Carbon beam dosimetry using VIP polymer gel and MRI

Radio-fluorogenic nanoclay gel dosimeters with reduced linear energy transfer dependence for carbon-ion beam radiotherapy

PURPOSE

The precise assessment of the dose distribution of high linear energy transfer (LET) radiation remains a challenge, because the signal of most dosimeters will be saturated due to the high ionization density. Such measurements are particularly important for heavy-ion beam cancer therapy. On this basis, the present work examined the high LET effect associated with three-dimensional gel dosimetry based on radiation-induced chemical reactions. The purpose of this study was to create an ion beam radio-fluorogenic gel dosimeter with a reduced effect of LET.

METHODS

Nanoclay radio-fluorogenic gel (NC-RFG) dosimeters were prepared, typically containing 100 μM dihydrorhodamine 123 (DHR123) and 2.0 wt% nanoclay together with catalytic additives promoting Fenton or Fenton-like reactions. The radiological properties of NC-RFG dosimeters having different compositions in response to a carbon-ion beam were investigated using a fluorescence gel scanner.

RESULTS

An NC-RFG dosimeter capable of generating a fluorescence intensity distribution reflecting the carbon-ion beam dose profile was obtained. It was clarified that the reduction of the unfavorable LET dependence results from an acceleration of the reactions between DHR123 and H₂ O₂ , which is a molecular radiolysis product. The effects of varying the preparation conditions on the radiological properties of these gels were also examined. The optimum H₂ O₂ catalyst was determined to include 1 mM Fe³⁺ ions, and the addition of 100 mM pyridine was also found to increase the sensitivity.

CONCLUSIONS

This technique allows the first-ever evaluation of the depth-dose profile of a carbon-ion beam at typical therapeutic levels of several Gy without LET effect.

Whole Three-Dimensional Dosimetry of Carbon Ion Beams with an MRI-Based Nanocomposite Fricke Gel Dosimeter Using Rapid T1 Mapping Method

MRI-based gel dosimeters are attractive systems for the evaluation of complex dose distributions in radiotherapy. In particular, the nanocomposite Fricke gel dosimeter is one among a few dosimeters capable of accurately evaluating the dose distribution of heavy ion beams. In contrast, reduction of the scanning time is a challenging issue for the acquisition of three-dimensional volume data. In this study, we investigated a three-dimensional dose distribution measurement method for heavy ion beams using variable flip angle (VFA), which is expected to significantly reduce the MRI scanning time. Our findings clarified that the whole three-dimensional dose distribution could be evaluated within the conventional imaging time (20 min) and quality of one cross-section.

Gel dosimetry for three dimensional proton range measurements in anthropomorphic geometries

Proton beams used for radiotherapy have potential for superior sparing of normal tissue, although range uncertainties are among the main limiting factors in the accuracy of dose delivery. The aim of this study was to benchmark an N-vinylpyrrolidone based polymer gel to perform three-dimensional measurement of geometric proton beam characteristics and especially to test its suitability as a range probe in combination with an anthropomorphic phantom. For single proton pencil beams as well as for 3×3cm² mono-energy layers depth dose profiles, lateral dose distribution at different depths and proton range were evaluated in simple cubic gel phantoms at different energies from 75 to 115MeV and different dose levels. In addition, a 90MeV mono-energetic beam was delivered to an anthropomorphic 3D printed head phantom, which was filled with gel. Subsequently, all phantoms underwent magnetic resonance imaging using an axial pixel size of 0.68-0.98mm and with slice thicknesses of 2 or 3mm to derive a 3-dimensional distribution of the T₂ relaxation time, which correlates with radiation dose. Indices describing lateral dose distribution and proton range were compared against predictions from a treatment planning system (TPS, for cubic and head phantoms) and Monte Carlo simulations (MC, for the head phantom) after manual rigid co-registration with the T₂ relaxation time datasets. For all pencil beams, the FWHM agreement with TPS was better than 1mm or 7%. For the mono-energetic layer, the agreement with TPS in this respect was even better than 0.3mm in each case. With respect to range, results from gel measurements differed no more than 0.9mm (1.6%) from values predicted by TPS. In case of the anthropomorphic phantom, deviations with respect to a nominal range of about 61mm as well as in FWHM were slightly higher, namely within 1.0mm and 1.1mm respectively. Average deviations between gel and TPS/MC were similar (-0.3mm±0.4mm/-0.2±0.5mm). In conclusion, polymer gel dosimetry was found to be a valuable tool to determine geometric proton beam properties three-dimensionally and with high spatial resolution in simple cubic as well as in a more complex anthropomorphic phantom. Post registration range errors of the order of 1mm could be achieved. The additional registration uncertainty (95%) was 1mm.