Open Journal of Radiology, 2013, 3, 133-135 Published Online September 2013 (
Bizarre Parosteal Osteochondromatous Proliferation of
the Skull in a Young Male
Radu Baz, Cosmin Niscoveanu*
Pozitron Medical Investigations, Constanta, Roma nia
Email: *
Received August 1, 2012; revised January 2, 2013; accepted January 9, 2013
Copyright © 2013 Radu Baz, Cosmin Niscoveanu. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Bizarre parosteal osteochondromatous proliferation (BPOP), as defined by Nora and colleagues in 1983 (also called
Nora lesion), is a rare lesion. About 160 cases of BPOP have been presented in the literature to date. The lesion is an
exophytic outgrowth from the cortical surface consisting of bone, cartilage and fibrous tissue. These types of lesions
have been reported mostly in the hands and feet. Localization at the level of the skull is extremely rare. We report a case
of a young man with multiple Nora’s lesions with atypical localization in the skull and mandible.
Keywords: BPOP; Nora; CT; Skull
1. Introduction
Nora’s lesion, also called bizarre parosteal osteochon-
dromatous proliferation (BPOP), is a rare benign osseous
tumor that presents exophytic cortical growth consisting
of bone, cartilage, and fibrous tissue. It was first de-
scribed by Nora and colleagues in 1983 [1]. It usually
affects the proximal and middle phalanges, and the
metacarpal or metatarsal bones. The hands are 4 times
more commonly affected than the feet; however, lesions
in other bones have been reported [2-4]. It usually affects
patients in their 20s or 30s, with no sex predilection. This
benign lesion can be mistaken for malignant processes
because of the high frequency of recurrence, the potential
rapid growth and atypical histologic appearance. We
report a case of a young man with multiple Nora’s le-
sions with atypical localization in the skull and mandi-
ble. From our knowledge this is the second case in the
literature with BPOP in the mandible [5], and also we did
not encounter other cases with such a spreading in the
skull and jaw.
2. Case Report
A 22-years old male was referred to our imaging center
for a computed tomography of the skull for paro tid space
masses which appeared a few months ago, being more
painful in the last weeks. He also reported a recent in-
crease in the size of the masses. At clinical examination,
the patient presented with mildly painful swelling at the
level of parotid and submandibular space, bilaterally.
Palpation revealed hard masses, 3 - 5 cm in diameter,
immobile, with irregular surface, more proeminent on the
left. The overlyin g skin was freely mobile. There was no
obvious soft tissue redness or increased temperature. No
other bone or joint pain was noticed, nor stiffness of the
temporo-mandibular joint. He claimed to have no history
of injury.
CT examination was performed on a Lightspeed VCT
64-slice system provided by GE Medical Systems. Non-
contrast scans (0.6 mm) of the entire skull were acquired
in a 2000:350 window. CT showed multiple infra- and
supracentimetric osteochondroma-like lesions with lobu-
lated well-defined borders and parosteal development at
the level of left temporal bone, left ethmoid, calvaria,
zygomatic arches, maxilla and mandible. Those masses
associated cortical bone thickening, without interruption,
and well delineation with compressive effect on soft tis-
sues (Figures 1-3).
The patient underwent surgical treatment of the big-
gest and most painful lesion (left mandibular) with re-
moval of the pseudocapsule and underlying periosteum
excision. Morphopathological result was that of a lobu-
lated, well-defined mass, with disorganized cartilage and
patchy ossification, large chondrocytes and osteoblasts,
without atypias.
Clinical and imaging findings were consistent with
diagnosis of bizarre parosteal osteochondromatous pro-
liferation, certified by morphopathological result.
*Corresponding author.
opyright © 2013 SciRes. OJRad
Figure 1. Axial CT-scans showing multiple lobulated well-
defined lesions at the level of mandibular angles (a,b); left
mandibular ramus (c,d); zigomatic arches (e) and ethmoi-
dal bone (f).
Figure 2. Coronal CT reformats showing the same man-
dibular lesions (a, b, c, d) and smaller ones at the level of
calvaria (c,d), with heterogeneous structure and without
continuation with normal cortical bone.
3. Discussion
Bizarre parosteal osteochondromatous proliferation
(Nora’s disease) is a rare benign lesion usually seen in
young adults, although it can appear in any age group. It
Figure 3. Volume rendering images indicating multiple
hard masses in calvaria, maxillar bones and mandible
affects men and women in equal proportions. Some au-
thors believe the etiology is due to minor trauma but
there is still no imaging or pathologic evidence to support
this. Patients usually present with the history of a slowly
growing mass or pain, as in our patient.
The following specific radiological findings help to
distinguish BPOP from many of its mimickers including
solitary osteochondroma:
1. The tumor appears to arise from the surface of the
cortex directly.
2. There is no continuity between the central part of
the tumor and the medulla of the underlying bone.
3. There is no flaring of the cortex of the underlying
bone [6].
On computed tomography scan, BPOP is typically well
delineated without continuity with the cortex of the me-
dullar canal, like in osteochondromas. The lesion may
exhibit a spiculated or irregular surface leading to a pos-
sible misdiagnosis of malignancy such as osteosarcoma.
In contrast to malignant lesions, BPOP exhibits no perio-
steal reaction and has normal radiological underlying
bone and adjacent soft tissu e.
BPOP has been described as a lesion with a nodular
surface covered by glistening cartilage and a distinct blue
tint of the bone within on gross examination. The pe-
riphery of the lesion contains lobulated hypercellular
cartilage and fibrous tissue with occasional large chon-
drocytes. There are uniform osteoblasts on the bony tra-
beculae. Chondrocytes may be binucleated and prolifera-
Copyright © 2013 SciRes. OJRad
Copyright © 2013 SciRes. OJRad
tion of bizarre (even spindle shaped) fibroblast in the
intertrabecular spaces of disorganized bone has been re-
ported. Typically the cartilage lobules show mild atypia
and moderate mitotic activity is seen in the spindle cells.
This often conf uses the diagnosis and the lesion n eeds to
be distinguished from conventional osteosarcoma, low-
grade parosteal osteosarcoma, and chondrosarcoma. There
is no “columnation” of the cartilage as in osteochon-
droma, rather the cartilage is disorganized and irregular,
with patc hy areas of ossification [7].
The recommended treatment of choice is excising the
pseudocapsule over the lesion, any periosteal tissue be-
neath the lesion, and decorticating any abnormal appear-
ing areas in the underlying host bone, as this procedure
has less recurrence rate than simple excision of th e lesion
These lesions have a remarkable tendency to recur:
recurrence rates between 29% and 55% in a 2-year in-
terval have been reported, and almost half of those pa-
tients have had a second recurrence. [9] Nora and col-
leagues presented 35 cases of BPOP with 18 (51%) local
recurrences [1]. Meneses and colleagues reported a re-
currence rate of 55% in a series of 65 patients [2] and
Dhondt and colleagues reported a recurrence rate of 29%
in 24 patients [10]. However, despite a high tendency to
recur and a sometimes atypical histological appearance,
no malignant transformation, metastases, deaths or asso-
ciated systemic diseases have been described so far in
patients with BPOP.
4. Conclusion
Nora’s lesion represents a distinct radio-pathological
entity. It should be considered whenever the diagnosis of
osteochondroma is made. Being a rare lesion, the diag-
nosis of Nora’s lesion is challenging for most radiolo-
gists and pathologists as illustrated by this case and oth-
ers. The particularity of our case is the distinct localiza-
tion of the lesions, with very few cases of BPOP of the
skull bein g r ep or t ed.
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