Evaluation of end-to-end 3D absorbed dose distribution in Y-SIRT and SBRT combination therapy using MAGIC-f polymer gel dosimeter.

BACKGROUND

Selective Internal Radiation Therapy (SIRT) with Y-microspheres, and Stereotactic Body Radiation Therapy (SBRT), are well-established treatment strategies for liver malignancies. Combining these two modalities has proven to be an effective and safe approach for addressing undertreated tumor regions from initial SIRT through a boost dose from SBRT. The complex dosimetry process, which includes image-based dosimetry of Y-SIRT and SBRT dose painting, requires precise dosimetry verification. In this study, gel dosimetry is proposed as a valuable novel tool to capture the 3D absorbed dose contributions from both treatments with high spatial resolution.

METHODS

The MAGIC-f polymer gel dosimeter was used in three experiments: external beam radiation therapy (EBRT), Y-SIRT, and combination therapy. For EBRT, eight calibration vials filled with gel were irradiated with absorbed doses ranging from 0 to 21 Gy. In the Y-SIRT experiments, eleven vials were filled with gel containing varying concentrations of Y-citrate, delivering doses from 0 to 44 Gy after 68 h. For combination therapy, eight vials received 0-7 Gy from Y-citrate after 68 h, followed by an additional 0-7 Gy from SBRT (final doses 0-14 Gy). A phantom, designed to simulate a tumor within a normal liver, was created with the sphere receiving 10 Gy after 68 h from SIRT and another 10 Gy from SBRT using 6MV photons (in total 20 Gy). PET/CT imaging was conducted before SBRT, and the Medical Internal Radiation Dose (MIRD) schema was used for dose calculations. For each experiment, MR T2-weighted imaging was performed using a 3 Tesla scanner, and R2 maps (1/s) were analyzed to establish a dose-response relationship between absorbed dose (Gy) and R2. The gel's sensitivity to each irradiation was measured, and phantom's dose maps were evaluated using mean absorbed dose, dose volume histograms (DVHs), line profiles, and isodose maps.

RESULTS

The dose response of the gel was linear within the irradiated ranges for EBRT and combination therapy. The linear range for Y-SIRT was between 0 and 16.75 Gy. The sensitivity of the gel was 0.380, 0.758, and 0.713 s⁻¹/Gy for EBRT, Y-SIRT, and combination experiments, respectively. In the phantom irradiated with combination therapy, a saturated area was observed in the central core of the sphere, surrounded by an underestimation area with a diameter of 4.5 mm. The mean absorbed dose values within the sphere were 9.83 Gy, 9.71 Gy, and 18.58 Gy from SBRT, SIRT, and combination therapy, respectively. For the cylinder, these values were 1.29 Gy, 0.61 Gy, and 2.68 Gy, respectively. The DVHs, line profiles, and isodose lines for the combination therapy demonstrated the cumulative effects of the absorbed dose from both treatments.

CONCLUSION

This is the first study demonstrating the feasibility of using MAGIC-f gel dosimetry to directly measure 3D dose distributions from combined Y-SIRT and SBRT. Compared to PET/CT-based dosimetry, the gel method offers superior spatial resolution and enables objective physical verification of complex dose distributions. This technique has strong potential for quality assurance in theranostic protocols, particularly for beta emitters and, potentially, alpha-emitting radiotracers in future applications.