Dirk Albers

Comparison of dose calculation algorithms for treatment planning in external photon beam therapy for clinical situations.

A study of the performance of five commercial radiotherapy treatment planning systems (TPSs) for common treatment sites regarding their ability to model heterogeneities and scattered photons has been performed. The comparison was based on CT information for prostate, head and neck, breast and lung cancer cases. The TPSs were installed locally at different institutions and commissioned for clinical use based on local procedures. For the evaluation, beam qualities as identical as possible were used: low energy (6 MV) and high energy (15 or 18 MV) x-rays. All relevant anatomical structures were outlined and simple treatment plans were set up. Images, structures and plans were exported, anonymized and distributed to the participating institutions using the DICOM protocol. The plans were then re-calculated locally and exported back for evaluation. The TPSs cover dose calculation techniques from correction-based equivalent path length algorithms to model-based algorithms. These were divided into two groups based on how changes in electron transport are accounted for ((a) not considered and (b) considered). Increasing the complexity from the relatively homogeneous pelvic region to the very inhomogeneous lung region resulted in less accurate dose distributions. Improvements in the calculated dose have been shown when models consider volume scatter and changes in electron transport, especially when the extension of the irradiated volume was limited and when low densities were present in or adjacent to the fields. A Monte Carlo calculated algorithm input data set and a benchmark set for a virtual linear accelerator have been produced which have facilitated the analysis and interpretation of the results. The more sophisticated models in the type b group exhibit changes in both absorbed dose and its distribution which are congruent with the simulations performed by Monte Carlo-based virtual accelerator.

On the dosimetric behaviour of photon dose calculation algorithms in the presence of simple geometric heterogeneities: comparison with Monte Carlo calculations

A comparative study was performed to reveal differences and relative figures of merit of seven different calculation algorithms for photon beams when applied to inhomogeneous media. The following algorithms were investigated: Varian Eclipse: the anisotropic analytical algorithm, and the pencil beam with modified Batho correction; Nucletron Helax-TMS: the collapsed cone and the pencil beam with equivalent path length correction; CMS XiO: the multigrid superposition and the fast Fourier transform convolution; Philips Pinnacle: the collapsed cone. Monte Carlo simulations (MC) performed with the EGSnrc codes BEAMnrc and DOSxyznrc from NRCC in Ottawa were used as a benchmark. The study was carried out in simple geometrical water phantoms (rho = 1.00 g cm(-3)) with inserts of different densities simulating light lung tissue (rho = 0.035 g cm(-3)), normal lung (rho = 0.20 g cm(-3)) and cortical bone tissue (rho = 1.80 g cm(-3)). Experiments were performed for low- and high-energy photon beams (6 and 15 MV) and for square (13 x 13 cm2) and elongated rectangular (2.8 x 13 cm2) fields. Analysis was carried out on the basis of depth dose curves and transverse profiles at several depths. Assuming the MC data as reference, gamma index analysis was carried out distinguishing between regions inside the non-water inserts or inside the uniform water. For this study, a distance to agreement was set to 3 mm while the dose difference varied from 2% to 10%. In general all algorithms based on pencil-beam convolutions showed a systematic deficiency in managing the presence of heterogeneous media. In contrast, complicated patterns were observed for the advanced algorithms with significant discrepancies observed between algorithms in the lighter materials (rho = 0.035 g cm(-3)), enhanced for the most energetic beam. For denser, and more clinical, densities a better agreement among the sophisticated algorithms with respect to MC was observed.

Dosimetric evaluation of a 2D pixel ionization chamber for implementation in clinical routine

In this paper we present the results of a dosimetric evaluation of a 2D ionization chamber array with the objective of its implementation for quality assurance in clinical routine. The pixel ionization chamber MatriXX (Scanditronix Wellhofer, Germany) consists of 32x32 chambers with a distance of 7.6 mm between chamber centres. The effective depth of measurement under the surface of the detector was determined. The dose and energy dependence, the behaviour of the device during its initial phase and its time stability as well as the lateral response of a single chamber of the detector in cross-plane and diagonal directions were analysed. It could be shown, that the detectors response is linear with dose and energy independent. Taking the lateral response into account, two different dose profiles, for a pyramidal and an IMRT dose distribution, were applied to compare the data generated by a treatment planning system with measurements. From these investigations it can be concluded that the detector is a suitable device for quality assurance and 2D dose verifications.

Evaluation of prognostic factors and two radiation techniques in patients treated with surgery followed by radio(chemo)therapy or definitive radio(chemo)therapy for locally advanced head-and-neck cancer

Background and Purpose:Conventional radiotherapy (RT) still is the standard technique for head-and-neck cancer in many centers worldwide, whereas other centers replaced this technique by 3-D conformal RT, which is associated with more appropriate dose distributions. Comparative studies regarding outcome and toxicity are lacking. This study compared both techniques for overall survival (OS), metastases-free survival (MFS), loco-regional control (LC), and toxicity in stage III/IV head-and-neck cancer.Patients and Methods:Data of 345 patients irradiated for stage III/IV squamous cell head-and-neck cancer were retrospectively analyzed. Patients received conventional RT (group A, n = 166) or 3-D conformal RT (group B, n = 179). Both techniques were compared for outcomes and toxicity. Eleven further potential prognostic factors were investigated: age, gender, performance status, tumor site, grading, T-stage, N-stage, AJCC-stage, chemotherapy, surgery, pre-RT hemoglobin.Results:3-year-OS was 62% in group A and 57% in group B (p = 0.15). 3-year-MFS was 67% and 76% (p = 0.46), 3-year-LC was 65% and 68%, respectively (p = 0.71). On multivariate analysis, gender (p = 0.005), performance status (p < 0.001), T-stage (p = 0.002), and N-stage (p < 0.001) were associated with OS. MFS was influenced by performance status (p < 0.001) and N-stage (p < 0.001), LC by gender (p = 0.021), T-stage (p < 0.001), and pre-RT hemoglobin level (≥ 12 better than < 12 g/dl, p = 0.004). Grade 2–3 xerostomia was less frequent with 3-D conformal RT (43% vs. 58%, p = 0.06). Otherwise, toxicities were similar.Conclusion:Both RT techniques resulted in similar treatment outcomes. Because xerostomia was less with 3-D conformal RT, this technique appeared beneficial for patients, in whom one parotid gland can be spared. Outcome was associated with gender, performance status, tumor stage, and pre-RT hemoglobin.Hintergrund und Ziel:Die konventionelle Strahlentherapie ist in vielen Institutionen weltweit noch die Standardtechnik bei Kopf-Hals-Tumoren. Andere Institutionen haben diese Technik durch die 3D-konformale Strahlentherapie ersetzt. Vergleichende Studien fehlen. Diese Studie vergleicht beide Techniken hinsichtlich Gesamtüberleben (OS), metastasenfreien Überlebens (MFS), lokoregionaler Kontrolle (LC) und Toxizität bei Patienten mit einem Plattenepithelkarzinom der Kopf-Hals-Region im Stadium III/IV.Patienten und Methodik:Daten von 345 Patienten wurden retrospektiv analysiert (Tabelle 1). Die Patienten erhielten eine konventionelle (Gruppe A, n = 166) oder 3D-konformale (Gruppe B, n = 179) Strahlentherapie (Abbildungen 1 und 2). Elf weitere potentielle Prognosefaktoren wurden untersucht: Alter, Geschlecht, Allgemeinzustand, Tumorsitz, Grading, T-Stadium, N-Stadium, AJCC-Stadium, Chemotherapie, Operation, Hämoglobin vor Strahlentherapie (Tabellen 2 bis 4).Ergebnisse:Die 3-Jahres-Überlebensraten waren 62% in Gruppe A und 57% in Gruppe B (p = 0,15, Abbildung 3). Die MFS-Raten waren 67% und 76% (p = 0,46, Abbildung 4), die LC-Raten 65% und 68% (p = 0,71, Abbildung 5). In der Multivarianzanalyse waren Geschlecht (p = 0,005), Allgemeinzustand (p < 0,001), T-Kategorie (p = 0,002) und N-Kategorie (p < 0,001) mit dem OS assoziiert. Allgemeinzustand (p < 0,001) und N-Stadium (p < 0,001) waren mit dem MFS assoziiert, Geschlecht (p = 0,021), T-Stadium (p < 0,001) und Hämoglobin vor Strahlentherapie (≥ 12 g/dl besser als < 12 g/dl, p = 0,004) mit der LC. Xerostomie Grad 2–3 war seltener nach 3D-konformaler Strahlentherapie (43% vs. 58%, p = 0,06). Ansonsten waren Akut- und Spättoxizität in beiden Gruppen ähnlich (Abbildungen 6 und 7).Schlussfolgerung:Beide Bestrahlungstechniken führten zu ähnlichen Behandlungsergebnissen. Da die Xerostomie nach 3Dkonformaler Strahlentherapie geringer ausgeprägt war, scheinen Patienten, bei denen eine Parotis geschont werden kann, von dieser Technik zu profitieren. Die Behandlungsergebnisse wurden durch das Geschlecht, den Allgemeinzustand, das Tumorstadium und den prä-strahlentherapeutischen Hämoglobinwert beeinflusst.

Comparison of electron IMRT to helical photon IMRT and conventional photon irradiation for treatment of breast and chest wall tumours

BACKGROUND AND PURPOSE Conventional irradiation of breast and chest wall tumours may cause high doses in underlying organs. Intensity-modulated radiation therapy (IMRT) with photons achieves high conformity between treated and tumour volume but is associated with considerable low-dose effects which may induce secondary malignancies. We compare treatment plans of electron IMRT to helical photon IMRT and conventional irradiation. MATERIAL AND METHODS Treatment planning for three patients (breast, chest wall plus lymph nodes, sarcoma of medial chest wall/sternum) was performed using XiO 4.3.3 (CMS) for conventional photon irradiation, Hi-Art 2.2.2.05 (TomoTherapy) for helical photon IMRT, and a self-designed programme for electron IMRT. RESULTS The techniques resulted in similar mean and maximum target doses. Target coverage by the 95%-isodose was best with tomotherapy. Mean ipsilateral lung doses were similar with all techniques. Electron IMRT achieved best sparing of heart, and contralateral breast. Compared with photon IMRT, electron IMRT allowed better sparing of contralateral lung and total healthy tissue. CONCLUSIONS Electron IMRT is superior to conventional irradiation, as it allows satisfying target coverage and avoids high doses in underlying organs. Its advantage over photon IMRT is better sparing of most organs at risk (low-dose effects) which reduces the risk of radiation-induced malignancies.

Design of a computer-controlled multileaf collimator for advanced electron radiotherapy

A multileaf collimator for electrons (eMLC) has been designed that fulfils the technical requirements for providing advanced irradiation techniques with electrons. In the present work, the basic design parameters of leaf material, leaf height, leaf width and number of leaves as well as leaf overtravel and leaf shape were determined such that an eMLC with motorized leaves can be manufactured by a company specialized in MLC technology. For this purpose, a manually driven eMLC with variable source-to-collimator distance (SCD) was used to evaluate the chosen leaf specification and investigate the impact of the SCD on the off-axis dose distribution. In order to select the final SCD of the eMLC, a compromise had to be found between maximum field size, minimum beam penumbra and necessary distance between eMLC and isocentre to eliminate patient realignments during gantry rotation. As a result, the eMLC is placed according to the target position at 72 and 84 cm SCD, respectively. This feature will be achieved by interchangeable distance holders. At these SCDs, the corresponding maximum field sizes at 100 cm source-to-isocentre distance are 20 x 20 cm and 17 x 17 cm, respectively. Finally, the off-axis dose distribution at the maximum opening of the eMLC was improved by fine-tuning the settings of the accelerator jaws and introducing trimmer bars above the eMLC. Following this optimization, a prototype eMLC consisting of 2 x 24 computer-controlled brass leaves is manufactured by 3D Line Medical Systems.

Towards accurate dose accumulation for Step-&-Shoot IMRT: Impact of weighting schemes and temporal image resolution on the estimation of dosimetric motion effects.

PURPOSE Breathing-induced motion effects on dose distributions in radiotherapy can be analyzed using 4D CT image sequences and registration-based dose accumulation techniques. Often simplifying assumptions are made during accumulation. In this paper, we study the dosimetric impact of two aspects which may be especially critical for IMRT treatment: the weighting scheme for the dose contributions of IMRT segments at different breathing phases and the temporal resolution of 4D CT images applied for dose accumulation. METHODS Based on a continuous problem formulation a patient- and plan-specific scheme for weighting segment dose contributions at different breathing phases is derived for use in step-&-shoot IMRT dose accumulation. Using 4D CT data sets and treatment plans for 5 lung tumor patients, dosimetric motion effects as estimated by the derived scheme are compared to effects resulting from a common equal weighting approach. Effects of reducing the temporal image resolution are evaluated for the same patients and both weighting schemes. RESULTS The equal weighting approach underestimates dosimetric motion effects when considering single treatment fractions. Especially interplay effects (relative misplacement of segments due to respiratory tumor motion) for IMRT segments with only a few monitor units are insufficiently represented (local point differences >25% of the prescribed dose for larger tumor motion). The effects, however, tend to be averaged out over the entire treatment course. Regarding temporal image resolution, estimated motion effects in terms of measures of the CTV dose coverage are barely affected (in comparison to the full resolution) when using only half of the original resolution and equal weighting. In contrast, occurence and impact of interplay effects are poorly captured for some cases (large tumor motion, undersized PTV margin) for a resolution of 10/14 phases and the more accurate patient- and plan-specific dose accumulation scheme. CONCLUSIONS Radiobiological consequences of reported single fraction local point differences >25% of the prescribed dose are widely unclear and should be subject to future investigation. Meanwhile, if aiming at accurate and reliable estimation of dosimetric motion effects, precise weighting schemes such as the presented patient- and plan-specific scheme for step-&-shoot IMRT and full available temporal 4D CT image resolution should be applied for IMRT dose accumulation.

A New Electron IMRT Technique for Breast Cancer: Comparison to Photon IMRT and Conventional Irradiation Based on Static and Dynamic Dose Measurements

Background/Purpose: In radiotherapy of breast cancer, heart and lung receive high doses. When intensity-modulated radiotherapy (IMRT) with photons is applied, parts of the breast volume can be missed due to breathing motion or tissue swelling. Additionally, larger volumes of normal tissue are exposed to a low dose which may induce secondary malignancies. This study compares breast irradiation plans of a new electron IMRT technique to plans achieved with photon IMRT and conventional radiotherapy and investigates the impact of breathing motion on dose delivery.

High Accuracy of Virtual Simulation with the Laser System Dorado CT4

Purpose:To evaluate the accuracy of virtual simulation, which is less time-consuming than physical simulation, with the new laser system Dorado CT4 in 96 prostate cancer patients.Patients and Methods:Virtual simulation was based on a spiral scan with 8 mm reconstruction index and 8 mm slice thickness in 64 patients (group A), and 3 mm reconstruction index and 3 mm slice thickness in 32 patients (group B). Both groups were evaluated for impact on maximum difference (Δmax) regarding the isocenters obtained from virtual simulation versus those obtained from physical simulation.Results:In the entire cohort, mean differences were as follows: Δmax 5.7 ± 3.5 mm, Δx (left/right) 2.8 ± 2.9 mm, Δy (anterior/posterior) 4.5 ± 3.8 mm, and Δz (cranial/caudal) 2.1 ± 2.2 mm. In group A, mean values were Δmax 6.2 ± 3.8 mm, Δx 2.9 ± 3.1 mm, Δy 4.9 ± 4.2 mm, and Δz 2.3 ± 2.3 mm. In group B, mean values were Δmax 4.8 ± 2.8 mm, Δx 2.7 ± 2.7 mm, Δy 3.7 ± 2.7 mm, and Δz 1.7 ± 2.0 mm. Time of radiotherapy (primary vs. salvage RT) and radiation regimen (external-beam radiotherapy [EBRT] vs. high-dose-rate brachytherapy [HDR-BT] plus EBRT) had no significant impact on Δmax.Conclusion:Virtual simulation with the new laser system Dorado CT4 was very precise for both primary and salvage RT in the treatment of prostate cancer patients. High precision was achieved for both EBRT and HDR-BT plus EBRT. Virtual simulation should be performed with a planning CT with 3 mm reconstruction index and 3 mm slice thickness for high accuracy.Hintergrund und Ziel:Diese Studie untersucht die Präzision der virtuellen Simulation mit dem neuen Lasersystem Dorado CT4 bei 96 Patienten mit Prostatakarzinom.Patienten und Methodik:Die virtuelle Simulation (Abbildung 1) basierte bei 64 Patienten (Gruppe A) auf einem Spiral-CT mit 8 mm Schichtdicke und 8 mm Rekonstruktionsindex, bei 32 Patienten (Gruppe B) auf einem Spiral-CT mit 3 mm Schichtdicke und 3 mm Rekonstruktionsindex (Tabelle 1). Beide Gruppen wurden hinsichtlich der maximalen Differenz (Δmax) zwischen den Isozentren der virtuellen Simulation und den Isozentren der konventionellen Simulation verglichen.Ergebnisse:Im Gesamtkollektiv waren die mittleren Differenzen folgendermaßen: Δmax 5,7 ± 3,5 mm, Δx (links/rechts) 2,8 ± 2,9 mm, Δy (anterior/posterior) 4,5 ± 3,8 mm, Δz (kranial/kaudal) 2,1 ± 2,2 mm (Tabelle 2). In Gruppe A ergaben sich folgende Mittelwerte: Δmax 6,2 ± 3,8 mm, Δx 2,9 ± 3,1 mm, Δy 4,9 ± 4,2 mm, Δz 2,3 ± 2,3 mm. Die Mittelwerte in Gruppe B waren 4,8 ± 2,8 mm für Δmax, 2,7 ± 2,7 mm für Δx, 3,7 ± 2,7 mm für Δy und 1,7 ± 2,0 mm für Δz. Der Zeitpunkt der Strahlentherapie (primäre vs. Salvage-RT) und das RT-Regime (perkutane RT vs. High-Dose-Rate-Brachytherapie [HDR-BT] plus perkutane RT) hatten keinen signifikanten Einfluss auf Δmax (Tabelle 3).Schlussfolgerung:Die virtuelle Simulation mit dem Lasersystem Dorado CT4 bei der RT von Patienten mit Prostatakarzinom ist sehr präzise, und zwar unabhängig vom Zeitpunkt der RT (primäre RT, Salvage-RT) und vom RT-Regime (perkutane RT, HDR-BT plus perkutane RT). Schichtdicke und Rekonstruktionsindex des für die virtuelle Simulation verwendeten Planungs-CT sollten 3 mm betragen, um eine hohe Präzision zu erreichen.

Comparison of different evaluation programs for the verification of patient irradiations with tomotherapy

Three different evaluation programs used to verify patient irradiations with tomotherapy were checked. All programs compare a planned dose distribution with the measured dose distribution using radiochromic film. The evaluation programs differ in quality, time effort and user friendliness.