Spatial fractionation of the dose in heavy ions therapy: An optimization study

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Spatially fractionated radiotherapy (SFRT), including minibeam or Grid therapy, combined with charged particle therapy, is a novel approach to enhance the therapeutic index for radioresistant tumors.
• The complex dose distributions resulting from nuclear fragmentation in heavy ion SFRT necessitate advanced dosimetric assessments beyond classical methods.

Purpose of the Study:

• To determine the optimal irradiation configuration for heavy ion spatially fractionated radiotherapy (SFRT).
• To guide future biological experiments by evaluating ion species, beam width, center-to-center distances, and linear energy transfer (LET).
• To address the need for a comprehensive dosimetric description considering nuclear fragmentation in SFRT.

Main Methods:

• Monte Carlo simulations using GATE 6.2 were employed to model dose distributions for various ions, beam widths, and spacings.
• 3D maps of dose-averaged LET were calculated.
• A novel parameter, the peak-to-valley-LET ratio (PVLR), was introduced for a more complete physical evaluation.

Main Results:

• Beam widths exceeding 400 μm are recommended to maintain entrance-to-target dose ratios comparable to conventional irradiations.
• Large center-to-center distances (3500 μm) promote tissue sparing by increasing the peak-to-valley dose ratio (PVDR) and reducing the contribution of heavy nuclear fragments.
• Valleys exhibit lower LET values with large center-to-center distances, leading to more homogeneous dose distributions within the target.

Conclusions:

• Heavy ion minibeam radiotherapy (MBRT) demonstrates advantageous dose distributions.
• Submillimetric beams, due to reduced lateral scattering, maintain entrance-to-target dose ratios similar to conventional radiotherapy.
• Preferring large center-to-center distances (3500 μm) optimizes normal tissue sparing by utilizing lighter nuclear fragments in the valleys, thus reducing dose-averaged LET.
• Neon (Ne) ions emerged as the most promising species, balancing high PVDR and PVLR in normal tissues with high LET values (~100 keV/μm) and a favorable oxygen enhancement ratio in the target.