Gastrointestinal Cancer

PV QA 1 - Poster Viewing Q&A 1

SU_1_2002 - Three-Dimensional Printing of Adaptive Bolus for Squamous Cell Carcinoma of the Anus

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

Three-Dimensional Printing of Adaptive Bolus for Squamous Cell Carcinoma of the Anus
M. L. Cardenas, and J. R. Perks; University of California Davis, Sacramento, CA

Purpose/Objective(s): The standard of care for non-metastatic squamous cell carcinoma of the anus (SCCA) is concurrent chemoradiation (CRT). Gross tumor may extend below the anal verge and appropriate dosing to the planning treatment volume (PTV) may require bolus of the perianal skin. Through internal re-planning studies, our group has established the dosimetric superiority of three-dimensional (3D)-printed bolus over wet gauze for SCCA treatment. Wet gauze may still be used for initial fractions while a 3D-printed bolus is being fabricated depending on the urgency treatment. During the course of CRT, tumor may regress resulting in changes in air gap volume between bolus and perianal skin causing dosimetric uncertainty compared to the simulation computed tomography (CT). We sought to quantify such changes in air gap volume under, first, wet gauze bolus, and then, adapted 3D-printed bolus for a patient undergoing CRT for SCCA.

Materials/Methods: Daily cone-beam computed tomography (CBCT) scans were analyzed for skin-to-bolus air gap volumes during a 30 fraction treatment for SCCA. For the first 6 fractions, perianal skin was bolused with wet-gauze. For the final 24 fractions, adapted 3-D printed bolus was used. CBCT scans were acquired on an 88 slice scanner at 120 kVP with a 40 cm x 40 cm field of view, 1 mm x 1 mm in-plane resolution, and 1.2 mm slice thickness with the patient in the prone position. Adapted 3-D printed bolus was fabricated with Tango Plus malleable material of 1.12 g/cm3 density using Polyjet technology (Stratasys Ltd., Eden Prairie, MN) to conform to the intragluteal cleft about the anal oriface per simulation CT. Air gap volumes were contoured between bolus and perianal skin for each CBCT. Volume measurements from fractions 1-6, 7-12, and 13-30 were grouped together for statistical analysis. Sample means were compared by analysis of variance (ANOVA). Statistical calculations were done in Microsoft Excel (Microsoft, Redmond, WA).

Results: Thirty CBCT scans were acquired for one patient undergoing a 30 fraction treatment with 5-fluorouracil/mitomycin C-based CRT for SCCA clinically stage 3C/T3N1M0. Bolused perianal skin was included in the PTV treated to 54 Gy. Bolus thickness was 0.5 cm. The mean air gap volumes (±95% confidence interval) for fractions 1-6, 7-12, 13-30 were 4.7±1.7 ml, 8.5±0.6ml, and 12.6±1.9ml, respectively. On ANOVA, a significant difference in air gap volume was demonstrated under 3-D printed bolus in fractions 7-12 and fractions 13-30 (p = 0.027).

Conclusion: We utilize image-based adapted 3D-printed bolus for the treatment of SCCA with PTV involvement of the perianal skin. This current study quantifies the changes in air gap volumes under the bolus which occurs through the course of CRT as a result of tumor regression. This change may result in dosimetric uncertainty. Repeat 3D-printing of adaptive boluses during the course of treatment with re-planning can reduce this uncertainty.

Author Disclosure: M.L. Cardenas: None. J.R. Perks: President; Julian Perks.

Michael Cardenas, MD

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