In vivo rectal dosimeter with MRI marker

Area of Science:

• Medical Physics
• Radiotherapy
• Dosimetry

Background:

• Precise rectal dose evaluation is crucial for hypofractionated external beam radiotherapy in prostate cancer.
• A novel radiochromic polyurethane-based rectal dosimeter with a custom MRI marker and patient-specific applicator was developed.
• This system aims for in vivo dose verification (IDV) during gated MR-image guided radiotherapy (MR-IGRT).

Purpose of the Study:

• To fabricate and evaluate a radiochromic rectal dosimeter for in vivo dose verification.
• To assess the feasibility of using a custom MRI marker and patient-specific applicator for MR-IGRT.
• To compare in vivo measured doses with Treatment Planning System (TPS) calculated doses.

Main Methods:

• A radiochromic polyurethane active layer (leucomalachite green and tartrazine) was used.
• Detachable PMMA applicators in five sizes and elastomeric MRI markers were developed.
• Dose-response calibration, post-irradiation fading assessment, and dose uncertainty analysis were performed.
• In vivo measurements were conducted on two patients and compared with TPS doses.

Main Results:

• Vyta Flex 20 was chosen as the optimal MRI marker.
• The dosimeter demonstrated a sensitivity of 0.00253 cGy-1 with dose uncertainties of 1.6-2.1%.
• In vivo verification showed mean dose differences of 3.7% and 5.9% compared to TPS calculations, with measured doses generally higher.

Conclusions:

• The fabricated dosimeter system is a feasible proof-of-concept for in vivo rectal dose verification in MR-IGRT.
• Further studies with larger cohorts and more fractions are needed to confirm reproducibility and clinical utility.
• Refinements in positioning, especially in high-dose-gradient regions, are necessary.