Assessing dose rate distributions in VMAT plans

Dose rate is an essential factor in radiobiology. As modern radiotherapy delivery techniques such as volumetric modulated arc therapy (VMAT) introduce dynamic modulation of the dose rate, it is important to assess the changes in dose rate. Both the rate of monitor units per minute (MU rate) and collimation are varied over the course of a fraction, leading to different dose rates in every voxel of the calculation volume at any point in time during dose delivery. Given the radiotherapy plan and machine specific limitations, a VMAT treatment plan can be split into arc sectors between Digital Imaging and Communications in Medicine control points (CPs) of constant and known MU rate. By calculating dose distributions in each of these arc sectors independently and multiplying them with the MU rate, the dose rate in every single voxel at every time point during the fraction can be calculated. Independently calculated and then summed dose distributions per arc sector were compared to the whole arc dose calculation for validation. Dose measurements and video analysis were performed to validate the calculated datasets. A clinical head and neck, cranial and liver case were analyzed using the tool developed. Measurement validation of synthetic test cases showed linac agreement to precalculated arc sector times within ±0.4 s and doses ±0.1 MU (one standard deviation). Two methods for the visualization of dose rate datasets were developed: the first method plots a two-dimensional (2D) histogram of the number of voxels receiving a given dose rate over the course of the arc treatment delivery. In similarity to treatment planning system display of dose, the second method displays the dose rate as color wash on top of the corresponding computed tomography image, allowing the user to scroll through the variation over time. Examining clinical cases showed dose rates spread over a continuous spectrum, with mean dose rates hardly exceeding 100 cGy min(-1) for conventional fractionation. A tool to analyze dose rate distributions in VMAT plans with sub-second accuracy was successfully developed and validated. Dose rates encountered in clinical VMAT test cases show a continuous spectrum with a mean less than or near 100 cGy min(-1) for conventional fractionation

VoiceS: voice quality after transoral CO2 laser surgery versus single vocal cord irradiation for unilateral stage 0 and I glottic larynx cancer-a randomized phase III trial [study protocol].

BACKGROUND Surgery and radiotherapy are well-established standards of care for unilateral stage 0 and I early-stage glottic cancer (ESGC). Based on comparative studies and meta-analyses, functional and oncological outcomes after both treatment modalities are similar. Historically, radiotherapy (RT) has been performed by irradiation of the whole larynx. However, only the involved vocal cord is being treated with recently introduced hypofractionated concepts that result in 8 to 10-fold smaller target volumes. Retrospective data argues for an improvement in voice quality with non-inferior local control. Based on these findings, single vocal cord irradiation (SVCI) has been implemented as a routine approach in some institutions for ESGC in recent years. However, prospective data directly comparing SVCI with surgery is lacking. The aim of VoiceS is to fill this gap. METHODS In this prospective randomized multi-center open-label phase III study with a superiority design, 34 patients with histopathologically confirmed, untreated, unilateral stage 0-I ESGC (unilateral cTis or cT1a) will be randomized to SVCI or transoral CO2-laser microsurgical cordectomy (TLM). Average difference in voice quality, measured by using the voice handicap index (VHI) will be modeled over four time points (6, 12, 18, and 24 months). Primary endpoint of this study will be the patient-reported subjective voice quality between 6 to 24 months after randomization. Secondary endpoints will include perceptual impression of the voice via roughness - breathiness - hoarseness (RBH) assessment at the above-mentioned time points. Additionally, quantitative characteristics of voice, loco-regional tumor control at 2 and 5 years, and treatment toxicity at 2 and 5 years based on CTCAE v.5.0 will be reported. DISCUSSION To our knowledge, VoiceS is the first randomized phase III trial comparing SVCI with TLM. Results of this study may lead to improved decision-making in the treatment of ESGC. TRIAL REGISTRATION ClinicalTrials.gov NCT04057209. Registered on 15 August 2019. Cantonal Ethics Committee KEK-BE 2019-01506

Verbesserte VMAT-Planung für Kopf- und Halstumore mit einem fortschrittlichen Optimierungs-Algorithmus

OBJECTIVE In this study, the "Progressive Resolution Optimizer PRO3" (Varian Medical Systems) is compared to the previous version "PRO2" with respect to its potential to improve dose sparing to the organs at risk (OAR) and dose coverage of the PTV for head and neck cancer patients. MATERIALS AND METHODS For eight head and neck cancer patients, volumetric modulated arc therapy (VMAT) treatment plans were generated in this study. All cases have 2-3 phases and the total prescribed dose (PD) was 60-72Gy in the PTV. The study is mainly focused on the phase 1 plans, which all have an identical PD of 54Gy, and complex PTV structures with an overlap to the parotids. Optimization was performed based on planning objectives for the PTV according to ICRU83, and with minimal dose to spinal cord, and parotids outside PTV. In order to assess the quality of the optimization algorithms, an identical set of constraints was used for both, PRO2 and PRO3. The resulting treatment plans were investigated with respect to dose distribution based on the analysis of the dose volume histograms. RESULTS For the phase 1 plans (PD=54Gy) the near maximum dose D2% of the spinal cord, could be minimized to 22±5 Gy with PRO3, as compared to 32±12Gy with PRO2, averaged for all patients. The mean dose to the parotids was also lower in PRO3 plans compared to PRO2, but the differences were less pronounced. A PTV coverage of V95%=97±1% could be reached with PRO3, as compared to 86±5% with PRO2. In clinical routine, these PRO2 plans would require modifications to obtain better PTV coverage at the cost of higher OAR doses. CONCLUSION A comparison between PRO3 and PRO2 optimization algorithms was performed for eight head and neck cancer patients. In general, the quality of VMAT plans for head and neck patients are improved with PRO3 as compared to PRO2. The dose to OARs can be reduced significantly, especially for the spinal cord. These reductions are achieved with better PTV coverage as compared to PRO2. The improved spinal cord sparing offers new opportunities for all types of paraspinal tumors and for re-irradiation of recurrent tumors or second malignancies.Zielsetzung In dieser Studie wird der „Progressive Resolution Optimizer PRO3“ (Varian Medical Systems) mit der Vorgängerversion „PRO2“ gezielt auf sein Potential bei Kopf-Hals-Tumoren untersucht. Im Vordergrund stehen die Dosisabdeckung des PTV und die Schonung der Risikoorgane (OAR). Methoden Für acht Patienten wurden VMAT-Pläne mit beiden Algorithmen erstellt. Die Gesamtdosis im PTV von 60-72 Gy ist in allen Fällen auf 2-3 Phasen verteilt, die nacheinander abgestrahlt werden. Die Studie ist fokussiert auf die erste Bestrahlungsphase, die in allen Fällen eine komplexe PTV-Struktur mit Planungsdosis von 54 Gy hatte und teilweise mit den Parotiden überlappte. Grundlage für die Optimierung sind die ICRU83-Richtlinien für das PTV und das Planungsziel, das Rückenmark und die Parotiden außerhalb des PTV maximal zu schonen. In einem ersten Schritt wurde durch interaktive Planung mit PRO3 ein möglichst optimaler Plan generiert. Die dabei gewählten Grenzwerte („dose constraints“) für PTV und Risikoorgane wurden in einem zweiten Schritt für den Vergleich der Optimierungsalgorithmen verwendet. Die Optimierung wurde also für diese nun fest vorgegebenen Dosisgrenzwerte sowohl mit PRO3 wiederholt als auch mit PRO2 durchgeführt, ohne zusätzliche Interaktion des Planers. Die resultierenden beiden Bestrahlungspläne wurden anhand der Dosis-Volumen-Histogramme verglichen. Ergebnisse Ein markanter Unterschied in Planqualität ergibt sich aus dem Vergleich der Rückenmarkdosis in der ersten Bestrahlungsphase mit 54 Gy Dosis im PTV. Die maximum-nahe Dosis D2% im Rückenmark konnte mit PRO3 auf 22±5 Gy reduziert werden, während die Optimierung mit identischen Randbedingungen bei PRO2 eine Dosis D2% von 32±12 Gy ergab. Auch die mittlere Dosis für die Parotiden war kleiner mit PRO3, allerdings mit einem weniger großen Unterschied. Eine PTV-Abdeckung von V95% = 97±1% konnte mit PRO3 erreicht werden. Mit PRO2 lag diese Abdeckung dagegen nur bei V95% = 86±5%. Die Mehrzahl der so generierten PRO2-Pläne würde in klinischer Routine nicht akzeptiert und müsste durch die Anpassung der Randbedingungen modifiziert werden, so dass die Dosis auf die Risikoorgane entsprechend höher wird. Schlussfolgerung Ein Vergleich der Optimierungsalgorithmen PRO2 und PRO3 wurde bei acht Patienten mit Kopf-Hals-Tumoren durchgeführt. Hierbei wurde mit dem Algorithmus PRO3 eine deutlich bessere Planqualität erzielt als mit PRO2. Die Verbesserung führt zu einer tieferen Belastung der Risikoorgane, mit dem deutlichsten Unterschied beim Rückenmark. Diese Schonung geht nicht zu Lasten der PTV-Abdeckung. Vielmehr wurde bei allen PRO3-Plänen eine bessere PTV-Abdeckung erzielt als mit PRO2. Die verbesserte Möglichkeit der Rückenmarkschonung, auch für komplexe PTV, eröffnet neue Möglichkeiten der Bestrahlung paraspinaler Tumore. Durch die Schonung des Rückenmarks ist zudem eine erneute Bestrahlung bei wiederauftretenden Tumoren möglich

TH-EF-BRB-02: Feasibility of Optimization for Dynamic Trajectory Radiotherapy.

PURPOSE Over the last years, volumetric modulated arc therapy (VMAT) has been widely introduced into clinical routine using a coplanar delivery technique. However, VMAT might be improved by including dynamic couch and collimator rotations, leading to dynamic trajectory radiotherapy (DTRT). In this work the feasibility and the potential benefit of DTRT was investigated. METHODS A general framework for the optimization was developed using the Eclipse Scripting Research Application Programming Interface (ESRAPI). Based on contoured target and organs at risk (OARs), the structures are extracted using the ESRAPI. Sampling potential beam directions, regularly distributed on a sphere using a Fibanocci-lattice, the fractional volume-overlap of each OAR and the target is determined and used to establish dynamic gantry-couch movements. Then, for each gantry-couch track the most suitable collimator angle is determined for each control point by optimizing the area between the MLC leaves and the target contour. The resulting dynamic trajectories are used as input to perform the optimization using a research version of the VMAT optimization algorithm and the ESRAPI. The feasibility of this procedure was tested for a clinically motivated head and neck case. Resulting dose distributions for the VMAT plan and for the dynamic trajectory treatment plan were compared based on DVH-parameters. RESULTS While the DVH for the target is virtually preserved, improvements in maximum dose for the DTRT plan were achieved for all OARs except for the inner-ear, where maximum dose remains the same. The major improvements in maximum dose were 6.5% of the prescribed dose (66 Gy) for the parotid and 5.5% for the myelon and the eye. CONCLUSION The result of this work suggests that DTRT has a great potential to reduce dose to OARs with similar target coverage when compared to conventional VMAT treatment plans. This work was supported by Varian Medical Systems. This work was supported by Varian Medical Systems

Dose calculation of dynamic trajectory radiotherapy using Monte Carlo.

PURPOSE Using volumetric modulated arc therapy (VMAT) delivery technique gantry position, multi-leaf collimator (MLC) as well as dose rate change dynamically during the application. However, additional components can be dynamically altered throughout the dose delivery such as the collimator or the couch. Thus, the degrees of freedom increase allowing almost arbitrary dynamic trajectories for the beam. While the dose delivery of such dynamic trajectories for linear accelerators is technically possible, there is currently no dose calculation and validation tool available. Thus, the aim of this work is to develop a dose calculation and verification tool for dynamic trajectories using Monte Carlo (MC) methods. METHODS The dose calculation for dynamic trajectories is implemented in the previously developed Swiss Monte Carlo Plan (SMCP). SMCP interfaces the treatment planning system Eclipse with a MC dose calculation algorithm and is already able to handle dynamic MLC and gantry rotations. Hence, the additional dynamic components, namely the collimator and the couch, are described similarly to the dynamic MLC by defining data pairs of positions of the dynamic component and the corresponding MU-fractions. For validation purposes, measurements are performed with the Delta4 phantom and film measurements using the developer mode on a TrueBeam linear accelerator. These measured dose distributions are then compared with the corresponding calculations using SMCP. First, simple academic cases applying one-dimensional movements are investigated and second, more complex dynamic trajectories with several simultaneously moving components are compared considering academic cases as well as a clinically motivated prostate case. RESULTS The dose calculation for dynamic trajectories is successfully implemented into SMCP. The comparisons between the measured and calculated dose distributions for the simple as well as for the more complex situations show an agreement which is generally within 3% of the maximum dose or 3mm. The required computation time for the dose calculation remains the same when the additional dynamic moving components are included. CONCLUSION The results obtained for the dose comparisons for simple and complex situations suggest that the extended SMCP is an accurate dose calculation and efficient verification tool for dynamic trajectory radiotherapy. This work was supported by Varian Medical Systems