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

ABSTRACT

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.