MSOR2 Presentation Time: 5:05 PM

Abstract

Purpose

Personalized 3D-printed brachytherapy (BT) implants and advanced dose painting methods for better-targeting tumor geometry, could improve BT outcomes¹. One challenge in this approach is the necessity of measuring the dose profile for each device, before implantation. Polymer gel dosimeters (PGD) could meet the dosimetry challenges of personalized BT devices. However, PGDs have been mainly used in external beam therapy applications and at high-energy photons; by comparison for low-dose-rate (LDR) BT, low-energy photons (LEP) raise issues of water equivalency, energy response^2a and longer exposure times. In this study, a homogeneous PGD formulation based on methacrylic acid (MAA, the reactive monomer) was optimized to meet these challenges. A dosimetry assessment methodology was elaborated for LDR-BT devices, involving the measurement of gel response to LEP using MRI (3 magnetic fields: 1, 1.5, 3T) and experimental dose profiles were generated in PGDs upon exposure to 3D-printed plaques containing ¹²⁵I seeds and numerically plotted in 3D.

Materials and Methods

Single Seed Experiment (SSE): a MAGIC-f gel formulation³ was optimized by adding 0.75 %w/w paraformaldehyde (PF) as gelatin crosslinker. Gelatin and PF were dissolved in pure water and mixed at 45°C. CopperII sulfate pentahydrate, L-ascorbic acid and MAA were added at 37°C. The solution was gelified in a glass container at 4°C overnight. Formed gels were exposed to a ¹²⁵I seed (OncoSeed 6711, activity: 2.13 mCi; n=3) placed in a fixed glass tube for 54h at RT (Fig1a). The gels were MRI-scanned (T₂-w) with 0.9 × 0.9 × 0.9 mm³ voxel size using 1T, 1.5T and 3T MRI (TR/TE= 3585(1T)-4350(1.5T)-5410(3T)/22-352ms). T₂ (=1/ R₂) maps were generated across the vial axial plane. The TOPAS Monte Carlo (MC) code toolkit was used to calculate the absolute dose deposited in the gel with a similar seed model and the R₂-dose calibration curve was plotted. Gel water equivalency was assessed by MC, comparing the percentage depth dose (PDD) delivered to water and gel phantoms when exposed to ¹²⁵I seeds. Gel thermal and temporal stability were assessed up to 70 °C and over 7 days, respectively. Dose profile visualization: A two-part box and a plaque with 3 holes (1 × 1.5 mm diam/depth) were 3D-printed (Apium P220 printer) in PEEK as the gel container and ¹²⁵I seed holder, respectively (Fig1e). The gel was prepared (SSE protocol) and exposed to the plaque for 54h (avg. activity: 1.58 mCi; n=3) followed by 3T T₂-w MRI scanning. The dose was visualized in 3D using the calibration curve and the Python code using Plotly Library.

Results

The results showed that MAGIC-f gel is water equivalent (gel/water PDD ∼1), thermally (up to 70 °C) and temporally stable for 7 days at RT. The SEE results showed detection range of 10-20, 7-25 and 8-22 Gy and sensitivity of 0.6, 0.35 and 0.44 Gy^-1s^-1 for 1T, 1.5T, 3T MRI scans, respectively (Fig1d); that are at least 3 times more sensitivity to LEP than for the only equivalent study reported in the literature^2b. While 1T MRI scanning gives the highest gel sensitivity, lower dose values and broader dose range could be detected with 1.5 and 3T MRI. Fig1i indicates the 3D dose distribution delivered by the 3D-printed LDR plaque validating the gel functionality to visualize the dose profile with high resolution in 3D.

Conclusions

A MAGIC-f gel formulation optimized for LDR-BT applications is thermally and temporally stable and water equivalent. It can detect small dose changes induced by LEP, with high sensitivity and resolution. This gel is a promising tool for developing the clinical workflow including dose profile assessments, required for 3D-printed personalized LDR-BT devices.[1] Lescot et al Adv Health Mater 12.25 2023 [2] Pantelis et al Phys Med Biol (a) 49.15 2004 & (b) 50.18 2005 [3] Fernandes et al JPCS 164.1 2009