Main Session
Sep 28
PQA 01 - Radiation and Cancer Physics, Sarcoma and Cutaneous Tumors

2108 - Development and Validation of an Individualized PTV Margin Customization Approach Using Fan-Beam Computed Tomography-Guided Adaptive Radiotherapy (FBCT-gART) and Four-Dimensional Computed Tomography (4DCT) in Pancreatic Cancer Patients

02:30pm - 04:00pm PT
Hall F
Screen: 26
POSTER

Presenter(s)

Wenjing Huo, RT - Department of Radiation Oncology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, Shandong

W. Huo1, L. Xu1, S. He2, and J. Yue1; 1Department of Radiation Oncology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, 2United Imaging Research Institute of Intelligent Imaging, Beijing, China

Purpose/Objective(s):

Online adaptive radiotherapy reduces the planning target volume (PTV) margins by minimizing interfractional setup errors and organ motion. This study aims to propose an individualized margin customization strategy that combines fan-beam computed tomography-guided adaptive radiotherapy (FBCT-gART) with four-dimensional computed tomography (4DCT) for patients with locally advanced pancreatic cancer (LAPC). The dosimetric advantages of this approach were compared to conventional margin techniques in LAPC patients undergoing radiotherapy.

Materials/Methods:

Four patients with LAPC underwent FBCT-gART at 60 Gy/15 fractions. A 3 mm PTV margin (PTV_standard) was expanded from the clinical target volume (CTV).

During the first three treatments, three 4DCT scans are acquired: 4DCT-1 (post positioning), 4DCT-2 (pre-beam out), and 4DCT-3 (post-beam out). Subsequently, three sets of 4DCT scans are obtained weekly, with only 4DCT-1 (post-positioning) acquired in the remaining fractions. Deviations between 4DCT-2 and 4DCT-3 are used to calculate the average surface distance(ASD)of CTV and the Internal Gross Tumor Volume (IGTV) during beam delivery.

Data from the first three treatment fractions are used as the training cohort. The mean ASD is defined as the baseline value (A). Two edge extension thresholds are added to this value to account for contouring errors inherent in daily adaptive contouring: A + 0.5 mm and A + 1 mm. The minimum PTV margin must ensure that IGTV and CTV coverage in 4DCT-3 exceed 95%. The IGTV/CTV coverage is calculated using the following formula:

Coverage_(IGTV/CTV) =(the geometric overlap between PTV and IGTV/CTV)/(the volume of IGTV/CTV)

Data from the remaining fractions served as a validation cohort to assess the efficacy of individualized margins through evaluation of IGTV and CTV coverage.

Absolute differences in PTV volume are quantified. Statistical comparisons between the two approaches are performed using a Wilcoxon matched-pairs signed rank test, with significance defined as p < 0.05.

Results:

Compared to PTV_standard, individualized margins resulted in a 15.75% reduction in PTV volume. Among the patients, 75% (3/4) exhibited reduced mean dose (Dmean) to the liver and duodenum, as well as reduced dose received by 2cm3 (D2 cc) to the small bowel and duodenum (p < 0.05). Additionally, 50% of patients (2/4) showed reductions in the maximum dose (Dmax) to the duodenum, stomach, small intestine, and spinal cord (p < 0.05). Similarly, 50% of patients demonstrated decreased Dmean to the stomach and duodenum (p < 0.05).

Conclusion:

The use of individualized margins in LAPC patients reduces PTV size, decreases radiation dose to adjacent non-target tissues, and maintains adequate target coverage.