Monitor Units Research Articles

Dosimetric evaluation and gradient analysis of various MLC leaf-width effects in external beam radiation therapy: TrueBeam Vs Halcyon

IntroductionThis study investigates dosimetric influence and gradient-analysis of leaf-width of various Multi-leaf-collimators (MLC) from Truebeam and Halcyon linear accelerators. MethodsThe leaf-width effects of Millennium120 and HD120MLC from Truebeam and SX1 and SX2 from Halcyon were studied using virtual phantom in Eclipse16.1.2 TPS. Target structures consist of bore and wave cylinders of equally spaced projected Planning target volume (PTV) of diameters ranging from 1-to-5 cm and 20 cm length. Treatment plans for all PTVs, and four different MLCs configurations from both machines were created utilizing 6MVFFF beam to deliver dose of 50Gy/25# using IMRT and VMAT. Plans were evaluated using plan quality indices including dose conformity, homogeneity, gradient radius, and Monitor-unit (MU). Also, dose gradients were analyzed by estimating distinct integral volume VD% and paired-t-tests were performed to evaluate statistical differences. ResultsAll the plan satisfied the minimum criteria of D95 %≥prescription dose and V107 %≤2cc. Mean conformity and homogeneity indices for SX1MLC(CI = 0.680,HI = 0.022) were found significantly higher and lower than SX2(CI = 0.746,HI = 0.016), Millennium120 (CI = 0.739,HI = 0.012), and HD120MLC(CI = 0.745,HI = 0.017), respectively. However, CI and HI for SX2, Millennium120, and HD120MLC found comparable. Gradient radius enclosing 50 % of isodose observed maximum and minimum for SX1 and HD120MLC, respectively. Plan MUs for Truebeam MLCs were found approximately 25 % higher than Halcyon MLCs. Dose distribution generated using SX2 and Millennium120 found comparable, however p ≤ 0.05 ascertained substantial differences among SX1, Millennium120, and HD120MLC. ConclusionMLC leaf-width influences intensity-modulation and significantly alters dosimetric outcome depending on the magnitude of the leaf-width. HD120MLC does not show much significant advantages over SX2 and Millennium120 except gradient control. Implications for practiceHalcyon SX1MLC produces least effective plan compared to all other MLC types. However, SX2 and Millennium120 can produce plans of comparable quality except plan MUs difference.

An evaluation of ankle and foot bolus in paediatric modulated arc total body irradiation (MATBI).

This retrospective planning study aimed to evaluate the role of bolus in achieving dose uniformity in the ankles and feet in paediatric patients undergoing Modulated Arc Total Body Irradiation (MATBI) treatment and to identify patient factors that may negate or warrant its use. The clinically treated plans of 20 paediatric patients who received MATBI treatment utilising ankle and foot bolus (Bolus plan) were compared with two retrospectively generated plans; a plan with bolus removed and no re-optimisation (No Bolus plan), and a re-optimised plan without bolus attempting to achieve equal dosimetry to the clinical plan via monitor unit adjustment (MU plan). Descriptive statistics were used to evaluate the dose uniformity criteria of ±10% coverage of the reference dose (RD) for each subregion of the ankle and foot for the three plans. The impact of patient height, weight, and age at the time of treatment was evaluated using Spearman's correlation. Variation in doses >10% RD was minimal across the three plans, with an average D1cc difference < 0.4Gy. For the ankle and foot regions in the Bolus plans, the volume receiving at least 90% of the RD (V90) was on average > 92%. In No Bolus and MU plans, there was an average reduction of 24.5% and 23.2% V90 coverage respectively in the toes. Spearman's correlation suggests height has the strongest relationship to D1cc. This study validated the continued use of ankle and foot bolus to achieve dosimetric goals for paediatric MATBI treatments, particularly V90 coverage across all heights.

Dosimetry and efficiency comparison of knowledge-based and manual planning using volumetric modulated arc therapy for craniospinal irradiation.

Craniospinal irradiation (CSI) poses a challenge to treatment planning due to the large target, field junction, and multiple organs at risk (OARs) involved. The aim of this study was to evaluate the performance of knowledge-based planning (KBP) in CSI by comparing original manual plans (MP), KBP RapidPlan initial plans (RPI), and KBP RapidPlan final plans (RPF), which received further re-optimization to meet the dose constraints. Dose distributions in the target were evaluated in terms of coverage, mean dose, conformity index (CI), and homogeneity index (HI). The dosimetric results of OARs, planning time, and monitor unit (MU) were evaluated. All MP and RPF plans met the plan goals, and 89.36% of RPI plans met the plan goals. The Wilcoxon tests showed comparable target coverage, CI, and HI for the MP and RPF groups; however, worst plan quality was demonstrated in the RPI plans than in MP and RPF. For the OARs, RPF and RPI groups had better dosimetric results than the MP group (P < 0.05 for optic nerves, eyes, parotid glands, and heart). The planning time was significantly reduced by the KBP from an average of 677.80 min in MP to 227.66 min (P < 0.05) and 307.76 min (P < 0.05) in RPI, and RPF, respectively. MU was not significantly different between these three groups. The KBP can significantly reduce planning time in CSI. Manual re-optimization after the initial KBP is recommended to enhance the plan quality.

Predicting the error magnitude in patient-specific QA during radiotherapy based on ResNet.

The error magnitude is closely related to patient-specific dosimetry and plays an important role in evaluating the delivery of the radiotherapy plan in QA. No previous study has investigated the feasibility of deep learning to predict error magnitude. The purpose of this study was to predict the error magnitude of different delivery error types in radiotherapy based on ResNet. A total of 34 chest cancer plans (172 fields) of intensity-modulated radiation therapy (IMRT) from Eclipse were selected, of which 30 plans (151 fields) were used for model training and validation, and 4 plans including 21 fields were used for external testing. The collimator misalignment (COLL), monitor unit variation (MU), random multi-leaf collimator shift (MLCR), and systematic MLC shift (MLCS) were introduced. These dose distributions of portal dose predictions for the original plans were defined as the reference dose distribution (RDD), while those for the error-introduced plans were defined as the error-introduced dose distribution (EDD). Different inputs were used in the ResNet for predicting the error magnitude. In the test set, the accuracy of error type prediction based on the dose difference, gamma distribution, and RDD + EDD was 98.36%, 98.91%, and 100%, respectively; the root mean squared error (RMSE) was 1.45-1.54, 0.58-0.90, 0.32-0.36, and 0.15-0.24; the mean absolute error (MAE) was 1.06-1.18, 0.32-0.78, 0.25-0.27, and 0.11-0.18, respectively, for COLL, MU, MLCR and MLCS. In this study, error magnitude prediction models with dose difference, gamma distribution, and RDD + EDD are established based on ResNet. The accurate prediction of the error magnitude under different error types can provide reference for error analysis in patient-specific QA.

Dosimetric comparison of O-ring versus conventional C-arm linear accelerator for volumetric modulated arc therapy (VMAT) of head and neck cancer and carcinoma cervix patients.

The objective of this study is dosimetric comparison between the O-ring Halcyon and C-arm Clinac iX linac for volumetric modulated arc therapy (VMAT) plans for head & neck (H&N) cancer and carcinoma cervix patients. Total 60 patients of H&N cancer and carcinoma cervix were enrolled prospectively from March 2021 to March 2023. VMAT plans with 6 MV photons for Halcyon and Clinac iX were generated and compared for each patient by dose volume histogram for planning target volume coverage and organ at risk (OAR) sparing. There were no differences in between both the linacs for PTV D2% and D98%, homogeneity index, conformity index, Dmax (maximum dose) and Dmean (mean dose) of OAR. Halcyon had significantly shorter treatment time compared to Clinac iX. Halcyon delivered higher integral dose and monitor units. O-ring Halcyon produces VMAT plans comparable to other C-arm linacs for H&N and carcinoma cervix patients.

Spot-scanning proton therapy for early breast cancer in free breathing versus deep inspiration breath-hold.

Proton therapy for breast cancer is usually given in free breathing (FB). With the use of deep inspiration breath-hold (DIBH) technique, the location of the heart is displaced inferiorly, away from the internal mammary nodes and, thus, the dose to the heart can potentially be reduced. The aim of this study was to explore the potential benefit of proton therapy in DIBH compared to FB for highly selected patients to reduce exposure of the heart and other organs at risk. We aimed at creating proton plans with delivery times feasible with treatment in DIBH. Sixteen patients with left-sided breast cancer receiving loco-regional proton therapy were included. The FB and DIBH plans were created for each patient using spot-scanning proton therapy with 2-3 fields, robust and single field optimization. For the DIBH plans, minimum monitor unit per spot and spot spacing were increased to reduce treatment delivery time. All plans complied with target coverage constraints. The median mean heart dose was statistically significant reduced from 1.1 to 0.6 Gy relative biological effectiveness (RBE) by applying DIBH. No statistical significant difference was seen for mean dose and V17Gy RBE to the ipsilateral lung. The median treatment delivery time for the DIBH plans was reduced by 27% compared to the FB plans without compromising the plan quality. The median absolute reduction in dose to the heart was limited. Proton treatment in DIBH may only be relevant for a subset of these patients with the largest reduction in heart exposure.

A generalized model for monitor units determination in ocular proton therapy using machine learning: A proof-of-concept study

Objective. Determining and verifying the number of monitor units is crucial to achieving the desired dose distribution in radiotherapy and maintaining treatment efficacy. However, current commercial treatment planning system(s) dedicated to ocular passive eyelines in proton therapy do not provide the number of monitor units for patient-specific plan delivery. Performing specific pre-treatment field measurements, which is time and resource consuming, is usually gold-standard practice. This proof-of-concept study reports on the development of a multi-institutional-based generalized model for monitor units determination in proton therapy for eye melanoma treatments. Approach. To cope with the small number of patients being treated in proton centers, three European institutes participated in this study. Measurements data were collected to address output factor differences across the institutes, especially as function of field size, spread-out Bragg peak modulation width, residual range, and air gap. A generic model for monitor units prediction using a large number of 3748 patients and broad diversity in tumor patterns, was evaluated using six popular machine learning algorithms: (i) decision tree; (ii) random forest, (iii) extra trees, (iv) K-nearest neighbors, (v) gradient boosting, and (vi) the support vector regression. Features used as inputs into each machine learning pipeline were: Spread-out Bragg peak width, range, air gap, fraction and calibration doses. Performance measure was scored using the mean absolute error, which was the difference between predicted and real monitor units, as collected from institutional gold-standard methods. Main results. Predictions across algorithms were accurate within 3% uncertainty for up to 85.2% of the plans and within 10% uncertainty for up to 98.6% of the plans with the extra trees algorithm. Significance. A proof-of-concept of using machine learning-based generic monitor units determination in ocular proton therapy has been demonstrated. This could trigger the development of an independent monitor units calculation tool for clinical use.

Assessing quality assurance of multi-leaf collimator using the structural similarity index.

This study aims to evaluate the viability of utilizing the Structural Similarity Index (SSI*) as an innovative imaging metric for quality assurance (QA) of the multi-leaf collimator (MLC). Additionally, we compared the results obtained through SSI* with those derived from a conventional Gamma index test for three types of Varian machines (Trilogy, Truebeam, and Edge) over a 12-week period of MLC QA in our clinic. To assess sensitivity to MLC positioning errors, we designed a 1cm slit on the reference MLC, subsequently shifted by 0.5-5mm on the target MLC. For evaluating sensitivity to output error, we irradiated five 25cm×25cm open fields on the portal image with varying Monitor Units (MUs) of 96-100. We compared SSI* and Gamma index tests using three linear accelerator (LINAC) machines: Varian Trilogy, Truebeam, and Edge, with MLC leaf widths of 1, 0.5, and 0.25mm. Weekly QA included VMAT and static field modes, with Picket fence test images acquired. Mechanical uncertainties related to the LINAC head, electronic portal imaging device (EPID), and MLC during gantry rotation and leaf motion were monitored. The Gamma index test started detecting the MLC shift at a threshold of 4mm, whereas the SSI* metric showed sensitivity to shifts as small as 2mm. Moreover, the Gamma index test identified dose changes at 95MUs, indicating a 5% dose difference based on the distance to agreement (DTA)/dose difference (DD) criteria of 1mm/3%. In contrast, the SSI* metric alerted to dose differences starting from 97MUs, corresponding to a 3% dose difference. The Gamma index test passed all measurements conducted on each machine. However, the SSI* metric rejected all measurements from the Edge and Trilogy machines and two from the Truebeam. Our findings demonstrate that the SSI* exhibits greater sensitivity than the Gamma index test in detecting MLC positioning errors and dose changes between static and VMAT modes. The SSI* metric outperformed the Gamma index test regarding sensitivity across these parameters.

Evaluation of conventional IMRT and VMAT strategies for postmastectomy radiation therapy after immediate implant-based reconstruction using the new ESTRO-ACROP contouring guidelines.

This study evaluated the usability of conventional templates based on the new contour guidelines of the European Society of Radiation and Oncology and Advisory Committee in Radiation Oncology Practice (ESTRO-ACROP) for treatment plans of postmastectomy radiotherapy after immediate implant-based reconstruction. Intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans generated with two different treatment planning systems (TPSs, Eclipse and Monaco) were examined. Six computed tomography scans of patients aged 35-54years were retrospectively analysed who had undergone mastectomy and breast reconstruction using silicone implants after being diagnosed with left breast cancer. Six radiation oncologists participated in this study, and each of them contoured the target volume of one left breast using conventional contour (CTV-CONV) and new contour (CTV-ESTRO) methods. This study showed that compared with CTV-CONV, using CTV-ESTRO with objectives and cost functions similar to those of TPSs worsened the target volume coverage and increased the total number of monitor units. Considering the organs at risk, CTV-ESTRO tended to increase the mean dose delivered to the contralateral lung. It is concluded that the approach used for the new ESTRO-ACROP contour method cannot be applied in a manner similar to that for the conventional breast contour method, implying that the new ESTRO-ACROP contour method may require more time for improving plans for a given treatment.

Single isocenter versus dual isocenter treatment using flattening filter-free and jaw-tracking volumetrically modulated arc therapy for boot-shaped lung cancer: Evaluation of dosimetric and feasibility.

To determine whether a dual-isocenter volumetrically modulated arc therapy (VMAT) technique results in lower normal pulmonary dosage compared to a traditional single isocenter technique for boot-shaped lung cancer. A cohort of 15 patients with advanced peripheral or central lung cancer who had metastases in the mediastinum and supraclavicular lymph nodes was randomly selected for this retrospective study. VMAT plans were generated for each patient using two different beam alignment techniques with the 6-MV flattening filter-free (FFF) photon beam: single-isocenter jaw-tracking VMAT based on the Varian TrueBeam linear accelerator (S-TV), and dual-isocenter VMAT based on both TrueBeam (D-TV) and Halcyon linear accelerator (D-HV). For all 45 treatment plans, planning target volume (PTV) dose coverage, conformity/homogeneity index (CI/HI), mean heart dose (MHD), mean lung dose (MLD) and the total lung tissue receiving 5, 20, 30Gy (V5, V20, V30) were evaluated. The monitor units (MUs), delivery time, and plan quality assurance (QA) results were recorded. The quality of the objectives of the three plans was comparable to each other. In comparison with S-TV, D-TV and D-HV improved the CI and HI of the PTV (p<0.05). The MLD was 13.84±1.44Gy (mean±SD) for D-TV, 14.22±1.30Gy and 14.16±1.42Gy for S-TV and D-HV, respectively. Lungs-V5Gy was 50.78±6.24%, 52.00±7.32% and 53.36±8.48%, Lungs-V20Gy was 23.72±2.27%, 26.18±2.86% and 24.96±3.09%, Lungs-V30Gy was 15.69±1.76%, 17.20±1.72% and 16.52±2.07%. Compared to S-TV, D-TV provided statistically significant better protection for the total lung, with the exception of the lungs-V5. All plans passed QA according the gamma criteria of 3%/3mm. Taking into account the dosimetric results and published clinical data on radiation-induced pulmonary injury, dual-isocenter jaw-tracking VMAT may be the optimal choice for treating boot-shaped lung cancer.

Beam control system and output fine-tuning for safe and precise delivery of FLASH radiotherapy at a clinical linear accelerator.

We have previously adapted a clinical linear accelerator (Elekta Precise, Elekta AB) for ultra-high dose rate (UHDR) electron delivery. To enhance reliability in future clinical FLASH radiotherapy trials, the aim of this study was to introduce and evaluate an upgraded beam control system and beam tuning process for safe and precise UHDR delivery. The beam control system is designed to interrupt the beam based on 1) a preset number of monitor units (MUs) measured by a monitor detector, 2) a preset number of pulses measured by a pulse-counting diode, or 3) a preset delivery time. For UHDR delivery, an optocoupler facilitates external control of the accelerator's thyratron trigger pulses. A beam tuning process was established to maximize the output. We assessed the stability of the delivery, and the independent interruption capabilities of the three systems (monitor detector, pulse counter, and timer). Additionally, we explored a novel approach to enhance dosimetric precision in the delivery by synchronizing the trigger pulse with the charging cycle of the pulse forming network (PFN). Improved beam tuning of gun current and magnetron frequency resulted in average dose rates at the dose maximum at isocenter distance of >160 Gy/s or >200 Gy/s, with or without an external monitor chamber in the beam path, respectively. The delivery showed a good repeatability (standard deviation (SD) in total film dose of 2.2%) and reproducibility (SD in film dose of 2.6%). The estimated variation in DPP resulted in an SD of 1.7%. The output in the initial pulse depended on the PFN delay time. Over the course of 50 measurements employing PFN synchronization, the absolute percentage error between the delivered number of MUs calculated by the monitor detector and the preset MUs was 0.8 ± 0.6% (mean ± SD). We present an upgraded beam control system and beam tuning process for safe and stable UHDR electron delivery of hundreds of Gy/s at isocenter distance at a clinical linac. The system can interrupt the beam based on monitor units and utilize PFN synchronization for improved dosimetric precision in the dose delivery, representing an important advancement toward reliable clinical FLASH trials.

Effect of prescription isodose line on tissue sparing in linear accelerator-based stereotactic radiosurgery treating multiple brain metastases using dynamic conformal arcs.

Linear accelerator-based stereotactic radiosurgery (SRS) has become a mainstay for simultaneous management of multiple intracranial targets. Recent improvements in treatment planning systems (TPS) have enabled treatment of multiple brain metastases using dynamic conformal arcs (DCA) and a single treatment isocenter. However, as the volume of healthy tissue receiving at least 12Gy (V12) is linked to the probability of developing radionecrosis, balancing target coverage while minimizing V12 is a critical factor affecting SRS plan quality. Current TPS allow users to adjust various parameters influencing plan optimization. The purpose of this work is to quantify the effect of negative margins on V12 for cranial SRS plans managing multiple brain metastases. Using the Brainlab Elements v3.0 TPS (Brainlab, Munich, Germany), we calculated V10, V12, V15, monitor units, and conformity index for seventeen SRS plans treating 2-10 metastases on our Elekta Versa HD (Elekta, Stockholm, Sweden) linear accelerator. We compared plans optimized using 70%-90% prescription isodose lines (IDL) in 5% increments. Irrespective of the number of treated metastases, optimization at a lower prescription IDL reduced V10, V12, and V15 and increased MU compared to the 90% IDL (p<0.01). However, comparing the 70% and 75% IDL optimizations, there was little difference in tissue sparing. The conformity index showed no consistent trends at different IDLs due to a significant spread in case data. For our plans treating up to 10 metastases, diminishing returns for tissue sparing at IDLs below 80% paired with increasing treatment MU and dosimetric hot spot made optimization at lower IDLs less favorable. In our clinic, after consulting with a physician, it was determined that optimization at the 80% IDL achieved the best balance of V12, treatment MU, and maximum dose. Clinics implementing LINAC-based SRS programs may consider using similar evaluations to develop their own clinical protocols.

Technical note: A small animal irradiation platform for investigating the dependence of the FLASH effect on electron beam parameters.

The recent rediscovery of the FLASH effect, a normal tissue sparing phenomenon observed in ultra-high dose rate (UHDR) irradiations, has instigated a surge of research endeavors aiming to close the gap between experimental observation and clinical treatment. However, the dependences of the FLASH effect and its underpinning mechanisms on beam parameters are not well known, and large-scale in vivo studies using murine models of human cancer are needed for these investigations. To commission a high-throughput, variable dose rate platform providing uniform electron fields(≥15cm diameter) at conventional (CONV) and UHDRs for in vivo investigations of the FLASH effect and its dependences on pulsed electron beam parameters. A murine whole-thoracic lung irradiation (WTLI) platform was constructed using a 1.3cm thick Cerrobend collimator forming a 15 × 1.6 cm2 slit. Control of dose and dose rate were realized by adjusting the number of monitor units and couch vertical position, respectively. Achievable doses and dose rates were investigated using Gafchromic EBT-XD film at 1cm depth in solid water and lung-density phantoms. Percent depth dose (PDD) and dose profiles at CONV and various UHDRs were also measured at depths from 0 to 2cm. A radiation survey was performed to assess radioactivation of the Cerrobend collimator by the UHDR electron beam in comparison to a precision-machined copper alternative. This platform allows for the simultaneous thoracic irradiation of at least three mice. A linear relationship between dose and number of monitor units at a given UHDR was established to guide the selection of dose, and an inverse-square relationship between dose rate and source distance was established to guide the selection of dose rate between 20 and 120Gy·s-1 . At depths of 0.5 to 1.5cm, the depth range relevant to murine lung irradiation, measured PDDs varied within±1.5%. Similar lateral dose profiles were observed at CONV and UHDRs with the dose penumbrae widening from 0.3mm at 0cm depth to 5.1mm at 2.0cm. The presence of lung-density plastic slabs had minimal effect on dose distributions as compared to measurements made with only solid water slabs. Instantaneous dose rate measurements of the activated copper collimator were up to two orders of magnitude higher than that of the Cerrobend collimator. A high-throughput, variable dose rate platform has been developed and commissioned for murine WTLI electron FLASH radiotherapy. The wide field of our UHDR-enabled linac allows for the simultaneous WTLI of at least three mice, and for the average dose rate to be modified by changing the source distance, without affecting dose distribution. The platform exhibits uniform, and comparable dose distributions at CONV and UHDRs up to 120Gy·s-1 , owing to matched and flattened 16 MeV CONV and UHDR electron beams. Considering radioactivation and exposure to staff, Cerrobend collimators are recommended above copper alternatives for electron FLASH research. This platform enables high-throughput animal irradiation, which is preferred for experiments using a large number of animals, which are required to effectively determine UHDR treatment efficacies.

Optimization of stereotactic radiotherapy (SRT) planning for brain metastases from lung cancer by Cyberknife system: reducing dose distribution inner ear.

Objective: Through retrospective statistical analysis of radiation distribution in inner ear avoidance for brain metastases from lung cancer by the CyberKnife (CK) system, it can provide a reference for stereotactic radiotherapy (SRT) planning and treatment optimization. Methods: Computed tomography/magnetic resonance imaging data of 44 patients with one brain metastases lesion from lung cancer were used to re-plan and analyze, who had been treated by CK system from April 2021 to April 2022. The prescribed doses of 14-30 Gy in 1-3 fractions was simultaneously delivered to the metastatic lesions. The SRT plans for the same patients were replaned under with and without inner ear avoidance setting. The plan parameters and dose distribution differences were compared between plans. Results: All plans met the dose restrictions. There were no significant differences in the coverage (Coverage), conformity index (CI), mean dose (Dmean), the maximum dose (Dmax) and minimum dose (Dmin) of planning target volume (PTV). With inner ear avoidance setting, the Dmax and Dmean of inner ear area decreased by 13.76% and 12.15% (p<0.01), respectively. The total number of machine nodes and monitor units (MU) increased by 4.63% and 1.06%. Conclusions: During the SRT plan designing for brain metastases from lung cancer, the dose distribution in inner ear area could be reduced by avoidance setting, and the patient's hearing would be well protected.

Plan complexity metrics for head and neck VMAT competition plans

Previous plan competitions have largely focused on dose metric assessments. However, whether the submitted plans were realistic and reasonable from a quality assurance (QA) perspective remains unclear. This study aimed to investigate the relationship between aperture-based plan complexity metrics (PCM) in volumetric modulated arc therapy (VMAT) competition plans and clinical treatment plans verified through patient-specific QA (PSQA). In addition, the association of PCMs with plan quality was examined. A head and neck (HN) plan competition was held for Japanese institutions from June 2019 to July 2019, in which 210 competition plans were submitted. Dose distribution quality was quantified based on dose-volume histogram (DVH) metrics by calculating the dose distribution plan score (DDPS). Differences in PCMs between the two VMAT treatment plan groups (HN plan competitions held in Japan and clinically accepted HN VMAT plans through PSQA) were investigated. The mean (± standard deviation) DDPS for the 98 HN competition plans was 158.5 ± 20.6 (maximum DDPS: 200). DDPS showed a weak correlation with PCMs with a maximum r of 0.45 for monitor unit (MU); its correlation with some PCMs was "very weak." Significant differences were found in some PCMs between plans with the highest 20% DDPSs and the remaining plans. The clinical VMAT and competition plans revealed similar distributions for some PCMs. Deviations in PCMs for the two groups were comparable, indicating considerable variability among planners regarding planning skills. The plan complexity for HN VMAT competition plans increased for high-quality plans, as shown by the dose distribution. Direct comparison of PCMs between competition plans and clinically accepted plans showed that the submitted HN VMAT competition plans were realistic and reasonable from the QA perspective. This evaluation may provide a set of criteria for evaluating plan quality in plan competitions.

Acute Toxicity of Total Body Irradiation Using Volumetric Arc Therapy With a Focus on the Effect of Lung Dose Rate

Purpose/ObjectivesTo report adverse effects of high dose TBI delivered using a VMAT technique and to assess pulmonary toxicity at dose rates of 40 and 100 monitor units per minute (MU/min). Materials/MethodsThis retrospective study included patients >18 years old who received ≥8 Gy TBI using a VMAT technique. The TBI dose was prescribed to a planning target volume (PTV) consisting of a 0.5cm retraction of the body with the lungs subtracted. The objective function specified PTV coverage goals of D100%≥90% and Dmax<130%. A lung dose control structure consisting of a 1 cm retraction of the lung volume was limited to Dmean<75%. Treatments were initially delivered with a dose rate of 40 MU/min for the thoracic isocenters and 100 MU/min for the other isocenters. Beginning in January 2021, a dose rate of 100 MU/min was used for all isocenters. All treatments were administered in 2 Gy fractions delivered twice-daily. Acute toxicity was assessed for 30 days following TBI. ResultsA total of 29 patients were included in this analysis who received TBI between January 2019 and October 2021. Prescription dose ranged from 8 – 12 Gy. Mean lung dose was 7.9 Gy (SD: 1.4 Gy) for patients treated at 40 MU/min and for patients treated at 100 MU/min 7.1 Gy (SD: 1.3 Gy). Mucositis was the most common grade 3 toxicity and occurred in 10 (34%) patients. Only 1 instance of pneumonitis was observed and occurred in a patient who received a mean lung dose of 10.1 Gy delivered at 40 MU/min. ConclusionsIn this cohort of patients who received high dose TBI using a VMAT technique, the composite rate of acute toxicity was not unexpectedly high. We did not observe an increase in lung toxicity after increasing the dose rate of the thoracic isocenters from 40 MU/min to 100 MU/min.

Multi-institutional investigation into the robustness of intra-cranial multi-target stereotactic radiosurgery plans to delivery errors.

The use of modulated techniques for intra-cranial stereotactic radiosurgery (SRS) results in highly modulated fields with small apertures, which may be susceptible to uncertainties in the delivery device. This study aimed to quantify the impact of simulated delivery errors on treatment plan dosimetry and how this is affected by treatment planning system (TPS), plan geometry, delivery technique, and plan complexity. A beam modelling error was also included as context to the dose uncertainties due to treatment delivery errors. Delivery errors were assessed for multiple-target brain SRS plans obtained through the Trans-Tasman Radiation Oncology Group (TROG) international treatment planning challenge (2018). The challenge dataset consisted of five intra-cranial targets, each with a prescription of 20Gy. Of the final dataset of 54 plans, 51 were created using the volumetric modulated arc therapy (VMAT) technique and three used intensity modulated arc therapy (IMRT). Thirty-five plans were from the Varian Eclipse TPS, 17 from Elekta Monaco TPS, and one plan each from RayStation and Philips Pinnacle TPS. The errors introduced included: monitor unit calibration errors, multi-leaf collimator (MLC) bank offset, single MLC leaf offset, couch rotations, and collimator rotations. Dosimetric leaf gap (DLG) error was also included as a beam modelling error. Dose to targets was assessed via dose covering 98% of planning target volume (PTV) (D98%), dose covering 2% of PTV (D2%), and dose covering 99% of gross tumor volume (GTV) (D99%). Dose to organs at risk (OARs) was assessed using the volume of normal brain receiving 12Gy (V12Gy), mean dose to normal brain, and maximum dose covering 0.03cc brainstem (D0.03cc). Plan complexity was also assessed via edge metric, modulation complexity score (MCS), mean MLC gap, mean MLC speed, and plan modulation (PM). PTV D98% showed high robustness on average to most errors with the exception of a bank shift of 1.0mm and large rotational errors ≥1.0° for either the couch or collimator. However, in some cases, errors close to or within generally accepted machine tolerances resulted in clinically relevant impacts. The greatest impact upon normal brain V12Gy, mean dose to normal brain, and D0.03cc brainstem was found for DLG error in alignment with other recent studies. All delivery errors had on average a minimal impact across these parameters. Comparing plans from the Monaco TPS and the Eclipse TPS, showed a lesser increase to V12Gy, mean dose to normal brain, and D0.03cc brainstem for Monaco plans (p<0.01) when DLG error was simulated. Monaco plans also correlated to lower plan complexity. Using Spearman's correlation coefficient (r) a strong negative correlation (r≤-0.8) was found between the mean MLC gap and dose to OARs for DLG errors. Reducing MLC complexity and using larger mean MLC gaps is recommended to improve plan robustness and reduce sensitivity to delivery and modelling errors. For cases in which the calculated dose distribution or dose indices are close to the clinically acceptable limits, this is especially important.

Treatment Optimization in Linac-Based SBRT for Localized Prostate Cancer: A Single-Arc versus Dual-Arc Plan Comparison.

This study aimed to comprehensively present data on treatment optimization in linac-based SBRT for localized prostate cancer at a single institution. Moreover, the dosimetric quality and treatment efficiency of single-arc (SA) versus dual-arc (DA) VMAT planning and delivery approaches were compared. Re-optimization was performed on twenty low-to-intermediate-risk- (36.25 Gy in 5 fractions) and twenty high-risk (42.7 Gy in 7 fractions) prostate plans initially administered with the DA FFF-VMAT technique in 2021. An SA approach was adopted, incorporating new optimization parameters based on increased planning and clinical experience. Analysis included target coverage, organ-at-risk (OAR) sparing, treatment delivery time, and the pre-treatment verification's gamma analysis-passing ratio. The SA optimization technique has consistently produced superior plans. Rectum and bladder mean doses were significantly reduced, and comparable target coverage and homogeneity were achieved in order to maintain a urethra protection strategy. The mean SA treatment delivery time was reduced by 22%; the mean monitor units increased due to higher plan complexity; and dose measurements demonstrated optimal agreement with calculations. The substantial reduction in treatment delivery time decreased the probability of prostate motion beyond the applied margins, suggesting potential decrease in treatment-related toxicity and improved target coverage in prostate SBRT. Further investigations are warranted to assess the long-term clinical outcomes.

Impact of ovary-sparing treatment planning on plan quality, treatment time and gamma passing rates in intensity-modulated radiotherapy for stage I/II cervical cancer.

This study aimed to investigate the impact of ovary-sparing intensity-modulated radiotherapy (IMRT) on plan quality, treatment time, and gamma passing rates for stage I/II cervical cancer patients. Fifteen stage I/II cervical cancer patients were retrospectively enrolled, and a pair of clinically suitable IMRT plans were designed for each patient, with (Group A) and without (Group B) ovary-sparing. Plan factors affecting plan quality, treatment time, and gamma passing rates, including the number of segments, monitor units, percentage of small-area segments (field area < 20 cm2), and percentage of small-MU segments (MU < 10), were compared and statistically analyzed. Key plan quality indicators, including ovarian dose, target dose coverage (D98%, D95%, D50%, D2%), conformity index, and homogeneity index, were evaluated and statistically assessed. Treatment time and gamma passing rates collected by IBA MatriXX were also compared. The median ovarian dose in Group A and Group B was 7.61 Gy (range 6.71-8.51 Gy) and 38.52 Gy (range 29.84-43.82 Gy), respectively. Except for monitor units, all other plan factors were significantly lower in Group A than in Group B (all P < .05). Correlation coefficients between plan factors, treatment time, and gamma passing rates that were statistically different were all negative. Both Groups of plans met the prescription requirement (D95% ≥ 45.00 Gy) for clinical treatment. D98% was smaller for Group A than for Group B (P < .05); D50% and D2% were larger for Group A than for Group B (P < .05, P < .05). Group A plans had worse conformity index and homogeneity index than Group B plans (P < .05, P < .05). Treatment time did not differ significantly (P > .05). Gamma passing rates in Group A were higher than in Group B with the criteria of 2%/3 mm (P < .05) and 3%/2 mm (P < .05). Despite the slightly decreased quality of the treatment plans, the ovary-sparing IMRT plans exhibited several advantages including lower ovarian dose and plan complexity, improved gamma passing rates, and a negligible impact on treatment time.

Dosimetry of intensity-modulated radiation therapy and volumetric-modulated arc therapy techniques after modified radical mastectomy for breast cancer and hypofractionated intensity-modulated radiotherapy.

The aim of this study was to compare the advantages and disadvantages of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) in patients with left-sided breast cancer who underwent hypofractionated IMRT after a modified radical mastectomy. Twenty patients who required adjuvant radiotherapy after modified radical mastectomy were randomly selected, and a specified dose of 43.5 Gy/15 F was used to plan for IMRT or VMAT. Dose-volume histograms (DVHs) were utilized to evaluate the dose distribution of the planning target volumes (PTVs) and organs at risk (OARs). VMAT demonstrated a greater and more uniform dose distribution of PTVs and lower number of monitor units. No significant differences were found in V5 of the affected lung and heart between the two techniques (P > 0.05). The V10, V20, V30, and Dmean of the affected lung and V10, V20, V30, V40, Dmean, and Dmax of the whole heart were better in the VMAT than in the IMRT (P < 0.05). The Dmean and Dmax of the left anterior descending (LAD) branch of the coronary artery of the heart were better in the VMAT (P < 0.05), and the use of the VMAT effectively reduced the cardiopulmonary dose. A significant advantage of V30 and Dmean was also found in VMAT (P < 0.05). These findings indicate that VMAT has higher clinical significance than IMRT, because it improved the dose distribution in the target area, reduced the cardiopulmonary dose, protected the OARs (e.g. thyroid), and shortened the treatment duration.