Three-dimensional BANG^TMgel dosimetry in conformal carbon ion radiotherapy

Abstract

In this study we applied BANG polymer-gel dosimetry using magnetic resonance imaging (MRI) to densely ionizing radiation such as carbon ion beams. BANG polymer gels were irradiated with a quadratic field of monoenergetic 12C ions at different beam energies in the range of 135 MeV u(-1) to 410 MeV u(-1). They were irradiated at the radiotherapy facility of the GSI, Darmstadt, Germany. Our object was to examine the saturation effect for densely ionizing radiation that occurs at high values of linear energy transfer (LET). The examination yielded the first effectiveness values that will be discussed in the following sections. A solid sphere and a hollow sphere were both irradiated with a horizontal pencil beam from the raster scanning facility at energies of 268 MeV u(-1) (solid sphere) and 304 MeV u(-1) (hollow sphere) respectively. MR dosimetry measurements were compared with data from a planning system. As far as quality is concerned, there is good agreement between the measured dose distributions of both samples and the dose maps from the planning software. The measured MR signals cannot be converted into absolute dose, since the relative efficiency is still unknown for mixed radiation fields of primary carbon ions and it is known only to a limited extent for nuclear fragments with different energies from highly energetic photon radiation. Model calculations are in progress in order to facilitate conversions of measured MR signals into dose.

A scintillating gas detector for 2D dose measurements in clinical carbon beams

“…In order to overcome this limitation, several methods are being developed to measure the dose in three or two dimensions at once. MRI gel dosimetry (Ramm et al 2000, Maryansky et al 1993, Olsson et al 1990 provides 3D dose information but it has the disadvantage that a magnetic resonance imaging unit is needed for evaluation. 3D arrays of ionization chambers (Karger et al 1999) present reliable dosimetric properties, but do not have satisfactory spatial resolution (∼5-6 mm).…”

MAGIC polymer gel for dosimetric verification in boron neutron capture therapy

“…This finding contradicts our previous results, which showed a decrease in the dose– response of polymer gel to high‐LET radiation. The BANG‐1‐type polymer gel has been reported to underestimate the absorbed dose at the Bragg peak of the proton beam by $20\%-30\%$ ( 4 , 5 , 11 ) and by $10\%-40\%$ at carbon ion beams of various energies (6) . The BANG‐3‐type gels underestimate dose under the same conditions by 50% and $40\%-60\%$ respectively.…”

“…The BANG‐3‐type gels underestimate dose under the same conditions by 50% and $40\%-60\%$ respectively. ( 4 – 6 , 11 ) …”

“…Since their introduction, ( 1 , 2 ) ferrous sulfate (Fricke) and polymer gels have been shown to be useful tools in measuring dose distributions by magnetic resonance imaging (MRI) in special applications of radiotherapy such as intensity‐modulated radiotherapy or stereotactic radiosurgery (3) . Gel dosimeters have also been applied to proton beams, ( 4 , 5 ) high‐energy carbon ion beams, (6) and epithermal neutron beams, all of which are used in therapeutic applications. ( 7 – 12 ) The advantages of gel dosimetry include tissue‐like elemental composition, high spatial resolution, capability for three‐dimensional (3D) dose measurements, and possibility of preparing dosimeters of varying sizes and geometries.…”

“…The various components and their relative share of total absorbed dose are of interest because the polymer gel response to absorbed dose depends on linear energy transfer (LET). ( 4 – 6 , 11 ) Studies have shown that gel response decreases with high‐LET particles.…”