Treatment Planning System Calculations Research Articles

Comprehensive stereotactic radiosurgery platform characterization: A novel end-to-end approach with anthropomorphic 3D dosimetry.

Stereotactic radiosurgery (SRS) is a widely employed strategy for intracranial metastases, utilizing linear accelerators and volumetric modulated arc therapy (VMAT). Ensuring precise linear accelerator performance is crucial, given the small planning target volume (PTV) margins. Rapid dose falloff is vital to minimize brain radiation necrosis. Despite advances in SRS planning, tools for end-to-end testing of SRS treatments are lacking, hindering confidence in the procedure. This study introduces a novel end-to-end three-dimensional (3D) anthropomorphic dosimetry system for characterization of a radiosurgery platform, aiming to measure planning metrics, dose gradient index (DGI), brain volumes receiving at least 10 and 12Gy (V10, V12), as well as assess delivery uncertainties in multitarget treatments. The study also compares metrics from benchmark plans to enhance understanding and confidence in SRS treatments. The developed anthropomorphic 3D dosimetry system includes a modified Stereotactic End-to-End Verification (STEEV) phantom with a customized insert integrating 3D dosimeters and a fiber optic CT scanner. Labview and MATLAB programs handle optical scanning, image preprocessing, and dosimetric analysis. SlicerRT is used for 3D dose visualization and analysis. A film stack insert was used to validate the 3D dosimeter measurements at specific slices. Benchmark plans were developed and measured to investigate off-axis errors, dose spillage, small field dosimetry, and multi-target delivery. The accuracy of the developed 3D dosimetry system was rigorously assessed using radiochromic films. Two two-dimensional (2D) dose planes, extracted from the 3D dose distribution, were compared with film measurements, resulting in high passing rates of 99.9% and 99.6% in gamma tests. The mean relative dose difference between film and 3D dosimeter measurements was -1%, with a standard deviation of 2.2%, well within dosimeter uncertainties. In the first module, evaluating single-isocenter multitarget treatments, a 1.5mm dose distribution shift was observed when targets were 7cm off-axis. This shift was attributed to machine mechanical errors and image-guided system uncertainties, indicating potential limitations in conventional gamma tests. The second module investigated discrepancies in intermediate-to-low dose spillage, revealing higher measured doses in the connecting region between closely positioned targets. This discrepancy was linked to uncertainties in treatment planning system (TPS) modeling of out-of-field dose and multileaf collimator (MLC) characteristics, resulting in lower DGI values and higher V10 and V12 values compared to TPS calculations. In the third module, irradiating multiple targets showed consistent V10 and V12 values within 1 cm3 agreement with dose calculations. However, lower DGI values from measurements compared to calculations suggested intricacies in the treatment process. Conducting vital end-to-end testing demonstrated the anthropomorphic 3D dosimetry system's capacity to assess overall treatment uncertainty, offering a valuable tool for enhancing treatment accuracy in radiosurgery platforms. The study introduces a novel anthropomorphic 3D dosimetry system for end-to-end testing of a radiosurgery platform. The system effectively measures plan quality metrics, captures mechanical errors, and visualizes dose discrepancies in 3D space. The comprehensive evaluation capability enhances confidence in the commissioning and verification process, ensuring patient safety. The system is recommended for commissioning new radiosurgery platforms and remote auditing of existing programs.

The impact of the titanium cranial hardware in proton single-field uniform dose plans.

Neurosurgical cranial titanium mesh and screws are commonly encountered in postoperative radiation therapy. However, only a limited number of reports are available in the context of proton therapy, resulting in a lack of consensus among the proton centers regarding the protocol for handling the hardware. This study is to examine the impact of the hardware in proton plans. The results serve as evidence for proton centers to generate standard operating procedures to manage the hardware in proton treatment. Plans with different gantry angles and material overrides are generated on the CT images of a phantom made of the hardware. The dose distributions of the plans with and without material override, at different depths are compared. Films and ionization chambers are used to measure the plans and the measurements are compared to the treatment planning system (TPS) calculations by gamma analysis. There are some overdose and underdose regions downstream of the hardware. The overdose and underdose values are within a few percent of the prescribed dose when multiple fields with large hinge angles are used. The gamma analysis results show that the measurements agree with the TPS calculations within limits that are clinically relevant. The study has demonstrated the influence of the hardware on proton plans. Based on the result of this study, a standard operating procedure of managing the hardware has been implemented in our clinic.

Correction method for depth and field size dependence in Octavius 1000 SRS patient-specific QA

Stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) are advanced techniques capable of delivering higher doses of radiation precisely to small and complex targets. As these higher doses are delivered, the significance of patient-specific quality assurance becomes increasingly crucial to ensure both the effectiveness and safety of treatment. This study investigates the depth and field size dependence in Octavius 1000 SRS patient-specific quality assurance (PSQA), which is vital for ensuring accuracy in modern radiotherapy treatments. It comprehensively compares the performance of the Octavius 1000 SRS array against standard treatment planning system calculations, focusing on various field sizes and depths. An extensive measurement process was performed, wherein output factors were analyzed across a range of field sizes, from very small (1 × 1 cm2) to larger fields (10 × 10 cm2), at different depths (5 cm, 10 cm, and 15 cm). Our results highlighted significant variations in the measurement accuracy, dependent on the field size and depth, which underscored the challenge involved in ensuring precise dosimetry in PSQA for small fields. The largest discrepancy observed was 2.65% for a 1 × 1 cm2 field size at a depth of 15 cm. Based on these findings, a novel correction method tailored for small field sizes and varying depths was proposed. This method enhances the accuracy and reliability of PSQA in advanced radiotherapy to ensure that the delivered dose conforms closely to the planned dose, which is a critical aspect in treatments targeting small and complex tumor volumes. Through the proposed correction, this study aims to bridge the gap between calculated and measured dose distributions, thus contributing significantly to the refinement of PSQA processes in radiotherapy.

Dosimetric validation for prospective clinical trial on GRID collimator-based spatially fractionated radiation therapy: Dose metrics consistency and heterogeneous pattern reproducibility.

The purpose of this study is to characterize the dosimetric properties of a commercial brass GRID collimator for high energy photon beams including 15 and 10 MV. Then, the difference in dosimetric parameters of GRID beams among different energies and linacs was evaluated. A water tank scanning system was used to acquire the dosimetric parameters, including the percentage depth dose (PDD), beam profiles, peak to valley dose ratios (PVDRs), and output factors (OFs). The profiles at various depths were measured at 100cm source to surface distance (SSD), and field sizes of 10×10 cm2 and 20×20 cm2 on three linacs. The PVDRs and OFs were measured and compared with the treatment planning system (TPS) calculations. Compared with the open beam data, there were noticeable changes in PDDs of GRID fields across all the energies. The GRID fields demonstrated a maximal of 3mm shift in dmax (Truebeam STX, 15MV, 10×10 cm2). The PVDR decreased as beam energy increases. The difference in PVDRs between Trilogy and Truebeam STx using 6MV and 15MV was 1.5%±4.0% and 2.1%±4.3%, respectively. However, two Truebeam linacs demonstrated less than 2% difference in PVDRs. The OF of the GRID field was dependent on the energy and field size. The measured PDDs, PVDRs, and OFs agreed with the TPS calculations within 3% difference. The TPS calculations agreed with the measurements when using 1mm calculation resolution. The dosimetric characteristics of high-energy GRID fields, especially PVDR, significantly differ from those of low-energy GRID fields. Two Truebeam machines are interchangeable for GRID therapy, while a pronounced difference was observed between Truebeam and Trilogy. A series of empirical equations and reference look-up tables for GRID therapy can be generated to facilitate clinical applications.

The IAEA remote audit of small field dosimetry for testing the implementation of the TRS-483 code of practice.

The TRS‑483, an IAEA/AAPM International Code of Practice on dosimetry of small static photon fields, underwent testing via an IAEA coordinated research project (CRP). Alongside small field output factors (OFs) measurements using active dosimeters by CRP participants, the IAEA Dosimetry Laboratory received a mandate to formulate a remote small field dosimetry audit method using its passive dosimetry systems. This work aimed to develop a small field dosimetry audit methodology employing radiophotoluminescent dosimeters (RPLDs) and radiochromic films. The methodology was subsequently evaluated through a multicenter pilot study with CRP participants. The developments included designing and manufacturing a dosimeter holder set and the characterization of an RPLD system for measurements in small photon fields using the new holder. The audit included verification of small field OFs and lateral beam profiles for small fields. At first, treatment planning system (TPS) calculated OFs were checked against a reference data set that was available for conventional linacs. Second, calculated OFs were verified through the RPLD measurement of point doses in a machine-specific reference field, 4cm × 4cm, 2cm × 2cm, and 1cm × 1cm, corresponding size circularfields or nearest achievable field sizes. Lastly, profile checks in in-plane and cross-plane directions were done for the two smallest fields by comparing film measurements with TPS calculations at 20%, 50%, and 80% isodose levels. RPLD correction factors for small field measurements were approximately unity. However, they influenced the dose determination's overall uncertainty in small fields, estimated at 2.30% (k=1 level). Considering the previous experience in auditing reference beam output following the TRS-398 Code of Practice, the acceptance limit of 5% for the ratio of the dose determined by RPLD to the dose calculated by TPS, DRPLD/DTPS, was considered adequate. The multicenter pilot study included 15 participants from 14 countries (39 beams). Consistent with the previous findings, the results of the OF check against the reference data confirmed that TPSs tend to overestimate OFs for the smallest fields included in this exercise. All except three RPLD measurement results were within the acceptance limit, and the spread of results increased for smaller field sizes. The differences between the film measured and TPS calculated dose profiles were within 3mm for most of the beams checked; deviated results revealed problems with TPS commissioning and calibration of the treatment unit collimation systems. The newly developed small field dosimetry audit methodology proved effective and successfully complemented the CRP OF measurements by participants with RPLD audit results.

Validation of the Elekta iBEAM Evo couchtop modeling in the Monaco treatment planning system

IntroductionRecently, carbon fiber (CF) has prevailed as the primary material used in radiotherapy couchtops. Modern couchtops incorporate the CF sandwich design, in which two thin CF plates sandwich an air-equivalent polymeric foam. Developments in radiotherapy necessitate irradiation from posterior angles through the couchtop. However, the presence of the couchtop needs proper modeling in the Treatment Planning System (TPS) due to attenuation; otherwise, the tumor dose is reduced. In the current study, an effort was made with the intent of finding the optimum electron density (ED) values for Elekta's iBEAM Evo couchtop components (CF and Foam Core (FC)) for its proper modeling in Monaco TPS. Also, the attenuation of the beam due to the couchtop's presence was investigated. Materials and MethodsA cylindrical phantom with an ionization chamber positioned at the isocenter was utilized for the measurements. The phantom was placed centrally on the iBEAM Evo couchtop and was irradiated with an Elekta Infinity linear accelerator's 6, 10, and 15 MV photon beams. The gantry angle was set at 0o and from 120o to 180o with an increment of 10o. The same procedure was designed and followed in Monaco TPS. Measured and calculated dose values were compared by calculating percentage deviation (PD). Attenuation has also been calculated using the measurements of the experimental setup and the Monaco calculations. ResultsThe values of ED that provided the optimum agreement between measured and Monaco-calculated dose values while minimizing PD were 0.55 g/cm3 for CF, and 0.1 g/cm3 for FC. The maximum values of PD for the beams of 6, 10, and 15 MV were -0.62%, +1,78%, and +2.35% , respectively, for a 5x5 cm2 field size. Furthermore, Monaco predicted attenuation from 1.83% to 6.26% (calculated values), while from the measurements, an attenuation from 1.44% to 5.75% (measured values) regarding the posterior angles was found. Thus, good agreement was verified between the TPS calculations and experimental measurements. ConclusionsElekta's iBEAM Evo couchtop modeling was successfully validated in Monaco TPS. The couchtop's presence alters the patient's dose regarding irradiation from the posterior angles. Due to the attenuation of the beam, proper incorporation, modeling, and validation of the couchtop are necessary to improve the radiotherapy outcome and ensure that each patient receives the optimal treatment.

Electron streaming dose measurements and calculations on a 1.5 T MR-Linac.

To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)-linac. Skin dose due to the ESE on an MR-linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x-y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22cm2 fields, gantry at 90°. Out-of-field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed±20.0cm from the isocenter along the x-direction. A high-resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1cm dose grid with 0.2% statistical uncertainty (0.1cm, 0.2%) and a 0.2cm dose grid with 3.0% statistical uncertainty (0.2cm, 3.0%). OSLD, film, the 0.1cm, 0.2%, and 0.2cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as -75.3%, -7.0%, -24.7%, and -41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements. These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

Characterization of a commercial PIN diode for radiotherapy photon beam dosimetry

The dosimetric characterization of a commercial diode (BPW-34) was performed using a True Beam accelerator (Varian), with flattening filter (FF) photon beams of 6 and 15 MV and flattening filter free (FFF) of 6 MVFFF. The main performance characteristics, namely dose response, energy and dose rate dependence, field output factor, and percentage depth dose profile (PDD), were compared with those available in the literature and, whenever possible, benchmarked against the Varian Eclipse treatment planning system (TPS) predictions. Regardless of the energy, the results showed that the dose response curves are linear (R2 = 1) with nonlinearity parameters less than 0.1% and 0.3% of repeatability. Furthermore, the average dose rate effect (0.7%) is almost negligible within the range of 20–600 MU/min (6 MV, 15 MV) and 400–1400 MU/min (6 MVFFF). Despite these good results, the charge sensitivity measurements evidenced that the diode response depends slightly on the energy, being within 5% for 6 MV–15 MV. It is important to underline that all these results adhere to the standard radiotherapy dosimetry protocols. Moreover, the general output field factor measurements and the percentage depth dose profiles, which are in excellent agreement with the Eclipse TPS calculations, also demonstrated that the diode BPW-34 is a low-budget alternative radiotherapy photon beam dosimeter.

Imaging and characterization of optical emission from ex vivo tissue during conventional and UHDR PBS proton therapy

Objective. Imaging of optical photons emitted from tissue during radiotherapy is a promising technique for real-time visualization of treatment delivery, offering applications in dose verification, treatment monitoring, and retrospective treatment plan comparison. This research aims to explore the feasibility of intensified imaging of tissue luminescence during proton therapy (PT), under both conventional and ultra-high dose rate (UHDR) conditions. Approach. Conventional and UHDR pencil beam scanning (PBS) PT irradiation of fresh ex vivo porcine tissue and tissue-mimicking plastic phantom was imaged using intensified complementary metal-oxide-semiconductor(CMOS) cameras. The optical emission from tissue was characterized during conventional irradiation using both blue and red-sensitive intensifiers to ensure adequate spectral coverage. Spectral characterization was performed using bandpass filters between the lens and sensor. Imaging of conventional proton fields (240 MeV, 10 nA) was performed at 100 Hz frame rate, while UHDR PBS proton delivery (250 MeV, 99 nA) was recorded at 1 kHz frame rate. Dependence of optical emission yield on proton energy was studied using an optical tissue-mimicking plastic phantom and a range shifter. Finally, we demonstrated fast beam tracking capability of fast camera towards in vivo monitoring of FLASH PT. Main results. Under conventional treatment dose rates optical emission was imaged with single spot resolution. Spot profiles were found to agree with the treatment planning system calculation within >90% for all spectral bands and spot intensity was found to vary with spectral filtration. The resultant polychromatic emission presented a maximum intensity at 650 nm and decreasing signal at lower wavelengths, which is consistent with expected attenuation patterns of high fat and muscle tissue. For UHDR beam imaging, optical yield increased with higher proton energy. Imaging at 1 kHz allowed continuous monitoring of delivery during porcine tissue irradiation, with clear identification of individual dwell positions. The number of dwell positions matched the treatment plan in total and per row showing adequate temporal capability of iCMOS imaging. Significance. For the first time, this study characterizes optical emission from tissue during PT and demonstrates our capability of fast optical tracking of pencil proton beam on the tissue anatomy in both conventional and UHDR setting. Similar to the Cherenkov imaging in radiotherapy, this imaging modality could enable a seamless, independent validation of PT treatments.

Technical note: Cryostat transmission characterization for MR linac - temporal stability, clinical impact and change implementation.

In the Unity MR linac (Elekta AB, Stockholm, Sweden), the radiation beam traverses the cryostat and the coil support structure. The resulting beam attenuation must be considered for output calibration and its variation with gantry angle must be characterized in the treatment planning system (TPS). The aim of this work was to investigate the impact of a change of the cryostat transmission characterization (CTC) curve, due to the helium level modification, on clinical treatment plan dosimetry and to report on the experience with the CTC curve update. Twenty stereotactic body radiotherapy (SBRT) treatment plans: 10 prostate and 10 oligo-metastatic cancer plans, prepared with a beam model incorporating the CTC curve acquired at installation time, were re-calculated using the model implementing CTC curve post helium top-up. To account for the CTC change as well as to align our system to the recent reference conditions recommendations, the new model was commissioned with the emphasis on the specifics associated with the treatment plan adaptation and the existence of the offline and online TPS components. Average CTV mean dose reduction by 0.45% in prostate cases and average GTV mean dose reduction by 0.22% in oligo-metastatic cases was observed. Updated model validation showcased good agreement between measurements and TPS calculations. The agreement between CTC measurements demonstrates its temporal constancy and robustness of the measurement method employed. A helium fill level change was shown to affect the CTC and led to a small but systematic dose calculation inaccuracy. Finally, model validation and end-to-end testing results presented, underscore the minimal impact of transitioning to the new beam model and new reference conditions.

Commissioning and Validation of CT Number to SPR Calibration in Carbon Ion Therapy Facility

PurposeWe performed computed tomography (CT)—stopping power ratio (SPR) calibration in a carbon-ion therapy facility and evaluated SPR estimation accuracy. Materials and MethodsA polybinary tissue model method was used for the calibration of CT numbers and SPR. As a verification by dose calculation, we created a virtual phantom to which the CT-SPR calibration table was applied. Then, SPR was calculated from the change in the range of the treatment planning beam when changing to 19 different CT numbers, and the accuracy of the treatment planning system (TPS) calculation of SPR values from the CT-SPR calibration table was validated. As a verification by measurement, 5 materials (water, milk, olive oil, ethanol, 40% K2HPO4) were placed in a container, and the SPR was obtained by measurement from the change in the range of the beam that passed through the materials. ResultsThe results of the dose calculations of the TPS showed that the results agreed within 1% for the lower CT numbers up to 1000 HU, but there was a difference of 3.0% in the higher CT number volume. The difference between the SPR calculated by TPS and the SPR caused by the difference in the energy of the incident particles agreed within 0.51%. The accuracy of SPR estimation was measured, and the error was within 2% for all materials tested. ConclusionThese results indicate that the SPR estimation errors are within the range of errors that can be expected in particle therapy. From commissioning and verification results, the CT-SPR calibration table obtained during this commissioning process is clinically applicable.

LETd Optimization Verification With an SOI Microdosimeter

A first of its kind experimental verification of dose-averaged linear energy transfer (LETd) optimized treatment plans for proton therapy has been carried out using a silicon-on-insulator microdosimeter at the Massachusetts General Hospital (MGH), Boston, USA. Three clinical treatment plans of a typical ependymoma structure set were designed using the standard clinical approach, the proposed protocol approach, and a one-field approach. The plans were then reoptimized to reduce the LETd-weighted dose in the brain stem. All six plans were delivered in a solid water phantom and the experimental yD‾ measured. After LETd optimization, a reduction in yD‾ was found within the brain stem by an average of 12%, 19%, and 4% for the clinical, protocol, and one-field plans, respectively, while maintaining adequate coverage of the tumor structure. The experimental LETd-weighted doses were in agreement with the treatment planning system calculations and Monte Carlo simulations and reinforced the improvement of the optimization. This work demonstrates the first experimental verification of the clinical implementation of LETd optimization for patient treatment with proton therapy.

Deep learning-based tools to distinguish plan-specific from generic deviations in EPID-based in vivo dosimetry.

Dose distributions calculated with electronic portal imaging device (EPID)-based in vivo dosimetry (EIVD) differ from planned dose distributions due to generic and plan-specific deviations. Generic deviations are characteristic to a class of plans. Examples include limitations in EIVD dose reconstruction, inaccuracies in treatment planning system (TPS) calculations and systematic machine deviations. Plan-specific deviations have an unpredictable character. Examples include discrepancies between the patient model used for dose calculation and the patient position or anatomy during delivery, random machine deviations, and data transfer, human or software errors. During the inspection work performed with traditional γ-evaluation statistical methods: (i) generic deviations raise alerts that need to be inspected but that rarely lead to action as their root cause is usually understood and (ii) the detection of relevant plan-specific deviations may be hindered by the presence of generic deviations. To investigate whether deep learning-based tools can help in identifying γ-alerts raised by generic deviations and in improving the detectability of plan-specific deviations. A 3D U-Net was trained as an autoencoder to reconstruct underlying patterns of generic deviations in γ-distributions. The network was trained for four treatment disease sites differently affected by generic deviations: volumetric modulated arc therapy (VMAT) lung (no known deviations), VMAT prostate (TPS inaccuracies), VMAT head-and-neck (EIVD limitations) and intensity modulated radiation therapy (IMRT) breast (large EIVD limitations). The network was trained with virtual non-transit γ-distributions: 60 train/10 validation for the VMAT sites and 30 train/10 validation for IMRT breast. It was hypothesized that in vivo γ-distributions obtained in the presence of plan-specific deviations would differ from those seen during training. For each disease site, the sensitivity of γ-analysis and the network to detect (synthetically introduced) patient-related deviations was compared by receiver operator characteristic analysis. The investigated deviations were patient positioning errors, weight gain or loss, and tumor volume changes. The clinical relevance was illustrated qualitatively with 793 in vivo clinical cases (141 lung, 136 head-and-neck, 209 prostate and 307 breast). Error detectability of patient-related deviations was better with the network than with γ-analysis. The average area under the curve values over all sites were 0.86±0.12(1SD) and 0.69±0.25(1SD), respectively. Regarding in vivo clinical results, the percentage of cases differently classified by γ-analysis and the network was 1%, 19%, 18% and 64% for lung, head-and-neck, prostate, and breast, respectively. In head-and-neck and breast cases, 45 γ-only alerts were examined, of which 43 were attributed to EPID dose reconstruction limitations. For prostate, all 15 investigated γ-only alerts were due to known TPS inaccuracies. All 59 investigated network alerts were explained by either patient-related deviations or EPID acquisition incidents. Some patient-related deviations detected by the network were not detected by γ-analysis. Deep learning-based tools trained to reconstruct underlying patterns of generic deviations in γ-distributions can be used to (i) automatically identify false positives within the set of γ-alerts and (ii) improve the detection of plan-specific deviations, hence minimizing the likelihood of false negatives. The presented method provides clear additional value to the γ-alert management process for large scale EIVD systems.

Implementation of a Patient Specific QA Protocol for a Novel Dedicated Stereotactic Radiosurgery Linear Accelerator

To describe the patient specific QA (Quality Assurance) protocol implemented for plan integrity verification, in stereotactic radiosurgery (SRS) and radiotherapy (SRT) treatments performed on a novel dedicated frameless image guided radiosurgery system using conical collimator, and a combination of yoked gimbals to cover near2pi solid angle. Startup and commissioning results obtained for planned vs measured dose distributions with several detectors are presented. The patient specific QA protocol includes: A) Review of the approved plan based on AAPM TG 275 report recommendations B) 1D verification using small volume ionization chambers inside an anthropomorphic head phantom C) 2D dose verification of a coronal and sagittal dose plan performed with high resolution 2D Array and respective phantom apparatus D) In-house independent MU (Monitor Unit) calculation using the same formalism of the vendor TPS (Treatment Planning Systems) RayTracing dose-calculation algorithm E) Monte Carlo based secondary dose check & plan QA. A retrospective analysis of the results of the first 15 patients treated is presented, focusing on plan complexity vs QA results. The 1D results obtained for the 15 SRS plans were within ±5% for all reported cases, with a mean percent difference of -1,25%, confirming an overall good agreement and, as expected, a partial volume effect in plans with smaller collimators. For the 2D dose verifications, with a 10% dose threshold, gamma passing rates of 97,5% (coronal) and 96,78 (sagittal) with 3% 1mm criteria, 95,1% (coronal) and 94,3% (sagittal) for 2% 1 mm criteria and 90,7% (coronal) and 90,0% (sagittal) for 1% 1mm criteria were obtained. Moreover, the results showed a correlation between lesion volume or number of collimators used with gamma passing rates. All MU verification results were within ±0,3% and provides an efficient risk mitigation approach for this new delivery technique. The verification results of the first 15 treated SRS plans confirmed point dose and planar measurements in agreement with TPS calculations, with superior results for planes with smaller lesions and fewer collimators. This also represents an integral validation of the image-based alignment system and fine treatment couch movements, as treatments are intrinsically multi-isocentric.

Development and validation of MonteRay, a fast Monte Carlo dose engine for carbon ion beam radiotherapy.

Monte Carlo (MC) simulations are considered the gold-standard for accuracy in radiotherapy dose calculation; so far however, no commercial treatment planning system (TPS) provides a fast MC for supporting clinical practice in carbon ion therapy. To extend and validate the in-house developed fast MC dose engine MonteRay for carbon ion therapy, including physical and biological dose calculation. MonteRay is a CPU MC dose calculation engine written in C++ that is capable of simulating therapeutic proton, helium and carbon ion beams. In this work, development steps taken to include carbon ions in MonteRay are presented. Dose distributions computed with MonteRay are evaluated using a comprehensive validation dataset, including various measurements (pristine Bragg peaks, spread out Bragg peaks in water and behind an anthropomorphic phantom) and simulations of a patient plan. The latter includes both physical and biological dose comparisons. Runtimes of MonteRay were evaluated against those of FLUKA MC on a standard benchmark problem. Dosimetric comparisons between MonteRay and measurements demonstrated good agreement. In terms of pristine Bragg peaks, mean errors between simulated and measured integral depth dose distributions were between -2.3% and +2.7%. Comparing SOBPs at 5, 12.5 and 20cm depth, mean absolute relative dose differences were 0.9%, 0.7% and 1.6% respectively. Comparison against measurements behind an anthropomorphic head phantom revealed mean absolute dose differences of with global 3%/3mm 3D-γ passing rates of 99.3%, comparable to those previously reached with FLUKA (98.9%). Comparisons against dose predictions computed with the clinical treatment planning tool RayStation 11B for a meningioma patient plan revealed excellent local 1%/1mm 3D-γ passing rates of 98% for physical and 94% for biological dose. In terms of runtime, MonteRay achieved speedups against reference FLUKA simulations ranging from 14× to 72×, depending on the beam's energy and the step size chosen. Validations against clinical dosimetric measurements in homogeneous and heterogeneous scenarios and clinical TPS calculations have proven the validity of the physical models implemented in MonteRay. To conclude, MonteRay is viable as a fast secondary MC engine for supporting clinical practice in proton, helium and carbon ion radiotherapy.

Generation and comparison of 3D dosimetric reference datasets for COMS eye plaque brachytherapy using model-based dose calculations.

A joint Working Group of the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) was created to aid in the transition from the AAPM TG-43 dose calculation formalism, the current standard, to model-based dose calculations. This work establishes the first test cases for low-energy photon-emitting brachytherapy using model-based dose calculation algorithms (MBDCAs). Five test cases are developed: (1) a single model 6711 125 I brachytherapy seed in water, 13 seeds (2) individually and (3) in combination in water, (4) the full Collaborative Ocular Melanoma Study (COMS) 16 mm eye plaque in water, and (5) the full plaque in a realistic eye phantom. Calculations are done with four Monte Carlo (MC) codes and a research version of a commercial treatment planning system (TPS). For all test cases, local agreement of MC codes was within ∼2.5% and global agreement was ∼2% (4% for test case 5). MC agreement was within expected uncertainties. Local agreement of TPS with MC was within 5% for test case 1 and ∼20% for test cases 4 and 5, and global agreement was within 0.4% for test case 1 and 10% for test cases 4 and 5. Dose distributions for each set of MC and TPS calculations are available online (https://doi.org/10.52519/00005) along with input files and all other information necessary to repeat the calculations. These data can be used to support commissioning of MBDCAs for low-energy brachytherapy as recommended by TGs 186 and 221 and AAPM Report 372. This work additionally lays out a sample framework for the development of test cases that can be extended to other applications beyond eye plaque brachytherapy.

Retrospective Analysis of Treatment Time Calculation Agreement Between TPS and Manual Method and Source Strength Verification for 13 HDR Ir-192 Sources In Flexitron Brachytherapy Unit: A Single Institute Experience

Introduction: Source strength measurement and verification during every source exchange in high dose rate (HDR) Brachytherapy 192Ir is primary part of quality assurance program as recommended by national regulatory body Atomic Energy Regulatory Board, Mumbai, India. The purpose of this study was to evaluate RAKR (in-house measurement) of 192Ir using re-entrant well-type ionization chamber with stated RAKR values as provided by the manufacturer in source certificate and to check the accuracy of the Oncentra treatment planning system calculation with manual method of computation at different intervals for 13 HDR Ir-192 sources. Materials And Methods: We have retrospectively evaluate the in-house measured RAKR with the calibrated Standard Imaging HDR 1000 PLUS Well chamber /electrometer CDX 2000B for 13 HDR Ir192 sources with stated RAKR and the treatment time calculation accuracy of Oncentra TPS verified by manual method of computation using TG-43 formalism. Results: The study shows, that the measured RAKR and stated RAKR were well within the tolerance limits set by the national and international recommendations i.e. ±3 % for 13 HDR Ir192 sources. The verification of TPS treatment time calculation with manual calculation for single source as well as for multi-source lies within ±2%. Conclusions: There is good agreement between in-house measured and vendor quoted values of RAKR which shows that the chamber has been stable to better than ±3 % and manual calculations shows agreement with the TPS outcome within ±2% for 13 HDR Ir192 sources. The manual method of computation works reliably and reassures the TPS functioning.

A method for selecting reference beam model of VMAT plans with three 6MV beam-matched linear accelerators during radiation oncology

Our objective was to provide a method for selecting reference beam model and evaluating the dosimetric accuracy of volumetric modulated arc therapy (VMAT) plans delivered on three Elekta beam-matched linacs during radiation oncology. Beam data was measured on three beam-matched linacs including Synergy1, Synergy2 and VersaHD. For eighteen lung and esophagus cases, fifty-four plans were generated using VMAT technique with three linac beam models respectively for point dose measurement and three-dimensional dose measurement. Each VMAT plan was executed sequentially on three linacs respectively. Measurement results were compared with treatment planning system (TPS) calculation results for all VMAT plans. Among three beam-matched linacs, discrepancy in beam output factor, percentage depth dose at 5 cm, 10 cm, 20 cm depth and MLC leaf offset are all within 1% except 20 × 20 cm2 and 30 × 30 cm2 field sizes, and discrepancy in beam profile is all within 2%. With comparison between measurement result and TPS calculation result, the absolute dose deviations are within the range of ± 3%, and the gamma passing rates are all over 95% for all VMAT plans, which are within the tolerance of clinical acceptability. Compared with all plans delivered on Synegy1 and VersaHD, the point dose discrepancy between measured results and TPS calculated results for plans delivered on Synergy2 is smallest, and the gamma passing rate between measured results and TPS calculated results for plans delivered on Synergy2 is highest. The beam-matched linacs demonstrate good agreement between measurement result and TPS calculation result for VMAT plans. The method can be used for selecting reference beam model for VMAT plans.

Biliary intervention combined with lenvatinib+ PD-1 inhibitor of Klatskin tumor complicated with malignant obstructive jaundice.

e14620 Background: Hilar Cholangiocarcinoma (Klatskin tumor) is a rare and highly aggressive cancer that occurs at or near the junction of the right and left hepatic ducts. Gemcitabine + cisplatin has been established as an effective first-line treatment. However, this malignancy is commonly complicated by malignant biliary obstruction which may gradually damage patients’ liver function and may limit the chemotherapy. In addition, Cumulative clinical evidence showed that tyrosine kinase inhibitor (TKI) combined with immunotherapy is represented as a promising alternative. This study aimed to evaluate the clinical value of biliary intervention combined with the Lenvatinib+PD-1 inhibitor (Tislelizumab/Sintilimab) for the Klatskin tumor. Methods: From January 2020 to December 2022, 21 Klatskin tumor patients with malignant hilar biliary obstruction (MHBO) were included. All included patients received biliary decompression by metal stent + Iodine-125 seed strand (ISS) implantation. The number of 125I seeds to be implanted was determined according to the plan of the preoperative computerized treatment planning system (TPS). The procedure-related outcomes were evaluated regarding successful drainage and complications. In addition, the survival of patients who received successful biliary drainage and patients who received sequential systemic anticancer treatment were analyzed. Successful biliary drainage was defined as the serum total bilirubin level dropped by 75% of the preoperative value within 4 weeks. Major complications referred to those that result in death or serious adverse effects including sepsis, uncontrollable biliary or intestinal bleeding, and acute renal failure. The time between stent insertion and death was defined as patient survival. Results: 71.4% achieved successful drainage, significant improvements were observed from before to one month after the procedure in liver function values, including total bilirubin (TBIL), and direct bilirubin (DBIL) (all P<0.001). Minor complications occurred in 5 patients (23.8%) in all patients, including 1 patient (4.8%) who experienced cholangitis, and 4 patients (19%) who suffered abdominal pain. No major procedure-related complication was observed. The median overall survival(mOS) of 21 patients was 6.1 months. Patients who received successful biliary drainage (n=15) had longer mOS than those who received unsuccessful biliary drainage(n=6) [7.4 months vs. 3.5 months, respectively, P<0.001]. Patients who received sequential Lenvatinib+ Pembrolizumab after effective drainage(n=9) had longer mOS than those who only received palliative therapy(n=6) [7.4 months vs. 5.2 months, respectively, P=0.028]. Conclusions: Biliary intervention + ISS combined with the following Lenvatinib+ PD-1 inhibitor is safe and feasible for cholangiocarcinoma patients complicated with obstructive jaundice.

Implementation of novel measurement-based patient-specific QA for pencil beam scanning proton FLASH radiotherapy.

Several studies have shown pencil beam scanning (PBS) proton therapy is a feasible and safe modality to deliver conformal and ultra-high dose rate (UHDR) FLASH radiation therapy. However, it would be challenging and burdensome to conduct the quality assurance (QA) of the dose rate along with conventional patient-specific QA (psQA). To demonstrate a novel measurement-based psQA program for UHDR PBS proton transmission FLASH radiotherapy (FLASH-RT) using a high spatiotemporal resolution 2D strip ionization chamber array (SICA). The SICA is a newly designed open-air strip-segmented parallel plate ionization chamber, which is capable of measuring spot position and profile through 2mm-spacing-strip electrodes at a 20kHz sampling rate (50μs per event) and has been characterized to exhibit excellent dose and dose rate linearity under UHDR conditions. A SICA-based delivery log was collected for each irradiation containing the measured position, size, dwell time, and delivered MU for each planned spot. Such spot-level information was compared with the corresponding quantities in the treatment planning system (TPS). The dose and dose rate distributions were reconstructed on patient CT using the measured SICA log and compared to the planned values in volume histograms and 3D gamma analysis. Furthermore, the 2D dose and dose rate measurements were compared with the TPS calculations of the same depth. In addition, simulations using different machine-delivery uncertainties were performed, and QA tolerances were deduced. A transmission proton plan of 250MeV for a lung lesion was planned and measured in a dedicated ProBeam research beamline (Varian Medical System) with a nozzle beam current between 100 to 215nA. The worst gamma passing rates for dose and dose rate of the 2D SICA measurements (four fields) compared to TPS prediction (3%/3mm criterion) were 96.6% and 98.8%, respectively, whereas the SICA-log reconstructed 3D dose distribution achieved a gamma passing rate of 99.1% (2%/2mm criterion) compared to TPS. The deviations between SICA measured log, and TPS were within 0.3ms for spot dwell time with a mean difference of 0.069±0.11 s, within 0.2mm for spot position with a mean difference of -0.016±0.03mm in the x-direction, and -0.036±0.059mm in the y-direction, and within 3% for delivered spot MUs. Volume histogram metric of dose (D95) and dose rate (V40Gy/s ) showed minimal differences, within less than 1%. This work is the first to describe and validate an all-in-one measurement-based psQA framework that can fulfill the goals of validating the dose rate accuracy in addition to dosimetric accuracy for proton PBS transmission FLASH-RT. The successful implementation of this novel QA program can provide future clinical practice with more confidence in the FLASH application.