Y. Roed

Investigation of magnetic field effects on the dose–response of 3D dosimeters for magnetic resonance – image guided radiation therapy applications

BACKGROUND AND PURPOSE The strong magnetic field of integrated magnetic resonance imaging (MRI) and radiation treatment systems influences secondary electrons resulting in changes in dose deposition in three dimensions. To fill the need for volumetric dose quality assurance, we investigated the effects of strong magnetic fields on 3D dosimeters for MR-image-guided radiation therapy (MR-IGRT) applications. MATERIAL AND METHODS There are currently three main categories of 3D dosimeters, and the following were used in this study: radiochromic plastic (PRESAGE®), radiochromic gel (FOX), and polymer gel (BANG™). For the purposes of batch consistency, an electromagnet was used for same-day irradiations with and without a strong magnetic field (B0, 1.5T for PRESAGE® and FOX and 1.0T for BANG™). RESULTS For PRESAGE®, the percent difference in optical signal with and without B0 was 1.5% at the spectral peak of 632nm. For FOX, the optical signal percent difference was 1.6% at 440nm and 0.5% at 585nm. For BANG™, the percent difference in R2 MR signal was 0.7%. CONCLUSIONS The percent differences in responses with and without strong magnetic fields were minimal for all three 3D dosimeter systems. These 3D dosimeters therefore can be applied to MR-IGRT without requiring a correction factor.

The potential of polymer gel dosimeters for 3D MR-IGRT quality assurance

Advances in radiotherapy technology have enabled more accurate delivery of radiation doses to anatomically complex tumor volumes, while sparing surrounding tissues. The most recent advanced treatment modality combines a radiation delivery system (either Cobalt-60 therapy heads or linear accelerator) with a diagnostic magnetic resonance (MR) scanner to perform MR-image guided radiotherapy (MR-IGRT). For a radiation treatment plan to be delivered successfully with MR-IGRT the compliance with previously established criteria to validate the passing of such plans has to be confirmed. Due to the added strong magnetic field a new set of quality assurance standards has to be developed. Ideal detectors are MR-compatible, can capture complex dose distributions and can be read out with MRI. Polymer gels were investigated as potential three dimensional MR-IGRT quality assurance detectors.

EP-1513: Polymer gels enable volumetric dosimetry of dose distributions from an MR-guided linac

MR Imaging Coronal MR images were acquired at different times after irradiation with the MR component of the MR-linac using a 2D T2weighted spin echo sequence with TR = 1000 ms and TE = 20, 40, 60, 80 and 100 ms. Spin-spin relaxation rate (R2) maps were generated and averaged line profiles across the dosimetric volume were analyzed. Sigmoidal curves were fit to the data from which both the 80/20 penumbra and the field edge position were calculated.

SU-G-JeP2-11: Polymer Gels as 3D QA Devices in Magnetic Resonance-Guided Radiation Therapy

PURPOSE To demonstrate the potential of polymer gels to serve as 3D QA devices in MR-guided radiotherapy. METHODS Custom-designed polymer gel dosimeters (MGS Research, Madison, CT) were centered on the penumbras of a radiation field, parallel to the magnetic field lines inside a 1.5 T MR scanner integrated with a 7 MV linac (MR-Linac). 15 Gy were delivered perpendicular to the magnetic field. Coronal MR images were acquired with a 2D balanced-Fast Field Echo (b-FFE) sequence in real-time during irradiation and with a 2D spin echo sequence at different times after irradiation. Signal intensities (SI) were measured inside and outside the radiation field on b-FFE images obtained during irradiation. Spin-spin relaxation rate (R2) maps were calculated from images after irradiation and penumbra widths were calculated from these maps. RESULTS A difference in SI between areas of the dosimeter inside and outside of the radiation field could be measured immediately after the beam was turned on. The difference in SI increased throughout the duration of the exposure. R2 values from immediately after irradiation were 83% of those measured 24h post-irradiation. At 1h after irradiation, the R2 values were 87% of those at 24h indicating that polymerization of the gel was still ongoing. At 24h postirradiation the width of the 80/20 penumbra on the left field edge measured 6.5 mm and on the right 5.6 mm. CONCLUSION The MR-Linac allows for MR imaging during treatment and measurements of 3D dose distributions with steep dose gradients without having to transfer dosimeters for read-out. The results showed promise for polymer gels as relative 3D QA devices to verify radiation treatment plans delivered by an MR-Linac. To our knowledge, this was the first time that polymer gels were used for real-time image acquisition. Further investigations into the application of polymer gel dosimetry on MR-Linac are ongoing.

Gel dosimetry enables volumetric evaluation of dose distributions from an MR-guided linac

Magnetic resonance-guided radiation therapy (MRgRT) benefits from performing treatment response assessments at the beginning and end of treatment and also during the treatment itself allowing for more normal tissue sparing and better tumor conformality. Gel dosimeters have been shown to demonstrate 3D dose distributions from conventional treatment units. This qualitative study assessed the value of gel dosimetry to measure volumetric dose distributions delivered by an MR-linac (MRL) while using the MR component for readout.Polymer gels in custom-designed glass cylinders of 5 cm diameter and 4 cm height were obtained from MGS Research Inc. (Madison, CT). In addition, Fricke-type dosimeters were prepared in-house with 0.05 mM xylenol orange. For this preliminary study, the dosimeters were irradiated in air, with a part of each dosimeter outside the treatment field to act as a reference. MR imaging was performed with the MRL to observe the change in paramagnetic properties pre and post irradiation using a T2...

SU‐E‐T‐716: Suitability Study of a Unique 3D Dosimeter for Commissioning Radiation Treatment Machines

Purpose: To determine the suitability of a methacrylic acid-based MAGAT gel dosimeter to serve as a 3D commissioning tool Methods: Different sized MAGAT gel dosimeters were fabricated in-house to perform dose response, depth dose and dose rate measurements using an orthovoltage treatment unit. Dose response data were obtained by irradiating dosimeters to six dose levels at three different effective beam energies. Depth dose measurements were taken using one dose at one energy. Dose rate dependence was measured by varying only distances for the gel irradiations while dose level and energy remained unchanged. A Bruker Biospecs 4.7T MRI scanner was used to acquire spin-spin relaxation rates on axial slices at different depths throughout each dosimeter 24h after irradiation. Results: The dose response curves exhibited a separation when compared for the measured energies. MAGAT depth dose data, normalized to ion chamber data at 2 cm depth, showed an increasing divergence with increasing depth. The dose rate decreased when distances were increased and demonstrated a significant dose rate dependence of the measurements. Consequently, dose rate corrections were applied to all measurements, thus, aligning the dose response curves for all three energies and reducing the divergence between MAGAT and ion chamber depth dose data. Conclusion: The MAGAT dosimeter was intended to serve as a 3D commissioning tool for testing radiation treatment plans of small fields with steep dose gradients and complex treatment volumes. A suitable detector for commissioning needs to be dependent on dose but independent of dose rate. This study, however, showed a dose rate dependence of up to 30% eliminating the current composition of MAGAT as a suitable dosimeter. Further investigations of chemical compositions need to be performed.