A. Bel

A verification procedure to improve patient set-up accuracy using portal images

The purpose of this study was to establish which level of geometrical accuracy can be obtained during radiotherapy, using portal image analysis, with a minimum number of patient set-up measurements and corrections. A set-up verification and correction procedure using decision rules for improving the set-up of a patient during radiotherapy was investigated by means of a computer simulation. In this simulation study, set-up deviations were assumed to be the sum of random and systematic deviations and varying ratios of random and systematic deviations were studied. The distribution of random deviations (SD equal to sigma) was assumed to be equal for all patients of a specific treatment site. Set-up deviations are measured during the first N consecutive fractions after the start of the treatment or after a patient set-up correction. A set-up is corrected when the deviation averaged over these measurements is larger than an N-dependent action level. This action level is specified by alpha/square root of N, in which alpha is a variable initial action level parameter. After the start of the treatment or after each correction, Nmax measurements are made to decide on a possible (further) correction. By varying alpha and Nmax, the relation between the overall accuracy and the workload has been analyzed. It was possible to obtain a resulting overall accuracy level which is almost independent of the initial distribution of systematic deviations.(ABSTRACT TRUNCATED AT 250 WORDS)

Initial experience with intensity-modulated conformal radiation therapy for treatment of the head and neck region

PURPOSE The efficacy of a conventional, noninvasive fixation technique in combination with a commercially available system for conformal radiotherapy by intensity modulation of the treatment beam has been studied. METHODS AND MATERIALS A slice-by-slice arc-rotation approach was used to deliver a conformal dose to the target and patient fixation was performed by means of thermoplastic casts. Eleven patients have been treated, of which 9 were for tumors of the head and neck region and 2 were for intracranial lesions. A procedure for target localization and verification of patient positioning suitable for this particular treatment technique has been developed based on the superposition of digitized portals with plots generated from the treatment-planning system. A dosimetric verification of the treatment procedure was performed with an anthropomorphic phantom: both absolute dose measurements (alanine and thermoluminescent detectors) and relative dose distribution measurements (film dosimetry) have been applied. The dose delivered outside the target has also been investigated. RESULTS The dose verification with the anthropomorphic phantom yielded a ratio between measured and predicted dose values of 1.0 for different treatment schedules and the calculated dose distribution agreed with the measured dose distribution. Day-to-day variations in patient setup of 0.3 cm (translations) and 2.0 degrees (rotations) were considered acceptable for this particular patient population, whereas the verification protocol allowed detection of 0.1 cm translational errors and 1.0 rotational errors. CONCLUSIONS The noninvasive fixation technique in combination with an adapted verification protocol proved to be acceptable for conformal treatment of the head and neck region. Dose measurements, in turn, confirmed the predicted dose values to the target and organs at risk within uncertainty. Daily monitoring becomes mandatory if an accuracy superior to 0.1 cm and 1.0 degree is required for patient setup.

Setup deviations in wedged pair irradiation of parotid gland and tonsillar tumors, measured with an electronic portal imaging device

The first aim of this study was to quantify estimated translational setup deviations of patients treated with a wedged pair of oblique beams for parotid gland and tonsillar tumors, using portal imaging. The second aim was to design an off-line setup verification procedure, to improve the setup accuracy, if necessary. Thirty-one patients were treated with two conformal fields (anterior-oblique and posterior-oblique). The patients were immobilized with a head cast. For the last 10 patients, the rigidity of the cast was improved while, in addition, wax molds with metal markers were placed into the outer ear for image correlation. Portal images were acquired about weekly. Setup deviations were analyzed, using anatomical structures and, when available, metal markers for image matching. The consistency of the deviations was determined by the correlation between deviations in the cranio-caudal direction, as measured from both beams. When the deviations were consistent, the translational setup deviation during a treatment session could be described by a three-dimensional (3D) vector. A setup verification procedure was designed using a computer simulation. The statistics of the 3D setup deviations were used as input. The output consisted of the resulting setup accuracy and workload (i.e., the number of setup corrections and portal images). Using the anatomical structures for image correlation, the deviations in the cranio-caudal direction were not correlated, either for the old or the improved cast. However, by using the metal markers, the deviations were correlated and a 3D analysis could be performed. The standard deviations, averaged over the three directions, were equal to 1.8 and 1.4 mm for the distribution of systematic and random deviations, respectively. Application of a setup verification procedure, with 0.7 corrections on the average per patient, could potentially reduce the percentage of 3D systematic deviations larger than 4 mm from 30 to 2%. It can be concluded that it was not possible to obtain consistent translational setup deviations, due to rotations. To quantify 3D translational setup deviations, it was necessary to use additional metal markers, which were visible in the portal images of both beams. A further improvement of the setup accuracy is possible by using an off-line setup verification procedure.

Improving locoregional hyperthermia delivery using the 3-D controlled AMC-8 phased array hyperthermia system: A preclinical study

Background: The aim of this study is preclinical evaluation of our newly developed regional hyperthermia system providing 3-D SAR control: the AMC-8 phased array consisting of two rings, each with four 70 MHz waveguides. It was designed to achieve higher tumour temperatures and improve the clinical effectiveness of locoregional hyperthermia. Methods: The performance of the AMC-8 system was evaluated with simulations and measurements aiming at heating a centrally located target region in rectangular (30 × 30 × 110 cm) and elliptical (36 × 24 × 80 cm) homogeneous tissue equivalent phantoms. Three properties were evaluated and compared to its predecessor, the 2-D AMC-4 single ring four waveguide array: (1) spatial control and (2) size of the SAR focus, (3) the ratio between maximum SAR outside the target region and SAR in the focus. Distance and phase difference between the two rings were varied. Results: (1) Phase steering provides 3-D SAR control for the AMC-8 system. (2) The SAR focus is more elongated compared to the AMC-4 system, yielding a lower SAR level in the focus when using the same total power. This is counter-balanced by (3) a superficial SAR deposition which is half of that in the AMC-4 system, yielding a more favourable ratio between normal tissue and target SAR and allowing higher total power and up to 30% more SAR in the focus for 3 cm ring distance. Conclusion: The AMC-8 system is capable of 3-D SAR control and its SAR distribution is more favourable than for the 2-D AMC-4 system. This result promises improvement in clinical tumour temperatures.

Accelerated ray tracing for radiotherapy dose calculations on a GPU

PURPOSE The graphical processing unit (GPU) on modern graphics cards offers the possibility of accelerating arithmetically intensive tasks. By splitting the work into a large number of independent jobs, order-of-magnitude speedups are reported. In this article, the possible speedup of PLATOs ray tracing algorithm for dose calculations using a GPU is investigated. METHODS A GPU version of the ray tracing algorithm was implemented using NVIDIAs CUDA, which extends the standard C language with functionality to program graphics cards. The developed algorithm was compared based on the accuracy and speed to a multithreaded version of the PLATO ray tracing algorithm. This comparison was performed for three test geometries, a phantom and two radiotherapy planning CT datasets (a pelvic and a head-and-neck case). For each geometry, four different source positions were evaluated. In addition to this, for the head-and-neck case also a vertex field was evaluated. RESULTS The GPU algorithm was proven to be more accurate than the PLATO algorithm by elimination of the look-up table for z indices that introduces discretization errors in the reference algorithm. Speedups for ray tracing were found to be in the range of 2.1-10.1, relative to the multithreaded PLATO algorithm running four threads. For dose calculations the speedup measured was in the range of 1.5-6.2. For the speedup of both the ray tracing and the dose calculation, a strong dependency on the tested geometry was found. This dependency is related to the fraction of air within the patients bounding box resulting in idle threads. CONCLUSIONS With the use of a GPU, ray tracing for dose calculations can be performed accurately in considerably less time. Ray tracing was accelerated, on average, with a factor of 6 for the evaluated cases. Dose calculation for a single beam can typically be carried out in 0.6-0.9 s for clinically realistic datasets. These findings can be used in conventional planning to enable (nearly) real-time dose calculations. Also the importance for treatment optimization techniques is evident.

Interfractional position variation of pancreatic tumors quantified using intratumoral fiducial markers and daily cone beam computed tomography

PURPOSE The aim of this study was to quantify interfractional pancreatic position variation using fiducial markers visible on daily cone beam computed tomography (CBCT) scans. In addition, we analyzed possible migration of the markers to investigate their suitability for tumor localization. METHODS AND MATERIALS For 13 pancreatic cancer patients with implanted Visicoil markers, CBCT scans were obtained before 17 to 25 fractions (300 CBCTs in total). Image registration with the reference CT was used to determine the displacement of the 2 to 3 markers relative to bony anatomy and to each other. We analyzed the distance between marker pairs as a function of time to identify marker registration error (SD of linear fit residuals) and possible marker migration. For each patient, we determined the mean displacement of markers relative to the reference CT (systematic position error) and the spread in displacements (random position error). From this, we calculated the group systematic error, Σ, and group random error, σ. RESULTS Marker pair distances showed slight trends with time (range, -0.14 to 0.14 mm/day), possibly due to tissue deformation, but no shifts that would indicate marker migration. The mean SD of the fit residuals was 0.8 mm. We found large interfractional position variations, with for 116 of 300 (39%) fractions a 3-dimensional vector displacement of >10 mm. The spread in displacement varied significantly (P<.01) between patients, from a vector range of 9.1 mm to one of 24.6 mm. For the patient group, Σ was 3.8, 6.6, and 3.5 mm; and σ was 3.6, 4.7 and 2.5 mm, in left-right, superior-inferior, and anterior-posterior directions, respectively. CONCLUSIONS We found large systematic displacements of the fiducial markers relative to bony anatomy, in addition to wide distributions of displacement. These results for interfractional position variation confirm the potential benefit of using fiducial markers rather than bony anatomy for daily online position verification for pancreatic cancer patients.

Influence of daily setup measurements and corrections on the estimated delivered dose during IMRT treatment of prostate cancer patients

PURPOSE To evaluate the impact of marker-based position verification, using daily imaging and an off-line correction protocol, by calculating the delivered dose to prostate, rectum and bladder. METHODS Prostate cancer patients (n=217) were treated with IMRT, receiving 35 daily fractions. Plans with five beams were optimized taking target coverage (CTV, boost) and organs-at-risk (rectum and bladder) into account. PTV margins were 8mm. Prostate position was verified daily using implanted fiducial gold markers by imaging the first segment of all the five beams on an EPID. Setup deviations were corrected off-line using an adapted shrinking-action-level protocol. The estimated delivered dose, including daily organ movements, was calculated using a version of PLATOs dose engine, enabling batch processing of large numbers of patients. The dose was calculated +/- inclusion of setup corrections, and was evaluated relative to the original static plan. The marker-based measurements were considered representative for all organs. RESULTS Daily organ movements would result in an underdosage of 2-3Gy to CTV and boost volume relative to the original plan, which was prevented by daily setup corrections. The dose to rectum and bladder was on average unchanged, but a large spread was introduced by organ movements, which was reduced by including setup corrections. CONCLUSIONS Without position verification and setup corrections, margins of 8mm would be insufficient to account for position uncertainties during IMRT of prostate cancer. With the daily off-line correction protocol, the remaining variations are accommodated adequately.

Optimization in hyperthermia treatment planning: The impact of tissue perfusion uncertainty

PURPOSE Hyperthermia treatment planning (HTP) potentially provides a valuable tool for monitoring and optimization of treatment. However, one of the major problems in HTP is that different sources of uncertainty degrade its reliability. Perfusion uncertainty is one of the largest uncertainties and hence there is an ongoing debate whether optimization should be limited to power-based strategies. In this study a systematic analysis is carried out addressing this question. METHODS The influence of perfusion uncertainty on optimization was analyzed for five patients with cervix uteri carcinoma heated with the AMC-8 70 MHz phased-array waveguide system. The effect of variations (up to +/- 50%) in both the muscle and tumor perfusion level was investigated. For every patient, reference solutions were calculated using constrained temperature-based optimization for 25 different and known perfusion distributions. Reference solutions were compared to those found by temperature-based optimization using standard perfusion values and four SAR-based optimization methods. The effect of heterogeneity was investigated by creating 5 x 100 perfusion distributions for different levels of local variation (+/- 25% and +/- 50%) and scale (1 and 2 cm). Here the performance of the temperature-based optimization method was compared to a SAR-based method that showed good performance in the previous analysis. RESULTS Solutions found with temperature-based optimization using a deviating perfusion distribution during optimization were found within 1.0 degrees C from the true optimum. For the SAR-based methods, deviations up to 2.9 degrees C were found. The spread found in these deviations was comparable, typically 0.5-1.0 degrees C. When applying intramuscle variation to the perfusion, temperature-based optimization proved to be the best strategy in 95% of the evaluated cases applying +/- 50% local variation. CONCLUSIONS Temperature-based optimization proves to be superior to SAR-based optimization both under variation of perfusion level as well as under the application of intratissue variation. The spread in achieved temperatures is comparable. These results are valid under the assumption of constant perfusion at hyperthermic levels. Although similar results are expected from models including thermoregulation, additional analysis is required to confirm this. In view of uncertainty in tissue perfusion and other modeling uncertainties, the authors propose feedback guided temperature-based optimization as the best candidate to improve thermal dose delivery during hyperthermia treatment.

Behavior of Lipiodol Markers During Image Guided Radiotherapy of Bladder Cancer

PURPOSE To investigate the stability of a novel type of markers used in partial bladder tumor irradiation and tumor deformation as indicated by the markers. MATERIALS AND METHODS In 15 patients with solitary bladder cancer, lipiodol was injected in the bladder wall during flexible cystoscopy to identify the tumor. A planning CT scan was made, followed by daily cone-beam CT (CBCT) scans during treatment. To study the accuracy of using these markers for image guidance, uncertainties U1 and U2 were calculated, which were defined as the difference between submask registration (covering single marker) and the average of all submask registrations and the difference between the submask registration and the general mask registration (including all markers), respectively. Finally, to study tumor deformation, the relative movement of each marker pair was correlated with the relative bladder volume (RBV). RESULTS The analyzed patients had 2.3 marker injections on average. The lipiodol spot size was 0.72 +/- 1.1 cm(3). The intensity of spots in both CT and CBCT was significantly higher than the surrounding bladder tissue. The uncertainties U1 and U2 were comparable, and the uncertainties in left-right direction (0.14-0.19 cm) were smaller than those in cranial-caudal and anterior-posterior directions (0.19-0.32 cm). The relative marker movement of within-zone marker pairs was much smaller (and has less dependence on the RBV) than across-zones marker pairs. CONCLUSIONS Lipiodol markers are a feasible method to track bladder tumor by using online CBCT. Tumor deformation is observed, especially for tumors that cross the defined bladder zones.

Uncertainty in hyperthermia treatment planning: the need for robust system design

Hyperthermia treatment planning (HTP) is an important tool to improve the quality of hyperthermia treatment. It is a practical way of designing new hyperthermia systems and can be used to optimize the phase and amplitude settings to achieve optimal heating. One of the main challenges to be dealt with however is the uncertainty in the modeling parameters. The role of dielectric and combined dielectric and perfusion uncertainty on optimization was investigated by means of HTP for six different systems: the 70 MHz AMC-4 (AMC: Academic Medical Center) and AMC-8 system, a 130 MHz version of the AMC-8 system, a three-ring AMC-12 system operating at 130 MHz, the BSD SigmaEye applicator and a dipole applicator with three rings each containing six dipole pairs operated at 150 MHz. For five patients with cervix uteri carcinoma, a patient model was created based on a hyperthermia planning CT. Variation of tissue parameters resulted in 16 dielectric models for every patient. In addition, four thermal models were created to study the combined effect of perfusion and dielectric uncertainty. The impact of dielectric uncertainty on optimization is found to be clearly dependent on the number of channels and increased from 0.5 °C for four channels to 1.5 °C for the 18-channel system. As a result, the potential gain relative to the AMC-4 system for the 70 MHz AMC-8 system was found to be largely compromised, while for the remaining systems a robust improvement in T(90) was observed. The dipole applicator showed the best target heating for two out of five patients, while for three others heating efficacy was comparable to the 130 MHz AMC-12 system or the 130 MHz AMC-8 system (one patient). Considering the increase in complexity when the number of channels is increased from 12 to 18, the AMC-12 system is considered as a good compromise between heating efficacy and robustness while still being a manageable heating system in clinical practice.