(P 40) LONG TERM OUTCOMES OF NEOADJUVANT IMATINIB AND SURGICAL RESECTION OF RECTAL GASTROINTESTINAL STROMAL TUMOURS

Objective: Rectal gastrointestinal stromal tumours (GISTs) are rare neoplasms that present unique challenges in terms of diagnosis and management. This study aimed to evaluate the long-term outcomes after surgical resection for rectal GISTs.

Methods: A retrospective analysis was conducted of patients with histologically confirmed rectal GISTs treated with surgery at a single institution between 2009-2019.

Results: 18 patients were included. The median age was 51 (range 34-68) years and 61% (n= 11) were female. Neoadjuvant imatinib was used in 11 patients with a median reduction in tumour size of 3 (0-5.3) cm and time to maximum response of 13 (5-39) months. 7 patients had upfront surgery. Median tumour size overall was 6.9 cm (2-14cm) and was similar between the neoadjuvant imatinib 6.8 cm (pre-treatment) and upfront surgery cohort 7 cm.

Adjuvant imatinib was used in 11 patients (7 of the neoadjuvant and 5 in the upfront surgery cohort). Median tumour size of patients not given adjuvant imatinib was 2.6 (range 2 - 8) cm. Median tumour mitotic count per 50 high-power fields (50 hpf) was 10 (0-50) overall with 19 (2-50) in the upfront surgery group and 8 (0-40) in the neoadjuvant cohort. After neoadjuvant imatinib there was a statistically significant decrease in tumour mitotic rate p= 0.02 (two-tailed paired t test), median 0 (0-5) 50 hpf.

Median follow up was 124 (range 60-179) months, with a median disease free survival (DFS) 117 (4-172) months. The local recurrence rate was 17% (3/18) overall with 9% (1/11) in the neoadjuvant and 29% (2/7) in the upfront surgery cohort. Median time to local relapse was 35 (range 4-64) months. At 5yrs DFS was 83% overall, with 91% and 72% for the neoadjuvant and upfront surgery, respectively. At 7 years the DFS was 71% overall, with 80% and 57 % for the neoadjuvant and upfront surgery, respectively. The overall survival at 7 years was 100% and 83% at 10 years.

Conclusion: Management of rectal GISTs remains challenging due to the complex location of these tumours and the lack of standardized treatment approaches. Our findings suggest that patients treated with surgical resection can derive durable survival. In our series, the outcome following neoadjuvant imatinib was favourable.