As radiotherapy using ultra-high dose rates has gained new interest, the dosimetric challenges arising at these conditions needs to be addressed. Ionization chambers suffer from a large decrease in ion collection efficiency due to ion recombination, making on-line dosimetry difficult. In this work we present experimental setups and dosimetric procedures for FLASH irradiation of cells, zebrafish embryos and small animals using a 10 MeV electron beam at a modified clinical linear accelerator, and describe the dosimetric steps required to initiate clinical trials. The dosimetric equipment used for our pre-clinical experiments consisted of radiochromic film, thermoluminescent dosimeters, a Farmer-type ionization chamber and phantom material mimicking the experimental setup for irradiation. In preparation for small animal irradiation, dose profiles and depth dose curves were measured for all collimator sizes. The average dose rates were ≥620 Gy/s, ≥640 Gy/s and ≥400 Gy/s for cells, zebrafish embryos and small animals, respectively.

Linac calibration is done in water, but patients are comprised primarily of soft tissue. Conceptually, and specified in NRG/RTOG trials, dose should be reported as dose-to-muscle to describe the dose to the patient. Historically, the dose-to-water of the linac calibration was often converted to dose-to-muscle for patient calculations through manual application of a 0.99 dose-to-water to dose-to-muscle correction factor, applied during the linac clinical reference calibration. However, many current treatment planning system (TPS) dose calculation algorithms approximately provide dose-to-muscle (tissue), making application of a manual scaling unnecessary. There is little guidance on when application of a scaling factor is appropriate, resulting in highly inconsistent application of this scaling by the community. In this report we provide guidance on the steps necessary to go from the linac absorbed dose-to-water calibration to dose-to-muscle in patient, for various commercial TPS algorithms. If the TPS does not account for the difference between dose-to-water and dose-to-muscle, then TPS reference dose scaling is warranted. We have tabulated the major vendors' TPS in terms of whether they approximate dose-to-muscle or calculate dose-to-water and recommend the correction factor required to report dose-to-muscle directly from the TPS algorithm. Physicists should use this report to determine the applicable correction required for specifying the reference dose in their TPS to achieve this goal and should remain attentive to possible changes to their dose calculation algorithm in the future.

In the novel and promising radiotherapy technique known as FLASH, ultra-high dose-rate electron beams are used. As a step towards clinical trials, dosimetric advances will be required for accurate dose delivery of FLASH. The purpose of this study was to determine whether a built-in transmission chamber of a clinical linear accelerator can be used as a real-time dosimeter to monitor the delivery of ultra-high-dose-rate electron beams. This was done by modeling the drop-in ion-collection efficiency of the chamber with increasing dose-per-pulse values, so that the ion recombination effect could be considered. The raw transmission chamber signal was extracted from the linear accelerator and its response was measured using radiochromic film at different dose rates/dose-per-pulse values, at a source-to-surface distance of 100 cm. An increase of the polarizing voltage, applied over the transmission chamber, by a factor of 2 and 3, improved the ion-collection efficiency, with corresponding increased efficiency at the highest dose-per-pulse values by a factor 1.4 and 2.2, respectively. The drop-in ion-collection efficiency with increasing dose-per-pulse was accurately modeled using a logistic function fitted to the transmission chamber data. The performance of the model was compared to that of the general theoretical Boag models of ion recombination in ionization chambers. The logistic model was subsequently used to correct for ion recombination at dose rates ranging from conventional to ultra-high, making the transmission chamber useful as a real-time monitor for the dose delivery of FLASH electron beams in a clinical setup.

ObjectivesThe purpose of this study was to modify a clinical linear accelerator, making it capable of electron beam ultra-high dose rate (FLASH) irradiation. Modifications had to be quick, reversible, and without interfering with clinical treatments.MethodsPerformed modifications: (1) reduced distance with three setup positions, (2) adjusted/optimized gun current, modulator charge rate and beam steering values for a high dose rate, (3) delivery was controlled with a microcontroller on an electron pulse level, and (4) moving the primary and/or secondary scattering foils from the beam path.ResultsThe variation in dose for a five-pulse delivery was measured to be 1% (using a diode, 4% using film) during 10 minutes after a warm-up procedure, later increasing to 7% (11% using film). A FLASH irradiation dose rate was reached at the cross-hair foil, MLC, and wedge position, with ≥30, ≥80, and ≥300 Gy/s, respectively. Moving the scattering foils resulted in an increased output of ≥120, ≥250, and ≥1000 Gy/s, at the three positions. The beam flatness was 5% at the cross-hair position for a 20 × 20 and a 10 × 10 cm2 area, with and without both scattering foils in the beam. The beam flatness was 10% at the wedge position for a 6 and 2.5 cm diametric area, with and without the scattering foils in the beam path.ConclusionsA clinical accelerator was modified to produce ultra-high dose rates, high enough for FLASH irradiation. Future work aims to fine-tune the dose delivery, using the on-board transmission chamber signal and adjusting the dose-per-pulse.

BACKGROUND AND PURPOSE:Hypofractionated radiotherapy of prostate cancer reduces the overall treatment time but increases the per-fraction beam-on time due to the higher fraction doses. This increased fraction treatment time results in a larger uncertainty of the prostate position. The purpose of this study was to investigate the effect of prostate motion during flattening filter free (FFF) Volumetric Modulated Arc Therapy (VMAT) in ultrahypofractionation of prostate cancer radiotherapy with preserved plan quality compared to conventional flattened beams. MATERIALS AND METHODS:Nine prostate patients from the Scandinavian HYPO-RT-PC trial were re-planned using VMAT technique with both conventional and flattening filter free beams. Two fractionation schedules were used, one hypofractionated (42.7 Gy in 7 fractions), and one conventional (78.0 Gy in 39 fractions). Pre-treatment verification measurements were performed on all plans and the treatment time was recorded. Measurements with simulated prostate motion were performed for the plans with the longest treatment times. RESULTS:All the 10FFF plans fulfilled the clinical gamma pass rate, 90% (3%, 2 mm), during all simulated prostate motion trajectories. The 10MV plans only fulfilled the clinical pass rate for three of the trajectories. The mean beam-on-time for the hypofractionated plans were reduced from 2.3 min to 1.0 min when using 10FFF compared to 10MV. No clinically relevant differences in dose distribution were identified when comparing the plans with different beam qualities. CONCLUSION:Flattening-filter free VMAT reduces treatment times, limiting the dosimetric effect of organ motion for ultrahypofractionated prostate cancer with preserved plan quality.

commercial motion management surrogate in clinical use in our clinic due to its superior versatility in positioning, improved skin sparing and identical performance.

often underestimated by healthcare professionals.This makes the technique potentially hazardous for patient, surgeons and assistants.A safe and effective use of the technique requires theoretical knowledge and practical skills.In this abstract, we describe the initiative to formulate competence requirements for using electrosurgery that are general applicable and can be used as input for training and examination.Methods.A group of medical physicists located in eight Dutch hospitals used educational methods to analyze the use of electrosurgery in the operation room and determine competence requirements.Additionally, the group investigated resources like the Dutch risk profile for electrosurgery, risk analyses of individual hospitals, supplier manuals, user protocols, (nearly) incident reports and available local training materials.For each requirement, an explanation was given and it was pointed out if the requirement is verifiable in practice.The result was presented to users of electrosurgery such as urologists and surgeons.Furthermore, a member consultation of the association of general medical physicists took place.Outcomes of consultations were used to improve and complete the requirements.Results.The final requirements are clustered in electricity, technology, tissue effect, instruments and their use, settings, minimal invasive surgery, risks (patient electrode patches, alternative current flow, fire, long term use, contamination of electrode) and surgical smoke.It appeared to be important for physicians to have a good perception of the behavior of electrical current in the human body, without elaborating in too much physics and mathematical equations.Best results are obtained applying a combination of theoretical explanation followed by practicing in a skills lab.Conclusions.Safe patient care requires qualified medical staff.The approach which combined physical knowledge and educational methods successfully resulted in competence requirements.These requirements are used by hospitals as input for teaching materials and to state a minimum level of qualification for using electrosurgery safely and effectively in patient care.

The calculation of absorbed dose within patients during external photon beam radiotherapy is reviewed. This includes the modelling of the radiation source i.e. in most cases a linear accelerator (beam modelling) and examples of dose calculation algorithms applied within the patient i.e. the dose engine. For the first part - the beam modelling, the different sources in the treatment head as target, filters and collimators etc are discussed as well as their importance for the photon and electron fluence reaching the patient. The consequences of removing the flattening filter, which several vendors now have made commercially available, is also shown. The pros and cons regarding different dose engines ability to consider density changes within the patient will is covered (type a and b models). Engines covered are, for example, pencil-beam models, collapsed cone superposition/-convolution models and combinations of these, as well as a glimpse on Monte Carlo methods for radiotherapy. The different models' ability to calculate dose to medium (tissue) and or water is. Finally, the role of commissioning data especially measurements in today's model based dose calculation is presented.

The purpose of this report is to review and compile what have been and can be learnt from incidents and accidents in radiation oncology, especially in external beam and brachytherapy. Some major accidents from the last 20 years will be discussed. The relationship between major events and minor or so-called near misses is mentioned, leading to the next topic of exploring the knowledge hidden among them. The main lessons learnt from the discussion here and elsewhere are that a well-functioning and safe radiotherapy department should help staff to work with awareness and alertness and that documentation and procedures should be in place and known by everyone. It also requires that trained and educated staff with the required competences are in place and, finally, functions and responsibilities are defined and well known.

To evaluate the performance of two treatment planning systems (TPS), using radiotherapy beams both with and without flattening filter, for extreme hypo-fractionated prostate cancer treatment. The TPSs were evaluated regarding plan quality and robustness with respect to prostate motion. With hypofractionation, the treatment time per fraction is increased, making prostate motion more significant. Flattening filter free treatments can decrease the treatment time due to the higher dose rate, and therefore reduce the effect of prostate motion. Prostate cancer patients treated with extreme hypofractionation (7x6.1 Gy=42.7 Gy) in the experimental arm of the randomized Scandinavian HYPO-RT-PC trial were used in this study. The TPSs used were Varian Eclipse (Varian Medical Systems) and RayStation (RaySearch Laboratories). The treatment plan comparison was made using the Pareto front method. Several VMAT plans were optimized with varying importance assigned to rectal sparing and PTV coverage objectives. The rectal volume receiving more than 90%, (Rectum, V90%), of the prescribed dose was plotted as a function of the PTV under dosage, (PTV, V95%). Two different beam qualities were used on each TPS, 10MV with flattening filter (FF) and 10MV flattening filter free (FFF). All plans were made for a Varian TrueBeam accelerator. The Pareto optimal plans, where one objective cannot be improved without deteriorating the other, formed a front for each TPS and beam quality. Three plans for each Pareto front were selected for measurement from three different regions in the fronts. These were defined as follows: PTV more important than Rectum (PTV>>Rect), Rectum more important than PTV (Rect>>PTV) and the knee of the Pareto front. The latter is the point where the target coverage starts to drastically decrease without significant improvement in rectum dose. The measurements were performed using the Delta4 Phantom+ (ScandiDos) and motion was simulated using the HexaMotion device with realistic motion patterns. The measurements were evaluated with gamma analysis using the local clinical protocol at our clinic (90% pass rate at 3%/2mm). The Pareto front plan comparison showed very small differences between the two TPSs and beam qualities. Eclipse had slightly better target coverage in the PTV>>Rect region but RayStation had lower rectum doses in the Rect>>PTV region. All plans fulfilled the dose-volume constraints of the trial protocol. However, all fronts showed that the rectum dose could be decreased even more without losing target coverage. The static measurements showed 100% pass rate for all plans. For the in-motion measurements, the FF plans had a pass rate below the clinically acceptable 90% pass rate while the FFF plans were above 90%. The beam-on time was decreased from 2.5 min to 1 min. Both TPSs delivered plans with good quality for both FF and FFF beams. The FFF plans proved to be clinically acceptable even with prostate motion for both TPSs.

We present a clinical distance measure for Pareto front evaluation studies in radiotherapy, which we show strongly correlates (r = 0.74 and 0.90) with clinical plan quality evaluation. For five prostate cases, sub-optimal treatment plans located at a clinical distance value of > 0.32 (0.28-0.35) from fronts of Pareto optimal plans, were assessed to be of lower plan quality by our (12) observers (p < .05). In conclusion, the clinical distance measure can be used to determine if the difference between a front and a given plan (or between different fronts) corresponds to a clinically significant plan quality difference.

BACKGROUND AND PURPOSE:Substantial inter-observer variations in target delineation have been presented previously. Target delineation for paediatric cases is difficult due to the small number of children, the variation in paediatric targets, the number of study protocols, and the individual patient's specific needs and demands. Uncertainties in target delineation might lead to under-dosage or over-dosage. The aim of this work is to apply the concept of a consensus volume and good quality treatment plans to visualise and quantify inter-observer target delineation variations in dosimetric terms in addition to conventional geometrically based volume concordance indices. MATERIAL AND METHODS:Two paediatric cases were used to demonstrate the potential of adding dose metrics when evaluating target delineation diversity; Hodgkin's disease (case 1) and rhabdomyosarcoma of the parotid gland (case 2). The variability in target delineation (PTV delineations) between six centres was quantified using the generalised conformity index, CIgen, generated for volume overlap. The STAPLE algorithm, as implemented in CERR, was used for both cases to derive a consensus volumes. STAPLE is a probabilistic estimate of the true volume generated from all observers. Dose distributions created by each centre for the original target volumes were then applied to this consensus volume. RESULTS:A considerable variation in target segmentation was seen in both cases. For case 1 the variation was 374-960 cm3 (average 669 cm3) and for case 2; 65-126 cm3 (average 109 cm3). CIgen were 0.53 and 0.70, respectively. The DVHs in absolute volume displayed for the delineated target volume as well as for the consensus volume adds information on both "compliant" target volumes as well as outliers which are hidden with just the use of concordance indices. CONCLUSIONS:The DVHs in absolute volume add valuable and easily understood information to various indices for evaluating uniformity in target delineation.

Advanced dose calculation algorithms have demonstrated excellent performance against measurements for complex treatments and heterogeneous phantoms.Thus, it is natural to consider those as the best candidates for treatment planning.Because the dose calculation is more accurate, so will be the treatment and its outcome improved.This seems intuitively obvious.However, a broader view on our clinical practice may temper this conclusion.In our clinical practice, we are using dose prescriptions from past experience that was typically based on less accurate dose calculation algorithms.Also, we are using safety margins for geometrical uncertainties that are based on hypothesis that simplify considerably the physics of dose deposition, but yet seem to provide adequate coverage and safety for the majority of the patients.We will show during this debate that changing the dose calculation algorithm considering our present practice will not necessary have a positive impact for the patients.Therefore, the introduction of such algorithms in clinics should be made cautiously.

________________________________________________________________________________(PTV) was created adding a margin of 5 mm to the ITV .The dose distribution was optimized on the average CT prescribing a dose of 20 Gy per fraction delivering a total dose of 60 Gy to the PTV.The plans were calculated in a Phillips Pinnacle 9.10 planning system using conformal 3DRT and heterogeneity correction.The parameters obtained in the average CT optimized plan, were copied to the different image sets with identical monitor units to analyze the differences. Results:The average GTV volume was 1.6 ± 1.1 cc.The ITV size is twice the lesion size in most of the cases except in those with higher breathing amplitude.The ITVs outlined in the average CT were smaller than those outlined in the 4DCT ranging from 0.1 cc, where there hardly was lesion movement, to 0.6 cc.The differences between the volumes were usually found in the cranio-caudal direction due to the higher movement of the lesion in this direction.The ITVs outlined in the MIP CT were equivalent to the 4DCT except in the cases where there was a higher density organ in the vicinity of the tumor.Respect to dose distribution, the dose of the organs at risk shows no significant differences in the different image sets.The V100 of the ITV presents significant variations up to 15% due to the variation in electron densities depending on the CT mode chosen.The V100 of the GTV calculated in each phase is greater than 97%. Conclusion:We recommend using the ten phases of the 4DCT study for proper delineation of ITV.If the institution does not have the technology the CT average (low pitch CT) could be used selecting the appropriate window level and increasing margins.There is no significant difference in dose to organs at risk between the images modalities studied.Optimized planning in the average CT provides adequate coverage of GTV at different breathing phases.

metric show a somewhat larger influence of the calculation algorithm used compared to the edge area metric. Conclusion:Different dose calculation algorithms can influence on the correlation between aperture-based complexity metric scores and complexity of the treatment field.The impact is different for different metrics.

Dose mimicking allows for the automatic creation and optimization of coplanar/non-coplanar three dimensional conformal radiation therapy (3DCRT), step-and-shoot intensity-modulated radiation therap...

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