[en] In the present study, the impact of respiratory gating on the beam characteristics of a linear accelerator is investigated. The main focus is put on the influence of the duty cycle. Measurements were performed on a linear accelerator type Oncor (Siemens) with photon energies 6 MV and 15 MV, equipped with the Anzai gating system AZ-733V. Depth dose curves and beam profiles were found not to be significantly altered by gating even for duly cycles down to 5% for realistic respiration frequencies (dose variations all <2.5%). However, for very small duty cycles, the absolute dosimetry changes significantly (dose deviations > 10%), leading to clinically relevant underdoses. The crucial parameter for the dosimetry is the number of monitor units per gate. Our results imply that treatment planning for respiratory gating can be performed on the basis of data obtained under ungated operation if and only if the absolute gates sizes during treatment are sufficiently large. The limiting values for the gate sizes have to be determined individually for each accelerator. (orig.)

[en] The new TomoDirect™ modality offers a non-rotational option with discrete beam angles. We have investigated this mode for TBI with the intention to test the feasibility and to establish it as a clinical routine method. Special foci were directed onto treatment planning, dosimetric accuracy and practical aspects. TBI plans were calculated with TomoDirect™ for a Rando™ phantom and all patients with an intended fractionated total body irradiation between November 2013 and May 2014 (n = 8). Finally, four of these patients were irradiated with TomoDirect™. Additionally we studied variations in the modulation factor, pitch, field width of Y-jaws and dose grid during optimization. Dose measurements were performed using thermoluminescent rods in the Rando™ phantom, with the Delta4® and with ionization chambers in a solid water phantom. For all eight calculated plans with a prescribed dose of 12 Gy Dmean was 12.09-12.33 Gy (12,25 ± 0.08 Gy), D98 11.2-11.6 Gy (11.45 ± 0.12 Gy) and D2 12.6-13.1 Gy (12.94 ± 0.13 Gy). Dmean of inner lungs was 8.73 ± 0.22 Gy on the left side and 8.69 ± 0.27 Gy on the right side. When single planning parameters are varied with otherwise constant parameters, the modulation factor showed the greatest impact on dose homogeneity and treatment time. The impact of the pitch was marginally, and almost equal homogeneity can be obtained with field width of Y-jaws 5 cm and 2.5 cm. Measurements with thermoluminescent rods (n = 25) in the Rando™ phantom showed a mean dose deviation between measured and calculated dose of 0.66 ± 2.26%. 18 of 25 TLDs had a deviation below 3%, seven of 25 TLDs between 3% and 5%. TBI with TomoDirect™ allows a superior homogeneity compared to conventional methods, where lung blocks are widely accepted. The treatment is performed only in supine position and is robust and comfortable for the patient. TomoDirect™ allows the implementation of organ-specific dose prescriptions. So the discussion about the balance between the need for aggressive treatment and limited toxicity can be renewed with the new potentials of TomoDirect™ - for children as well as for adults – and possibly yield a better clinical outcome in the future

[en] This project compares the different patient-related quality assurance systems for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques currently used in the central Germany area with an independent measuring system. The participating institutions generated 21 treatment plans with different combinations of treatment planning systems (TPS) and linear accelerators (LINAC) for the QUASIMODO (Quality ASsurance of Intensity MODulated radiation Oncology) patient model. The plans were exposed to the ArcCHECK measuring system (Sun Nuclear Corporation, Melbourne, FL, USA). The dose distributions were analyzed using the corresponding software and a point dose measured at the isocenter with an ionization chamber. According to the generally used criteria of a 10 % threshold, 3 % difference, and 3 mm distance, the majority of plans investigated showed a gamma index exceeding 95 %. Only one plan did not fulfill the criteria and three of the plans did not comply with the commonly accepted tolerance level of ±3 % in point dose measurement. Using only one of the two examined methods for patient-related quality assurance is not sufficiently significant in all cases. (orig.)