Predicting the Achievable Ablative Radiation Dose Coverage to Gross Tumor Volumes in Pancreatic Cancer Patients

Abstract

<h3>Purpose/Objective(s)</h3> Ablative radiation therapy (RT) in pancreas cancer is associated with durable control of the primary tumor leading to favorable survival outcomes. A key planning goal is to achieve ablative dose (> 100 Gy BED) to the gross tumor volume (GTV), while still respecting organ-at-risk (OAR) dose constraints. However, the GTVs are typically located near radiosensitive OARs such as small bowel, large bowel, and stomach. This makes predicting V100 coverage to the GTV difficult to assess a priori. Each patient's GTV and OAR location, shape, and volume vary significantly and introduces uncertainty to the achievable GTV V100. Currently, achievable GTV V100 coverage is determined through trial and error as the planner explores tradeoffs between OAR sparing and prescription dose coverage. Iterative planning is time and labor intensive. In this study, we present a customized patient anatomy dependent prediction formula that can be used to guide the planning goals and improve planning speed and quality. <h3>Materials/Methods</h3> The minimum GTV coverage prediction formula was empirically derived as below Gf=fGTV - [fs × (1-fGTV)/3] + [(1-fs) × (1-fGTV)/20] ± 4% Gf: GTV V100 prediction value OARs=Small bowel + Large bowel + Stomach+ Duodenum OARs_5.5mm: OARs with 5.5mm 3D expansion fGTV: VGTV- OARs_5.5mm / VGTV fs: SOAR / SGTV VGTV: GTV volume VGTV- OARs_5.5mm: GTV volume subtract OARs with 5.5mm 3D margin SGTV: total number CT slices with GTV SOAR: total number CT slices with OARs within 3cm from the GTV Using this formula, 30 patients were randomly selected for testing efficacy. Patient plans were prescribed with dose painting to 3750cGy and 6750cGy in 15 fractions with GTVs intending to receive 6750cGy. Plans were generated using 2 or 3 6X VMAT full arcs with jaw tracking. If the original GTV coverage differed by 4% below the predicted GTV coverage, it was replanned with same treatment field geometry to verify the reliability of the prediction value. <h3>Results</h3> For the 30 patients, the GTV size ranged from 10.8 cc to 183.9cc. The fGTV and fs factor ranged from 0.61 -1 and 0 - 1. The GTV coverage for 17 out of the 30 patients was within the predicted GTV coverage range. The remaining patients had GTV coverage more than 4% below the predicted value with differences ranging from 5.1% to 22.3%. For these 13 patients, the GTV coverage improved to within the predicting formula range by adding a high priority minimum GTV coverage dose constraint to the optimization parameters. <h3>Conclusion</h3> The presented formula can sufficiently predict achievable GTV coverage while accommodating patient-specific anatomy and GTV volume sizes. The prediction formula is a stable tool to assist planners during the optimization process while maintaining plan quality. Future developments will include adjusting the formula to accommodate different prescription doses.