Discussion :
As PPB is the archetype of DICER1- associated neoplasms, it is
not surprising that several of the extrapulmonary DICER1 mutated
sarcomas share aspects of its morphologic template from a cystic to
solid macroscopic appearance, to one or more of the histologic patterns.
The basic collage consists of a variable intermixture of
rhabdomyosarcoma usually embryonal type, islands and nests of blastema,
primitive spindle cell pattern with presence of cambium layer, nodules
of fetal to sarcomatous cartilage and anaplastic tumor cells. The
various sarcomatous patterns are heterogeneously represented, so that
blastema is the dominant pattern in one tumor and spindle cells sarcoma
in another. This similarity to PPB was noted by Schultz et al and they
proposed the designation ‘pleuropulmonary blastoma-like peritoneal
sarcoma’. 8
Warren et al noted in their literature review that mostDICER1- associated sarcomas had a component of undifferentiated
small round blue cells with rhabdomyoblastic differentiation (in 64/71
cases; 90.1%), poorly differentiated spindle cell component with
anaplasia (45/50; 90%) and chondroid/myxoid differentiation (39/45;
86.7%). This similarity was seen in spite of the different nomenclature
used for the reported cases and the varied sites of presentation. Though
this was not addressed, the morphologic descriptions have a striking
resemblance to PPB. Rhabdomyoblastic differentiation was the most common
feature in all these cases, (including in the 3 cases reported by them),
while the other features were present in variable frequency.
Appreciating the commonality in morphology, they proposed a unified
terminology for these tumours – ‘DICER1 -associated sarcoma’,
which was further supported by McCluggage et al. 7,10
In our 3 cases, similarities with PPB type III include primitive spindle
cell pattern, cambium-like arrangement of tumour cells, nodules of fetal
to sarcomatous cartilage and a variable population of anaplastic tumor
cells. Chondroid differentiation and anaplastic component was lacking in
one case. small focus of immature cartilage was identified in only one
out of 11 sections from tumour. Although chondroid differentiation may
be focal and could be missed in a biopsy, however, in our experience,
when present it is quite specific for DICER1 sarcoma and should prompt
testing for DICER1 pathogenic variation. None of the tumors
however had a neoplastic epithelial component and rhabdomyoblastic
differentiation was the only constant feature in all 3 cases. However,
this rhabdomyoblastic differentiation was somewhat patchy in all 3
cases, both morphologically and on immunohistochemical staining (with
desmin, myogenin and myoD1), which raised our suspicion and helped avoid
misclassification as typical rhabdomyosarcoma.
Another morphologic variation not described in the Warren review, is the
rare multi-patterned DICER1 -associated tumour with primitive
neuroepithelial elements, designated as ‘pre-sacral malignant teratoid
neoplasm in association with pathogenic DICER1 variation’.9 We also noted neuropil with possible focal
neuroblastic-like component in Case 3, but in all these tumours and
ours, the predominant component was ERMS. Pancaldi et al reported aDICER1 associated PNET with EWSR1 rearrangement which did not
show any ERMS component. 11 A unique observation in
our case was the positivity of SALL4 and synaptophysin in the recurrent
high grade sarcoma, which was negative for myoid markers. Warren and
McCluggage have separately reported 2 ovarian tumours, where the small
spindle cells were SALL4 positive, but dually positive for myoid markers
(desmin and myogenin). 7,12
The DICER1 -associated sarcomas seem to have a predilection for
the female genital tract and brain. 7,8,12,13 However,
rare cases have been reported in males as well, including the
para-testicular region. 14
The PPB-like morphologic spectrum ranging from benign to overtly
malignant sarcoma, is also noted in DICER1 associated renal
cystic nephromas and anaplastic sarcoma of kidney, both of which also
show rhabdomyoblastic differentiation. 7,15
Like in cystic nephromas and type 1 PPB, the spindle cell morphology in
some DICER1 associated tumours can be very bland and hypocellular
in some tumours and can be misinterpreted on a small biopsy as fibrotic
tissue (as with the recurrent vaginal tumour case 3), even though the
cells express RMS markers on IHC. This has also been highlighted by Yoon
et al. 16 Routine use of desmin to evaluate genital
tract spindle cell lesions in young females, may help correctly identify
these tumours.
The association of rhabdomyosarcomas of the female genital tract withDICER1 has been reported variably in literature, but it is not
clear yet if identification of any ERMS element should prompt
consideration of a DICER1 -related neoplasm and justify testing,
especially in the absence of immature cartilage or primitive mesenchyme.
This question is most pertinent for children where ERMS is known to be
one of the commonest childhood sarcomas. While Apellaniz-Ruiz in their
review showed that nearly all gynecologic ERMSs (except vaginal tumours)
were DICER1 -associated, Doros et al detected DICER1mutation in only 3.8% (2/52) cases of sporadic ERMS and the Children’s
oncology group cohort of pediatric rhabdomyosarcomas showedDICER1 germline mutation in 4.4%. 10,17-20
DICER1 -associated central nervous system sarcomas also have a
high association with rhabdomyosarcomatous differentiation.8,21,22 Our CNS case mimicked mesenchymal
chondrosarcoma, though it expressed ERMS markers.
We found the presence of gain of chromosome 8 in cases 1 and 2 (FISH was
attempted in case 3, but failed quality measures). Review of literature
suggests a very high incidence of gain of chromosome 8 with PPB.23 Interestingly, association of gain of chromosome 8
is also reported in a high proportion of cases of ERMS and some cases of
mesenchymal chondrosarcomas. 24,25 This molecular
association has not yet been well explored in DICER1 -related
sarcomas, needs further investigation. A recent landmark paper of a DNA
methylation based sarcoma classifier found that DICER1 mutated
sarcomas cluster independent (methylation class sarcoma RMS-like) from
conventional embryonal and MYOD1 mutated rhabdomyosarcomas and may
represent a distinct group of tumours. This distinction is further
sharpened as these tumours do not respond to standard RMS protocol
chemotherapy. All the DICER1 mutated sarcomas in their cohort
showed gain of chromosome 8 as well. 26
Treatment guidelines are still not standard for these tumours, though
most reports have used an anthracycline and alkylator based protocol
with varied response. 8 These tumours seem to benefit
from initially a more intensive chemotherapy backbone followed by a
prolonged maintenance chemotherapy strategy, in spite of the presence of
RMS differentiation, thus making the accurate diagnosis of DICER1mutated sarcoma, vital. All our patients had aggressive disease but
responded to the alternating VDC/PEI backbone followed by maintenance
chemotherapy. The decision for VDC/PEI for the CNS case was based on a
multidisciplinary tumor board review with expert consensus from around
the world and then it became our standard for the subsequent tumors in
view of the excellent responses observed serially. Satisfactory local
control with surgery and/or radiotherapy seems imperative for
maintaining remission. Prognosis can be favorable, provided the tumours
are detected early. 1,2,27
The risk of developing a tumour associated with germline DICER1variation is greatest in early childhood (1st decade)
and decreases with increasing age and surveillance protocols are
developed accordingly. Correct and early diagnosis of DICER1syndrome is imperative to ensure appropriate surveillance protocols are
followed in these patients for improving survival. Apart from
macrocephaly in some individuals, this cancer predisposition syndrome is
typically not associated with specific phenotypic characteristics,
making clinical identification difficult. Hence histologic
identification is the key and the onus rests on the pathologist to
correctly identify the morphologic clues to guide testing forDICER1 mutation, especially in resource constrained settings. The
presence of rhabdomyoblastic differentiation along with primitive
blastemal and mesenchymal component, with or without chondroid
differentiation are the key features which should raise suspicion,
especially in pediatric sarcomas of the female genitourinary tract or
brain.
Disclosure / Conflict of interest : The authors D. Ashley Hill
and KrisAnn Schultz are supported by National Institutes of Health grant
NCI R01CA143167 and NCI R37CA24494. The International PPB/DICER1Registry gratefully acknowledges philanthropic support from the Pine
Tree Apple Classic Fund and Children’s Minnesota Foundation.
The other authors have no conflict of interest and nothing to disclose.
Ethical approval : Institutional review board approval
((EC/WV/TMC/41/20) has been taken for this study.
Informed consent - Informed consent was taken from patients’
parents before this study
Funding – None. Testing for DICER1 was supported by the
DICER1/ Pleuro-Pulmonary Blastoma society. Courier charges were borne by
Tata Medical Center.