arXiv subjects

Faustino Gómez

Publications and source records attributed to Faustino Gómez.

4 recordsLinked to original sources

arXiv2026

Charge collection efficiency of thimble ionization chambers exposed to ultra-high dose per pulse

Background: Commercially available ionization chambers (ICs) exposed to ultra-high dose per pulse (UHDP) exhibit deviations from a linear dose response due to volume recombination. Simulation models have been developed to describe the charge collection efficiency (CCE) but focused on parallel-plate ICs. This study aims to measure and simulate the CCE and polarity effect of thimble ICs in UHDP. Methods: The response of two PinPoint3D T31022 (PP3D) and two PinPoint T31023 (PP) ICs was investigated experimentally at the national metrology institute of Germany (PTB). The ICs were irradiated using the UHDP reference electron beam with dose per pulse up to 9.3 Gy for different voltages. A novel finite element code capable of simulating 1D and 2D geometries was developed. Results: Thimble ICs exhibit a pronounced polarization effect when irradiated with UHDP. When the sign of the collected charge is negative, the CCE is larger than when it is positive. The major contribution to the polarity effect can be attributed to the polarity-dependent charge transport and recombination. Experimental and simulated CCE (polarity effect correction factor) agrees within 1.4 % (7.0 %) and 1.6 % (3.2 %) for the PP3D and PP ICs, respectively. The CCE of parallel-plate and thimble ICs is related through a geometrical rule. Conclusions: The PP IC shows greater CCE due to its smaller external radius. The numerical model is able to satisfactory reproduce the actual CCE and polarity effect for these two chambers. At UHDP, thimble ICs should be used with caution due to their large polarity effect.

physics.med-ph↗

arXiv2025

Evaluation of the two-voltage method for parallel-plate ionization chambers irradiated with pulsed beams

Air-vented ionization chambers exposed to clinical radiation beams may suffer from recombination during the drift of the charge carriers towards the electrodes. Thus, dosimetry protocols recommend the use of a correction factor, usually denominated saturation factor ($k_{\rm sat}$), to correct the ionization chamber readout for the incomplete collection of charge. The two-voltage method is the recommended methodology for the calculation of the saturation factor, however, it is based on the early Boag model, which only takes into account the presence of positive and negative ions in the ionization chamber and does not account for the electric field screening or the free electron contribution to the signal. The numerical simulation shows a better agreement with the experimental data than the current analytical theories in terms of charge collection efficiency. The classical two-voltage method, systematically overestimates the saturation factor, with differences increasing with dose per pulse also present at low dose per pulse. These results may have implications for the dosimetry with ionization chambers in therapy modalities that use a dose per pulse higher than conventional radiotherapy such as intraoperative radiotherapy but also in conventional dose per pulse for ionization chambers that suffer from significant charge recombination.

physics.med-ph↗

arXiv2025

Analysis of the sensitivity of tumor control probability in molecular radiotherapy to uncertainties in the dose rate curves

In this work, we have investigated the sensitivity of the effectiveness (TCP) of molecular radiotherapy (MRT) treatment to uncertainties of the dose rate curves that may appear when reconstructing those curves. We generated different dose rate curves from experimental data, imposing the constraint of equal dose for each of them. Then, we computed TCPs and looked for correlations between metrics measuring the differences between the dose rate curves and differences in TCP. Finally, according to these results, we estimated the range of tolerable uncertainties in the dose rate curves. The study was performed for different radiopharmaceuticals and different radiosensitive parameters that can affect the dose rate response ($α/β$, sub-lethal repair rate). The best correlation between differences in the dose rate curves and TCP was found for a metric that computes averaged linear differences between the curves. With this metric, we quantified differences in dose rate curves that would lead to differences in TCP of 0.02, a parameter denoted $m_{1,\: 0.02}$ that is a surrogate of the dependence of the TCP on the dose rate profile. The results showed that the sensitivity of the TCP to dose rate variations decreases (i.e. larger values of $m_{1,\: 0.02}$) with increasing $α/β$ and sub-lethal damage repair rate of the tumor cells, and increasing biological half-life of the dose rate curves. The radiobiological effect of a MRT treatment on a tumor depends on the absorbed dose and the dose rate profile. Ideally, both magnitudes should be measured with accuracy in order to progress towards the optimization of individualized MRT treatments. Our study suggests that this would be more important for tumors with low $α/β$ and moderately slow sub-lethal damage repair treated with fast-decaying radiopharmaceuticals.

physics.med-ph↗

arXiv2023

An in-silico study of conventional and FLASH radiotherapy iso-effectiveness: Radiolytic oxygen depletion and its potential impact on tumor control probability

FLASH radiotherapy (FLASH-RT) has shown the potential to spare normal tissue while seemingly maintaining the effectiveness of conventional radiotherapy (CONV-RT). It has been suggested that the protective effect arises from the radiolytic oxygen depletion (ROD) caused by FLASH-RT, but it is not entirely clear why this protective effect is not observed in tumors. Iso-effectiveness has been experimentally observed in time-volume curves of preclinical tumors irradiated with FLASH and conventional radiotherapy, but it may not translate to clinical trials, where tumor control probability (TCP) is typically the investigated endpoint. In this work, we used mathematical models to investigate the iso-effectiveness of FLASH-RT/CONV-RT on tumors, focusing on the role of ROD. We used a spatiotemporal reaction-diffusion model, including ROD, to simulate tumor oxygenation. From those oxygen distributions we obtained surviving fractions (SFs), using the linear-quadratic model with oxygen enhancement ratios (OER). We then used the calculated SFs to describe the evolution of preclinical tumor volumes through a mathematical model of tumor response. We also calculated TCPs using the Poisson-LQ approach. Our study suggests that ROD causes differences in SF between FLASH-RT and CONV-RT, especially in low $α$/$β$ and poorly oxygenated cells. These changes do not lead to significant differences in the evolution of preclinical tumors. However, when extrapolating this effect to TCP curves, we observed important differences between both techniques (TCP is lower in FLASH-RT). Nonetheless, it cannot be discarded that other effects not modeled in this work could contribute to tumor control and maintain the iso-effectiveness of FLASH-RT.

physics.med-ph↗