ESTRO 2023 - Abstract Book

S1702

Digital Posters

ESTRO 2023

Conclusion The expected increase in the adaptation rate at lower robustness levels and the benefit to the patient in the form of a reduced dose to OARs was confirmed in this study. In this way the most cost-effective trade-off workflow between plan robustness and the adaptation rate for proton therapy of lung cancer patients can be identified. The achieved OAR sparing can have significant clinical benefits in cases where lower robustness levels suffice for ensuring target coverage. This work also allows the identification of the optimal time point for adaptive treatment.

PO-1946 Measurement of LET distributions by CR-39 for a carbon ion beam in the microporous structure

J. Zhao 1 , S. Yang 2 , B. Chen 3

1 Shanghai Proton and heavy ion Center, Medical physics, Shanghai, China; 2 Fudan university Institute of Radiation medicine, Radiation dosimetry, Shanghai, China; 3 Fudan university Insitute of Radiation medicine, radiation dosimetry, Shanghai, China Purpose or Objective The depth dose distribution of the carbon ion beam is degraded in a porous structure compared to a homogeneous structure with the same density. The changes would also happen to the distribution of the fragments, which is the critical parameter of the relative biological effect (RBE) model. In this study, the CR-39 detector was used to measure the primary and secondary particles of the carbon ion beam in a porous structure. The LET spectrum obtained by FLUKA calculation in a microporous model was compared to the measurements. Materials and Methods The Gammex phantom LN300 with microporous structure was used to construct the measurement phantom (30 × 30 × 40 cm3).The CR-39 (BARYOTRAK, Fukuvi Chemical Industry Co., Ltd., Japan) with an area of 30 × 30 mm2 and thickness of 0.9 mm was used as the detector, and the detectors were placed between two materials at different depths according to the measurement requirements. The irradiated CR-39 was etched, and the nuclear track image was scanned with an in-house developed automatic optical microscope system (resolution 0.402 µm/ pixel). The LET was calculated according to the track etching sensitivity. The LET spectrum calculations in the microporous structures and homogeneous material were also simulated in FLUKA. The calculation results were compared to the measurements. Results Figure 1 shows the LET spectrum distribution obtained by simulation and measurement at different depths after the carbon ion beam passes through the microporous structure. The detection threshold of CR39 is about ten keV/µm. The solid black line in the figure is the measurement result of the LET spectrum, which is consistent with the red simulation result. (g) is the DDD curve obtained by simulation, marking the position of energy spectrum measurement. The secondary debris yield measured at the incident flat area (a) Depth = 14.3 mm is higher than the simulation result, which may be due to the low debris yield, leading to large statistical fluctuations and errors. The measurements match the FLUKA calculation results. Figure 2 shows the LET spectra of primary and secondary particles in the microporous structure model and homogeneous materials obtained by FLUKA simulation. (a-e) and (i-m) are the LET spectrum distribution after the carbon ion beam passes through the microporous structure and homogeneous materials, respectively. (h) and (n) are the schematic diagram of the micropore model and DDD and the calculated depth position. Comparing (a-e) and (i-m), the maximum value of LET of carbon ions passing through the micropore structure at the BP position is smaller than that passing through the homogeneous materials, and the number of secondary particles is reduced.