Henk Huizenga

Inclusion of geometrical uncertainties in radiotherapy treatment planning by means of coverage probability

PURPOSE Following the ICRU-50 recommendations, geometrical uncertainties in tumor position during radiotherapy treatments are generally included in the treatment planning by adding a margin to the clinical target volume (CTV) to yield the planning target volume (PTV). We have developed a method for automatic calculation of this margin. METHODS AND MATERIALS Geometrical uncertainties of a specific patient group can normally be characterized by the standard deviation of the distribution of systematic deviations in the patient group (Sigma) and by the average standard deviation of the distribution of random deviations (sigma). The CTV of a patient to be planned can be represented in a 3D matrix in the treatment room coordinate system with voxel values one inside and zero outside the CTV. Convolution of this matrix with the appropriate probability distributions for translations and rotations yields a matrix with coverage probabilities (CPs) which is defined as the probability for each point to be covered by the CTV. The PTV can then be chosen as a volume corresponding to a certain iso-probability level. Separate calculations are performed for systematic and random deviations. Iso-probability volumes are selected in such a way that a high percentage of the CTV volume (on average > 99%) receives a high dose (> 95%). The consequences of systematic deviations on the dose distribution in the CTV can be estimated by calculation of dose histograms of the CP matrix for systematic deviations, resulting in a so-called dose probability histogram (DPH). A DPH represents the average dose volume histogram (DVH) for all systematic deviations in the patient group. The consequences of random deviations can be calculated by convolution of the dose distribution with the probability distributions for random deviations. Using the convolved dose matrix in the DPH calculation yields full information about the influence of geometrical uncertainties on the dose in the CTV. RESULTS The model is demonstrated to be fast and accurate for a prostate, cervix, and lung cancer case. A CTV-to-PTV margin size which ensures at least 95% dose to (on average) 99% of the CTV, appears to be equal to about 2Sigma + 0.7sigma for three all cases. Because rotational deviations are included, the resulting margins can be anisotropic, as shown for the prostate cancer case. CONCLUSION A method has been developed for calculation of CTV-to-PTV margins based on the assumption that the CTV should be adequately irradiated with a high probability.

Do Patients With Localized Prostate Cancer Treatment Really Want More Aggressive Treatment

PURPOSE Examine whether patients with prostate cancer choose the more aggressive of two radiotherapeutic options, whether this choice is reasoned, and what the determinants of the choice are. PATIENTS AND METHODS One hundred fifty patients with primary prostate cancer (T(1-3)N(0)M(0)) were informed by means of a decision aid of two treatment options: radiotherapy with 70 Gy versus 74 Gy. The latter treatment is associated with more cure and more toxicity. The patients were asked whether they wanted to choose, and if so which treatment they preferred. They also assigned importance weights to the probability of various outcomes, such as survival, cure and adverse effects. Patients who wanted to choose their own treatment (n = 119) are described here. RESULTS The majority of these patients (75%) chose the lower radiation dose. Their choice was highly consistent (P < or = .001), with the importance weights assigned to the probability of survival, cure (odds ratio [OR] = 6.7 and 6.9) and late GI and genitourinary adverse effects (OR = 0.1 and 0.2). The lower dose was chosen more often by the older patients, low-risk patients, patients without hormone treatment, and patients with a low anxiety or depression score. CONCLUSION Most patients with localized prostate cancer prefer the lower radiation dose. Our findings indicate that many patients attach more weight to specific quality-of-life aspects (eg, GI toxicity) than to improving survival. Treatment preferences of patients with localized prostate cancer can and should be involved in radiotherapy decision making.

Pathological fracture prediction in patients with metastatic lesions can be improved with quantitative computed tomography based computer models.

PURPOSE In clinical practice, there is an urgent need to improve the prediction of fracture risk for cancer patients with bone metastases. The methods that are currently used to estimate fracture risk are dissatisfying, hence affecting the quality of life of patients with a limited life expectancy. The purpose of this study was to assess if non-linear finite element (FE) computer models, which are based on Quantitative Computer Tomography (QCT), are better than clinical experts in predicting bone strength. MATERIALS AND METHODS Ten human cadaver femurs were scanned using QCT. In one femur of each pair a hole (size 22, 40, or 45 mm diameter) was drilled at the anterior or medial side to simulate a metastatic lesion. All femurs were mechanically tested to failure under single-limb stance-type loading. The failure force was calculated using non-linear FE-models, and six clinical experts were asked to rank the femurs from weak to strong based on X-rays, gender, age, and the loading protocol. Kendall Tau correlation coefficients were calculated to compare the predictions of the FE-model with the predictions of the clinicians. RESULTS The FE-failure predictions correlated strongly with the experimental failure force (r(2)=0.92, p<0.001). For the clinical experts, the Kendall Tau coefficient between the experimental ranking and predicted ranking ranged between tau=0.39 and tau=0.72, whereas this coefficient was considerably higher (tau=0.78) for the FE-model. CONCLUSION This study showed that the use of a non-linear FE-model can improve the prediction of bone strength compared to the prediction by clinical experts.

A model to determine the initial phase space of a clinical electron beam from measured beam data.

Advanced electron beam dose calculation models for radiation oncology require as input an initial phase space (IPS) that describes a clinical electron beam. The IPS is a distribution in position, energy and direction of electrons and photons in a plane in front of the patient. A method is presented to derive the IPS of a clinical electron beam from a limited set of measured beam data. The electron beam is modelled by a sum of four beam components: a main diverging beam, applicator edge scatter, applicator transmission and a second diverging beam. The two diverging beam components are described by weighted sums of monoenergetic diverging electron and photon beams. The weight factors of these monoenergetic beams are determined by the method of simulated annealing such that a best fit is obtained with depth-dose curves measured for several field sizes at two source-surface distances. The resulting IPSs are applied by the phase-space evolution electron beam dose calculation model to calculate absolute 3D dose distributions. The accuracy of the calculated results is in general within 1.5% or 1.5 mm; worst cases show differences of up to 3% or 3 mm. The method presented here to describe clinical electron beams yields accurate results, requires only a limited set of measurements and might be considered as an alternative to the use of Monte Carlo methods to generate full initial phase spaces.

Clinical validation of the normalized mutual information method for registration of CT and MR images in radiotherapy of brain tumors

Image registration integrates information from different imaging modalities and has the potential to improve determination of target volume in radiotherapy planning. This paper describes the implementation and validation of a 3D fully automated registration procedure in the process of radiotherapy treatment planning of brain tumors. Fifteen patients with various brain tumors received computed tomography (CT) and magnetic resonance (MR) brain imaging before the start of radiotherapy. First, the normalized mutual information (NMI) method was used for image registration. Registration accuracy was estimated by performing statistical analysis of coordinate differences between CT and MR anatomical landmarks along the x‐, y‐ and z‐axes. Second, a visual validation protocol was developed to validate the quality of individual registration solutions, and this protocol was tested in a series of 36 CT‐MR registration procedures with intentionally applied registration errors. The mean coordinate differences between CT and MR landmarks along the x‐ and y‐axes were in general within 0.5 mm. The mean coordinate differences along the z‐axis were within 1.0 mm, which is of the same magnitude as the applied slice thickness in scanning. In addition, the detection of intentionally applied registration errors by employment of a standardized visual validation protocol resulted in low false‐negative and low false‐positive rates. Application of the NMI method for the brain results in excellent automatic registration accuracy, and the method has been incorporated into the daily routine at our institution. A standardized validation protocol ensures the quality of individual registrations by detecting registration errors with high sensitivity and specificity. This protocol is proposed for the validation of other linear registration methods. PACS numbers: 87.53.Xd, 87.57.Gg

Film dosimetry of clinical electron beams

Film dosimetry is used widely to obtain relative dose distributions of clinical electron beams in phantoms. Nevertheless, measurement results obtained with film dosimetry may lack precision and reliability. In this paper well defined and reproducible methods in film dosimetry are discussed. By application of these methods, film dosimetry appears to be adequate in measuring relative dose distributions of clinically applied electron beams, with an accuracy of 1% to 2% of the dose maximum, in water and plastics as well as in heterogeneously composed material.

Numerical calculation of energy deposition by high-energy electron beams: III. Three-dimensional heterogeneous media

The phase space time evolution model of Huizenga and Storchi and Morawska-Kaczyńska and Huizenga has been modified to accommodate calculations of energy deposition by arbitrary electron beams in three-dimensional heterogeneous media. This is a further development aimed at the use of the phase space evolution model in radiotherapy treatment planning. The model presented uses an improved method to control the evolution of the phase space state. This new method results in a faster algorithm, and requires less computer memory. An extra advantage of this method is that it allows the pre-calculation of information, further reducing calculation times. Typical results obtainable with this model are illustrated with the cases of (i) a 20 MeV pencil beam in a water phantom, (ii) a 20 MeV 5 x 5 cm2 beam in a water phantom containing two air cavities, and (iii) a 20 MeV 5 x 5 cm2 beam in a water phantom containing an aluminium region. The results of the dose distribution calculations are in good agreement with and require significantly less computation time than results obtained with Monte Carlo methods.

Scattered radiation from applicators in clinical electron beams

In radiotherapy with high-energy (4-25 MeV) electron beams, scattered radiation from the electron applicator influences the dose distribution in the patient. In most currently available treatment planning systems for radiotherapy this component is not explicitly included and handled only by a slight change of the intensity of the primary beam. The scattered radiation from an applicator changes with the field size and distance from the applicator. The amount of scattered radiation is dependent on the applicator design and on the formation of the electron beam in the treatment head. Electron applicators currently applied in most treatment machines are essentially a set of diaphragms, but still do produce scattered radiation. This paper investigates the present level of scattered dose from electron applicators, and as such provides an extensive set of measured data. The data provided could for instance serve as example input data or benchmark data for advanced treatment planning algorithms which employ a parametrized initial phase space to characterize the clinical electron beam. Central axis depth dose curves of the electron beams have been measured with and without applicators in place, for various applicator sizes and energies, for a Siemens Primus, a Varian 2300 C/D and an Elekta SLi accelerator. Scattered radiation generated by the applicator has been found by subtraction of the central axis depth dose curves, obtained with and without applicator. Scattered radiation from Siemens, Varian and Elekta electron applicators is still significant and cannot be neglected in advanced treatment planning. Scattered radiation at the surface of a water phantom can be as high as 12%. Scattered radiation decreases almost linearly with depth. Scattered radiation from Varian applicators shows clear dependence on beam energy. The Elekta applicators produce less scattered radiation than those of Varian and Siemens, but feature a higher effective angular variance. The scattered radiation decreases somewhat with increasing field size and is spread uniformly over the aperture. Experimental results comply with the results of simulations of the treatment head and electron applicator, using the BEAM Monte Carlo code, and Siemens, but feature a higher effective angular variance. The scattered radiation decreases somewhat with increasing field size and is spread uniformly over the aperture. Experimental results comply with the results of simulations of the treatment head and electron applicator, using the BEAM Monte Carlo code.

In vivo determination of the accuracy of field matching in breast cancer irradiation using an electronic portal imaging device

The purpose of this study was to investigate the accuracy of field matching in patients treated by irradiation of the breast and adjacent lymph nodes. Field matching is performed by the radiographers during each session on a match line drawn on the patients skin. Field edge positions were assessed in the cranial match plane of tangential breast fields and supraclavicular-axillary fields using an electronic portal imaging device and match line markers placed on the skin of the patients. The mean gap/overlap of the four fields for individual patients during each treatment session, derived from 374 marker projections, was +0.5 mm indicating that no systematic gap or overlap was observed. The uncertainty in the position of the four fields with respect to the match plane ranges from 3.1 to 5.1 mm (1 SD) for the individual patients. Gaps and overlaps between fields were also related to an absolute match line position, found by comparison of simulator and portal images, showing a small systematic uncertainty of 2.4 mm and a standard deviation of 3.3 mm. It can be concluded that the use of an electronic portal imaging device in combination with match line markers is a good method to quantify the accuracy of field matching in vivo. The results showed good stability and reproducibility in the field matching region for this treatment technique of breast cancer irradiation.

Film dosimetry for radiotherapy treatment planning verification of a 6 MV tangential breast irradiation

Film dosimetry has been applied to measure relative dose distributions in an anthropomorphic polystyrene breast phantom having cork lungs, simulating a radiation therapy treatment with 6 MV opposing tangential beams. Measured relative dose distributions showed good reproducibility (about 1.5%, 1 SD) and good agreement (< 2%) with calculations performed with a three-dimensional treatment planning system. These results demonstrate that film dosimetry is a useful tool for treatment planning verification.