Keywords
Accessory spleen - computed tomography - intrapancreatic accessory spleen - magnetic
resonance imaging - pancreas - spleen
Introduction
An accessory spleen is a benign and relatively common congenital entity resulting
from failure of fusion of the splenic anlage during embryologic splenic development.[1] The incidence is about 10–30%[2] with splenic hilum (80%) being the most common site followed by the pancreatic tail
(17%).[3] The intrapancreatic accessory spleen (IPAS) poses a diagnostic challenge on imaging
and often gets misdiagnosed as pancreatic tumor,[4] which can lead to unnecessary surgery or biopsy. Thus, accurate diagnosis of IPAS
might save the patient from unwarranted intervention.
Case History
A 55-year-old male presented to our institution with complaint of epigastric pain
for 10–15 days which was intermittent and dull aching. On the physical examination,
the abdomen was non-tender. An ultrasound examination done elsewhere suggested a well-defined
mass lesion at the tail of the pancreas. The serum amylase and lipase levels, complete
blood count, and differential white blood cell count were normal. Contrast-enhanced
CT scan of abdomen was done at our institute, which showed a well-defined, enhancing
mass lesion in the pancreatic tail. The enhancement of mass matched to that of the
spleen in arterial as well as venous phases [Figure 1]. MRI with contrast was done for further assessment using multi-sequence abdomen
protocol. It was observed that the signal characteristics of the mass were similar
to that of the spleen. In the arterial phase after gadovist injection, the pancreatic
mass showed “zebra pattern” enhancement which is typical for the spleen [Figure 2]. Diffusion weighted imaging was done at the b values of 0, 400, and 800 s/mm2 and ADC map was generated. Six circular regions of interest (ROI) of the same size
were drawn in the pancreatic mass, the spleen and normal pancreas in the ADC map and
the mean ADC values were recorded [Figure 3]. In the next step, we used unpaired T-test on this data set (using SPSS version
19 for Microsoft Windows) and came to the conclusion that there was no significant
difference between ADC values from the pancreatic mass and normal spleen (P = 0.753) while the ADC value of pancreatic mass differed significantly from that
of normal pancreatic tissue (P = 0.0002). With these findings on CT and MRI, we inferred that the mass was an IPAS.
Patient was followed up on MRI after 6 months and no change in the lesion was noted
[Figure 4].
Figure 1 (A-C): (A) On precontrast axial CT image IPAS (arrow) is isodense compared to pancreas and
spleen. (B) On contrast-enhanced axial CT image obtained during the arterial phase,
the IPAS (arrow) is located in the tail of pancreas and it shows isoattenuation compared
to the spleen. (C) On contrast-enhanced axial CT images obtained and portal venous
phase IPAS (arrow) is located in the tail of pancreas and it shows isoattenuation
compared to the spleen
Figure 2 (A-D): (A) Fat saturated T2 weighted (SPACE) axial MRI image shows that the IPAS (arrow)
is isointense to spleen while hyperintense than the pancreas. (B) T1 weighted axial
MRI image shows isointensity of the IPAS (arrow) with orthotopic spleen and hypointensity
with respect to the pancreas. (C) In the post-contrast arterial phase, the IPAS shows
similar zebra pattern enhancement with that of the spleen. (D) In the post-contrast
venous phase, the IPAS (arrow) shows similar enhancement with that of the spleen
Figure 3: Axial ADC map generated after diffusion-weighted MRI showing the ROIs in the spleen,
IPAS and the pancreas with respective ADC values. No significant difference was found
between the ADC values of IPAS and spleen (P ≥ 0.05), while a significant difference was there between the ADC values of IPAS
and pancreas (P ≤ 0.05)
Figure 4: Follow up MRI after 6 months: T1 weighted axial image shows, the lesion (arrow) was
still of the same size and signal intensity bolstering the diagnosis of IPAS
Discussion
An accessory spleen is a benign entity with its common locations near the splenic
hilum and in the tail of the pancreas. Despite of its typical locations, confident
radiological diagnosis warrants careful observation of its radiological appearance.
This is critical as often an IPAS may be confused with pancreatic tumors like adenoma,
carcinoma, and islet cell tumor.[3], [5] When a focal lesion within the pancreas shows similar signal characteristics and
the same degree of enhancement as that of the spleen in all phases, possibility of
an IPAS should be considered.[6], [7] Multiple imaging modalities like contrast-enhanced ultrasound and CT, superparamagnetic
iron oxide-enhanced MR imaging, and nuclear medicine are available to distinguish
IPAS from other types of pancreatic lesions. Among these, contrast-enhanced ultrasound,
superparamagnetic iron oxide-enhanced MRI and nuclear medicine are very sensitive
and specific for the diagnosis of IPAS, however they depend on phagocytosis of macrophages
due to which the splenic visualization becomes difficult if there is minimal functioning
of splenic tissue and more time may be required to acquire the essential images.[8] Other drawbacks include lower spatial resolution of scintigraphy than CT or MRI,
particularly in smaller lesions or lesions overlapping with the spleen. Ultrasound
is operator dependent and has a limited sonic window to fully examine the pancreatic
tail.[6], [7], [9] In our case, the CT density and MR signals of the mass were equal to those of spleen
before and after contrast administration. Also, the mass showed the same degree of
enhancement as the spleen on postcontrast CT and MRI, including the typical zebra
pattern of arterial phase enhancement.[10] Ding et al.[11] reported that no significant difference in the apparent diffusion coefficient was
identified between the IPAS and orthotopic spleen (P > 0.05). Jang et al.[8] in their study of 42 patients had found that small pseudopapillary tumor which is
the principal differential diagnosis for IPAS differs significantly from the spleen
in signal intensity on the diffusion-weighted and contrast-enhanced sequences while
the IPAS shows the similar signal to that of the spleen. In agreement with the aforementioned
studies, we observed that the mean ADC value of the pancreatic mass had no significant
difference with that of the spleen while the ADC value of normal pancreatic parenchyma
was much higher with a significant difference. Takayama et al.[9] suggested that to aid the clinical judgement, either biopsy or follow-up should
be done to avoid unnecessary pancreatectomy. In this case, in view of the CT density
and MR signals of the mass being equal to those of the spleen, the diagnostic possibility
of an intrapancreatic accessory spleen was considered. Follow-up MRI was done after
6 months in which the lesion was stable.
In conclusion, the IPAS is a diagnosis which should be kept in mind as a differential
while reviewing a case with pancreatic mass as it would prevent unnecessary intervention
due to misdiagnosis of a pancreatic neoplasm. CT attenuation, MRI signal characteristics,
post-contrast enhancement, and comparison of ADC values of the pancreatic mass with
spleen and pancreas; all can lead to imaging diagnosis of IPAS, and the patient can
be further followed up with imaging instead of immediate intervention in the form
of biopsy or surgery.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms.
In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
