Flattening filter-free accelerators: a report from the AAPM Therapy Emerging Technology Assessment Work Group

Recalculation of scatter fractions for homogeneous and heterogeneous geometries using Geant4 Monte Carlo simulations

The primary aim of this study was to determine the scatter fraction from patient, particularly in the presence of patient heterogeneities, using Monte Carlo simulations. The Geant4 toolkit was used to estimate the scatter fractions of 6, 10, 15, and 24 MV circular photon beams (area ~400 cm2). For scatter fraction calculation in a cubic water phantom at 100 cm from a point source, concentric spheres were designed, with the inner sphere radius ~1 m and the outer sphere was either 1.015 or 1.025 cm to allow dose build-up. The scatter fractions were calculated in water and heterogeneous medium (i.e. a slab of either lung, stainless steel, or aluminum) in the range of scattering angles (i.e. 3°-150°). Higher energy beams (i.e. 24 MV) exhibit a rapid fall-off in scatter fraction compared to lower energy beams (i.e. 6 MV). For angles below 35°, higher energy beams have the largest scatter fraction. Beyond 60°, smallest energy beams show the largest scatter fraction. The scatter fraction deviates by up to 48% from published data.

Evaluation of radiation treatment plan quality in head and neck cancer: a comparative analysis of RapidArc technique with flattening filter and flattening filter-free photon beams

AIM

This study aims to evaluate the treatment plan quality for oral cavity cancers in the head and neck region using the RapidArc (RA) technique with both flattening filter (FF) and flattening filter-free (FFF) photon beams.

MATERIALS AND METHODS

In this analytical study, treatment plans for 12 patients originally planned with a 6 MV FF photon beam were recreated using the RA technique with a 6 MV FFF photon beam. Identical beam parameters and planning objectives were maintained for both sets of plans to facilitate comparison. All plans were evaluated based on planning indices and doses to organs at risk (OAR).

RESULTS

A significant dose variation was found in the minimum (Dmin) and mean (Dmean) doses of the high-risk planning target volume between FF and FFF photon beam RA plans. However, the dose distribution for the low-risk planning target volume was equivalent between the two techniques. The FFF-RA plans demonstrated superior conformity and homogeneity indices compared to the FF plans, with these differences being statistically significant. In addition, the FF-RA plans showed higher doses to the parotid glands, eyes and lenses than the FFF plans. The FFF plans also showed significantly shorter beam-on treatment times and a higher gamma passing index rate compared to the FF plans.

CONCLUSION

In contrast to the FF photon beam, an FFF photon beam-oriented RA plan provides significant OAR sparing without losing the quality of the treatment plan. High monitor units and beam on time are major highlights of the RA plan with FFF beam.

Assessing focal spot alignment in clinical linear accelerators: a comprehensive evaluation with triplet phantoms

A fundamental parameter to evaluate the beam delivery precision and stability on a clinical linear accelerator (linac) is the focal spot position (FSP) measured relative to the collimator axis of the radiation head. The aims of this work were to evaluate comprehensive data on FSP acquired on linacs in clinical use and to establish the ability of alternative phantoms to detect effects on patient plan delivery related to FSP. FSP measurements were conducted using a rigid phantom holding two ball-bearings at two different distances from the radiation source. Images of these ball-bearings were acquired using the electronic portal imaging device (EPID) integrated with each linac. Machine QA was assessed using a radiation head-mounted PTW STARCHECK phantom. Patient plan QA was investigated using the SNC ArcCHECK phantom positioned on the treatment couch, irradiated with VMAT plans across a complete 360° gantry rotation and three X-ray energies. This study covered eight Elekta linacs, including those with 6 MV, 18 MV, and 6 MV flattening-filter-free (FFF) beams. The largest range in the FSP was found for 6 MV FFF. The FSP of one linac, retrofitted with 6 MV FFF, displayed substantial differences in FSP compared to 6 MV FFF beams on other linacs, which all had FSP ranges less than 0.50 mm and 0.25 mm in the lateral and longitudinal directions, respectively. The PTW STARCHECK phantom proved effective in characterising the FSP, while the SNC ArcCHECK measurements could not discern FSP-related features. Minor variations in FSP may be attributed to adjustments in linac parameters, component replacements necessary for beam delivery, and the wear and tear of various linac components, including the magnetron and gun filament. Consideration should be given to the ability of any particular phantom to detect a subsequent impact on the accuracy of patient plan delivery.

Intensity Modulated Therapy for Patients With Breast Cancer. Practical Guidelines and Tips for an Effective Treatment Planning Strategy

Purpose

Practical guidelines and tips for effective and robust radiation therapy treatment planning for patients with breast cancer are addressed for fixed-field intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) techniques. The concepts described here are general and valid on all treatment planning systems. However, some details shown here have been applied to the Varian platforms used at the authors' institutions.

Methods and Materials

The specific aspects of using C-arm- or O-ring-mounted linear accelerators are covered in the document, as well as tips for dealing with certain resource constraints, target cropping, and skin flash aiming to reduce risks of skin toxicity and to manage (residual after breath control) respiration motion or edema.

Results

A decision tree is presented, and practical solutions for cases where a target volume is contoured or not and where volumetric modulated arc therapy or fixed-beam intensity modulation should be applied and details about the technical implementation (tangential IMRT, butterfly IMRT or VMAT, and large partial VMAT arcs) are discussed. Target cropping and skin flash implications are discussed in detail, and links to plan robustness are outlined.

Conclusions

Practical guidelines for breast planning are presented and summarized with a decision tree and technical summaries.

Stereotactic body radiation therapy for prostate cancer: a dosimetric comparison of IMRT and VMAT using flattening filter and flattening filter-free beams

This retrospective study was performed to evaluate plan quality and treatment delivery parameters of stereotactic body radiation therapy (SBRT) for prostate cancer. The study utilized different isocentric modulated techniques: intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) using 6 MV flattening filter (FF) and 10 MV flattening filter-free beams (FFF). Fifteen retrospective prostate cancer patients were selected for this study. Sixty plans were created with an SBRT-prescribed dose of 36.25 Gy delivered in five fractions. Planning target volume (PTV) coverage, plan quality indices, doses delivered to organs at risk (OARs), and treatment delivery parameters were compared for all plans. It turned out that VMAT plans, particularly those using the FFF beam, provided superior target conformality and a steeper dose gradient as compared to IMRT plans. Additionally, VMAT plans showed better OARs sparing compared to IMRT plans. However, IMRT plans delivered a lower maximum dose to the target than VMAT plans. Importantly, the VMAT plans resulted in reduced treatment delivery parameters, including beam on time (BOT), monitor unit (MU), and modulation factor (MF), compared to IMRT plans. Furthermore, a statistically significant difference was observed in BOT and mean body dose between FF and FFF beams, with FFF beams showing superior performance. Considering all results, VMAT using 10 MV (FFF) is suggested for treating prostate cancer patients with SBRT. This offers the fastest delivery in addition to maintaining the highest plan quality.

Impact of target degradation on the 6FFF output of a Varian TrueBeam Linac

The dosimetric output of a 6FFF beam, produced from a Varian TrueBeam linac exhibited an unexpected downward trend over time that was contrary to well-established expectations. To elucidate the cause of this uncharacteristic trend, a review of the linac's quality control results over its lifetime was performed, including, constancy checks of the dosimetric output, beam energy, flatness and symmetry, and percentage depth dose characteristics. These results were supplemented with a comprehensive series of measurements including flatness and symmetry measurements with a 1D-diode array, high-resolution measurements of the photon beam's build-up region with a parallel-plate chamber and measurement of the beam's output as a function of the x-ray target position. The review of the linac's QC results and supplemental tests identified no deviations in the linac's performance from its commissioning and baseline measurements. However, the 6FFF beam output exhibited a significant dependence on the target location relative to its default position, increasing by 5.43 % with a 0.5 mm target translation, indicating that target degradation was the cause of the atypical output trend. The change in output behaviour was believed to be the result of primary electrons escaping the degraded target and interacting with the linac's monitor chamber. Replacement of the x-ray target caused the 6FFF output to realign with expected trends. Target degradation was uncovered due to a robust quality control trending database and awareness of typical output behaviour. These results demonstrate the importance of data trending to identify component failure and provide centres with knowledge to recognise this potential fault.

MR-linac MLC positioning QA by digitally stitching dual double-exposed films

PURPOSE

The picket fence (PF) test is highly recommended for multi-leaf collimator (MLC) quality assurance. However, since the electronic portal imaging device (EPID) on the Elekta Unity only covers a small area, it is not feasible to perform the PF test for the entire MLC. Here, we propose a technique for the PF test by stitching two double-exposed films.

METHODS

Two EBT3 films were used to encompass the entire MLC, with each one covering one half of the area. Two fields were employed to apply double exposure: a PF pattern consisting of 11 2 mm wide pickets and a 2.84 cm x 22 cm open field. The edges of the open field defined by the diaphragms were used to correct film rotation as well as align them horizontally. The PF pattern was also measured with the EPID where the pickets were used to align the films vertically. Individual leaf positions were detected on the merged film for quantitative analysis. Various MLC positioning errors were introduced to evaluate the technique's sensitivity.

RESULTS

The merged films covered 72 leaf pairs properly (four leaf pairs on both sides were outside the treatment couch). With the EPID, the leaf positioning accuracy was -0.02 ± 0.07 mm (maximum: 0.29 mm) and the picket width variation was 0.00 ± 0.03 mm (maximum: 0.11 mm); with the films, the position accuracy and width variation were -0.03 ± 0.13 mm (maximum: 0.80 mm) and 0.00 ± 0.13 mm (maximum: 0.74 mm), respectively. The EPID was able to detect errors of 0.5 mm or above with submillimeter accuracy; the films were only able to detect errors > 1.0 mm.

CONCLUSION

We developed a quantitative technique for the PF test on the Elekta Unity. The merged films covered nearly the entire MLC leaf banks. The technique exhibited clinically acceptable accuracy and sensitivity to MLC positioning errors.

Comparison Flattening Filter and Flattening Filter-Free Techniques in Small-Fields Dosimetry with Various Types of Detectors

PURPOSE

The aim of this study was to investigate the detector size effect on small-field dosimetry and compare the performance of 6MV WFF/FFF techniques.

METHODS

We investigated the detector size effect on small-field dosimetry and compared the performance of 6MV WFF/FFF techniques. PDD, profile curves, and absorbed dose were measured in water under reference conditions with 6MV (WFF/FFF) techniques. We employed Farmer FC65-P, CC13, CC01, and IBA Razor diode, with Versa Lineac. Subsequently, we replicated this assessment for small-fields under 5cmx5cm dimensions.

RESULTS

For both 6MV WFF/FFF, significant dose differences (Dmax=1.47cm), were ±4.55%, ±6.7, ±12.75% and ±33.3% for 4cmx4cm, 3cmx3cm, 2cmx2cm, and 1cmx1cm, respectively. The average difference relative to D10 was observed to be ±4.66%, ±5.73%, ±6.58%, and ±8.75% for the previous field sizes. Differences between WFF/FFF are neglected values at all field sizes>2.3%, also, the output of the largest detector FC65-P is lower at 55% in the smallest field size. Variation in the profile doesn't exceed a difference of >5% in flatness between WFF/FFF at depth10cm, across all fields, while symmetry is >1%, but radiation output is considerably lower at 55% for FC65-P chamber in 2cmx2cm, 1cmx1cm compared to the CC01 chamber and Razor diode. Significant differences in 1cmx1cm, where FC65-P chamber exhibits around 49% difference compared to Razor diode with 6MV (WFF/FFF).  Conclusions: Significant differences were observed in doses with various detectors. Detector-size influences the dose. WFF/FFF techniques show no major differences in small-fields dosimetry. Utilize some situations the advantage of FFF boasting a higher dose rate, consequently reducing treatment time to half.

Accuracy of Beamscan Software in Determining the Inflection Point from the FFF Beam Profile Using Several Array Detectors

OBJECTIVE

The goal of this study is to determine the accuracy of the PTW Beamscan program in determining the inflection point from Flattening Filter Free Beam Profile utilizing Multiple Detectors.

METHODS

True Beam Linear Accelerator with 6FFF and 10FFF Photon Energies and 10 cm, 15 cm and 20 cm Field Sizes were used for this study. Profile measurements were taken with PTW's 729, 1,500, and 1,600 and the Starcheck system, the Pinpoint 3D with Beamscan system, and Linac's EPID. The first-order derivative was utilized in both the Excel spreadsheet and Beamscan software to analyse raw measured data to locate inflection point and the FWHM was calculated. The accuracy of inflection points and FWHM between the Excel sheet calculation and the software program were investigated.

RESULTS

For 10X10 cm2 in the 729 Array, the greatest differences in FWHM were 5.16 mm and 5.04 mm for the X6 FFF and X10 FFF Energies, respectively. The largest difference was 2.26 mm for 1,600 SRS arrays with a 15×15 cm2 field size. The difference in FWHM between Manual and software analysis for 10X10 cm2 and 20X20 cm2 Field Sizes is in decreasing order for detectors from 729, 1,500, 1,600 SRS, Starcheck, Pinpoint 3D, and EPID. In contrast, there is no climbing or declining pattern detected in the difference in Field Width for the 15×15 cm2 Field Size. Similarly, for all detectors except the 1,600 SRS array, the peak of the first-order derivative occurs at the chamber position for a 15X15 cm2 field size.

CONCLUSION

The higher resolution of measurement yields more accuracy in inflection point and the FWHM. Irrespective of measurement resolution, the Beamscan software provided the FWHM closer to the respective nominal Field Size. Out of all detectors, results obtained with Excel Starcheck and EPID are good in agreement with values obtained by the software analysis. Thus, it is shown that Beamscan software is so accurate in determining inflection point of a FFF beam profile and used for routine profile analysis.

A comprehensive Study of the Out of Field Non Target Dose Associated with 6 and 10 MV Flattened and Flattening Filter Free X Ray Beam in a True Beam Linear Accelerator

AIM

To evaluate the out-of-field dose associated with flattened (FF) and flattening filter-free (FFF) 6 and 10 MV X-ray beams in a TrueBeam linear accelerator (Linac).

MATERIALS AND METHODS

Measurements were taken in a slab phantom using the metal oxide semiconductor field effect transistor (MOSFET) detector at varying depths (dmax, 5 cm, and 10 cm) for clinically relevant field sizes and up to 30 cm from the field edges for 6 and 10 MV FF and FFF beams in TrueBeam Linac. Dose calculation accuracy of the analytic anisotropic algorithm (AAA) and Acuros algorithm was investigated in the out-of-field region. Similarly, the out-of-field dose associated with volumetric modulated arc therapy (VMAT) head-and-neck plan delivered to a body phantom was evaluated.

RESULTS

The out-of-field dose for both FF and FFF photon beams (6 and 10 MV) decreased with increasing distance from the field boundary and size. Furthermore, regardless of FF in the field, higher-energy photon beams were associated with lower out-of-field dose. Both algorithms underestimated the dose in the out-of-field region, with AAA failing to calculate the out-of-field dose at 15 cm from the field edge and Acuros failing to calculate out-of-field radiation at 20 cm. At 5 cm from the field edge, an average of 50% underestimation was observed, and at 10 cm, an average of 60% underestimation was observed for both FF and FFF (6 and 10 MV) beams. The VMAT head-and-neck plan performed with the FFF beam resulted in a lower out-of-field dose than the FF beam for a comparable dose distribution.

CONCLUSION

Compared with flattened beams, the FFF modes on TrueBeam Linac exhibited a clinically relevant reduction in the out-of-field dose. Further dosimetric studies are warranted to determine the significant benefit of FFF beams across different cancer sites.

Evaluation of Ion Recombination Correction for Indigenously Developed Farmer Ion Chamber in Flattening Filter-Free Photon Beams

Purpose and Aim

Modern generation linear accelerator (linac) either generates X-rays with a flattening filter (WFF beam) or without flattening filter free (FFF beam). The FFF beams are associated with a significantly higher dose per pulse compared to WFF beams due to the absence of a flattening filter and the corresponding attenuation caused by it. This results in increased ion recombination and a larger saturation correction factor (ks). In accordance with the IAEA TRS 398 dosimetry protocol, ks is necessary for the accurate measurement of absorbed dose at a point in water. The objective of this study was to evaluate the ks for the indigenous FAR 65-GB ion chamber (IC) for the FFF X-rays.

Materials and Methods

The study was carried out on TrueBeam linac (Varian, A Siemens Healthineers company) which offers 6 MV WFF, 6 MV FFF, 10 MV WFF and 10 MV FFF beams. The two-voltage method was employed to measure ks in a solid water phantom at a depth of 10 cm for a FAR 65-GB and SNC 600c and 0.6cc PTW 30013 Farmer chambers at 100 cm and 150 cm source-to-chamber distances for a 10 cm × 10 cm field size.

Results

The ks values for the FAR 65-GB, PTW 30,013, and SNC 600c were 1.0055 (1.0113), 1.0051 (1.0071), and 1.0033 (1.0066) for the 6 MV WFF (FFF) beams, respectively, and 1.0066 (1.0178), 1.0061 (1.0137), and 1.0035 (1.0119) for the 10MV WFF (FFF) beams, respectively. The ks values calculated by two-voltage method matches with ks values obtained from Jaffe's plot. The chamber exhibited a linear dose-response up to 3000 cGy, beyond which a saturation effect was observed.

Conclusions

Our study reveals that this chamber is suitable for the reference dosimetry for the FFF beams.

Commissioning and clinical evaluation of a novel high-resolution quality assurance digital detector array for SRS and SBRT

PURPOSE

We aimed to perform the commissioning and clinical evaluation of myQA SRS detector array for patient-specific quality assurance (PSQA) of stereotactic radiosurgery (SRS)/ stereotactic body radiotherapy (SBRT) plans.

METHODS

To perform the commissioning of myQA SRS, its dose linearity, dose-rate dependence, angular dependence, and field-size dependence were investigated. Ten SBRT plans were selected for clinical evaluation: 1) Common clinical deviations based on the original SBRT plan (Plan0), including multileaf collimator (MLC) positioning deviation and treatment positioning deviation were introduced. 2) Compared the performance of the myQA SRS and a high-resolution EPID dosimetry system in PSQA measurement for the SBRT plans. Evaluation parameters include gamma passing rate (GPR) and distance-to-agreement (DTA) pass rate (DPR).

RESULTS

The dose linearity, angle dependence, and field-size dependence of myQA SRS system exhibit excellent performance. The myQA SRS is highly sensitive in the detection of MLC deviations. The GPR of (3%/1 mm) decreases from 90.4% of the original plan to 72.7%/62.9% with an MLC outward/inward deviation of 3 mm. Additionally, when the setup error deviates by 1 mm in the X, Y, and Z directions with the GPR of (3%/1 mm) decreasing by an average of -20.9%, -25.7%, and -24.7%, respectively, and DPR (1 mm) decreasing by an average of -33.7%, -32.9%, and -29.8%. Additionally, the myQA SRS has a slightly higher GPR than EPID for PSQA, However, the difference is not statistically significant with the GPR of (3%/1 mm) of (average 90.4%% vs. 90.1%, p = 0.414).

CONCLUSION

Dosimetry characteristics of the myQA SRS device meets the accuracy and sensitivity requirement of PSQA for SRS/SBRT treatment. The dose rate dependence should be adequately calibrated before its application and a more stringent GPR (3%/1 mm) evaluation criterion is suggested when it is used for SRS/SBRT QA.

Evaluation of the Minimum Segment Width and Fluence Smoothing Tools for Intensity-modulated Techniques in Monaco Treatment Planning System

Purpose

This study aims to minimize monitor units (MUs) of intensity-modulated treatments in the Monaco treatment planning system while preserving plan quality by optimizing the "Minimum Segment Width" (MSW) and "Fluence Smoothing" parameters.

Materials and Methods

We retrospectively analyzed 30 prostate, 30 gynecological, 15 breast cancer, 10 head and neck tumor, 11 radiosurgery, and 10 hypo-fractionated plans. Original prostate plans employed "Fluence Smoothing" = Off and were reoptimized with Low, Medium, and High settings. The remaining pathologies initially used MSW = 0.5 cm and were reoptimized with MSW = 1.0 cm. Plan quality, including total MU, delivery time, and dosimetric constraints, was statistically analyzed with a paired t-test.

Results

Prostate plans exhibited the highest MU variation when changing "Fluence Smoothing" from Off to High (average ΔMU = -5.1%; P < 0.001). However, a High setting may increase overall MU when MSW = 0.5 cm. Gynecological plans changed substantially when MSW increased from 0.5 cm to 1.0 cm (average ΔMU = -29%; P < 0.001). Organs at risk sparing and planning target volumes remained within 1.2% differences. Replanning other pathologies with MSW = 1.0 cm affected breast and head and neck tumor plans (average ΔMU = -168.38, average Δt = -11.74 s, and average ΔMU = -256.56, average Δt = -15.05 s, respectively; all with P < 0.004). Radiosurgery and hypofractioned highly modulated plans did not yield statistically significant results.

Conclusions

In breast, pelvis, head and neck, and prostate plans, starting with MSW = 1.0 cm optimally reduces MU and treatment time without compromising plan quality. MSW has a greater impact on MU than the "Fluence Smoothing" parameter. Plans with high modulation might present divergent behavior, requiring a case-specific analysis with MSW values higher than 0.5 cm.

Effect of simultaneous integrated boost concepts on photoneutron and distant out-of-field doses in VMAT for prostate cancer

BACKGROUND

A simultaneous integrated boost (SIB) may result in increased out-of-field (DOOF) and photoneutron (HPN) doses in volumetric modulated arc therapy (VMAT) for prostate cancer (PCA). This work therefore aimed to compare DOOF and HPN in flattened (FLAT) and flattening filter-free (FFF) 6‑MV and 10-MV VMAT treatment plans with and without SIB.

METHODS

Eight groups of 30 VMAT plans for PCA with 6 MV or 10 MV, with or without FF and with uniform (2 Gy) or SIB target dose (2.5/3.0 Gy) prescriptions (CONV, SIB), were generated. All 240 plans were delivered on a slab-phantom and compared with respect to measured DOOF and HPN in 61.8 cm distance from the isocenter. The 6‑ and 10-MV flattened VMAT plans with conventional fractionation (6- and 10-MV FLAT CONV) served as standard reference groups. Doses were analyzed as a function of delivered monitor units (MU) and weighted equivalent square field size Aeq. Pearson's correlation coefficients between the presented quantities were determined.

RESULTS

The SIB plans resulted in decreased HPN over an entire prostate RT treatment course (10-MV SIB vs. CONV -38.2%). Omission of the flattening filter yielded less HPN (10-MV CONV -17.2%; 10-MV SIB -22.5%). The SIB decreased DOOF likewise by 39% for all given scenarios, while the FFF mode reduced DOOF on average by 60%. A strong Pearson correlation was found between MU and HPN (r > 0.9) as well as DOOF (0.7 < r < 0.9).

CONCLUSION

For a complete treatment, SIB reduces both photoneutron and OOF doses to almost the same extent as FFF deliveries. It is recommended to apply moderately hypofractionated 6‑MV SIB FFF-VMAT when considering photoneutron or OOF doses.

Comparative Evaluation of Dosimetric Quality and Treatment Efficiency for Halcyon, TrueBeam, and TomoTherapy in Cervical-Thoracic Esophageal Cancer Radiotherapy

INTRODUCTION

This study primarily aims to investigate the suitability of Halcyon in the context of cervical-thoracic esophageal cancer by exploring the dosimetric quality and delivery efficiency of Halcyon plans with different arc configurations. Additionally, it compares these findings with the dosimetric indices and delivery efficiency of TrueBeam and TomoTherapy accelerators, focusing on their capability to optimize protection for organs at risk (OARs) while maintaining efficient treatment delivery strategies.

METHODS

This retrospective study involved 26 patients diagnosed with cervical-thoracic esophageal cancer, and new radiotherapy plans were created using Halcyon, TrueBeam, and TomoTherapy. Dose volume histogram (DVH) metrics and delivery efficiency for plans involving different arc numbers on Halcyon (2, 3, and 4 arcs) were compared with those from TrueBeam and TomoTherapy. T-tests were employed to evaluate differences in organ protection among the accelerators.

RESULTS

The Halcyon plans, especially those with 4 arcs, provided superior protection for organs at risk, including the heart, lungs, and spinal cord, while maintaining excellent delivery efficiency. Compared to TrueBeam 2arc plans and TomoTherapy helical plans, Halcyon plans with 3 arcs also showed slight advantages. Although TomoTherapy offered better uniformity in dose distribution, it did not demonstrate a clear advantage over the other accelerators in terms of OAR protection or treatment efficiency. Furthermore, despite the lack of clear advantages in TrueBeam 2arc plans with flattening filter (FF), TrueBeam with flattening filter free (FFF) plans may hold potential in the treatment.

CONCLUSION

Halcyon, particularly with 4 arcs, offers an optimal balance between reducing toxicity to organs at risk and maintaining treatment efficiency, making it a preferred choice in cervical thoracic esophageal cancer radiotherapy. The findings highlight the need for careful selection of radiotherapy accelerators based on specific clinical goals, with Halcyon showing potential advantages in scenarios where both treatment efficiency and OAR protection are paramount.

Reirradiation of head and neck squamous cell carcinomas: a pragmatic approach-part I: prognostic factors and indications to treatment

INTRODUCTION

Reirradiation (reRT) of locally recurrent/second primary tumors of the head and neck region is a potentially curative treatment for patients not candidate to salvage surgery. Aim of the present study is to summarize available literature on both prognostic factors and indications to curative reRT in this clinical setting.

MATERIALS AND METHODS

A narrative review of the literature was performed on two topics: (1) patients' selection according to prognostic factors and (2) dosimetric feasibility of reRT. Postoperative reRT and palliative intent treatments were out of the scope of this work.

RESULTS

Patient-tumor and treatment-related prognostic factors were analyzed, together with dosimetric parameters concerning target volume and organs at risk. Based on available evidence, a stepwise approach has been proposed aiming to provide a useful tool to identify suitable candidates for curative reRT in clinical practice. This was then applied to two clinical cases, proposed at the end of this work.

CONCLUSION

A second course of RT in head and neck recurrence/second primary tumors is a personalized approach that can be offered to selected patients only in centers with expertise and dedicated equipment following a multidisciplinary team discussion.

Dosimetric Evaluation of Semiflex Three-dimensional Chamber under Unflatten Beam in Comparison among Different Detectors

Purpose

The goal of this study is to investigate the dosimetric properties of a Semiflex three-dimensional (3D) chamber in an unflatten beam and compare its data from a small to a large field flattening filter-free (FFF) beam with different radiation detectors.

Methods

The sensitivity, linearity, reproducibility, dose rate dependency, and energy dependence of a Semiflex 3D detector in flattening filter and filter-free beam were fully investigated. The minimum radiation observed field widths for all detectors were calculated using lateral electronic charged particle equilibrium to investigate dosimetric characteristics such as percentage depth doses (PDDs), profiles, and output factors (OPFs) for Semiflex 3D detector under 6FFF Beam. The Semiflex 3D measured data were compared to that of other detectors employed in this study.

Results

The ion chamber has a dosage linearity deviation of +1.2% for <10 MU, a dose-rate dependency deviation of +0.5%, and significantly poorer sensitivity due to its small volume. There is a difference in field sizes between manufacturer specs and derived field sizes. The measured PDD, profiles, and OPFs of the Semiflex 3D chamber were within 1% of each other for all square field sizes set under linac for the 6FFF beam.

Conclusion

It was discovered to be an appropriate detector for relative dose measurements for 6 FFF beams with higher dose rates for field sizes more than or equal to 3 cm × 3 cm.

Radiotherapy dose escalation using pre-treatment diffusion-weighted imaging in locally advanced rectal cancer: a planning study

Objectives

Diffusion-weighted MRI (DWI) may provide biologically relevant target volumes for dose-escalated radiotherapy in locally advanced rectal cancer (LARC). This planning study assessed the dosimetric feasibility of delivering hypofractionated boost treatment to intra-tumoural regions of restricted diffusion prior to conventional long-course radiotherapy.

Methods

Ten patients previously treated with curative-intent standard long-course radiotherapy (50 Gy/25#) were re-planned. Boost target volumes (BTVs) were delineated semi-automatically using 40th centile intra-tumoural apparent diffusion coefficient value with expansions (anteroposterior 11 mm, transverse 7 mm, craniocaudal 13 mm). Biased-dosed combined plans consisted of a single-fraction volumetric modulated arc therapy flattening-filter-free (VMAT-FFF) boost (phase 1) of 5, 7, or 10 Gy before long-course VMAT (phase 2). Phase 1 plans were assessed with reference to stereotactic conformality and deliverability measures. Combined plans were evaluated with reference to standard long-course therapy dose constraints.

Results

Phase 1 BTV dose targets at 5/7/10 Gy were met in all instances. Conformality constraints were met with only 1 minor violation at 5 and 7 Gy. All phase 1 and combined phase 1 + 2 plans passed patient-specific quality assurance. Combined phase 1 + 2 plans generally met organ-at-risk dose constraints. Exceptions included high-dose spillage to bladder and large bowel, predominantly in cases where previously administered, clinically acceptable non-boosted plans also could not meet constraints.

Conclusions

Targeted upfront LARC radiotherapy dose escalation to DWI-defined is feasible with appropriate patient selection and preparation.

Advances in knowledge

This is the first study to evaluate the feasibility of DWI-targeted upfront radiotherapy boost in LARC. This work will inform an upcoming clinical feasibility study.

A novel lung SBRT treatment planning: Inverse VMAT plan with leaf motion limitation to ensure the irradiation reproducibility of a moving target

This study exposed the implementation of a novel technique (VMATLSL) for the planning of moving targets in lung stereotactic body radiation therapy (SBRT). This new technique has been compared to static conformal radiotherapy (3D-CRT), volumetric-modulated arc therapy (VMAT), and dynamic conformal arc (DCA). The rationale of this study was to lower geometric complexity (54.9% lower than full VMAT) and hence ensure the reproducibility of the treatment delivery by reducing the risk for interplay errors induced by respiratory motion. Dosimetry metrics were studied with a cohort of 30 patients. Our results showed that leaf speed limitation provided conformal number (CN) close to the VMAT (median CN of VMATLSL is 0.78 vs 0.82 for full VMAT) and was a significant improvement on 3D-CRT and DCA with segment-weight optimized (respectively 0.55 and 0.57). This novel technique is an alternative to VMAT or DCA for lung SBRT treatments, combining independence from the patient's breathing pattern, from the size and amplitude of the lesion, free from interplay effect, and with dosimetry metrics close to the best that could be achieved with full VMAT.

The effects of flattening filter-free beams and aperture shape controller on the complexity of conventional large-field treatment plans

PURPOSE

The purpose of this study was to investigate the impact of using flattening filter-free (FFF) beams and the aperture shape controller (ASC) on the complexity of conventional large-field treatment plans.

METHODS AND MATERIALS

A total of 24 head and neck (H&N) and 24 prostate with pelvic nodes treatment plans were used in this study. Each plan was reoptimized using the original clinical objectives with both flattened and FFF beams, as well as six different ASC settings. The dosimetric qualities of each plan cohort were evaluated using commonly used dose-volume histogram values, and plan complexities were assessed through metrics including monitor unit (MU)/Dose, change in gantry speed, multileaf collimator (MLC) speed, the edge area ratio metric (EM), and the equivalent square length.

RESULTS

No significant differences in dosimetric qualities were found between plans with flattened and FFF beams. The ASC settings did not have significant effects on dosimetric qualities in the H&N plan cohort, but the "very high" ASC setting resulted in poorer dosimetric results for the prostate plans. Plans with FFF beams had significantly higher MU/Dose compared to plans with flattened beams. The use of flattening filter (FF) had significant effects on the change in gantry speed, with flattened beams producing plans that required higher change in gantry speed. However, the FF did not have significant effects on MLC speed, EM, or equivalent square length. In contrast, ASC settings had significant effects on these three metrics; increasing the ASC level resulted in plans with decreasing MLC speed, lower edge area ratio, and higher equivalent square length.

CONCLUSION

This study demonstrated that using FFF beams with various ASC settings, except for the "very high" level, can produce plans with reduced complexities without compromising dosimetric qualities in conventional large-field treatment plans.

Dosimetric and Radiobiological Impact of Flattening Filter-Free Beam and Dose Calculation Algorithm Using RapidArc Plans for Cervical Cancer Treatment

Purpose

The aim of this study is to quantify the potential benefits of a flattening filter-free (FFF) beam and implement a dose-computation algorithm for cervical radiotherapy through dosimetric and radiobiological analyses using RapidArc.

Methods

Thirty-three patients were enrolled, and four RapidArc plans were created for each patient using a dual-arc flattening filter and 6-MV FFF photon beams for the two calculation algorithms. Homogeneity index (HI), conformity index (CI), target coverage, monitor units (MUs), and organ-at-risk (OAR) dosimetric characteristics were compared between the plans. Radiobiological characteristics and normal tissue complication probability (NTCP) scores were computed for the OAR using different biological models.

Results

No significant differences were observed in the Dmax, D98%, and CI in the planning target volume (PTV). Both computations estimated a significant difference in V95%, D2%, and HI for the PTV. Furthermore, the FFF beam showed a significant increase in the MUs and a significant reduction in V30% for the femoral heads. The NTCP score showed a significant increase in the late effects on the bladder, rectum, and bowel with FFF beams.

Conclusion

The current study recommends FFF beams for better conformity, comparable dose coverage for the target, and OAR sparing invariable to the dose computation algorithm. The difference in the NTCP score for OAR was minimal with the FFF beam.

How much rotational error is clinically acceptable for single-isocenter/two-lesion lung SBRT treatment on halcyon ring delivery system (RDS)?

PURPOSE

SBRT treatment of two separate lung lesions via single-isocenter/multi-target (SIMT) plan on Halcyon RDS could improve patient comfort, compliance, patient throughput, and clinic efficiency. However, aligning two separate lung lesions synchronously via a single pre-treatment CBCT scan on Halcyon can be difficult due to rotational patient setup errors. Thus, to quantify the dosimetric impact, we simulated loss of target(s) coverage due to small, yet clinically observable rotational patient setup errors on Halcyon for SIMT treatments.

METHODS

Seventeen previously treated 4D-CT based SIMT lung SBRT patients with two separate lesions (total 34 lesions, 50 Gy in five fractions to each lesion) on TrueBeam (6MV-FFF) were re-planned on Halcyon (6MV-FFF) using a similar arc geometry (except couch rotation), dose engine (AcurosXB algorithm), and treatment planning objectives. Rotational patient setup errors of [± 0.5⁰ to ± 3.0⁰] on Halcyon were simulated via Velocity registration software in all three rotation axes and recalculated dose distributions in Eclipse treatment planning system. Dosimetric impact of rotational errors was evaluated for target coverage and organs at risk (OAR).

RESULTS

Average PTV volume and distance to isocenter were 23.7 cc and 6.1 cm. Average change in Paddick's conformity indexes were less than -5%, -10%, and -15% for 1°, 2°, and 3°, respectively for yaw, roll, and pitch rotation directions. Maximum drop off of PTV(D100%) coverage for 2° rotation was -2.0% (yaw), -2.2% (roll), and -2.5% (pitch). With ±1° rotational error, no PTV(D100%) loss was found. Due to anatomical complexity: irregular and highly variable tumor sizes and locations, highly heterogenous dose distribution, and steep dose gradient, no trend for loss of target(s) coverage as a function of distance to isocenter and PTV size was found. Change in maximum dose to OAR were acceptable per NRG-BR001 within ±1.0° rotation, but were up to 5 Gy higher to heart with 2° in the pitch rotation axis.

CONCLUSION

Our clinically realistic simulation results show that rotational patient setup errors up to 1.0° in any rotation axis could be acceptable for selected two separate lung lesions SBRT patients on Halcyon. Multivariable data analysis in large cohort is ongoing to fully characterize Halcyon RDS for synchronous SIMT lung SBRT.

Linac primary barrier transmission: Flattening filter free and field size dependence

There is widespread consensus in the literature that flattening filter free (FFF) beams have a lower primary barrier transmission than flattened beams. Measurements presented here, however, show that for energy compensated FFF beams, the barrier transmission can be as much as 70% higher than for flattened beams. The ratio of the FFF barrier transmission to the flattened beam barrier transmission increases with increasing barrier thickness. The use of published FFF TVL data for energy compensated FFF beams could lead to an order of magnitude underestimate of the air kerma rate. There are little data in the literature on the field size dependence of the barrier transmission for flattened beams. Barrier transmission depends on the field size at the barrier, not at isocenter Measurements are presented showing the relative dependence of barrier transmission on the field size, measured at the barrier, for 6 MV and 10 MV beams. An analytical fitting formula is provided for the field size dependence. For field sizes greater than about 150 cm in side length, the field size dependence is minimal. For field sizes less than about 100 cm, the transmission declines rapidly as the field size decreases.

Treatment time of image-guided radiotherapy with a Halcyon 2.0 system

INTRODUCTION

The treatment fraction time is a key indicator of the external beam radiotherapy process. The Halcyon system was designed to improve the clinical workflow, according to the manufacturer (Varian Medical Systems). Few works studied the actual delivery efficiency of the Halcyon system. This work analyzed the treatment time on a Halcyon 2.0 unit for a variety of sites along a period of 9 months.

MATERIALS AND METHODS

Treatment time included patient setup, image acquisition, image-guided online couch correction, and radiation delivery time. Data were extracted from the ARIA oncology information system and were studied as a function of the treatment site, the delivery modality, and the time from the first day of treatments with the Halcyon 2.0 system in our institution.

RESULTS

A total of 8599 fractions were delivered during the analyzed period (69.5% from VMAT plans, and 30.5% from IMRT plans). The number of fractions by site ranged from 30 for anal canal to 1933 for prostate. Five sites (prostate, lung, pelvis with prostate, breast, and gynecological sites) accounted for the 84% of the fractions. After a 2-week adaptation period of the staff, the daily mean treatment time was reduced to less than 12 min. The mean treatment time of all the fractions amounted to 10.5 ± 3.8 min.

CONCLUSIONS

The Halcyon 2.0 allowed delivering online image-guided radiation therapy in all fractions with total treatment time consistently below the 12-min standard time slot, for most of the analyzed treatment sites.

Developing an AI-assisted planning pipeline for hippocampal avoidance whole brain radiotherapy

BACKGROUND AND PURPOSE

Hippocampal avoidance whole brain radiotherapy (HA-WBRT) is effective for controlling disease and preserving neuro-cognitive function for brain metastases. However, contouring and planning of HA-WBRT is complex and time-consuming. We designed and evaluated a pipeline using deep learning tools for a fully automated treatment planning workflow to generate HA-WBRT radiotherapy plans.

MATERIALS AND METHODS

We retrospectively collected 50 adult patients who received HA-WBRT. Using RTOG- 0933 clinical trial protocol guidelines, all organs-at-risk (OARs) and the clinical target volume (CTV) were contoured by experienced radiation oncologists. A deep-learning segmentation model was designed and trained. Next, we developed a volumetric-modulated arc therapy (VMAT) auto-planning algorithm for 30 Gy in 10 fractions. Automated segmentations were evaluated using the Dice similarity coefficient (DSC) and 95th-percentile Hausdorff distance (95 % HD). Auto-plans were evaluated by the percentage of PTV volume that receives 30 Gy (V_30Gy), conformity index (CI), and homogeneity index (HI) of planning target volume (PTV) and the minimum dose (D_100%) and maximum dose (D_max) for the hippocampus, D_max for the lens, eyes, optic nerve, brain stem, and chiasm.

RESULTS

We developed a deep-learning segmentation model and an auto-planning script. For the 10 cases in the independent test set, the overall average DSC and 95 % HD of contours were greater than 0.8 and less than 7 mm, respectively. All auto-plans met the RTOG- 0933 criteria. The HA-WBRT plan automatically created time was about 10 min.

CONCLUSIONS

An artificial intelligence (AI)-assisted pipeline using deep learning tools can rapidly and accurately generate clinically acceptable HA-WBRT plans with minimal manual intervention and increase efficiency of this treatment for brain metastases.

Evaluation of a complementary metal oxide semiconductor detector as a tool for stereotactic body radiotherapy plan quality assurance

Background and purpose

A sub-mm resolution Complementary Metal Oxide Semiconductor sensor has been developed for stereotactic radiotherapy quality assurance. Herein we evaluate its basic dosimetric performance and its application for linac C-arm stereotactic body radiotherapy (SBRT) plan quality assurance.

Materials and methods

The detector was integrated into its accompanying phantom or in Water Equivalent Plastic (WEP). The measurement reproducibility, stability, dose linearity and dependence on angularity, dose rate and field size were investigated. Clinical plan measurements were compared to our radiotherapy treatment planning system and radiochromic film. Sensitivity to introduced Multi Leaf Collimator (MLC) offsets was evaluated by simulating single MLC offsets in SBRT plans and comparing measurements to expected doses.

Results

Signal reproducibility was within ± 0.1 % and output calibration was stable over a 6 month period. Detector showed good linearity with dose (r² = 1). Signal decreased by 5 % when dose rate was decreased from 1300 MU/min to 300 MU/min. Output factors agreed within 0.5 % of chamber measurements for 1x1 cm field sizes or greater. Angularity measurements showed good agreement with reference. For measurement of planned clinical doses, gamma pass-rates were 98.5 % ± 2.3 % (treatment planning system reference, 2 %/2mm) and 99.2 % ± 1.0 % (film reference, 2 %,2mm). The detector also showed sensitivity to errors of 1 mm offsets in MLC positioning.

Conclusion

The detector performed well when used for pre-treatment SBRT plan quality assurance, offering a good alternative to radiochromic film.

Comparison of surface dose during whole breast radiation therapy on Halcyon and TrueBeam using Cherenkov imaging

The emergence of the Halcyon linear accelerator has allowed for increased patient throughput and improved treatment times for common treatment sites in radiation oncology. However, it has been shown that this can lead to increased surface dose in sites like breast cancer compared with treatments on conventional machines with flattened radiation beams. Cherenkov imaging can be used to estimate surface dose by detection of Cherenkov photons emitted in proportion to energy deposition from high energy electrons in tissue. Phantom studies were performed with both square beams in reference conditions and with clinical treatments, and dosimeter readings and Cherenkov images report higher surface dose (25% for flat phantom entrance dose, 5.9% for breast phantom treatment) from Halcyon beam deliveries than for equivalent deliveries from a TrueBeam linac. Additionally, the first Cherenkov images of a patient treated with Halcyon were acquired, and superficial dose was estimated.

The Study of Dosimetric Characteristics of the XHA600D Medical Linear Accelerator Based on a Monte Carlo Code

By investigating the influence of initial electrons on dosimetric characteristics, reasonable incident electron parameters for the nominal 6 MV photon beam of the XHA600D accelerator are finally established, i.e., a 6 MeV monoenergetic electron beam with a radial intensity FWHM of 2.5 mm and an angular divergency of 0.15°. Based on reasonable initial parameters, Percentage Depth Doses (PDDs), Off-Axis Ratios (OARs), total scatter factors, beam qualities, and penumbra widths of both flatteningfilter (FF) and flattening-filter-free (FFF) beams for fields ranging from 4 × 4 to 30 × 30 cm2 are simulated systematically with EGSnrc codes. Not only the simulated dosimetric properties are in excellent agreement with the measurements, but also the dosimetric discrepancies between FF and FFF beams are consistent with the laws of previous studies on other accelerators. Therefore, reasonable incident electron parameters are able to accurately verify the performance of the XHA600D accelerator and can be used for further dosimetry research.

Characterization of the Plastic Scintillator Detector System Exradin W2 in a High Dose Rate Flattening-Filter-Free Photon Beam

(1) Background: The Exradin W2 is a commercially available scintillator detector designed for reference and relative dosimetry in small fields. In this work, we investigated the performance of the W2 scintillator in a 10 MV flattening-filter-free photon beam and compared it to the performance of ion chambers designed for small field measurements. (2) Methods: We measured beam profiles and percent depth dose curves with each detector and investigated the linearity of each system based on dose per pulse (DPP) and pulse repetition frequency. (3) Results: We found excellent agreement between the W2 scintillator and the ion chambers for beam profiles and percent depth dose curves. Our results also showed that the two-voltage method of calculating the ion recombination correction factor was sufficient to correct for the ion recombination effect of ion chambers, even at the highest DPP. (4) Conclusions: These findings show that the W2 scintillator shows excellent agreement with ion chambers in high DPP conditions.

Fetal dose estimation for Virtual Tangential-fields Arc Therapy whole breast irradiation by optically stimulated luminescence dosimeters

Breast cancer is the most frequently diagnosed tumor in pregnant women and radiation therapy should carefully be weighted up because of the dose to the fetus. The aim of this study was to investigate fetal dose in a patient treated with Virtual Tangential-fields Arc Therapy (ViTAT), an innovative technique for whole breast irradiation. Optically stimulated luminescence detectors (OSLDs) were calibrated on a Varian TrueBeam linac, with both a 6X and 6XFFF beam quality, and used for out-of-field measurements. Fetal dose related with ViTAT technique was compared to the standard 3D conformal radiation therapy technique (3DCRT). The fetal dose delivered with a ViTAT technique planned with 6XFFF beam was also investigated. Measurements were taken on a phantom composed of Rando Alderson Phantom slices and solid water slabs. OSLDs were placed in a region identified by the height of the fundus from conception to the twentieth week using a custom made PMMA grid. Due to the higher number of monitor units, the peripheral dose of ViTAT delivered with 6X beams is higher than that of 3DCRT. However, nanoDots measurements prove that ViTAT can be used in place of 3DCRT while maintaining similar fetal dose levels if 6XFFF beams are used.

IBA myQA SRS Detector for CyberKnife Robotic Radiosurgery Quality Assurance

The IBA myQA® SRS high-resolution solid-state detector was evaluated in the context of robotic radiosurgery delivered using CyberKnife®. The performance was investigated for periodic machine delivery quality assurance (DQA) and patient-specific treatment verification. myQA® SRS is a 140 × 120 mm CMOS matrix with 400 µm resolution, allocated in a cylindrical ABS phantom topped by a hemispheric cap. Evaluations included: periodic DQA tests, angular response, dose-rate dependence and Iris variable aperture collimator field size measurements. For patient-specific QA various intracranial targets were studied (Gamma Index, 3%/1 mm agreement criteria), taking into account also the detector’s angular response. Results for periodic DQA were in accordance with the machine commissioning data. Dose-rate dependence was confirmed, and angular response tests resulted in a signal decay >5% when beams were delivered outside a ±50° amplitude cone with respect to the vertical direction. Concerning patient-specific QA, >50° angled beams elimination led to a Gamma Index passing rates improvement ranging between +3% and +115%. IBA myQA® SRS proved to be a suitable device for many CyberKnife® constancy DQA checks, providing high-resolution real-time results. Patient-specific Gamma tests showed high passing rates once angular dependence corrections were performed, even in high complexity treatments such as those for trigeminal neuralgia targets.

State-of-the-Art Imaging Techniques in Metastatic Spinal Cord Compression

Metastatic Spinal Cord Compression (MSCC) is a debilitating complication in oncology patients. This narrative review discusses the strengths and limitations of various imaging modalities in diagnosing MSCC, the role of imaging in stereotactic body radiotherapy (SBRT) for MSCC treatment, and recent advances in deep learning (DL) tools for MSCC diagnosis. PubMed and Google Scholar databases were searched using targeted keywords. Studies were reviewed in consensus among the co-authors for their suitability before inclusion. MRI is the gold standard of imaging to diagnose MSCC with reported sensitivity and specificity of 93% and 97% respectively. CT Myelogram appears to have comparable sensitivity and specificity to contrast-enhanced MRI. Conventional CT has a lower diagnostic accuracy than MRI in MSCC diagnosis, but is helpful in emergent situations with limited access to MRI. Metal artifact reduction techniques for MRI and CT are continually being researched for patients with spinal implants. Imaging is crucial for SBRT treatment planning and three-dimensional positional verification of the treatment isocentre prior to SBRT delivery. Structural and functional MRI may be helpful in post-treatment surveillance. DL tools may improve detection of vertebral metastasis and reduce time to MSCC diagnosis. This enables earlier institution of definitive therapy for better outcomes.

Feasibility Study of Stereotactic Radiosurgery Treatment of Glomus Jugulare Tumors via HyperArc VMAT

This study aims to report on the clinical validation and feasibility of utilizing a novel fully automated treatment planning and delivery system, HyperArc VMAT stereotactic radiosurgery (SRS) for glomus jugulare tumors (GJT). Independent dose verification of the HyperArc module via the MD Anderson's SRS head phantom irradiation and credentialing results showed compliance with the SRS treatment requirements per IROC MD Anderson's standard. Following the Alliance clinical trial, AAPM, RTOG protocols, and QUENTAC requirements, utilizing selected three-partial arc geometry of HyperArc module on TrueBeam Linac with 6MV-FFF beam, GJT SRS plans were generated for nine previously treated Gamma Knife (GK) radiosurgery patients using advanced Acuros-based algorithm to account for tissue inhomogeneity corrections and frameless immobilization with Q-fix mask and Encompass device insert. HyperArc VMAT produced highly conformal SRS dose distributions to GJT, a steep dose gradient around the GJT, and spared adjacent critical organs including the spinal cord (< 3.0 Gy). Due to faster patient setup and less MLC modulation through the target (average beam-on time, 6.2 minutes), the HyperArc VMAT plan can deliver a single high-dose of 18 Gy to the GJT in less than 15 minutes overall treatment time, significantly improving patient comfort and clinic workflow. Pretreatment portal dosimetry quality assurance results and independent dose verification via Monte Carlo-based physics second check met our clinical SRS protocol's requirements for treatment. Due to the highly conformal dose distribution, rapid dose fall-off, excellent sparing of adjacent critical organs, and highly precise and accurate treatment, clinical implementation of frameless HyperArc VMAT for GJT patients who may not have access to nor tolerate frame-based GK SRS treatment are underway.

Evaluation of SRS MapCHECK with StereoPHAN phantom as a new pre-treatment system verification for SBRT plans

Introduction: The aim of this study was to evaluate the new 2-Dimensional diode array SRS MapCHECK (SunNuclear, Melbourne, USA) with dedicated phantom StereoPHAN (SunNuclear, Melbourne, USA) for the pre-treatment verification of the stereotactic body radiotherapy (SBRT).

Material and methods: For the system, the short and mid-long stability, dose linearity with MU, angular dependence, and field size dependence (ratio of relative output factor) were measured. The results of verification for 15 pre-treatment cancer patients (5 brains, 5 lungs, and 5 livers) performed with SRS MapCHECK and EBT3 Gafchromic films were compared. All the SBRT plans were optimized with the Eclipse (v. 15.6, Varian, Palo Alto, USA) treatment planning system (TPS) using the Acuros XB (Varian, Palo Alto, USA) dose calculation algorithm and were delivered to the Varian EDGE® (Varian, Palo Alto, USA) accelerator equipped with a high-definition multileaf collimator. The 6MV flattening-filter-free beam (FFF) was used.

Results: Short and mid-long stability of SRS MapCHECK was very good (0.1%-0.2%), dose linearity with MU and dependence of the response of the detector on field size results were also acceptable (for dose linearity R2 = 1 and 6% difference between microDiamond and SRS MapCHECK response for the smallest field of 1 × 1 cm2). The angular dependence was very good except for the angles close to 90° and 270°. For pre-treatment plan verification, the gamma method was used with the criteria of 3% dose difference and 3 mm distance to agreement (3%/3 mm), and 2%/2 mm, 1%/1 mm, 3%/1 mm, and 2%/1 mm. The highest passing rate for all criteria was observed on the SRS MapCHECK system.

Conclusions: It is concluded that SRS MapCHECK with StereoPHAN has sufficient potential for pre-treatment verification of the SBRT plans, so that verification of stereotactic plans can be significantly accelerated.

Quantification of Nanoscale Dose Enhancement in Gold Nanoparticle-Aided External Photon Beam Radiotherapy

The recent progress in Nanotechnology has introduced Gold Nanoparticles (AuNPs) as promising radiosensitizing agents in radiation oncology. This work aims to estimate dose enhancement due to the presence of AuNPs inside an irradiated water region through Monte Carlo calculations. The GATE platform was used to simulate 6 MV photon histories generated from a TrueBeam^® linear accelerator with and without a Flattening Filter (FF) and model AuNPs clusters. The AuNPs size, concentration and distribution pattern were examined. To investigate different clinical irradiation conditions, the effect of field size, presence of FF and placement of AuNPs in water were evaluated. The range of Dose Enhancement Factors (DEF = Dose_Au/Dose_Water) calculated in this study is 0.99 ± 0.01-1.26 ± 0.02 depending on photon beam quality, distance from AuNPs surface, AuNPs size and concentration and pattern of distribution. The highest DEF is reported for irradiation using un-flattened photon beams and at close distances from AuNPs. The obtained findings suggest that dose deposition could be increased in regions that represent whole cells or subcellular targets (mitochondria, cell nucleus, etc.). Nevertheless, further and consistent research is needed in order to make a step toward AuNP-aided radiotherapy in clinical practice.

Ultra‐High dose rate radiation production and delivery systems intended for FLASH

Higher dose rates, a trend for radiotherapy machines, can be beneficial in shortening treatment times for radiosurgery and mitigating the effects of motion. Recently, even higher doses (e.g., 100 times greater) have become targeted because of their potential to generate the FLASH effect (FE). We refer to these physical dose rates as ultra‐high (UHDR). The complete relationship between UHDR and the FE is unknown. But UHDR systems are needed to explore the relationship further and to deliver clinical UHDR treatments, where indicated. Despite the challenging set of unknowns, the authors seek to make reasonable assumptions to probe how existing and developing technology can address the UHDR conditions needed to provide beam generation capable of producing the FE in preclinical and clinical applications. As a preface, this paper discusses the known and unknown relationships between UHDR and the FE. Based on these, different accelerator and ionizing radiation types are then discussed regarding the relevant UHDR needs. The details of UHDR beam production are discussed for existing and potential future systems such as linacs, cyclotrons, synchrotrons, synchrocyclotrons, and laser accelerators. In addition, various UHDR delivery mechanisms are discussed, along with required developments in beam diagnostics and dose control systems.

Out-of-field dose and its constituent components for a 1.5 T MR-Linac

This study aims to quantify the relative contributions of phantom scatter, collimator scatter and head leakage to the out-of-field doses (OFDs) of both static fields and clinical intensity-modulated radiation therapy (IMRT) treatments in a 1.5 T MR-Linac. The OFDs of static fields were measured at increasing distances from the field edge in an MR-conditional water phantom. Inline scans at depths of dmax (14 mm), 50 and 100 mm were performed for static fields of 5 × 5, 10 × 10 and 15 × 15 cm²under three different conditions: full scatter, with phantom scatter prevented, and head leakage only. Crossline scans at isocenter and offset positions were performed in full scatter condition. EBT3 radiochromic films were placed at 100 mm depth of solid water phantom to measure the OFD of clinical IMRT plans. All water tank data were normalized to Dmax of a 10 × 10 cm²field and the film results were presented as a fraction of the target mean dose.The OFD in the inline direction varied from 3.5% (15 × 15 cm², 100 mm depth, 50 mm distance) to 0.014% (5 × 5 cm², dmax, 400 mm distance). For all static fields, the collimator scatter was higher than the phantom scatter and head leakage at a distance of 100-400 mm. Head leakage remained the smallest among the three components, except at long distances (>375 mm) with small field size. Compared to the inline scans, the crossline scans at the isocenter showed higher doses at distances longer than 80 mm. All crossline profiles at longitudinal offset positions showed a cone shape with laterally shifted maxima. The OFD of IMRT deliveries varied with different target size. For prostate stereotactic body radiation therapy (SBRT) treatment, the OFD decreased from 2% to 0.03% at a distance of 50-500 mm. The OFDs have been measured for a 1.5 T MR-Linac. The presented dosimetric data are valuable for radiation safety assessments on patients treated with the MR-Linac, such as evaluating carcinogenic risk and radiation exposure to cardiac implantable electronic devices.

Planar EPID-Based Dosimetry for SRS and SRT Patient-Specific QA

The study’s purpose was to develop and validate Electronic Portal Imaging Device (EPID)-based dosimetry for Stereotactic Radiosurgery (SRS) and Stereotactic Radiation Therapy (SRT) patient-specific Quality Assurance (QA). The co-operation between extended Source-to-Imager Distance (SID) to reduce the saturation effect and simplify the EPID-based dosimetry model was used to perform patient-specific QA in SRS and SRT plans. The four parameters were included for converting the image to dose at depth 10 cm; dose-response linearity with MU, beam profile correction, collimator scatter and water kernel. The model accuracy was validated with 10 SRS/SRT plans. The traditional diode arrays with MapCHECK were also used to perform patient-specific QA for assuring model accuracy. The 150 cm-SID was found a possibility to reduce the saturation effect. The result of model accuracy was found good agreement between our EPID-based dosimetry and TPS calculation with GPR more than 98% for gamma criteria of 3%/3 mm, more than 95% for gamma criteria of 2%/2 mm, and the results related to the measurement with MapCHECK. This study demonstrated the method to perform SRT and SRT patient-specific QA using EPID-based dosimetry in the FFF beam by co-operating between the extended SID that can reduce the saturation effect and estimate the planar dose distribution with the in-house model.

Linac reported steering error insensitive to 6 MV FFF transverse beam position deviations

Clinically significant beam position deviations were observed for a 6 MV FFF beam during patient specific QA on an Elekta linear accelerator. There was no significant reported transverse steering error from the machine ion chamber, and routine linac QA practices, including cardinal angle Winston-Lutz test, did not identify the deviations. Subsequent investigation using an electronic portal imaging device (EPID) revealed clinically significant beam position deviations for small steering errors. This prompted investigation into further impact and possible solutions. Testing set-points were established by adjusting transverse steering current to achieve introduced 2 T steering errors. Tests at each set-point included scanning water tank profiles and EPID images. A proposed method for adjusting the 2 T error sensitivity was tested via adjusting the 2 T loop parameter such that a reported error corresponds to specific beam position deviations. The testing set-points resulted in positional deviations of greater than 3 mm for reported errors of less than 1. A new method for improving 2 T error sensitivity was implemented. This work has shown that existing vendor protocol for establishing beam steering error for 6 MV FFF beams can lead to beam position deviations without machine interlocks or significant reported steering errors. Thus, an alternative method of establishing steering error sensitivity based on positional deviations is presented.

Comparison of Moderate Hypofractionated Volumetric-Modulated Arc Therapy Plans With and Without Flattening Filter for Localized Prostate Cancer

Background/Aim The aim of this study was to compare volumetric-modulated arc therapy (VMAT) radiation plans between conventional VMAT with flattening filter (cFF-VMAT) and flattening filter-free VMAT (FFF-VMAT) for localized prostate cancer. Materials and methods Ten patients with localized prostate cancer who underwent cFF-VMAT at Yokosuka General Hospital Uwamachi, Yokosuka, Japan, from July 2020 to October 2020 were enrolled. Dose-volume histogram (DVH) parameters of the target volume, normal organs, monitor units (MU), and beam-on time (BOT) were compared between cFF-VMAT and FFF-VMAT plans. Results No significant difference was observed for DVH parameters for the target volume. No significant difference was observed in all parameters for the bladder and rectum between the cFF-VMAT and FFF-VMAT groups. The mean values of MU were 686 ± 52 and 784 ± 80 in cFF-VMAT and FFF-VMAT, respectively (p < 0.001). The mean BOT was 97.0 ± 6.6 s and 72.9 ± 1.4 s for cFF-VMAT and FFF-VMAT, respectively (p < 0.001). Conclusion DVH parameters of the target volume and normal organs were not significantly different between the cFF-VMAT and FFF-VMAT plans. In FFF-VMAT, MU was significantly higher, and the BOT was significantly shorter than those in cFF-VMAT.

Fast generation of lung SBRT plans with a knowledge-based planning model on ring-mounted Halcyon Linac

Purpose

To demonstrate fast treatment planning feasibility of stereotactic body radiation therapy (SBRT) for centrally located lung tumors on Halcyon Linac via a previously validated knowledge‐based planning (KBP) model to support offline adaptive radiotherapy.

Materials/methods

Twenty previously treated non‐coplanar volumetric‐modulated arc therapy (VMAT) lung SBRT plans (c‐Truebeam) on SBRT‐dedicated C‐arm Truebeam Linac were selected. Patients received 50 Gy in five fractions. c‐Truebeam plans were re‐optimized for Halcyon manually (m‐Halcyon) and with KBP model (k‐Halcyon). Both m‐Halcyon and k‐Halcyon plans were normalized for identical or better target coverage than clinical c‐Truebeam plans and compared for target conformity, dose heterogeneity, dose fall‐off, and dose tolerances to the organs‐at‐risk (OAR). Treatment delivery parameters and planning times were evaluated.

Results

k‐Halcyon plans were dosimetrically similar or better than m‐Halcyon and c‐Truebeam plans. k‐Halcyon and m‐Halcyon plan comparisons are presented with respect to c‐Truebeam. Differences in conformity index were statistically insignificant in k‐Halcyon and on average 0.02 higher (p = 0.04) in m‐Halcyon plans. Gradient index was on average 0.43 (p = 0.006) lower and 0.27 (p = 0.02) higher for k‐Halcyon and m‐Halcyon, respectively. Maximal dose 2 cm away in any direction from target was statistically insignificant. k‐Halcyon increased maximal target dose on average by 2.9 Gy (p < 0.001). Mean lung dose was on average reduced by 0.10 Gy (p = 0.004) in k‐Halcyon and increased by 0.14 Gy (p < 0.001) in m‐Halcyon plans. k‐Halcyon plans lowered bronchial tree dose on average by 1.2 Gy. Beam‐on‐time (BOT) was increased by 2.85 and 1.67 min, on average for k‐Halcyon and m‐Halcyon, respectively. k‐Halcyon plans were generated in under 30 min compared to estimated dedicated 180 ± 30 min for m‐Halcyon or c‐Truebeam plan.

Conclusion

k‐Halcyon plans were generated in under 30 min with excellent plan quality. This adaptable KBP model supports high‐volume clinics in the expansion or transfer of lung SBRT patients to Halcyon.

Dosimetric impact of FFF over FF beam using VMAT for brain neoplasms treated with radiotherapy

Background: This study was conducted to assess the dosimetric impact of FFF beam plans on high-grade brain neoplasms using the VMAT technique when compared with FF beam plans.

Material and Methods: Thirty patients with high-grade brain neoplasms, who had received radiotherapy using VMAT technique retrospectively were selected for this study. All the patients were planned for VMAT using 6MV_FF beam and the same plan was re-optimized using 6MV_FFF beam keeping the same dose constraint. Radiotherapy dose distribution on planning target volume (PTV) and organs at risk (OAR), target conformity index (CI), Homogeneity Index (HI), Low dose volume in the patient (V5, V10, V20, and V30), and Integral dose to the whole body in both plans were compared.

Results: The PTV coverage and OAR’s showed no significant differences in dose distribution between the FFF and FF beam VMAT planning. There was a reduction of the average maximum dose in the right eye, left eye, right optic nerve, and left optic nerve using FFF beams. The reduction in average low dose volume was observed in V5, V10, V20, V30, and Mean Dose. Also, a significant reduction was observed in the integral dose to the whole body using the FFF beam.

Conclusions: Using FFF beams with VMAT is doable for the treatment of high-grade brain neoplasms, and the delivery mode of the FFF beam in VMAT may yield similar results to FF beam which should be confirmed in a large scale prospective clinical trial.

Survey on utilization of flattening filter-free photon beams in Japan

To understand the current state of flattening filter-free (FFF) beam implementation in C-arm linear accelerators (LINAC) in Japan, the quality assurance (QA)/quality control (QC) 2018-2019 Committee of the Japan Society of Medical Physics (JSMP) conducted a 37-question survey, designed to investigate facility information and specifications regarding FFF beam adoption and usage. The survey comprised six sections: facility information, devices, clinical usage, standard calibration protocols, modeling for treatment planning (TPS) systems and commissioning and QA/QC. A web-based questionnaire was developed. Responses were collected between 18 June and 18 September 2019. Of the 846 institutions implementing external radiotherapy, 323 replied. Of these institutions, 92 had adopted FFF beams and 66 had treated patients using them. FFF beams were used in stereotactic radiation therapy (SRT) for almost all disease sites, especially for the lungs using 6 MV and liver using 10 MV in 51 and 32 institutions, respectively. The number of institutions using FFF beams for treatment increased yearly, from eight before 2015 to 60 in 2018. Farmer-type ionization chambers were used as the standard calibration protocol in 66 (72%) institutions. In 73 (80%) institutions, the beam-quality conversion factor for FFF beams was calculated from TPR20,10, via the same protocol used for beams with flattening filter (WFF). Commissioning, periodic QA and patient-specific QA for FFF beams also followed the procedures used for WFF beams. FFF beams were primarily used in high-volume centers for SRT. In most institutions, measurement and QA was conducted via the procedures used for WFF beams.

A novel restricted single-isocenter stereotactic body radiotherapy (RESIST) method for synchronous multiple lung lesions to minimize setup uncertainties

Treating multiple lung lesions synchronously using a single-isocenter volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) plan can improve treatment efficiency and patient compliance. However, due to set up uncertainty, aligning multiple lung tumors on a single daily cone beam CT (CBCT) image has shown clinically unacceptable loss of target(s) coverage. Herein, we propose a Restricted Single-Isocenter Stereotactic Body Radiotherapy (RESIST), an alternative treatment that mitigates patient setup uncertainties. Twenty-one patients with two lung lesions were treated with single-isocenter VMAT-SBRT using a 6MV-FFF beam to 54 Gy in 3 fractions (n = 7) or 50 Gy in 5 fractions (n = 14) prescribed to 70-80% isodose line. To minimize setup uncertainties, each plan was re-planned using the RESIST method, utilizing a single-isocenter placed at the patient's mediastinum. It allows for an individual plan to be created for each tumor, using the first plan as the base-dose for the second plan, while still allowing both tumors to be treated in the same session. The technique uses novel features in Eclipse, including dynamic conformal arc (DCA)-based dose and aperture shape controller before each VMAT optimization. RESIST plans provided better target dose conformity (p < 0.001) and gradient indices (p < 0.001) and lower dose to adjacent critical organs. Using RESIST to treat synchronous lung lesions with VMAT-SBRT significantly reduces plan complexity as demonstrated by smaller beam modulation factors (p < 0.001), without unreasonably increasing treatment time. RESIST reduces the chance of a geometric miss due by allowing CBCT matching of one tumor at a time. Placement of isocenter at the mediastinum avoids potential patient/gantry collisions, provides greater flexibility of noncoplanar arcs and eliminates the need for multiple couch movements during CBCT imaging. Efficacy of RESIST has been demonstrated for two lesions and can potentially be used for more lesions. Clinical implementation of this technique is ongoing.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

Out-of-field dose in stereotactic radiotherapy for paediatric patients

Background and purpose

Stereotactic radiotherapy combines image guidance and high precision delivery with small fields to deliver high doses per fraction in short treatment courses. In preparation for extension of these treatment techniques to paediatric patients we characterised and compared doses out-of-field in a paediatric anthropomorphic phantom for small flattened and flattening filter free (FFF) photon beams.

Method and materials

Dose measurements were taken in several organs and structures outside the primary field in an anthropomorphic phantom of a 5 year old child (CIRS) using thermoluminescence dosimetry (LiF:Mg,Cu,P). Out-of-field doses from a medical linear accelerator were assessed for 6 MV flattened and FFF beams of field sizes between 2 × 2 and 10 × 10 cm².

Results

FFF beams resulted in reduced out-of-field doses for all field sizes when compared to flattened beams. Doses for FFF and flattened beams converged for all field sizes at larger distances (>40 cm) from the central axis as leakage becomes the primary source of out-of-field dose. Rotating the collimator to place the MLC bank in the longitudinal axis of the patient was shown to reduce the peripheral doses measured by up to 50% in Varian linear accelerators.

Conclusion

Minimising out-of-field doses by using FFF beams and aligning the couch and collimator to provide tertiary shielding demonstrated advantages of small field, FFF treatments in a paediatric setting.

AAPM Task Group 198 Report: An implementation guide for TG 142 quality assurance of medical accelerators

The charges on this task group (TG) were as follows: (a) provide specific procedural guidelines for performing the tests recommended in TG 142; (b) provide estimate of the range of time, appropriate personnel, and qualifications necessary to complete the tests in TG 142; and (c) provide sample daily, weekly, monthly, or annual quality assurance (QA) forms. Many of the guidelines in this report are drawn from the literature and are included in the references. When literature was not available, specific test methods reflect the experiences of the TG members (e.g., a test method for door interlock is self-evident with no literature necessary). In other cases, the technology is so new that no literature for test methods was available. Given broad clinical adaptation of volumetric modulated arc therapy (VMAT), which is not a specific topic of TG 142, several tests and criteria specific to VMAT were added.

Assessment of the Sun Nuclear ArcCHECK to detect errors in 6MV FFF VMAT delivery of brain SABR using ROC analysis

Institutions use a range of different detector systems for patient-specific quality assurance (QA) measurements conducted to assure that the dose delivered by a patient's radiotherapy treatment plan matches the calculated dose distribution. However, the ability of different detectors to detect errors from different sources is often unreported. This study contains a systematic evaluation of Sun Nuclear's ArcCHECK in terms of the detectability of potential machine-related treatment errors. The five investigated sources of error were multileaf collimator (MLC) leaf positions, gantry angle, collimator angle, jaw positions, and dose output. The study encompassed the clinical treatment plans of 29 brain cancer patients who received stereotactic ablative radiotherapy (SABR). Six error magnitudes were investigated per source of error. In addition, the Eclipse AAA beam model dosimetric leaf gap (DLG) parameter was varied with four error magnitudes. Error detectability was determined based on the area under the receiver operating characteristic (ROC) curve (AUC). Detectability of DLG errors was good or excellent (AUC >0.8) at an error magnitude of at least ±0.4 mm, while MLC leaf position and gantry angle errors reached good or excellent detectability at error magnitudes of at least 1.0 mm and 0.6°, respectively. Ideal thresholds, that is, gamma passing rates, to maximize sensitivity and specificity ranged from 79.1% to 98.7%. The detectability of collimator angle, jaw position, and dose output errors was poor for all investigated error magnitudes, with an AUC between 0.5 and 0.6. The ArcCHECK device's ability to detect errors from treatment machine-related sources was evaluated, and ideal gamma passing rate thresholds were determined for each source of error. The ArcCHECK was able to detect errors in DLG value, MLC leaf positions, and gantry angle. The ArcCHECK was unable to detect the studied errors in collimator angle, jaw positions, and dose output.