Purpose: To quantitatively assess the dosimetric impact of three materials override methods for injectable iodinated hydrogel spacer utilized in stereotactic body proton therapy (SBPT) for pancreatic cancer
Methods: TraceIT is an absorbable, iodinated polyethylene glycol (PEG) hydrogel for tissue marking and spacing. A patient diagnosed with locally advanced pancreas cancer (LAPC) underwent endoscopic hydrogel injection and CT simulation with breath-hold. The CT with patient and spacer contours was imported to Raystation 9B for proton pencil beam scanning (PBS) planning. The patient was prescribed with 3300 cGy (RBE) in 5 fractions to the GTV. Two posterior beams were used, one of which has a distal end close to the spacer. The ground truth of the spacer composition was calculated based on previous publication and imported to the Raystation. SBPT plans with the same beam but different material override methods for spacer were recalculated with Monte Carlo algorithm: 1) HC plan assigning spacer with ground truth material; 2) W plan with spacer assigned with water; and 3) CT plan with CT Hounsfield Units of spacer used in planning. The target and organs-at-risk doses and Gamma index of 2 mm / 2% were used to quantify the dosimetric differences.
Results: All three plans achieve the same target coverage with D98 around 3316 cGy (RBE). The mean doses to the duodenum are 1364 cGy (RBE), 1359 cGy (RBE), and 1364 cGy (RBE) for plan 1), 2) and 3), respectively. For stomach, the mean dose is 256 cGy (RBE), 249 cGy (RBE) and 255 cGy (RBE), respectively. The Gamma index analysis confirms the plan differences for plans 1) and 2), and plans 1) and 3) are minimal.
Conclusion: With TraceIT as a spacer for proton dose planning, we found no significant dosimetric difference existing among the plans using three override methods
Funding Support, Disclosures, and Conflict of Interest: Research Supported by National Cancer Institute R37CA229417 and Augmenix