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503 Background: Neuroendocrine tumors (NETs) are infrequent neoplasms that originate in the neuroendocrine cells of the embryological endoderm. Few data exist about the epidemiology and clinical characteristics of these tumors in the Mexican population. Methods: We performed a retrospective analysis of patients with pathology-confirmed NET treated at an Oncology Reference Center in North-East Mexico from 2010 to 2019. Clinical characteristics were recorded and an evaluation of overall survival (OS) using the Kaplan-Meier method was performed. Results: Fifty-three patients were included in the final analysis. The mean age at diagnosis was 54 +/- 16.36 years and 53% were men. Primarily affected sites were the gastroenteropancreatic tract [GEP] (47%), lung and mediastinum (21%), unknown primary (21%), and others (11%). Regarding the clinical stage at presentation, 22.6% of patients with NETs had localized disease, while 22.6% had regional spread and 54.7% were considered to have metastatic disease at diagnosis. Median OS was 42 months for all patients with NETs. In the GEP group, the median OS was 64 months, whereas for the lung/ mediastinum group was 8 months and for the unknown primary group was 5 months. Median OS stratified by stage was as follows: Localized disease: Not reached (NR), locoregional (20 months), and metastatic disease (8 months). Conclusions: To our knowledge, this is the first study published in English literature reporting the clinical characteristics and survival of Mexican patients with NETs. It is necessary to expand the information regarding these neoplasms to improve access of these patients to standard treatments and therefore improve their outcomes.

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e15502 Background: Anal carcinoma is a rare neoplasm, representing < 1% of all neoplasms with rising incidence and mortality. More than 90% of cases are linked with high-risk human papillomavirus (HPV). Other factors such as HIV status, smoking and socioeconomic disparity may have important roles. Few data are available on the Mexican population. Methods: Retrospective analysis of patients with pathology-confirmed anal carcinoma treated at an Oncology Reference Center in North-East Mexico from 2010 to 2020. Using descriptive statistics, we report clinical and pathological features, time from symptom onset to diagnosis and treatment, as well as response and recurrence rate. Results: Forty-one patients were included in the final analysis. The mean age at diagnosis was 61.4 +/- 13.8 years and 58.5% were women. Smoking history was identified in 17 patients (41.5%). 9.8% were HIV-positive, however 63.4% of all patients had unknown status at cancer diagnosis. Mean duration of symptoms to diagnosis was 344 days (range 4-1435 days) and the median interval from symptom onset to registration for oncology consultation was 424 days (range 28-1695 days). Patients attended a median of 3 (range 1 to 7) previous clinical consultations before being referred to our center. It is noteworthy that 44% of patients had hemorrhoidal disease history. Squamous cell carcinoma was the most common histological subtype with 93% of all cases, arising from the anal canal in 78% of cases, while 22% corresponded to perianal cancer. Regarding the clinical stage at presentation, 2.4% of patients had localized disease, while 95% had regional spread and 2.4% presented with distant metastases at diagnosis. Treatment intent was curative in 85.4%. Treatment modalities included radiotherapy (97%), surgery (7%), chemotherapy (88%). Concurrent chemoradiotherapy was received by 85%. Most patients were treated with a total dose between 45 and 60 Gy to the tumor volume. Radiotherapy delays due to toxicity occurred in 31.7% of cases, these unplanned treatment interruptions correlated negatively with response rate (p < 0.001). The chemotherapy regimen used was 5-fluorouracil (5-FU) with mitomycin C (MMC) in 29%, capecitabine monotherapy in 31.7%, 5-FU with cisplatin in 12.5%, capecitabine with MMC in 5%, and capecitabine with cisplatin in 5%. Among patients treated with curative-intent, 46% had a complete response and 37% had a partial response. Relapse occurred in 10 cases (24%), the majority being local relapses (54%). Distant relapse occurred in 15% of all cases. The median disease-free interval (DFI) was 32.5 months (range 0-252 mo). Colostomy rate was 29%, the median colostomy-free survival (CFS) was 10 months (range 3-15 mo). The comparative analysis between MMC-based chemotherapy and chemo without MMC did not show difference in response rate, recurrence, DFS and CFS. Conclusions: Due to its rarity and complex management patients with anal carcinoma should be treated at experienced centers.