(P 178) TREATMENT RESPONSE AND SURVIVAL OUTCOMES IN GASTROINTESTINAL STROMAL TUMORS FOLLOWING NEOADJUVANT THERAPY

Author(s)

Raghad alshammari, MD

PGY-4
University of British Columbia
vancouver, British Columbia, Canada

Co-Author(s)

SY

Susie Youn, MD

University of British Columbia
Vancouver, British Columbia, Canada
LF

Lior Flor, MD

University of British Columbia
Vancouver, British Columbia, Canada
AM

Andrew McFadden, MD, FRCS

Surgical Oncologist
University of British Columbia
Vancouver, British Columbia, Canada
Alannah Smrke, MD

Medical Oncologist
University of British Columbia
Vancouver, British Columbia, Canada
TH

Trevor D Hamilton, MD, FRCSC, FACS

Surgical Oncologist
University of British Columbia
Vancouver, British Columbia, Canada

Objective: Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the gastrointestinal tract. Neoadjuvant therapy may be used to improve resectability. However, the optimal treatment duration before surgery and role of mutation-specific treatment decisions remain unclear. This study evaluates treatment response and survival outcomes in patients receiving neoadjuvant imatinib.

Methods: This retrospective cohort study used data from a Provincial Cancer Registry for patients diagnosed with GIST between 2010 and 2022. Of 925 charts reviewed, 79 patients who received neoadjuvant therapy followed by curative-intent surgical resection were included. Data collected included demographics, tumor characteristics, mutational profile, treatment duration, surgical outcomes, recurrence, and survival.

Results: Of the 79 patients, 73.4% had gastric and 17.7% had small intestine tumors. The median baseline tumor size was 10.6 cm, which decreased by a median of 3.5 cm after neoadjuvant therapy. Notably, 50.6% of patients achieved partial response based on the RECIST, 46.8% had stable tumor size, and 2.5% had tumor progression. The median duration of neoadjuvant therapy was 9.6 months. 53.2% of patients completed three years of combined neoadjuvant and adjuvant therapy. Mutational analysis was performed in 57% of patients (43/79 c-KIT, 2/79 PDGFRA mutations).
Multivisceral resection was required in 49.4% of cases, with an R0 resection rate of 96.2%. Recurrence occurred in 36.7% of patients during follow-up. The 5-year overall and relapse-free survival rates were 84.7% and 62.3%, respectively.
Multivariable Cox regression analysis revealed that non-gastric tumor site (hazard ratio [HR] 4.96, p = 0.008), R1 resection margins (HR 8.16, p = 0.019), and older age (HR 1.12 per year, p = 0.001) were associated with worse overall survival. A high mitotic rate (> 5/50 high-power fields; HR 3.37, p = 0.005) and increasing age (HR 1.05 per year, p = 0.016) were strong predictors of recurrence.

Conclusion: Neoadjuvant therapy in patients with large or locally advanced GISTs results in significant tumor response in most cases and is associated with favorable long-term recurrence and survival outcomes.