special variants including deciduoid
or signet ring cell mesothelioma. Brcic L, et al. noticed
the highest agreement for sarcomatoid and epithelioid
mesotheliomas and the lowest agreement for biphasic
ones 9. In this study, the most reproducible subtypes
consisted in solid, tubulo-papillary, and sarcomatoid
subtypes. Difficulties in classifying biphasic tumours
may be explained by the cut-off of 10%, which may be
difficult to be assessed unanimously or the difficulty of
highlighting the sarcomatoid component, which can be
confused with active fibroblasts. The ASCO guideline for
the treatment and diagnosis of malignant mesothelioma
made a recommendation to quantify epithelioid versus
sarcomatoid components in surgical, thoracoscopic or
open pleural biopsies 4. In opposition to these guidelines,
the reproducibility of classifying biphasic mesotheliomas
between MESOPATH pathologists and the International
Mesothelioma Panel pathologists was reported to be
moderate with a weighted Kappa value of 0.45 10. Brcic
L attributed the better reproducibility of tubulo-papillary,
pleomorphic and trabecular patterns to the “striking”
character of these patterns that can be easily identified and
may be over-estimated by the pathologists 9. The most
challenging pattern reported by the authors and which was
not reported a few years before was the acinar pattern. This
fact was also noticed in this study and can be explained
by the difficulties to differentiate trabecular pattern from
acinar one when dealing with slit-like spaces. Acinar and
adenomatoid patterns may be also difficult to distinguish
because of the definition of glands and of differentiating
them from microcysts. After the second round, Brcic et
al. reported the highest improvements in micropapillary,
deciduoid and solid patterns with the same concern for
the acinar pattern 11. The transitional subtype was
not reported in this study. It has been clearly defined by
Galateau Salle, et al. 6. This subtype was reported to be hardly distinguished from sarcomatoid subtype. Dacic S, et
al. reported a fair interobserver agreement for diagnosing
transitional subtype and an interobserver agreement
dependant on the percentage of specific foci when dealing
with sarcomatoid features. This agreement was excellent
when the proportion of sarcomatoid features accounted
for more than 75% 12. Some authors reported that the
sample size may be a limiting factor because they noticed
a low agreement between observers when dealing with
needle biopsies 11. These results were in contradiction
with those of Chirieac LR et al. who reported a high
concordance of the diagnosis made on needle biopsies and
surgical biopsies in a study on 759 cases 13. They put
emphasis on the high accuracy of biopsies in sarcomatoid
subtypes in comparison to epithelioid subtypes. Besides,
they reported that the accuracy of histologic classification
increases with the number of tissue blocks examined
with a 100% concordance when more than 9 biopsies
were included. Another limitation reported by Crzs et
al. was the pathologists’ expertise. In fact, they reported
moderate intraobserver agreement with the lowest value
attributed to the least experienced pathologist. This
fact was not reflected by this study’s results because the
highest intraobserver variability was attributed to the most
experienced pathologist. This study puts emphasis on the
difficulties of subtyping the epithelioid mesotheliomas.
These difficulties are added to the challenges described
when differentiating the mesotheliomas from the multiple
mimickers including lung or breast carcinomas 14. In
spite of all these difficulties, pathologists have to adopt
the subtyping of epithelioid mesotheliomas because
morphologic features reflect molecular pathways. Blum
Y, et al. reported that mesotheliomas may be decomposed
as a combination of epithelioid-like and sarcomatoid-like
components that reflect different oncogenic pathways and
whose proportions are highly related to the prognosis 15.
In conclusion, this study puts emphasis on the difficulty
of subtyping epithelioid mesotheliomas according to
their architecture features and grade. Training and more
accurate details in the definition of the different features
are needed in order to integrate these characteristics in the
classification of malignant mesotheliomas and to make
them relevant in predicting the prognosis of mesotheliomas.
CONFLICT of INTEREST
The authors declare no conflict of interest.
The authors declared that this study has received no
1) (IARC WHO Classification of Tumours) International Agency
for Research on Cancer - WHO Classification of Tumours of the
Lung, Pleura, Thymus and Heart-World Health Organization
2) Alcala N, Mangiante L, Le-Stang N, Gustafson CE, Boyault
S, Damiola F. Redefining malignant pleural mesothelioma
types as a continuum uncovers immune-vascular interactions.
3) Nicholson AG, Sauter JL, Nowak AK, Kindler HL, Gill RR,
Remy-Jardin M. EURACAN/IASLC proposals for updating
the histologic classification of pleural mesothelioma: Towards a
more multidisciplinary approach. J Thorac Oncol. 2019;1:12-20.
4) Kindler HL, Ismaila N, Hassan R. Treatment of malignant pleural
mesothelioma: American Society of Clinical Oncology Clinical
Practice Guideline Summary. J Oncol Pract. 2018;14:256-64.
5) Husain AN, Colby TV, Ordóńez NG, Allen TC, Attanoos RL,
Beasley MB. Guidelines for pathologic diagnosis of malignant
Mesothelioma: 2017 Update of the consensus statement from
the International Mesothelioma Interest Group. Arch Pathol Lab
6) Galateau Salle F, Le Stang N, Nicholson AG, Pissaloux D,
Churg A, Klebe S. New insights on diagnostic reproducibility
of biphasic mesotheliomas: A multi-institutional evaluation by
the International Mesothelioma Panel From the MESOPATH
Reference Center. J Thorac Oncol. 2018;13(8):1189-203.
7) Rosen LE, Karrison T, Ananthanarayanan V, Gallan AJ,
Adusumilli PS, Alchami FS. Nuclear grade and necrosis predict
prognosis in malignant epithelioid pleural mesothelioma: A
multi-institutional study. Mod Pathol. 2018;31:598-606.
8) Churg A, Galateau-Salle F, Roden AC, Attanoos R, von der Thusen
JH, Tsao MS. Malignant mesothelioma in situ: Morphologic
features and clinical outcome. Mod Pathol. 2019;2:10-25.
9) Brcic L, Vlacic G, Quehenberger F, Kern I. Reproducibility of
malignant pleural mesothelioma histopathologic subtyping.
Arch Pathol Lab Med. 2018;142:747-52.
10) Churg A, Galateau-Salle F. The separation of benign and
malignant mesothelial proliferations. Arch Pathol Lab Med.
11) Brčić L, Jakopović M, Brčić I, Klarić V, Milošević M, Šepac A.
Reproducibility of histological subtyping of malignant pleural
mesothelioma. Virchows Arch. 2014;465:679-85.
12) Dacic S, Le Stang N, Husain A, Weynand B, Beasley MB, Butnor
K. Interobserver variation in the assessment of the sarcomatoid
and transitional components in biphasic mesotheliomas. Mod
13) Chirieac LR, Hung YP, Foo WC, Hofer MD, VanderLaan
PA, Richards WG. Diagnostic value of biopsy sampling in
predicting histology in patients with diffuse malignant pleural
mesothelioma. Cancer. 2019;125:4164-71.
14) Le Stang N, Burke L, Blaizot G, Gibbs AR, Lebailly P, Clin B.
Differential diagnosis of epithelioid malignant mesothelioma
with lung and breast pleural metastasis: A systematic review
compared with a standardized panel of antibodies—A new
proposal that may influence pathologic practice. Arch Pathol Lab
15) Blum Y, Meiller C, Quetel L, Elarouci N, Ayadi M, Tashtanbaeva
D. Dissecting heterogeneity in malignant pleural mesothelioma
through histo-molecular gradients for clinical applications. Nat