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11534 Background: GISTs are the most common mesenchymal tumours of the gastrointestinal tract with an annual incidence of 10-15 per million. 10% of GISTS have activating mutations in the PDGFRA gene. We report our 13 year experience of all the PDGFRA-mutated GISTs in our regional centre in Cambridge. Methods: PDGFRA-mutant GISTs were identified from the Cambridge GIST database. Demographics, clinical and histopathological features, survival and response to tyrosine kinase inhibitors of all the patients from 2008-2021 were reviewed. Results: n = 50 (male:female 1.5:1) Median age 68 years (range 19-87). 8% of patients were under the age of 40 years. Tumour size ranged from 1-26 cm with a median of 5 cm. Mitotic index ranged from 1-51 with a median of 1 mitosis/5mm2. 52% of GISTs were located in the gastric body. Histological subtypes: 44% epithelioid, 36% mixed and 20% spindle cell. 38% of cases had high KIT expression (immunohistochemistry), whilst 48% had patchy expression and 14% were negative. Most were DOG1-positive (94%). 76% had radical surgery, 60% had laparoscopic resection. 24% were assessed as being high risk GISTs using the modified AFIP model contrasted to 13% being high risk with prognostic contour mapping. 13% developed metastatic disease with liver being the most common metastatic site. The table shows PDGFRA mutational analysis results: 58% had a D842V mutation. None had a KIT mutation. With a median follow up of 55.1 months, 82% were alive. 6 patients died from metastatic GIST and 3 from other causes. Median time to metastatic disease in resected GISTs was 30.1 months and median time from metastatic diagnosis to death was 18.5 months. Patients who presented with metastatic disease had poor survival compared with patients with localised disease ( p=0.001). 8 patients were treated with tyrosine kinase inhibitors including imatinib, sunitinib and regorafenib prior to 2020 with no objective responses. 3 patients were treated with Avapritinib since 2020 within the compassionate use programme. All 3 patients had partial responses and 2 patients are continuing Avapritinib with no grade 3 adverse events. Conclusions: We report the largest single centre PDGFRA-mutant GIST cohort from Europe. Male preponderance and exclusive gastric location were observed. KIT expression was patchy to negative in the majority of patients. 58% had PDGFRA D842V mutations for which Avapritinib has been recently approved. No objective responses were seen with imatinib, sunitinib or regorafenib. Our early experience with Avapritinib is promising.[Table: see text]

417 Background: Following relapse from first line chemotherapy, 30-60% of patients with germ cell tumors (GCTs) can be salvaged with further chemotherapy. Oxaliplatin-based therapies may offer reduced toxicity. Methods: Eligible patients with GCTs who progressed following cisplatin-based chemotherapy were recruited into this single-arm, phase II clinical trial (NCT01782339). Participants were recruited from six centers and received four 21-day cycles of: Actinomycin D 1mg/m2, high dose Methotrexate 5-8g/m2 day 1, Paclitaxel 80mg/m2 days 1, 8 and 15, Oxaliplatin 85mg/m2 days 4 and 8 with Pegfilgrastim support. Participants underwent an FDG-PET scan at baseline, prior to cycle 2 and on completion of treatment. The primary endpoint was objective response rate (ORR). Secondary endpoints were progression-free survival (PFS), overall survival (OS) and toxicity. Results: 44 patients received at least one dose of the study medication between November 2012 and February 2020. In this full analysis set (FAS) population, the median age was 38 (range 20-57), with 9 (20%) with seminoma, 23 (52%) non-seminoma and 12 (27%) of other histology. 31 (70%) patients had at least intermediate risk disease by IPFSG risk group criteria (n = 16, 36% intermediate risk, n = 6, 14% high risk, n = 9, 20% very high risk). 39 patients received 2 or more cycles of the treatment regimen and were deemed part of the subgroup of patients in the evaluable population. The ORR of the FAS population was 52.3% and 59.0% in the evaluable population. The metabolic response rate (MRR) was 52.3% in the FAS population and 59.0% in the evaluable population. The median radiological PFS in the FAS population was 17.0 months (95% CI, 5.7 months- not reached, NR) with a radiological PFS rate at two years of 38.7% (95% CI, 22.3%-54.9%). The median combined radiological or tumour marker PFS in the FAS population was 8.3 months (95% CI, 2.6 months-17.4 months) with a combined radiological or tumour marker PFS rate at two years of 28.9% (95% CI, 16.0%-43.1%). At a median follow-up of 26.8 months, the median OS in the FAS population was not reached (95% confidence interval [CI], 17.0 months-NR) with an OS rate at two years of 62.5% (95% CI, 46.1%-75.2%). Toxicity data and correlation between early metabolic response and outcomes will be presented at the meeting. Conclusions: The GAMMA chemotherapy regimen has demonstrated encouraging anti-tumor activity in relapsed GCT, despite recruiting a cohort of patients with relatively unfavorable outcomes. Clinical trial information: NCT01782339.

403 Background: Metastatic germ cell tumour (GCT) to the liver is considered rare and usually caries adverse outcomes. We aimed to determine the outcome of patients with metastatic GCT to the liver. Methods: We identified retrospectively 36 patients with metastatic germ cell tumour to the liver between the years 2001 and 2015, from which 34 were non-seminomatous germ cell tumours (NSGCT) and two seminomas. 35 patients had other sites of metastases including lungs, bones and brain. Elevated tumour markers were seen in the vast majority of patients (97.2%). 15 patients received treatment with dose intense regime including actinomycin-D, high-dose methotrexate, etoposide and cisplatin (GAMEC) every 14 days, 20 patients received the standard protocol of bleomycin, etoposide and cisplatin (BEP) every 21 days and one patient received POMB/ACE chemotherapy. 20 patients had an induction cycle of cisplatin, vincristine and bleomycin (Baby-BOP) prior to initial treatment. Results: 12 patients had radiological complete response (CR) and 19 patients had radiological partial response (PR) in the liver, with five patients having a CR in all sites with negative markers. Five patients underwent liver resection with no viable tumour seen. Three patients that underwent liver resection also had retroperitoneal lymph node dissection (RPLND) the histology from which was viable seminoma in one case, mature teratoma in one case and necrosis in the final patient.16 patients had marker negative PR, 10 patients had marker positive PR and 5 patients had a marker negative CR. 15 patients eventually relapsed and 10 died with only one liver relapse. Median Overall survival for patients that received BEP was 35.38 months (not reached for GAMEC) (p = 0.0147). The median progression free survival (PFS) for the BEP group was 24.45 months (not reached for GAMEC) (p = 0.22) and the 2-years PFS for the GAMEC and BEP groups were 73% and 55% respectively. Conclusions: Within this cohort, liver metastasis from germ-cell tumour had a good response to chemotherapy, with progression occurring mainly in extra-hepatic sites. There was also a suggestion that dose dense GAMEC regime may offer superior efficacy compared with BEP.These results also question the role of liver metastectomy after initial response to chemotherapy.

2534 Background: Phase I oncology trials are crucial in cancer drug development and have evolved over the years. We examined the trends in trial characteristics, dose-limiting toxicities (DLTs) and...

2607 Background: AZD2014 (A) is a dual mTORC1/mTORC2 inhibitor and a previous phase I study determined the maximal tolerated dose (MTD) of A to be 50 mg BD 7/7. We have shown high p-P70S6K levels i...

e21503 Background: Historically, GISTs were often misdiagnosed as other tumours. However, with the increasing awareness of GISTs, one needs to be careful in confirming the diagnosis since other tumours can simulate GISTs on endoscopy/imaging/histology. We report our 8 year experience of GIST mimics from the CGSG database. Methods: 250 cases discussed in the specialist multidisciplinary meeting from 2004 – 2012 were reviewed. Tumours initially suspected to be GISTs but turned out to be other tumours were included in this analysis. Patient demographics, endoscopic, imaging and histological features were analysed in 41 GIST mimics. Central review by specialist histopathologist was undertaken with immunohistochemical and molecular analysis to differentiate these tumours from GISTs. Results: GIST mimics N= 41. Median age 56 years (range 11 – 82 years), male:female ratio 1:1. Median size 8 cm (range 1.9 – 30 cm). Site of origin of tumours: stomach 34%, oesophagus/junction 15%, mesentery 12%, small bowel 10%, colon 5%, retroperitoneum 5%, pancreas 5%, liver 2% and indeterminate site of origin 12%. Suspicion of GIST was raised from imaging (46%), endoscopy (41%) and histology (39%). The histological subtypes of GIST mimics are summarised in the Table. Conclusions: Many benign and malignant tumours do mimic GISTs on imaging/endoscopy or histology. The diagnosis of GISTs should be made by a specialist multidisciplinary team with experience in imaging and histopathology. Distinguishing GISTs from their mimics is of critical importance both for prognosis and management. [Table: see text]