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# Theoretical study of physical parameters of fundamental importance in reference dosimetry for hadron therapy

V.B. Tessaro, F. Poignant, B. Gervais, M. Beuve, M.E. Galassi

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V.B. Tessaro, F. Poignant, B. Gervais, M. Beuve, M.E. Galassi. Theoretical study of physical parameters of fundamental importance in reference dosimetry for hadron therapy. 58 th Annual Conference of the Particle Therapy Cooperative Group (PTCOG58), Jun 2019, Manchester, United Kingdom. pp.178, 10.14338/IJPT.19-PTCOG-6.4. in2p3-04937385

HAL Id: in2p3-04937385

https://in2p3.hal.science/in2p3-04937385v1

Submitted on 10 Feb 2025

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PTC58-0393

# Theoretical study of physical parameters of fundamental importance in reference dosimetry for hadron therapy

V.B. Tessaro<fnref n="1" />, F. Poignant<fnref n="2" />, B. Gervais<fnref n="3" />, M. Beuve<fnref n="2" />, M.E. Galassi<fnref n="4" />

The reference dosimetry in hadron therapy is based in the detection of secondary electrons generated by the ionization of the gas contained in ionization chambers. To determine the dose in liquid water, conversion factors such as W-values (mean energy expended by the incident particle to form an electron-hole pair after complete dissipation of its initial energy) in the gas of the ionization chamber are necessary. This physical parameter represents an important source of uncertainties in hadron therapy [1]. In a recent work [2] we studied the W-values by electron, proton and antiproton impact on vapor and liquid water. We used two different methods to take into account the slowing down of the primary particle and all the secondary electrons generated: the Monte Carlo code MDM (which does an event-by-event tracking of all particles, the primary and the secondary electrons) and the Fowler Equation (based in the Continuous Slowing Down Approximation). The results are in very good agreement with experimental data for water vapor and with values obtained by other authors for liquid water. In the present work, we calculate W-values by proton impact on the gases that compose the air extending the theoretical methods developed in [2]. Results obtained for proton energies from 0.5 to 100 MeV are in very good agreement with experimental data. In future work, these models will be extended to calculate W-values for other ions used in hadron therapy. References: 1) IAEA-TRS 398 (2005).

2) Tessaro, Poignant, Gervais, Beuve, Galassi. Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2018.11.03 (2018)

PTC58-0471

# Disagreement of measured small-field output with treatment planning system for a Varian ProBeam system

J. Harms<fnref n="1" />, C.W. Chang<fnref n="1" />, R. Zhang<fnref n="1" />, Y. Lin<fnref n="1" />, K. Langen<fnref n="1" />, T. Liu<fnref n="1" />, M. McDonald<fnref n="1" />, L. Lin<fnref n="1" />

**Purpose:** For pencil-beam scanning protons systems, in-air non-Gaussian halo can significantly impact output at small field sizes and low energies. Since the halo is typically not modelled in treatment planning systems (TPS), this can potentially lead to significant differences in planned and delivered treatment. Here, we report the magnitude of such disagreements.

**Methods:** A CC04 small-volume ion chamber was used to measure absolute output from a ProBeam nozzle in water, and the results were validated with a diamond detector. Field sizes from 2-20 cm were employed with energies ranging from 70-240 MeV. Measurements were taken at the water surface and at half-range for each proton energy. Raystation 8A's clinical Monte Carlo algorithm was used for output modeling.

**Results:** The extent of the halo is shown in Fig. 1, where plots at Z300 show the spot size just after the protons leave the snout, and Z0 shows the spot size 300 mm downstream. Fig. 2 shows the output measurements exhibit a 4-5% disagreement with the TPS for the 2 cm field with 100 MeV, and an 8% disagreement in output for the 2 cm field with 70 MeV.

**Conclusions:** We found that the clinical TPS overestimated output by as much as 8% for small field sizes of 2 cm at extremely low energy of 70 MeV. The in-air halo of low energy extension to 2-3 cm diameter may potentially lead to underdosage of patients treated with small fields.

<footnote n="1">¹Universidad Nacional de Rosario- Instituto de Física Rosario and Université de Lyon 1, FCEIA - Departamento de Física-Escuela de Cs. Básicas- Biomedical Physics Group and Institut de Physique Nucléaire de Lyon, Rosario, Argentina</footnote>

<footnote n="2">²Université de Lyon 1, Institut de Physique Nucléaire de Lyon, Villeurbanne, France</footnote>

<footnote n="3">³CEA/CNRS/ENSICAEN/Université de Caen-Basse Normandie UCBN, Center de Recherche sur les Ions- les Matériaux et la Photonique CIMAP/GANIL, Caen, France</footnote>

<footnote n="4">⁴Universidad Nacional de Rosario and Instituto de Física de Rosario, FCEIA - Departamento de Física- Escuela de Cs. Básicas. Biomedical Physics Group, Rosario, Argentina</footnote>

<footnote n="1">¹Emory University, Department of Radiation Oncology and Winship Cancer Institute, Atlanta, USA</footnote>