Background and purpose: The promise of the MR-linac is that one can visualize all anatomical changes during the course of radiotherapy and hence adapt the treatment plan in order to always have the optimal treatment. Yet, there is a trade-off to be made between the time spent for adapting the treatment plan against the dosimetric gain. In this work, the various daily plan adaptation methods will be presented and applied on a variety of tumour sites. The aim is to provide an insight in the behavior of the state-of-the-art 1.5 T MRI guided on-line adaptive radiotherapy methods. Materials and methods: To explore the different available plan adaptation workflows and methods, we have simulated online plan adaptation for five cases with varying levels of inter-fraction motion, regions of interest and target sizes: prostate, rectum, esophagus and lymph node oligometastases (single and multiple target). The plans were evaluated based on the clinical dose constraints and the optimization time was measured. Results: The time needed for plan adaptation ranged between 17 and 485 s. More advanced plan adaptation methods generally resulted in more plans that met the clinical dose criteria. Violations were often caused by insufficient PTV coverage or, for the multiple lymph node case, a too high dose to OAR in the vicinity of the PTV. With full online replanning it was possible to create plans that met all clinical dose constraints for all cases. Conclusion: Daily full online replanning is the most robust adaptive planning method for Unity. It is feasible for specific sites in clinically acceptable times. Faster methods are available, but before applying these, the specific use cases should be explored dosimetrically.

Online adaptive radiotherapy using the 1.5 Tesla MR-linac is feasible for SBRT (5 Â 7 Gy) of pelvic lymph node oligometastases. The workflow allows full online planning based on daily anatomy. Session duration is less than 60 min. Quality assurance tests, including independent 3D dose calculations and film measurements were passed.

IntroductionThe main aim of the current study was to assess the dosimetric accuracy and clinical quality of volumetric modulated arc therapy (VMAT) plans for stereotactic (stage I) and conventional (stage III) lung cancer treatments planned with Eclipse version 10.0 Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB) algorithm.MethodsThe dosimetric impact of using AAA instead of AXB, and grid size 2.5 mm instead of 1.0 mm for VMAT treatment plans was evaluated. The clinical plan quality of AXB VMAT was assessed using 45 stage I and 73 stage III patients, and was compared with published results, planned with VMAT and hybrid-VMAT techniques.ResultsThe dosimetric impact on near-minimum PTV dose (D98%) using AAA instead of AXB was large (underdose up to 12.3%) for stage I and very small (underdose up to 0.8%) for stage III lung treatments. There were no significant differences for dose volume histogram (DVH) values between grid sizes. The calculation time was significantly higher for AXB grid size 1.0 than 2.5 mm (p < 0.01). The clinical quality of the VMAT plans was at least comparable with clinical qualities given in literature of lung treatment plans with VMAT and hybrid-VMAT techniques. The average mean lung dose (MLD), lung V20Gy and V5Gy in this study were respectively 3.6 Gy, 4.1% and 15.7% for 45 stage I patients and 12.4 Gy, 19.3% and 46.6% for 73 stage III lung patients. The average contra-lateral lung dose V5Gy-cont was 35.6% for stage III patients.ConclusionsFor stereotactic and conventional lung treatments, VMAT calculated with AXB grid size 2.5 mm resulted in accurate dose calculations. No hybrid technique was needed to obtain the dose constraints. AXB is recommended instead of AAA for avoiding serious overestimation of the minimum target doses compared to the actual delivered dose.

Abstract.A quantum cascade laser spectrometer was evaluated for eddy covariance flux measurements of CH 4 and N 2 O using three months of continuous measurements at a field site. The required criteria for eddy covariance flux measurements including continuity, sampling frequency, precision and stationarity were examined. The system operated continuously at a dairy farm on peat grassland in the Netherlands from 17 August to 6 November 2006. An automatic liquid nitrogen filling system for the infrared detector was employed to provide unattended operation of the system. The electronic sampling frequency was 10 Hz, however, the flow response time was 0.08 s, which corresponds to a bandwidth of 2 Hz. A precision of 2.9 and 0.5 ppb Hz −1/2 was obtained for CH 4 and N 2 O, respectively. Accuracy was assured by frequent calibrations using low and high standard additions. Drifts in the system were compensated by using a 120 s running mean filter. The average CH 4 and N 2 O exchange was 512 ngC m −2 s −1 (2.46 mg m −2 hr −1 ) and 52 ngN m −2 s −1 (0.29 mg m −2 hr −1 ). Given that 40% of the total N 2 O emission was due to a fertilizing event.

A B S T R A C TFluxes of methane (CH 4 ) and carbon dioxide (CO 2 ) estimated by empirical models based on small-scale chamber measurements were compared to large-scale eddy covariance (EC) measurements for CH 4 and to a combination of EC measurements and EC-based models for CO 2 . The experimental area was a flat peat meadow in the Netherlands with heterogeneous source strengths for both greenhouse gases. Two scenarios were used to assess the importance of stratifying the landscape into landscape elements before up-scaling the fluxes measured by chambers to landscape scale: one took the main landscape elements into account (field, ditch edge ditch), the other took only the field into account. Non-linear regression models were used to up-scale the chamber measurements to field emission estimates. EC CO 2 respiration consisted of measured night time EC fluxes and modeled day time fluxes using the Arrhenius model. EC CH 4 flux estimate was based on daily averages and the remaining data gaps were filled by linear interpolation. The EC and chamber-based estimates agreed well when the three landscape elements were taken into account with 16.5% and 13.0% difference for CO 2 respiration and CH 4 , respectively. However, both methods differed 31.0% and 55.1% for CO 2 respiration and CH 4 when only field emissions were taken into account when up-scaling chamber measurements to landscape scale. This emphasizes the importance of stratifying the landscape into landscape elements. The conclusion is that small-scale chamber measurements can be used to estimate fluxes of CO 2 and CH 4 at landscape scale if fluxes are scaled by different landscape elements. ß

No abstract

No abstract

Background. In radiotherapy, the magnitude of respiratory-induced tumor motion is often measured using a single four-dimensional computed tomography (4DCT). This magnitude is required to determine the internal target volume. The aim of this study was to compare the magnitude of respiratory-induced motion of pancreatic tumors on a single 4DCT with the motion on daily cone beam CT (CBCT) scans during a 3 -5-week fractionated radiotherapy scheme. In addition, we investigated changes in the respiratory motion during the treatment course. Material and methods. The mean peak-to-peak motion (i.e. magnitude of motion) of pancreatic tumors was measured for 18 patients using intratumoral gold fi ducials visible on CBCT scans made prior to each treatment fraction (10 -27 CBCTs per patient; 401 CBCTs in total). For each patient, these magnitudes were compared to the magnitude measured on 4DCT. Possible time trends were investigated by applying linear fi ts to the tumor motion determined from daily CBCTs as a function of treatment day.Results. We found a signifi cant (p Յ 0.01) difference between motion magnitude on 4DCT and on CBCT in superiorinferior, anterior-posterior and left-right direction, in 13, 9 and 12 of 18 patients, respectively. In the anteriorposterior and left-right direction no fractions had a difference Ն 5 mm. In the superior-inferior direction the difference was Ն 5 mm for 17% of the 401 fractions. In this direction, a signifi cant (p Յ 0.05) time trend in tumor motion was observed in 4 of 18 patients, but all trends were small ( Ϫ 0.17 -0.10 mm/day) and did not explain the large differences in motion magnitude between 4DCT and CBCT. Conclusion. A single measurement of the respiratory-induced motion magnitude of pancreatic tumors using 4DCT is often not representative for the magnitude during daily treatment over a 3 -5-week radiotherapy scheme. For this patient group it may be benefi cial to introduce breath-hold to eliminate respiratory-induced tumor motion.