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Description
Particle minibeam radiotherapy (pMBRT) delivers spatially fractionated, submillimetre beamlets that produce peak–valley dose distributions, enabling normal tissue sparing while maintaining high tumour doses [1]. However, the radiobiological mechanisms underlying pMBRT remain incompletely characterised [2], as conventional dosimetry cannot capture the stochastic energy deposition at cellular scales; Microdosimetry, including the SOI Mushroom microdosimeter developed by the Centre for Medical Radiation Physics, provides a means to characterise radiation quality and link it to radiobiological outcomes [3], [4]. Microdosimetry was used to characterise minibeam patterns in spatially fractionated particle therapy at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. A 150 MeV/u helium ion beam, with a range in water of approximately 15.86 cm, was delivered to a 6.5 cm thick brass collimator containing 7 slits, each of 0.5 mm width and 25 mm length. Peak and value measurements were conducted using a Strip SOI Mushroom microdosimeter at varying WET depths between 8.83 mm and 149 mm.
The results show broader energy distributions, with a tendency towards higher lineal energies and elevated dose mean lineal energy (yD) values in valley regions compared to the peak regions. yD increases with depth, in both peaks and valleys regions. For the same vertical position within a valley, the corresponding yD values are 8.58, 7.21, 6.95, 6.84, 9.39, 30.97 keV/μm at depths of 8.83 (closest to collimator), 20, 40, 90, 130 and 149 mm, respectively. For the same vertical position within a peak, the yD values are 6.74, 6.69, 6.23, 6.66, 29.85 keV/μm at depths of 8.83, 20, 40, 60, 149 mm, respectively. From depths of 40-50 mm onward, there is no essential difference in yD results in the valley region. Beyond a depth of 40 mm, yD becomes homogeneous, with values approaching 30 keV/μm at the Bragg Peak. There appears to be minimal variation in the yD and spectra depending on the SOI position within the slits. At the depth closest to the collimator, the average values of 6.06, 6.68, 5.88 keV/μm for the first three slits (peak region), and 9.37, 8.00 and 7.33 keV/μm for the corresponding first three valleys.
The SOI mushroom Strip microdosimeter successfully measured both peak and valley regions in spatially fractionated minibeam fields with high accuracy, demonstrating submillimetre spatial resolution. The ability to connect single sensitive volume arrays enables precise measurements within these regions. To evaluate the tissue-sparing effects, RBE weighted dose should also be considered, as the sparing effect observed in the entrance region decreases with increasing depth. Scattered helium ions, neutrons, and other secondary particles may also contribute to dose in the valley regions.
References
[1] S. Charyyev et al., “Optimisation of hexagonal pattern minibeams for spatially fractionated radiotherapy using proton beam scanning,” Medical Physics., vol. 47, no. 8, pp. 485–3495, Aug. 2020, doi: 10.1002/mp.14192.
[2] E. Lee et al., “Collimator design for spatially fractionated proton beams for radiobiology research,” Physics in Medicine & Biology., vol. 61, no. 14, pp. 5378–5392, Jun. 2016, doi: 10.1088/0031-9155/61/14/5378.
[3] H. Paganetti, “Proton relative biological effectiveness-uncertainties and opportunities,” International Journal of Particle Therapy., vol. 5, no. 1, pp. 2–14, Jun. 2018, doi: 10.14338/IJPT-18-00011.1.
[4] L. T. Tran et al., “Silicon 3D microdosimeters for advanced quality assurance in particle therapy,” Applied Sciences., vol. 12, no. 1, Art. no. 328, Nov. 2022, doi: 10.3390/app12010328.