Automated determination of the ion-recombination correction factor (ksat) in ultra-high dose rate electron radiation therapy.

BACKGROUND: Plane-parallel ionization chambers are the recommended secondary standard systems for clinical reference dosimetry of electrons. Dosimetry in high dose rate and dose-per-pulse (DPP) is challenging as ionization chambers are subject to ion recombination, especially when dose rate and/or DPP is increased beyond the range of conventional radiotherapy. The lack of universally accepted models for correction of ion recombination in UDHR is still an issue as it is, especially in FLASH-RT research, which is crucial in order to be able to accurately measure the dose for a wide range of dose rates and DPPs. PURPOSE: The objective of this study was to show the feasibility of developing an Artificial Intelligence model to predict the ion-recombination factor-ksat for a plane-parallel Advanced Markus ionization chamber for conventional and ultra-high dose rate electron beams based on machine parameters. In addition, the predicted ksat of the AI model was compared with the current applied analytical models for this correction factor. METHODS: A total number of 425 measurements was collected with a balanced variety in machine parameter settings. The specific ksat values were determined by dividing the output of the reference dosimeter (optically stimulated luminescence [OSL]) by the output of the AM chamber. Subsequently, a XGBoost regression model was trained, which used the different machine parameters as input features and the corresponding ksat value as output. The prediction accuracy of this regression model was characterized by R2-coefficient of determination, mean absolute error and root mean squared error. In addition, the model was compared with the Two-Voltage (TVA) method and empirical Petersson model for 19 different dose-per-pulse values ranging from conventional to UDHR regimes. The Akiake Information criterion (AIC) was calculated for the three different models. RESULTS: The XGBoost regression model reached a R2-score of 0.94 on the independent test set with a MAE of 0.067 and RMSE of 0.106. For the additional 19 random data points, the ksat values predicted by the XGBoost model showed to be in agreement, within the uncertainties, with the ones determined by the Petersson model and better than the TVA method for doses per pulse >3.5 Gy with a maximum deviation from the ground truth of 14.2%, 16.7%, and -36.0%, respectively, for DPP >4 Gy. CONCLUSION: The proposed method of using AI for ksat determination displays efficiency. For the investigated DPPs, the ksat values obtained with the XGBoost model were in concurrence with the ones obtained with the current available analytical models within the boundaries of uncertainty, certainly for the DPP characterizing UDHR. But the overall performance of the AI model, taking the number of free parameters into account, lacked efficiency. Future research should optimize the determination of the experimental ksat, and investigate the determination the ksat for DPPs higher than the ones investigated in this study, while also evaluating the prediction of the proposed XGBoost model for UDHR machines of different centers.

Multi-institutional generalizability of a plan complexity machine learning model for predicting pre-treatment quality assurance results in radiotherapy.

Background and purpose: Treatment plans in radiotherapy are subject to measurement-based pre-treatment verifications. In this study, plan complexity metrics (PCMs) were calculated per beam and used as input features to develop a predictive model. The aim of this study was to determine the robustness against differences in machine type and institutional-specific quality assurance (QA). Material and methods: A number of 567 beams were collected, where 477 passed and 90 failed the pre-treatment QA. Treatment plans of different anatomical regions were included. One type of linear accelerator was represented. For all beams, 16 PCMs were calculated. A random forest classifier was trained to distinct between acceptable and non-acceptable beams. The model was validated on other datasets to investigate its robustness. Firstly, plans for another machine type from the same institution were evaluated. Secondly, an inter-institutional validation was conducted on three datasets from different centres with their associated QA. Results: Intra-institutionally, the PCMs beam modulation, mean MLC gap, Q1 gap, and Modulation Complexity Score were the most informative to detect failing beams. Eighty-tree percent of the failed beams (15/18) were detected correctly. The model could not detect over-modulated beams of another machine type. Inter-institutionally, the model performance reached higher accuracy for centres with comparable equipment both for treatment and QA as the local institute. Conclusions: The study demonstrates that the robustness decreases when major differences appear in the QA platform or in planning strategies, but that it is feasible to extrapolate institutional-specific trained models between centres with similar clinical practice. Predictive models should be developed for each machine type.

Long-Term Results of a Phase 1 Dose Escalation Trial of Ablative Stereotactic Body Radiation Therapy.

PURPOSE: Stereotactic body radiation therapy is increasingly used for oligometastatic disease as well as palliation, but treatment protocols for nonspine bone and nodal metastases are lacking, with a wide variety of schedules applied. METHODS AND MATERIALS: A prospective dose-escalation trial was initiated, involving 90 patients, among whom 52 (58%) had primary prostate tumors, 13 had breast tumors (14%), and 25 (28%) had other primary tumor types. All visible lymph node or nonspine bone oligometastases were treated in 3 consecutive cohorts: 5 × 7.0 Gy, 3 × 10.0 Gy, or 1 × 20.0 Gy. RESULTS: Initial results revealed no dose-limiting toxicity after a median follow-up of 17.2 months. This update provides information on long-term toxicity, local failure (LF), and progression-free survival (PFS). After a median follow-up of 50 months, no new safety signals were observed. Grade 2 toxicity was 13%, 7% and 10% in the respective cohorts (P = .9), without grade 3 to 5 toxicities. LF rates were 9%, 3%, and 6% (P = .5) for the respective treatment groups, with an overall cumulative risk of LF of 7% (95% CI, 2-12) at 4 years. Median PFS was 16.5 months (95% CI, 9.8-21.5), and 4-year PFS was 21% (95% CI, 14-32). Median overall survival across groups was not reached (95% CI, 52.8 - not reached), 4-year OS was 68% (95% CI, 59-78). A subset of patients (23%) remained long-term disease-free, 37% had oligoprogressive disease at first recurrence and 40% developed polymetastatic relapse. CONCLUSIONS: The safe and effective use of dose-escalated single-fraction stereotactic body radiation therapy for bone and lymph node metastases is supported by this trial, especially considering patient-convenience and cost-effectiveness. Caution is needed when generalizing these outcomes beyond breast and prostate cancer, given their underrepresentation in our study.

Optically stimulated luminescence system as an alternative for radiochromic film for 2D reference dosimetry in UHDR electron beams.

Radiotherapy is part of the treatment of over 50% of cancer patients. Its efficacy is limited by the radiotoxicity to the healthy tissue. FLASH-RT is based on the biological effect that ultra-high dose rates (UHDR) and very short treatment times strongly reduce normal tissue toxicity, while preserving the anti-tumoral effect. Despite many positive preclinical results, the translation of FLASH-RT to the clinic is hampered by the lack of accurate dosimetry for UHDR beams. To date radiochromic film is commonly used for dose assessment but has the drawback of lengthy and cumbersome read out procedures. In this work, we investigate the equivalence of a 2D OSL system to radiochromic film dosimetry in terms of dose rate independency. The comparison of both systems was done using the ElectronFlash linac. We investigated the dose rate dependence by variation of the (1) modality, (2) pulse repetition frequency, (3) pulse length and (4) source to surface distance. Additionally, we compared the 2D characteristics by field size measurements. The OSL calibration showed transferable between conventional and UHDR modality. Both systems are equally independent of average dose rate, pulse length and instantaneous dose rate. The OSL system showed equivalent in field size determination within 3 sigma. We show the promising nature of the 2D OSL system to serve as alternative for radiochromic film in UHDR electron beams. However, more in depth characterization is needed to assess its full potential.

A geometry and dose-volume based performance monitoring of artificial intelligence models in radiotherapy treatment planning for prostate cancer.

Background and Purpose: Clinical Artificial Intelligence (AI) implementations lack ground-truth when applied on real-world data. This study investigated how combined geometrical and dose-volume metrics can be used as performance monitoring tools to detect clinically relevant candidates for model retraining. Materials and Methods: Fifty patients were analyzed for both AI-segmentation and planning. For AI-segmentation, geometrical (Standard Surface Dice 3 mm and Local Surface Dice 3 mm) and dose-volume based parameters were calculated for two organs (bladder and anorectum) to compare AI output against the clinically corrected structure. A Local Surface Dice was introduced to detect geometrical changes in the vicinity of the target volumes, while an Absolute Dose Difference (ADD) evaluation increased focus on dose-volume related changes. AI-planning performance was evaluated using clinical goal analysis in combination with volume and target overlap metrics. Results: The Local Surface Dice reported equal or lower values compared to the Standard Surface Dice (anorectum: (0.93 ± 0.11) vs (0.98 ± 0.04); bladder: (0.97 ± 0.06) vs (0.98 ± 0.04)). The ADD metric showed a difference of (0.9 ± 0.8)Gy for the anorectum D1cm3. The bladder D5cm3 reported a difference of (0.7 ± 1.5)Gy. Mandatory clinical goals were fulfilled in 90 % of the DLP plans. Conclusions: Combining dose-volume and geometrical metrics allowed detection of clinically relevant changes, applied to both auto-segmentation and auto-planning output and the Local Surface Dice was more sensitive to local changes compared to the Standard Surface Dice. This monitoring is able to evaluate AI behavior in clinical practice and allows candidate selection for active learning.

The use of in-vivo dosimetry to identify head and neck cancer patients needing adaptive radiotherapy.

BACKGROUND AND PURPOSE: Head and neck cancer (HNC) patients experiencing anatomical changes during their radiotherapy (RT) course may benefit from adaptive RT (ART). We investigated the sensitivity of an electronic portal imaging device (EPID)-based in-vivo dosimetry (EIVD) system to detect patients that require ART and identified its limitations. MATERIALS AND METHODS: A retrospective study was conducted for 182 HNC patients: laryngeal cancer without elective lymph nodes (group A), postoperative RT (group B) and primary RT including elective lymph nodes (group C). The effect of anatomical changes on the dose distribution and volumetric changes was quantified. The receiver operating characteristic curve was used to obtain the optimal cut-off value for the gamma passing rate (%GP) with a dose difference of 3% and a distance to agreement of 3 mm. RESULTS: Fifty HNC patients receiving ART were analyzed: 1 in group A, 10 in group B and 39 in group C. Failed fractions (FFs) occurred in 1/1, 6/10 and 23/39 cases before ART in group A, B and C respectively. In the four cases in group B without FFs, only minor dosimetric changes were observed. One of the cases in group C without FFs had significant dosimetric changes (false negative). Three cases received ART because of clinical reasons that cannot be detected by EIVD. The optimal cut-off value for the %GP was 95%/95.2% for old/new generation machines respectively. CONCLUSION: EIVD combined with 3D imaging techniques can be synergistic in the detection of anatomical changes in HNC patients who benefit from ART.

Assessing the impact of adaptations to the clinical workflow in radiotherapy using transit in vivo dosimetry.

Background and Purpose: Currently in-vivo dosimetry (IVD) is primarily used to identify individual patient errors in radiotherapy. This study investigated possible correlations of observed trends in transit IVD results, with adaptations to the clinical workflow, aiming to demonstrate the possibility of using the bulk data for continuous quality improvement. Materials and methods: In total 84,100 transit IVD measurements were analyzed of all patients treated between 2018 and 2022, divided into four yearly periods. Failed measurements (FM) were divided per pathology and into four categories of causes of failure: technical, planning and positioning problems, and anatomic changes. Results: The number of FM due to patient related problems gradually decreased from 9.5% to 6.6%, 6.1% and 5.6% over the study period. FM attributed to positioning problems decreased from 10.0% to 4.9% in boost breast cancer patients after introduction of extra imaging, from 9.1% to 3.9% in Head&Neck patients following education of radiation therapists on positioning of patients' shoulders, from 6.1% to 2.8% in breast cancer patients after introduction of ultrahypofractionated breast radiotherapy with daily online pre-treatment imaging and from 11.2% to 4.3% in extremities following introduction of immobilization with calculated couch parameters and a Surface Guided Radiation Therapy solution. FM related to anatomic changes decreased from 10.2% to 4.0% in rectum patients and from 6.7% to 3.3% in prostate patients following more patient education from dieticians. Conclusions: Our study suggests that IVD can be a powerful tool to assess the impact of adaptations to the clinical workflow and its use for continuous quality improvement.

Parameterisation of Respiratory Impedance in Lung Cancer Patients From Forced Oscillation Lung Function Test.

OBJECTIVE: This study aims to analyze the contribution and application of forced oscillation technique (FOT) devices in lung cancer assessment. Two devices and corresponding methods can be feasible to distinguish among various degrees of lung tissue heterogeneity. METHODS: The outcome respiratory impedance Zrs (in terms of resistance Rrs and reactance Xrs) is calculated for FOT and is interpreted in physiological terms by being fitted with a fractional-order impedance mathematical model (FOIM). The non-parametric data obtained from the measured signals of pressure and flow is correlated with an analogous electrical model to the respiratory system resistance, compliance, and elastance. The mechanical properties of the lung can be captured through Gr to define the damping properties and Hr to describe the elastance of the lung tissue, their ratio representing tissue heterogeneity ηr. RESULTS: We validated our hypotheses and methods in 17 lung cancer patients where we showed that FOT is suitable for non-invasively measuring their respiratory impedance. FOIM models are efficient in capturing frequency-dependent impedance value variations. Increased heterogeneity and structural changes in the lungs have been observed. The results present inter- and intra-patient variability for the performed measurements. CONCLUSION: The proposed methods and assessment of the respiratory impedance with FOT have been demonstrated useful for characterizing mechanical properties in lung cancer patients. SIGNIFICANCE: This correlation analysis between the measured clinical data motivates the use of the FOT devices in lung cancer patients for diagnosis of lung properties and follow-up of the respiratory function modified due to the applied radiotherapy treatment.

Lung-Heart Outcomes and Mortality through the 2020 COVID-19 Pandemic in a Prospective Cohort of Breast Cancer Radiotherapy Patients.

We investigated lung-heart toxicity and mortality in 123 women with stage I-II breast cancer enrolled in 2007-2011 in a prospective trial of adjuvant radiotherapy (TomoBreast). We were concerned whether the COVID-19 pandemic affected the outcomes. All patients were analyzed as a single cohort. Lung-heart status was reverse-scored as freedom from adverse-events (fAE) on a 1-5 scale. Left ventricular ejection fraction (LVEF) and pulmonary function tests were untransformed. Statistical analyses applied least-square regression to calendar-year aggregated data. The significance of outliers was determined using the Dixon and the Grubbs corrected tests. At 12.0 years median follow-up, 103 patients remained alive; 10-years overall survival was 87.8%. In 2007-2019, 15 patients died, of whom 11 were cancer-related deaths. In 2020, five patients died, none of whom from cancer. fAE and lung-heart function declined gradually over a decade through 2019, but deteriorated markedly in 2020: fAE dipped significantly from 4.6-4.6 to 4.3-4.2; LVEF dipped to 58.4% versus the expected 60.3% (PDixon = 0.021, PGrubbs = 0.054); forced vital capacity dipped to 2.4 L vs. 2.6 L (PDixon = 0.043, PGrubbs = 0.181); carbon-monoxide diffusing capacity dipped to 12.6 mL/min/mmHg vs. 15.2 (PDixon = 0.008, PGrubbs = 0.006). In conclusion, excess non-cancer mortality was observed in 2020. Deaths in that year totaled one-third of the deaths in the previous decade, and revealed observable lung-heart deterioration.

Characterisation and Quenching Correction for an Al2O3:C Optical Fibre Real Time System in Therapeutic Proton, Helium, and Carbon-Charged Beams.

Real time radioluminescence fibre-based detectors were investigated for application in proton, helium, and carbon therapy dosimetry. The Al2O3:C probes are made of one single crystal (1 mm) and two droplets of micro powder in two sizes (38 µm and 4 µm) mixed with a water-equivalent binder. The fibres were irradiated behind different thicknesses of solid slabs, and the Bragg curves presented a quenching effect attributed to the nonlinear response of the radioluminescence (RL) signal as a function of linear energy transfer (LET). Experimental data and Monte Carlo simulations were utilised to acquire a quenching correction method, adapted from Birks' formulation, to restore the linear dose-response for particle therapy beams. The method for quenching correction was applied and yielded the best results for the '4 µm' optical fibre probe, with an agreement at the Bragg peak of 1.4% (160 MeV), and 1.5% (230 MeV) for proton-charged particles; 2.4% (150 MeV/u) for helium-charged particles and of 4.8% (290 MeV/u) and 2.9% (400 MeV/u) for the carbon-charged particles. The most substantial deviations for the '4 µm' optical fibre probe were found at the falloff regions, with ~3% (protons), ~5% (helium) and 6% (carbon).

A new calculation method for the free electron fraction of an ionization chamber in the ultra-high-dose-per-pulse regimen.

The free electron fraction is the fraction of electrons, produced inside the cavity of an ionization chamber after irradiation, which does not bind to gas molecules and thereby reaches the electrode as free electrons. It is a fundamental quantity to describe the recombination processes of an ionization chamber, as it generates a gap of positive charges compared to negative ones, which certainly will not undergo recombination. The free electron fraction depends on the specific chamber geometry, the polarizing applied voltage and the gas thermodynamic properties. Therefore, it is necessary to evaluate such fraction in an accurate and easy way for any measurement condition. In this paper, a simple and direct method for evaluating the free electron fraction of ionization chambers is proposed. We first model the capture process of the electrons produced inside an ionization chamber after the beam pulse; then we present a method to evaluate the free electron fraction based on simple measurements of collected charge, by varying the applied voltage. Finally, the results obtained using an Advanced Markus chamber irradiated with a Flash Radiotherapy dedicated research Linac (ElectronFlash) to estimate the free electron fraction are presented. The proposed method allows the use of a conventional ionization chamber for measurements in ultra-high-dose-per-pulse (UHDP) conditions, up to values of dose-per-pulse at which the perturbation of the electric field due to the generated charge can be considered negligible.

Quality Assurance for AI-Based Applications in Radiation Therapy.

Recent advancements in artificial intelligence (AI) in the domain of radiation therapy (RT) and their integration into modern software-based systems raise new challenges to the profession of medical physics experts. These AI algorithms are typically data-driven, may be continuously evolving, and their behavior has a degree of (acceptable) uncertainty due to inherent noise in training data and the substantial number of parameters that are used in the algorithms. These characteristics request adaptive, and new comprehensive quality assurance (QA) approaches to guarantee the individual patient treatment quality during AI algorithm development and subsequent deployment in a clinical RT environment. However, the QA for AI-based systems is an emerging area, which has not been intensively explored and requires interactive collaborations between medical doctors, medical physics experts, and commercial/research AI institutions. This article summarizes the current QA methodologies for AI modules of every subdomain in RT with further focus on persistent shortcomings and upcoming key challenges and perspectives.

Point scintillator dosimetry in ultra-high dose rate electron "FLASH" radiation therapy: A first characterization.

FLASH radiation therapy is a novel technique combining ultra-high dose rates (UHDR) with very short treatment times to strongly decrease normal tissue toxicity while preserving the anti-tumoral effect. However, the radiobiological mechanisms and exact conditions for obtaining the FLASH-effect are still under investigation. There are strong indications that parameters defining the beam structure, such as dose per pulse, instantaneous dose rate and pulse repetition frequency (PRF) are of importance. UHDR irradiations therefore come with dosimetric challenges, including both dose assessment and temporal ones. In this work, a first characterization of 6 real-time point scintillating dosimeters with 5 phosphors (Al2O3:C,Mg; Y2O3:Eu; Al2O3:C; (C38H34P2)MnBr4 and (C38H34P2)MnCl4, was performed in an UHDR pulsed electron beam. The dose rate independence of the calibration was tested by calibrating the detector at conventional and UHDR. Dose rate dependence was observed, however, further investigation, including intermediate dose rates, is needed. Linearity of the response with dose was tested by varying the number of pulses and a linearity with R2> 0.9989 was observed up to at least 200 Gy. Dose per pulse linearity was investigated by variation of the pulse length and SSD. All point scintillators showed saturation effects up to some extent and the instantaneous dose rate dependence was confirmed. A PRF dependence was observed for the Al2O3:C,Mg and Al2O3:C- based point scintillators. This was expected as the luminescence decay time of these materials exceeds the inter-pulse time.

Postgraduate education in radiation oncology during the COVID-19 pandemic - What did we learn?

Introduction: During the COVID-19 pandemic the ESTRO School who provides international non-profit postgraduate education in Radiation Oncology and related disciplines, including Medical Physics and Radiation Technology, had to close down all live educational activities and turn online, although having only limited experience. The paper describes the experience, discusses the limitations and benefits of online education and suggests directions for the future. Materials and methods: Data about format and feedback from attendees and faculty members from the course activities held in 2019, 2020 and 2021 were made available from the ESTRO School. Results: In 2020, all but two out of thirty live courses that happened before the lockdown were canceled. Among the 18 courses scheduled in the second half of the year, seven went online with a short notice. Each course planned their activities quite differently, from compressed courses with consecutive full days online program to courses over several weeks with a few hours online a week. Both numbers of participants and different nationalities were higher than live courses in 2019 for the seven courses happening online, and courses were well evaluated by participants and faculties. Roughly-one-third of participants would prefer online courses in the future. Discussion: Although online education was well received by the majority, pros and cons exist and especially the personal discussions and networking were missed. Online education and live education are not comparable but can complement each other. Careful balancing these activities in the future is important and strategies for online andragogy are needed.

Machine learning-based treatment couch parameter prediction in support of surface guided radiation therapy.

Purpose: A fully independent, machine learning-based automatic treatment couch parameters prediction was developed to support surface guided radiation therapy (SGRT)-based patient positioning protocols. Additionally, this approach also acts as a quality assurance tool for patient positioning. Materials/Methods: Setup data of 183 patients, divided into four different groups based on used setup devices, was used to calculate the difference between the predicted and the acquired treatment couch value. Results: Couch parameters can be predicted with high precision µ = 0.90 , σ = 0.92 . A significant difference (p < 0.01) between the variances of Lung and Brain patients was found. Outliers were not related to the prediction accuracy, but are due to inconsistencies during initial patient setup. Conclusion: Couch parameters can be predicted with high accuracy and can be used as starting point for SGRT-based patient positioning. In case of large deviations (>1.5 cm), patient setup has to be verified to optimally use the surface scanning system.

A new solution for UHDP and UHDR (Flash) measurements: Theory and conceptual design of ALLS chamber.

Ultra-High dose-per-pulse regimens (UHDP), necessary to trigger the "FLASH" effect, still pose serious challenges to dosimetry. Dosimetry plays a crucial role, both to significantly improve the accuracy of the radiobiological experiments necessary to fully understand the mechanisms underlying the effect and its dependencies on the beam parameters, and to be able to translate such effect into clinical practice. The standard ionization chamber in UHDP region is significantly affected by the effects of the electric field generated by the enormous density of charges produced by the dose pulse. This work describes the theory and the conceptual design of a gas chamber (the ALLS chamber) which overcomes the above-mentioned problems.

Application of a novel diamond detector for commissioning of FLASH radiotherapy electron beams.

PURPOSE: A diamond detector prototype was recently proposed by Marinelli et al. (Medical Physics 2022, https://doi.org/10.1002/mp.15473) for applications in ultrahigh-dose-per-pulse (UH-DPP) and ultrahigh-dose-rate (UH-DR) beams, as used in FLASH radiotherapy (FLASH-RT). In the present study, such so-called flashDiamond (fD) was investigated from the dosimetric point of view, under pulsed electron beam irradiation. It was then used for the commissioning of an ElectronFlash linac (SIT S.p.A., Italy) both in conventional and UH-DPP modalities. METHODS: Detector calibration was performed in reference conditions, under 60 Co and electron beam irradiation. Its response linearity was investigated in UH-DPP conditions. For this purpose, the DPP was varied in the 1.2-11.9 Gy range, by changing either the beam applicator or the pulse duration from 1 to 4 µs. Dosimetric validation of the fD detector prototype was then performed in conventional modality, by measuring percentage depth dose (PDD) curves, beam profiles, and output factors (OFs). All such measurements were carried out in a motorized water phantom. The obtained results were compared with the ones from commercially available dosimeters, namely, a microDiamond, an Advanced Markus ionization chamber, a silicon diode detector, and EBT-XD GAFchromic films. Finally, the fD detector was used to fully characterize the 7 and 9 MeV UH-DPP electron beams delivered by the ElectronFlash linac. In particular, PDDs, beam profiles, and OFs were measured, for both energies and all the applicators, and compared with the ones from EBT-XD films irradiated in the same experimental conditions. RESULTS: The fD calibration coefficient resulted to be independent from the investigated beam qualities. The detector response was found to be linear in the whole investigated DPP range. A very good agreement was observed among PDDs, beam profiles, and OFs measured by the fD prototype and reference detectors, both in conventional and UH-DPP irradiation modalities. CONCLUSIONS: The fD detector prototype was validated from the dosimetric point of view against several commercial dosimeters in conventional beams. It was proved to be suitable in UH-DPP and UH-DR conditions, for which no other commercial real-time active detector is available to date. It was shown to be a very useful tool to perform fast and reproducible beam characterizations in standard clinical motorized water phantom setups. All of the previously mentioned demonstrate the suitability of the proposed detector for the commissioning of UH-DR linac beams for preclinical FLASH-RT applications.

Precision of image-guided spinal stereotactic ablative radiotherapy and impact of positioning variables.

Background and purpose: Spinal stereotactic ablative body radiotherapy (SABR) requires high precision. We evaluate the intrafraction motion during cone-beam computed tomography (CBCT) guided SABR with different immobilization techniques. Material and methods: Fifty-seven consecutive patients were treated for 62 spinal lesions with SABR with positioning corrected in six degrees of freedom. A surface monitoring system was used for patient set up and to ensure patient immobilization in 65% of patients. Intrafractional motion was defined as the difference between the last CBCT before the start of treatment and the first CT afterwards. Results: For all 194 fractions, the mean intrafractional motion was 0.1 cm (0-1.1 cm) in vertical direction, 0.1 cm (0-1.1 cm) in longitudinal direction and 0.1 cm (0-0.5 cm) in lateral direction. A mean pitch of 0.6° (0-4.3°), a roll of 0.5° (0-3.4°) and a rotational motion of 0.4° (0-3.9°) was observed. 95.5% of the translational errors and 95.4% of the rotational errors were within safety range. There was a significantly higher rotational motion for patients with arms along the body (p = 0.01) and without the use of the body mask (p = 0.05). For cervical locations a higher rotational motion was seen, although not significant (p = 0.1). The acquisition of an extra CBCT was correlated with a higher rotational (pitch) motion (p = 0 < 0.01). Conclusion: Very high precision in CBCT guided and surface-guided spinal SABR was observed in this cohort. The lowest intrafraction motion was seen in patients treated with arms above their head and a body mask. The use of IGRT with surface monitoring is an added value for patient monitoring leading to treatment interruption if necessary.

Comparing treatment uncertainty for ultra- vs. standard-hypofractionated breast radiation therapy based on in-vivo dosimetry.

Background and purpose: Postoperative ultrahypofractionated radiation therapy (UHFRT) in 5 fractions (fx) for breast cancer patients is as effective and safe as conventionally hypofractionated RT (HFRT) in 15 fx, liberating time for higher-level daily online Image-Guided Radiation Therapy (IGRT) corrections. In this retrospective study, treatment uncertainties occurring in patients treated with 5fx (5fx-group) were evaluated using electronic portal imaging device (EPID)-based in-vivo dosimetry (EIVD) and compared with the results from patients treated with conventionally HFRT (15fx-group) to validate the new technique and to evaluate if the shorter treatment schedule could have a positive effect on the treatment uncertainties. Materials and methods: EPID-based integrated transit dose images were acquired for each treatment fraction in the 5fx-group (203 patients) and on the first 3 days of treatment and weekly thereafter in the 15fx-group (203 patients). A total of 1015 EIVD measurements in the 5fx-group and 1144 in the 15fx-group were acquired. Of the latter group, 755 had been treated with online IGRT correction (i.e., Online-IGRT 15fx-group). Results: In the 15fx-group 12.0% of fractions failed (FFs) compared to 3.8% in the 5fx-group and 6.9% in the online-IGRT 15fx-group. Causes for FFs in the 15fx-group compared with the 5fx-group were patient positioning (7.4% vs. 2.2%), technical issues (3.1% vs. 1.2%) and breast swelling (1.4% vs. 0.5%). In the online-IGRT 15fx-group, 2.5% were attributed to patient positioning, 3.8% to technical issues and 0.5% to breast swelling. Conclusions: EIVD demonstrated that UHFRT for breast cancer results in less FFs compared to standard HFRT. A large proportion of this decrease could be explained by using daily online IGRT.