We collected as many publications written in English or Japanese involving GP and examined them to extract characteristic features of the tumor as presented in clinical and histopathological findings.
We focused on determining the significant factors associated with the progression of the tumor. Here we discuss the clinical, histopathological, and immunohistochemical findings in reference to matters that emerged from our research.
In the present study, we found gastrointestinal bleeding as the most common symptom, followed by abdominal pain. This fact has been previously accepted and our data confirm it. All patients without autopsies and having clinically incidental focus had surgical interventions, including 15 patients with endoscopic intervention. Indeed, one patient required additional surgical intervention due to a residue of the tumor at her first endoscopic procedure [36
], while other patients showed a good outcome without recurrence or metastasis. Furthermore, there was no record of a patient dying from GP, and patients with this tumor, therefore, have an extremely good prognosis. However, one patient has been reported as showing a recurrence due to a residue of a previous tumor at his initial surgical intervention [24
]. Accordingly, we emphasize the importance of both a histopathological assessment of extensive tumor components at the surgical margin and imaging examinations to monitor for recurrence or metastasis after the operation.
Although one patient received irradiation after surgical intervention [25
], we maintain that patients without residual tumor require no adjuvant therapy because no recurrence or metastasis has been reported in such patients. By contrast, it is still unclear whether a residual tumor can be controlled by irradiation or chemotherapy alone without surgical intervention.
The immunohistochemical findings on the tumors need consideration since the identification of three cellular components are essential for diagnosis. Epithelioid and ganglion-like cells showed a high positive rate for several kinds of immunohistochemical neuroendocrine markers, such as synaptophysin, chromogranin A, and NSE. In addition, epithelioid cells showed a high positive rate for PP. In contrast, spindle-shaped cells had the highest positive rate for S-100. These results are consistent with other previous publications. Furthermore, positive rates for each hormone, such as somatostatin, serotonin, gastrin, glucagon, and insulin in epithelioid cells, were significantly different and the meaning of this finding is worth investigating. However, these extracted immunohistochemical findings should be regarded as hints or suggestions because it is thought that results involving negative data have not been described in previous publications. In fact, our previous case report did not describe the immunohistochemical evaluation of each hormone according to negative reactivity.
Incidentally, we previously tried to establish the immunohistochemical prognostic indicators of GP using bcl-2, p53, and Ki-67, which are acceptable prognostic indicators in several kinds of neuroendocrine tumors [41
]. However, all of these indicators showed a negative reactivity.
Unfortunately, there were no other cases of GP that included an immunohistochemical evaluation using bcl-2 and p53, and the value of these factors as prognostic indicators of GP remains unclear.
In contrast, two cases without lymph node metastasis were described with the Ki-67 labeling index in GP [45
]. However, both showed extremely low Ki-67 labeling index values. Therefore, we suggest that immunohistochemical evaluation using Ki-67 may have a limited prognostic value in GP.
Finally, we gained insight into the progression of GP tumors and related factors. It has been accepted that GP usually arises from the submucosal or muscular layer, which may make the diagnosis difficult using a forceps biopsy prior to surgical intervention. In fact, we revealed that the diagnostic rate by biopsy before surgical intervention was only 11.4% (4/35). In addition, we showed that many more cases of GP exceeding the submucosal layer were reported (61.1%, 66/108) than expected, and GP exceeding the submucosal layer is a risk factor for lymph node metastasis. These facts emphasize the importance of imaging examinations prior to surgical interventions.
It is interesting to note that significant differences were found for gender between GP within the submucosal layer and exceeding the submucosal layer. On the basis of our investigation, a hypothesis emerged that asserted no significant relationship for gender and that female gender induces vertical growth of the tumor. To confirm part of our hypothesis, we focused on female-specific factors and initially evaluated tumor cells immunohistochemically using anti-estrogen and progesterone receptor antibodies. As a result, our immunohistochemical evaluation revealed that epithelioid cells showed positive reactivity for the progesterone receptor. Furthermore, some investigators reported that progesterone regulates neural differentiation [47
]. These facts suggest that the vertical growth of GP might be affected by progesterone exposure.
Additionally, it is interesting to note that normal pancreatic islet cells also showed positive reactivity for the progesterone receptor. It has been reported that normal pancreatic islet cells and pancreatic neuroendocrine tumors showed positive reactivity for the progesterone receptor [49
], and our literature survey demonstrated that epithelioid cells showed a high positive rate for PP (89.7%, 70/78) immunohistochemically. These facts indicate the relationship between GP and pancreatic islet cells. However, immunohistochemical evaluation for estrogen receptors differs between epithelioid cells and normal pancreatic islet cells, and our immunohistochemical evaluation was based on only one patient. Furthermore, it is strongly suspected that our data have been affected by publication bias.
To confirm these hypotheses, further investigation is required (e.g. compare the positivity between the metastatic cases [6
] and non-metastatic cases for the immunohistochemical antigen expression).