Similar additive effects of doxorubicin in combination with photon or proton irradiation in soft tissue sarcoma models.

High-precision radiotherapy with proton beams is frequently used in the management of aggressive soft tissue sarcoma (STS) and is often combined with doxorubicin (Dox), the first-line chemotherapy for STS. However, current treatment approaches continue to result in high local recurrence rates often occurring within the treatment field. This strongly indicates the need of optimized treatment protocols taking the vast heterogeneity of STS into account, thereby fostering personalized treatment approaches. Here, we used preclinical STS models to investigate the radiation response following photon (X) or proton (H) irradiation alone and in combination with different treatment schedules of Dox. As preclinical models, fibrosarcoma (HT-1080), undifferentiated pleiomorphic sarcoma (GCT), and embryonal rhabdomyosarcoma (RD) cell lines were used; the latter two are mutated for TP53. The cellular response regarding clonogenic survival, apoptosis, cell-cycle distribution, proliferation, viability, morphology, and motility was investigated. The different STS cell types revealed a dose-dependent radiation response with reduced survival, proliferation, viability, and motility whereas G2/M phase arrest as well as apoptosis were induced. RD cells showed the most radiosensitive phenotype; the linear quadratic model fit could not be applied. In combined treatment schedules, Dox showed the highest efficiency when applied after or before and after radiation; Dox treatment only before radiation was less efficient. GCT cells were the most chemoresistant cell line in this study most probably due to their TP53 mutation status. Interestingly, similar additive effects could be observed for X or H irradiation in combination with Dox treatment. However, the additive effects were determined more frequently for X than for H irradiation. Thus, further investigations are needed to specify alternative drug therapies that display superior efficacy when combined with H therapy.

Surface Chemistry and Specific Surface Area Rule the Efficiency of Gold Nanoparticle Sensitizers in Proton Therapy.

Gold nanoparticles (AuNPs) are currently the most studied radiosensitizers in proton therapy (PT) applicable for the treatment of solid tumors, where they amplify production of reactive oxygen species (ROS). However, it is underexplored how this amplification is correlated with the AuNPs' surface chemistry. To clarify this issue, we fabricated ligand-free AuNPs of different mean diameters by laser ablation in liquids (LAL) and laser fragmentation in liquids (LFL) and irradiated them with clinically relevant proton fields by using water phantoms. ROS generation was monitored by the fluorescent dye 7-OH-coumarin. Our findings reveal an enhancement of ROS production driven by I) increased total particle surface area, II) utilization of ligand-free AuNPs avoiding sodium citrate as a radical quencher ligands, and III) a higher density of structural defects generated by LFL synthesis, indicated by surface charge density. Based on these findings it may be concluded that the surface chemistry is a major and underexplored contributor to ROS generation and sensitizing effects of AuNPs in PT. We further highlight the applicability of AuNPs in vitro in human medulloblastoma cells.

Comprehensive investigation of lateral dose profile and output factor measurements in small proton fields from different delivery techniques.

BACKGROUND AND PURPOSE: As a part of the commissioning and quality assurance in proton beam therapy, lateral dose profiles and output factors have to be acquired. Such measurements can be performed with point detectors and are especially challenging in small fields or steep lateral penumbra regions as the detector's volume effect may lead to perturbations. To address this issue, this work aims to quantify and correct for such perturbations of six point detectors in small proton fields created via three different delivery techniques. METHODS: Lateral dose profile and output measurements of three proton beam delivery techniques (pencil beam scanning, pencil beam scanning combined with collimators, passive scattering with collimators) were performed using high-resolution EBT3 films, a PinPoint 3D 31022 ionization chamber, a microSilicon diode 60023 and a microDiamond detector 60019 (all PTW Freiburg, Germany). Detector specific lateral dose response functions K(x,y) acting as the convolution kernel transforming the undisturbed dose distribution D(x,y) into the measured signal profiles M(x,y) were applied to quantify perturbations of the six investigated detectors in the proton fields and correct the measurements. A signal theoretical analysis in Fourier space of the dose distributions and detector's K(x,y) was performed to aid the understanding of the measurement process with regard to the combination of detector choice and delivery technique. RESULTS: Quantification of the lateral penumbra broadening and signal reduction at the fields center revealed that measurements in the pencil beam scanning fields are only compromised slightly even by large volume ionization chambers with maximum differences in the lateral penumbra of 0.25 mm and 4% signal reduction at the field center. In contrast, radiation techniques with collimation are not accurately represented by the investigated detectors as indicated by a penumbra broadening up to 1.6 mm for passive scattering with collimators and 2.2 mm for pencil beam scanning with collimators. For a 3 mm diameter collimator field, a signal reduction at field center between 7.6% and 60.7% was asserted. Lateral dose profile measurements have been corrected via deconvolution with the corresponding K(x,y) to obtain the undisturbed D(x,y). Corrected output ratios of the passively scattered collimated fields obtained for the microDiamond, microSilicon and PinPoint 3D show agreement better than 0.9% (one standard deviation) for the smallest field size of 3 mm. CONCLUSION: Point detector perturbations in small proton fields created with three delivery techniques were quantified and found to be especially pronounced for collimated small proton fields with steep dose gradients. Among all investigated detectors, the microSilicon diode showed the smallest perturbations. The correction strategies based on detector's K(x,y) were found suitable for obtaining unperturbed lateral dose profiles and output factors. Approximation of K(x,y) by considering only the geometrical averaging effect has been shown to provide reasonable prediction of the detector's volume effect. The findings of this work may be used to guide the choice of point detectors in various proton fields and to contribute toward the development of a code of practice for small field proton dosimetry.

Optimization of proton pencil beam positioning in collimated fields.

BACKGROUND: The addition of static or dynamic collimator systems to the pencil beam scanning delivery technique increases the number of options for lateral field shaping. The collimator shape needs to be optimized together with the intensity modulation of spots. PURPOSE: To minimize the proton field's lateral penumbra by investigating the fundamental relations between spot and collimating aperture edge position. METHODS: Analytical approaches describing the effect of spot position on the resulting spot profile are presented. The theoretical description is then compared with Monte Carlo simulations in TOPAS and in the RayStation treatment planning system, as well as with radiochromic film measurements at a clinical proton therapy facility. In the model, one single spot profile is analyzed for various spot positions in air. Further, irradiation setups in water with different energies, the combination with a range shifter, and two-dimensional proton fields were investigated in silico. RESULTS: The further the single spot is placed beyond the collimating aperture edge ('overscanning'), the sharper the relative lateral dose fall-off and thus the lateral penumbra. Overscanning up to 5 mm $5\,\text{mm}$ reduced the lateral penumbra by about 20% on average after a propagation of 13 cm $13\,\text{cm}$ in air. This benefit from overscanning is first predicted by the analytical proofs and later verified by simulations and measurements. Corresponding analyses in water confirm the benefit in lateral penumbra with spot position optimization as observed theoretically and in air. The combination of spot overscanning with fluence modulation facilitated an additional improvement. CONCLUSIONS: The lateral penumbra of single spots in collimated scanned proton fields can be improved by the method of spot overscanning. This suggests a better sparing of proximal organs at risk in smaller water depths at higher energies, especially in the plateau of the depth dose distribution. All in all, spot overscanning in collimated scanned proton fields offers particular potential in combination with techniques such as fluence modulation or dynamic collimation for optimizing the lateral penumbra to spare normal tissue.

Validating a double Gaussian source model for small proton fields in a commercial Monte-Carlo dose calculation engine.

PURPOSE: The primary fluence of a proton pencil beam exiting the accelerator is enveloped by a region of secondaries, commonly called "spray". Although small in magnitude, this spray may affect dose distributions in pencil beam scanning mode e.g., in the calculation of the small field output, if not modelled properly in a treatment planning system (TPS). The purpose of this study was to dosimetrically benchmark the Monte Carlo (MC) dose engine of the RayStation TPS (v.10A) in small proton fields and systematically compare single Gaussian (SG) and double Gaussian (DG) modeling of initial proton fluence providing a more accurate representation of the nozzle spray. METHODS: The initial proton fluence distribution for SG/DG beam modeling was deduced from two-dimensional measurements in air with a scintillation screen with electronic readout. The DG model was either based on direct fits of the two Gaussians to the measured profiles, or by an iterative optimization procedure, which uses the measured profiles to mimic in-air scan-field factor (SF) measurements. To validate the DG beam models SFs, i.e. relative doses to a 10 × 10 cm2 field, were measured in water for three different initial proton energies (100MeV, 160MeV, 226.7MeV) and square field sizes from 1×1cm2 to 10×10cm2 using a small field ionization chamber (IBA CC01) and an IBA ProteusPlus system (universal nozzle). Furthermore, the dose to the center of spherical target volumes (diameters: 1cm to 10cm) was determined using the same small volume ionization chamber (IC). A comprehensive uncertainty analysis was performed, including estimates of influence factors typical for small field dosimetry deduced from a simple two-dimensional analytical model of the relative fluence distribution. Measurements were compared to the predictions of the RayStation TPS. RESULTS: SFs deviated by more than 2% from TPS predictions in all fields <4×4cm2 with a maximum deviation of 5.8% for SG modeling. In contrast, deviations were smaller than 2% for all field-sizes and proton energies when using the directly fitted DG model. The optimized DG model performed similarly except for slightly larger deviations in the 1×1cm2 scan-fields. The uncertainty estimates showed a significant impact of pencil beam size variations (±5%) resulting in up to 5.0% standard uncertainty. The point doses within spherical irradiation volumes deviated from calculations by up to 3.3% for the SG model and 2.0% for the DG model. CONCLUSION: Properly representing nozzle spray in RayStation's MC-based dose engine using a DG beam model was found to reduce the deviation to measurements in small spherical targets to below 2%. A thorough uncertainty analysis shows a similar magnitude for the combined standard uncertainty of such measurements.

Commissioning and validation of a novel commercial TPS for ocular proton therapy.

BACKGROUND: Until today, the majority of ocular proton treatments worldwide were planned with the EYEPLAN treatment planning system (TPS). Recently, the commercial, computed tomography (CT)-based TPS for ocular proton therapy RayOcular was released, which follows the general concepts of model-based treatment planning approach in conjunction with a pencil-beam-type dose algorithm (PBA). PURPOSE: To validate RayOcular with respect to two main features: accurate geometrical representation of the eye model and accuracy of its dose calculation algorithm in combination with an Ion Beam Applications (IBA) eye treatment delivery system. METHODS: Different 3D-printed eye-ball-phantoms were fabricated to test the geometrical representation of the corresponding CT-based model, both in orthogonal 2D images for X-ray image overlay and in fundus view overlaid with a funduscopy. For the latter, the phantom was equipped with a lens matching refraction of the human eye. Funduscopy was acquired in a Zeiss Claus 500 camera. Tantalum clips and fiducials attached to the phantoms were localized in the TPS model, and residual deviations to the actual position in X-ray images for various orientations of the phantom were determined, after the nominal eye orientation was corrected in RayOcular to obtain a best overall fit. In the fundus view, deviations between known and displayed distances were measured. Dose calculation accuracy of the PBA on a 0.2 mm grid was investigated by comparing between measured lateral and depth-dose profiles in water for various combinations of range, modulation, and field-size. Ultimately, the modeling of dose distributions behind wedges was tested. A 1D gamma-test was applied, and the lateral and distal penumbra were further compared. RESULTS: Average residuals between model clips and visible clips/fiducials in orthogonal X-ray images were within 0.3 mm, including different orientations of the phantom. The differences between measured distances on the registered funduscopy image in the RayOcular fundus view and the known ground-truth were within 1 mm up to 10.5 mm distance from the posterior pole. No clear benefit projection of either polar mode or camera mode could be identified, the latter mimicking camera properties. Measured dose distributions were reproduced with gamma-test pass-rates of >95% with 2%/0.3 mm for depth and lateral profiles in the middle of spread-out Bragg-peaks. Distal falloff and lateral penumbra were within 0.2 mm for fields without a wedge. For shallow depths, the agreement was worse, reaching pass-rates down to 80% with 5%/0.3 mm when comparing lateral profiles in air. This is caused by low-energy protons from a scatter source in the IBA system not modeled by RayOcular. Dose distributions modified by wedges were reproduced, matching the wedge-induced broadening of the lateral penumbra to within 0.4 mm for the investigated cases and showing the excess dose within the field due to wedge scatter. CONCLUSION: RayOcular was validated for its use with an IBA single scattering delivery nozzle. Geometric modeling of the eye and representation of 2D projections fulfill clinical requirements. The PBA dose calculation reproduces measured distributions and allows explicit handling of wedges, overcoming approximations of simpler dose calculation algorithms used in other systems.

Impact of Sterilization on the Colloidal Stability of Ligand-Free Gold Nanoparticles for Biomedical Applications.

Sterilization is a major prerequisite for the utilization of nanoparticle colloids in biomedicine, a process well examined for particles derived from chemical synthesis although highly underexplored for electrostatically stabilized ligand-free gold nanoparticles (AuNPs). Hence, in this work, we comprehensively examined and compared the physicochemical characteristics of laser-generated ligand-free colloidal AuNPs exposed to steam sterilization and sterile filtration as a function of particle size and mass concentration and obtained physicochemical insight into particle growth processes. These particles exhibit long-term colloidal stability (up to 3 months) derived from electrostatic stabilization without using any ligands or surfactants. We show that particle growth attributed to cluster-based ripening occurs in smaller AuNPs (∼5 nm) following autoclaving, while larger particles (∼10 and ∼30 nm) remain stable. Sterile filtration, as an alternative effective sterilizing approach, has no substantial impact on the colloidal stability of AuNPs, regardless of particle size, although a mass loss of 5-10% is observed. Finally, we evaluated the impact of the sterilization procedures on potential particle functionality in proton therapy, using the formation of reactive oxygen species (ROS) as a readout. In particular, 5 nm AuNPs exhibit a significant loss in activity upon autoclaving, probably dedicated to specific surface area reduction and surface restructuring during particle growth. The filtered analog enhanced the ROS release by up to a factor of ∼2.0, at 30 ppm gold concentration. Our findings highlight the need for carefully adapting the sterilization procedure of ligand-free NPs to the desired biomedical application with special emphasis on particle size and concentration.

Enhancement of Proton Therapy Efficiency by Noble Metal Nanoparticles Is Driven by the Number and Chemical Activity of Surface Atoms.

Proton-based radiotherapy is a modern technique for the treatment of solid tumors with significantly reduced side effects to adjacent tissues. Biocompatible nanoparticles (NPs) with high atomic numbers are known to serve as sensitizers and to enhance treatment efficacy, which is commonly believed to be attributed to the generation of reactive oxygen species (ROS). However, little systematic knowledge is available on how either physical effects due to secondary electron generation or the particle surface chemistry affect ROS production. Thereto, ligand-free colloidal platinum (Pt) and gold (Au) NPs with well-controlled particle size distributions and defined total surface area are proton-irradiated. A fluorescence-based assay is developed to monitor the formation of ROS using terephthalic acid as a cross-effect-free dye. The findings indicate that proton irradiation (PI)-induced ROS formation sensitized by noble metal NPs is driven by the total available particle surface area rather than particle size or mass. Furthermore, a distinctive material effect with Pt being more active than Au is observed which clearly indicates that the chemical reactivity of the NP surface is a main contributor to ROS generation upon PI. These results pave the way towards an in-depth understanding of the NP-induced sensitizing effects upon PI and hence a well-controlled enhanced therapy.

Technical note: Providing proton fields down to the few-MeV level at clinical pencil beam scanning facilities for radiobiological experiments.

PURPOSE: The adequate performance of radiobiological experiments using clinical proton beams typically requires substantial preparations to provide the appropriate setup for specific experiments. Providing radiobiologically interesting low-energy protons is a particular challenge, due to various physical effects that become more pronounced with larger absorber thickness and smaller proton energy. This work demonstrates the generation of decelerated low-energy protons from a clinical proton beam. METHODS: Monte Carlo simulations of proton energy spectra were performed for energy absorbers with varying thicknesses to reduce the energy of the clinical proton beam down to the few-MeV level corresponding to µ m-ranges. In this way, a setup with an optimum thickness of the absorber with a maximum efficiency of the proton fluence for the provisioning of low-energy protons is supposed to be found. For the specific applications of 2.5-3.3 MeV protons and α -particle range equivalent protons, the relative depth dose was measured and simulated together with the dose-averaged linear energy transfer (LETd) distribution. RESULTS: The resulting energy spectra from Monte Carlo simulations indicate an optimal absorber thickness for providing low-energy protons with maximum efficiency of proton fluence at an user-requested energy range for experiments. For instance, providing energies lower than 5 MeV, an energy spectrum with a relative total efficiency of 38.6 % to the initial spectrum was obtained with the optimal setup. The measurements of the depth dose, compared to the Monte Carlo simulations, showed that the dosimetry of low-energy protons works and protons with high LETd down to the range of α -particles can be produced. CONCLUSIONS: This work provides a method for generating all clinically and radiobiologically relevant energies - especially down to the few-MeV level - at one clinical facility with pencil beam scanning. Thereby, it enables radiobiological experiments under environmentally uniform conditions.

Model-based comparison of organ at risk protection between VMAT and robustly optimised IMPT plans.

The comparison between intensity-modulated proton therapy (IMPT) and volume-modulated arc therapy (VMAT) plans, based on models of normal tissue complication probabilities (NTCP), can support the choice of radiation modality. IMPT irradiation plans for 50 patients with head and neck tumours originally treated with photon therapy have been robustly optimised against density and setup uncertainties. The dose distribution has been calculated with a Monte Carlo (MC) algorithm. The comparison of the plans was based on dose-volume parameters in organs at risk (OARs) and NTCP-calculations for xerostomia, sticky saliva, dysphagia and tube feeding using Langendijk's model-based approach. While the dose distribution in the target volumes is similar, the IMPT plans show better protection of OARs. Therefore, it is not the high dose confirmation that constitutes the advantage of protons, but it is the reduction of the mid-to-low dose levels compared to photons. This work investigates to what extent the advantages of proton radiation are beneficial for the patient's post-therapeutic quality of life (QoL). As a result, approximately one third of the patients examined benefit significantly from proton therapy with regard to possible late side effects. Clinical data is needed to confirm the model-based calculations.