We give the complete twisted Yukawa couplings for all the Z n orbifold constructions in the most general case, i.e. when orbifold deformations are considered. This includes a certain number of tasks. Namely, determination of the allowed couplings, calculation of the explicit dependence of the Yukawa couplings values on the moduli expectation values (i.e. the parameters determining the size and shape of the compactified space), etc. The final expressions are completely explicit, which allows a counting of the different Yukawa couplings for each orbifold (with and without deformations). This knowledge is crucial to determine the phenomenological viability of the different schemes, since it is directly related to the fermion mass hierarchy. Other facts concerning the phenomenological profile of Z n orbifolds are also discussed, e.g. the existence of non-diagonal entries in the fermion mass matrices, which is related to a non-trivial structure of the Kobayashi-Maskawa matrix. Finally some theoretical results are given, e.g. the no-participation of (1,2) moduli in twisted Yukawa couplings. Likewise, (1,1) moduli associated with fixed tori which are involved in the Yukawa coupling, do not affect the value of the coupling.

Neutron production in radiotherapy facilities has been studied from the early days of modern linacs. Detailed studies are now possible using photoneutron capabilities of general-purpose Monte Carlo codes at energies of interest in medical physics. The present work studies the effects of modelling different accelerator head and room geometries on the neutron fluence and spectra predicted via Monte Carlo. The results from the simulation of a 15 MV Siemens PRIMUS linac show an 80% increase in the fluence scored at the isocentre when, besides modelling the components necessary for electron/photon simulations, other massive accelerator head components are included. Neutron fluence dependence on inner treatment room volume is analysed showing that thermal neutrons have a 'gaseous' behaviour and then a 1/V dependence. Neutron fluence maps for three energy ranges, fast (E > 0.1 MeV), epithermal (1 eV < E < 0.1 MeV) and thermal (E < 1 eV), are also presented and the influence of the head components on them is discussed.

UHDpulse -Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates is a recently started European Joint Research Project with the aim to develop and improve dosimetry standards for FLASH radiotherapy, very high energy electron (VHEE) radiotherapy and laser-driven medical accelerators. This paper gives a short overview about the current state of developments of radiotherapy with FLASH electrons and protons, very high energy electrons as well as laser-driven particles and the related challenges in dosimetry due to the ultra-high dose rate during the short radiation pulses. We summarize the objectives and plans of the UHDpulse project and present the 16 participating partners.

In order to obtain realistic and reliable Monte Carlo simulations of medical linac photon beams, an accurate determination of the parameters that define the primary electron beam that hits the target is a fundamental step. In this work we propose a new methodology to commission photon beams in Monte Carlo simulations that ensures the reproducibility of a wide range of clinically useful fields. For such purpose accelerated Monte Carlo simulations of 2 x 2, 10 x 10, and 20 x 20 cm2 fields at SSD = 100 cm are carried out for several combinations of the primary electron beam mean energy and radial FWHM. Then, by performing a simultaneous comparison with the correspondent measurements for these same fields, the best combination is selected. This methodology has been employed to determine the characteristics of the primary electron beams that best reproduce a Siemens PRIMUS and a Varian 2100 CD machine in the Monte Carlo simulations. Excellent agreements were obtained between simulations and measurements for a wide range of field sizes. Because precalculated profiles are stored in databases, the whole commissioning process can be fully automated, avoiding manual fine-tunings. These databases can also be used to characterize any accelerators of the same model from different sites.

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Neutron peripheral contamination in patients undergoing high-energy photon radiotherapy is considered as a risk factor for secondary cancer induction. Organ-specific neutron-equivalent dose estimation is therefore essential for a reasonable assessment of these associated risks. This work aimed to develop a method to estimate neutron-equivalent doses in multiple organs of radiotherapy patients. The method involved the convolution, at 16 reference points in an anthropomorphic phantom, of the normalized Monte Carlo neutron fluence energy spectra with the kerma and energy-dependent radiation weighting factor. This was then scaled with the total neutron fluence measured with passive detectors, at the same reference points, in order to obtain the equivalent doses in organs. The latter were correlated with the readings of a neutron digital detector located inside the treatment room during phantom irradiation. This digital detector, designed and developed by our group, integrates the thermal neutron fluence. The correlation model, applied to the digital detector readings during patient irradiation, enables the online estimation of neutron-equivalent doses in organs. The model takes into account the specific irradiation site, the field parameters (energy, field size, angle incidence, etc) and the installation (linac and bunker geometry). This method, which is suitable for routine clinical use, will help to systematically generate the dosimetric data essential for the improvement of current risk-estimation models.

This work is devoted to studying the influence of chamber response functions on the standard IMRT verification for the different detector technologies available on commercial devices. We have tested three of the most used 2D detector arrays for radiotherapy dosimetry verification, based on air-ionization chambers and diode detectors. The response function has been carefully simulated using the Monte Carlo method and measured through slit and pinhole collimators. Although the response function of air-ionization detectors is considerably different with respect to that of standard diodes, the impact on a verification based in the gamma function with tolerances 3 mm and 3% is quite limited. The results show that the standard air-ionization detector arrays perform in a similar way whenever the tolerances for the gamma function are not lowered below 1.5 mm and 1.5%. Additionally, the sensitivity of these devices to fluence perturbations was measured by intentionally modifying some leaf positions in the multileaf collimator. The wider response function of air-ionization chamber arrays made them slightly more sensitive to random fluence perturbations, although silicon diode arrays are more accurate to describe the dose distribution in a point by point basis.

Modern dosimetry protocols are based on the use of ionization chambers provided with a calibration factor in terms of absorbed dose to water. The basic formula to determine the absorbed dose at a user's beam contains the well-known beam quality correction factor that is required whenever the quality of radiation used at calibration differs from that of the user's radiation. The dosimetry protocols describe the whole ionization chamber calibration procedure and include tabulated beam quality correction factors which refer to 60Co gamma radiation used as calibration quality. They have been calculated for a series of ionization chambers and radiation qualities based on formulae, which are also described in the protocols. In the case of high-energy photon beams, the relative standard uncertainty of the beam quality correction factor is estimated to amount to 1%. In the present work, two alternative methods to determine beam quality correction factors are prescribed-Monte Carlo simulation using the EGSnrc system and an experimental method based on a comparison with a reference chamber. Both Monte Carlo calculations and ratio measurements were carried out for nine chambers at several radiation beams. Four chamber types are not included in the current dosimetry protocols. Beam quality corrections for the reference chamber at two beam qualities were also measured using a calorimeter at a PTB Primary Standards Dosimetry Laboratory. Good agreement between the Monte Carlo calculated (1% uncertainty) and measured (0.5% uncertainty) beam quality correction factors was obtained. Based on these results we propose that beam quality correction factors can be generated both by measurements and by the Monte Carlo simulations with an uncertainty at least comparable to that given in current dosimetry protocols.