Alexander Gemmel

A patient-specific planning target volume used in 'plan of the day' adaptation for interfractional motion mitigation.

We propose a patient-specific planning target volume (PTV) to deal with interfractional variations, and test its feasibility in a retrospective treatment-planning study. Instead of using one planning image only, multiple scans are taken on different days. The target and organs at risk (OARs) are delineated on each images. The proposed PTV is generated from a union of those target contours on the planning images, excluding voxels of the OARs, and is denoted the PTV ‘GP–OAR’ (global prostate–organs at risk). The study is performed using ‘plan of the day’ adaptive workflow, which selects a daily plan from a library of plans based on a similarity comparison between the daily scan and planning images. The daily plans optimized for GP–OAR volumes are compared with those optimized for PTVs generated from a single prostate contour (PTV SP). Four CT serials of prostate cancer patient datasets are included in the test, and in total 28 fractions are simulated. The results show that the daily chosen GP–OAR plans provide excellent target coverage, with V95 values of the prostate mostly > 95%. In addition, dose delivered to the OARs as calculated from applying daily chosen GP–OAR plans is slightly increased but comparable to that calculated from applying daily SP plans. In general, the PTV GP–OARs are able to cover possible target variations while keeping dose delivered to the OARs at a similar level to that of the PTV SPs.

Calculation and experimental verification of the RBE-weighted dose for scanned ion beams in the presence of target motion

We present an algorithm suitable for the calculation of the RBE-weighted dose for moving targets with a scanned particle beam. For verification of the algorithm, we conducted a series of cell survival measurements that were compared to the calculations. Calculation of the relative biological effectiveness (RBE) with respect to tumor motion was included in the treatment planning procedure, in order to fully assess its impact on treatment delivery with a scanned ion beam. We implemented an algorithm into our treatment planning software TRiP4D which allows determination of the RBE including its dependence on target tissue, absorbed dose, energy and particle spectra in the presence of organ motion. The calculations are based on time resolved computed tomography (4D-CT) and the corresponding deformation maps. The principal of the algorithm is illustrated in in silico simulations that provide a detailed view of the different compositions of the energy and particle spectra at different target positions and their consequence on the resulting RBE. The calculations were experimentally verified with several cell survival measurements using a dynamic phantom and a scanned carbon ion beam. The basic functionality of the new dose calculation algorithm has been successfully tested in in silico simulations. The algorithm has been verified by comparing its predictions to cell survival measurements. Four experiments showed in total a mean difference (standard deviation) of -1.7% (6.3%) relative to the target dose of 9 Gy (RBE). The treatment planning software TRiP is now capable to calculate the patient relevant RBE-weighted dose in the presence of target motion and was verified against cell survival measurements.

Influence of the delta ray production threshold on water-to-air stopping power ratio calculations for carbon ion beam radiotherapy

Previous calculations of the water-to-air stopping power ratio (s(w,)(air)) for carbon ion beams did not involve tracking of delta ray electrons, even though previous calculations with protons predict an effect up to 1%. We investigate the effect of the delta ray production threshold in s(w,)(air) calculations and propose an empirical expression which takes into account the effect of the delta ray threshold as well as the uncertainty in the mean ionization potentials (I-values) of air and water. The formula is derived from the results of Monte Carlo calculations using the most up-to-date experimental data for I-values and a delta ray production threshold of 10 keV. It allows us to reduce the standard uncertainty in s(w,)(air) below 0.8%, instead of the current 2% given in international protocols, which results in a reduction of the overall uncertainty for absolute dosimetry based on air-filled ionization chambers.

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets

A dynamic phantom has been developed to allow for measurement of 3D cell survival distributions and the corresponding distributions of the RBE-weighted dose (RBED) in the presence of motion. The phantom consists of two 96-microwell plates holding Chinese hamster ovary cells inside a container filled with culture medium and is placed on a movable stage. Basic biological properties of the phantom were investigated without irradiation and after irradiation with a carbon ion beam, using both a stationary (reference) exposure and exposure during motion of the phantom perpendicular to the beam with beam tracking. There was no statistically significant difference between plating efficiency measured in the microwells with and without motion (0.75) and values reported in the literature. Mean differences between measured and calculated cell survival for these two irradiation modes were within +/-5% of the target dose of 6 Gy (RBE).

Prediction methods for synchronization of scanned ion beam tracking

Beam tracking as a mitigation technique for treatment of intra-fractionally moving organs requires prediction to overcome latencies in the adaptation process. We implemented and experimentally tested a prediction method for scanned carbon beam tracking. Beam tracking parameters, i.e. the shift of the Bragg peak position in 3D, are determined prior to treatment in 4D treatment planning and applied during treatment delivery in dependence on the motion state of the target as well as on the scanning spot in the target. Hence, prediction is required for the organ motion trajectory as well as the scanning progress to achieve maximal performance. Prediction algorithms to determine beam displacements that overcome these latencies were implemented. Prediction times of 25 ms for target spot prediction were required for ~6 mm water-equivalent longitudinal beam shifts. The experimental tests proved feasibility of the implemented prediction algorithm.

A 3D model to calculate water-to-air stopping power ratio in therapeutic carbon ion fields

Air-filled ionization chambers (ICs) are extensively used in the dosimetry of charged particle radiotherapy [1]. The calibration procedure of ionization chambers for the determination of absorbed dose to water, which is the standard quantity used for dose determination in external radiotherapy [2] is known as ND,w formalism. In this formalism, the readout of the chamber is converted into absorbed dose to water via two factors: the calibration factor of the chamber, and a quality factor that accounts for the specificity of the beam. The water-to-air stopping power ratio, or sw,air, is one of the main components of these quality factors, and, in the case of carbon ion beams, its biggest source of uncertainty [2]. In a previous work by our group [3], an expression was proposed to calculate sw,air for carbon ion beams at different residual ranges, based on a set of Monte Carlo calculations and experimental measurements, namely: (1) where Rres is expressed in cm and calculated using a practical range at the 50% dose level Rres(z) = R50 – z, where z is the depth in water. This expression is based on a 1D analysis of dose and sw,air distributions, which is enough to model the variations in sw,air for homogeneous dose distributions, like the ones mostly used for calibration and quality assurance (QA) purposes. However, this 1D description might be insufficient in some cases. An example of this would be treatment plan verification with a matrix of ionization chambers [4], a protocol often used in scanning-beam facilities where a patient plan is shot into a water phantom and the deposited dose is measured at several points (see Fig. ​Fig.1).1). In such a case, the residual range Rres is not defined at every point, so the application of equation (1) is not possible.