ESTRO 2024 - Abstract Book

S2220

Clinical - Upper GI

ESTRO 2024

Purpose/Objective:

Treatment-induced lymphopenia after non-ablative radiation therapy (NA-RT) for pancreatic ductal adenocarcinoma (PDAC) is associated with spleen and vertebral body (VB) dose and can adversely impact overall survival (OS). Ablative radiation therapy (A-RT) can improve long-term clinical outcomes including potentially OS for select inoperable PDAC patients. However, radiation dose escalation might induce more significant post-treatment lymphopenia although to our knowledge this has not previously been evaluated.

Material/Methods:

We performed a single-institution retrospective analysis of inoperable PDAC patients treated on a 0.35T MR-Linac between 2018-2023 with online adaptive replanning as needed to ensure that organ-at-risk (OAR) constraints were met. Patients were treated with both arms down using a step-and-shoot technique and ~17-23 beam angles that predominantly were anterior and anterolateral while posterolateral angles were avoided that would treat through arms and edge of the couch. Dose calculation was performed with a Monte Carlo algorithm on a 2 mm3 isotropic resolution. Elective coverage became routine starting in early 2019, typically including at least a 7-10 mm margin around the celiac artery and superior mesenteric artery. The spleen and VB levels T11-L3, which were not avoidance structures, were retrospectively contoured and the dose to each on the original plan was evaluated. Absolute lymphocyte count (ALC) values and lymphopenia as per CTCAE version 5.0 were assessed ~1 month prior and ~1-3 months after A-RT. Univariable (UVA) and multivariable analyses (MVA) were performed to identify parameters associated with the incidence of post-treatment >grade 2 lymphopenia. 101 patients were evaluated with median age 71 years (range, 35-94 years) and median tumor size 3.6 cm (range, 1.3-6.8 cm). Tumors were usually in the head of pancreas (84.2%). Induction chemotherapy (IC) was routine (89.1%), commonly FOLFIRINOX (61.4%), with median duration 5.2 months (range, 0.2-14.0 months). The median dose prescribed to gross disease was 50 Gy (range, 40-50 Gy); nearly all (n=93; 92.1%) were treated with elective coverage prescribed to 33 Gy. The median mean spleen dose was 2.75 Gy (range, 0.38-16.79 Gy); median spleen V5Gy, V10Gy, V15Gy, and V20Gy were 12.8% (range, 0-77.5%), 1.9% (range, 0-66.2%), 0% (range, 0-54.5%), and 0% (range, 0-39.2%), respectively. The median mean VB dose was 9.39 Gy (range, 3.2-17.6 Gy); median VB V5Gy, V10Gy, V15Gy, and V20Gy were 63.6% (range, 22.3-86.1%), 48.5% (range, 3.8-77.1%), 26.1% (range, 0-62.6%), and 5.1% (range, 0-43.6%), respectively. The median ALC at a median 25.5 days before A-RT was 1.39 no./mL (range, 0.24-3.00 no./mL), which decreased to 0.85 no./mL (range, 0.20-2.45 no./mL) a median 6 weeks after A-RT. Grade 2 lymphopenia (G2L) and grade 3 lymphopenia (G3L) rates prior to vs. after A-RT were 6.9% vs. 27.7% (p<0.001) and 1.0% vs. 13.9% (p<0.001), respectively; there was no grade 4-5 lymphopenia. Parameters that were significant on MVA for post-RT G2L were male vs. female gender (OR: 0.29, 95% CI:0.105-0.804, p=0.017) and T1-3 vs. T4 (OR: 0.264, 95% CI:0.098-0.707, p=0.008); IC duration, target volume dose/size, spleen dose, and VB dose were not significant on univariate analysis (UVA). On UVA for post-RT G3L a trend was observed for male vs. female gender (OR:2.96, 95% CI:0.84-10.34, p=0.089), N0 vs. N1-2 (OR: 2.91, 95% CI:0.87-9.66, p=0.080), and VB V10Gy (OR: 91.33, 95% CI:0.67-99.99, p=0.071); the only significant parameter on MVA was T1-3 vs. T4 (OR: 0.27, 95% CI:0.10-0.73, p=0.010) while there was a trend towards significance for pre-RT ALC (OR: 0.40, 95% CI:0.15-1.04, p=0.061). Results:

Conclusion: