New possibilities for volumetric-modulated arc therapy using the Agility™ 160-leaf multileaf collimator.

PURPOSE: This study compares the quality of intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans optimized for an Elekta Agility(TM) (Elekta, Stockholm, Sweden) multileaf collimator (MLC; leaf width 5 mm) and an Elekta MLCi2 (leaf width 10 mm) for complex target volumes (anal, AC; head and neck, H&N and prostate cancer, PC). PATIENTS AND METHODS: For plan comparisons, 15 patients who had been treated with IMRT or VMAT using the MLCi2 were selected. For each patient, a retrospective treatment plan using the MLCi2 for the technique not applied was created, as were treatment plans for both techniques using the Agility(TM) MLC. Dose-volume histograms (DHVs) for planning target volumes (PTVs) and organs at risk (OARs) were compared. Further parameters relating to dose conformity, dose homogeneity and mean dose (Dmean) to the PTV, compliance with the intended OAR dose criteria and overall dose to normal tissue were analyzed. Verification measurements were performed and optimization and treatment times were compared. RESULTS: Compared to the MLCi2 plans, the Agility(TM) IMRT and VMAT plans show better or equivalent results in terms of PTV dose conformity and homogeneity. Compliance with the intended OAR dose criteria does not differ according to technique or MLC type. Slight differences are shown for dose distributions in OARs and normal tissue. Verification measurements show that all plans fulfill the acceptance criteria of a minimum of 95 % matched dose points for the 3 %/3 mm γ criterion. Optimization times for the VMAT plans increase compared to the IMRT plans, whereas treatment times decrease. CONCLUSION: With the MLCi2, treatment of complex target volumes with VMAT was only possible with compromises in terms of target coverage. Using the Agility(TM) MLC, even complex target volumes can be treated with VMAT without compromising target coverage or resulting in higher exposure of OARs or normal tissue.

Simple proposal for dosimetry with an Elekta iViewGT™ electronic portal imaging device (EPID) using commercial software modules.

BACKGROUND: An electronic portal imaging device (EPID) is used to control for patient setup and positioning during fractionated radiotherapy. Due to the rising complexity and conformity of irradiation techniques, the demand for an accurate verification of the dose delivered to the patient has also increased. The purpose of this study was to investigate a simple guidance for dosimetry with an Elekta iViewGT™ EPID using commercial software modules. MATERIAL AND METHODS: EPID measurements were performed using an Elekta iViewGT™ EPID on a linear accelerator with 6 MV x-ray beam. The EPID signal was studied for reproducibility, as well as characteristics as a function of dose, dose rate, and field size. A series of experiments, comparing the response of the flat panel imager and ionization chamber measurements of dose, determine the parameters for the calibration model. EPID measurements were also compared with calculations of the treatment planning system. RESULTS: We found a stable response of the EPID signal over a period of 14 months. It showed nonlinearity depending on dose up to 6.8%. There were low oscillations up to 1.2% depending on dose rate. For all fields, the calibrated flat panel profiles match the measured and calculated dose profiles with maximum deviation of 2-3% for the in-field region. In the high gradient areas, higher differences up to 6% were found. CONCLUSIONS: The gamma evaluation indicates good correlation between predicted and acquired EPID images. The EPID-based pretreatment IMRT verification method will help to improve the quality assurance procedure.

Silicon diodes as an alternative to diamond detectors for depth dose curves and profile measurements of photon and electron radiation.

BACKGROUND: Depth dose curves and lateral dose profiles should correspond to relative dose to water in any measured point, what can be more or less satisfied with different detectors. Diamond as detector material has similar dosimetric properties like water. Silicon diodes and ionization chambers are also commonly used to acquire dose profiles. MATERIAL AND METHODS: The authors compared dose profiles measured in an MP3 water phantom with a diamond detector 60003, unshielded and shielded silicon diodes 60008 and 60012 and a 0.125-cm(3) thimble chamber 233642 (PTW, Freiburg, Germany) for 6- and 25-MV photons. Electron beams of 6, 12 and 18 MeV were investigated with the diamond detector, the unshielded diode and a Markus chamber 23343. RESULTS: The unshielded diode revealed relative dose differences at the water surface below +10% for 6-MV and +4% for 25-MV photons compared to the diamond data. These values decreased to less than 1% within the first millimeters of water depth. The shielded diode was only required to obtain correct data of the fall-off zones for photon beams larger than 10 x 10 cm(2) because of important contributions of low-energy scattered photons. For electron radiation the largest relative dose difference of -2% was observed with the unshielded silicon diode for 6 MeV within the build-up zone. Spatial resolutions were always best with the small voluminous silicon diodes. CONCLUSION: Relative dose profiles obtained with the two silicon diodes have the same degree of accuracy as with the diamond detector.

In vivo dosimetry with semiconducting diodes for dose verification in total-body irradiation. A 10-year experience.

BACKGROUND AND PURPOSE: For total-body irradiation (TBI) using the translation method, dose distribution cannot be computed with computer-assisted three-dimensional planning systems. Therefore, dose distribution has to be primarily estimated based on CT scans (beam-zone method) which is followed by in vivo measurements to ascertain a homogeneous dose delivery. The aim of this study was to clinically establish semiconductor probes as a simple and fast method to obtain an online verification of the dose at relevant points. PATIENTS AND METHODS: In 110 consecutively irradiated TBI patients (12.6 Gy, 2 x 1.8 Gy/day), six semiconductor probes were attached to the body surface at dose-relevant points (eye/head, neck, lung, navel). The mid-body point of the abdomen was defined as dose reference point. The speed of translation was optimized to definitively reach the prescribed dose in this point. Based on the entrance and exit doses, the mid-body doses at the other points were computed. The dose homogeneity in the entire target volume was determined comparing all measured data with the dose at the reference point. RESULTS: After calibration of the semiconductor probes under treatment conditions the dose in selected points and the dose homogeneity in the target volume could be quantitatively specified. In the TBI patients, conformity of calculated and measured doses in the given points was achieved with small deviations of adequate accuracy. The data of 80% of the patients are within an uncertainty of +/- 5%. CONCLUSION: During TBI using the translation method, dose distribution and dose homogeneity can be easily controlled in selected points by means of semiconductor probes. Semiconductor probes are recommended for further use in the physical evaluation of TBI.