Improving interinstitutional and intertechnology consistency of pulmonary SBRT by dose prescription to the mean internal target volume dose.

PURPOSE: Dose, fractionation, normalization and the dose profile inside the target volume vary substantially in pulmonary stereotactic body radiotherapy (SBRT) between different institutions and SBRT technologies. Published planning studies have shown large variations of the mean dose in planning target volume (PTV) and gross tumor volume (GTV) or internal target volume (ITV) when dose prescription is performed to the PTV covering isodose. This planning study investigated whether dose prescription to the mean dose of the ITV improves consistency in pulmonary SBRT dose distributions. MATERIALS AND METHODS: This was a multi-institutional planning study by the German Society of Radiation Oncology (DEGRO) working group Radiosurgery and Stereotactic Radiotherapy. CT images and structures of ITV, PTV and all relevant organs at risk (OAR) for two patients with early stage non-small cell lung cancer (NSCLC) were distributed to all participating institutions. Each institute created a treatment plan with the technique commonly used in the institute for lung SBRT. The specified dose fractionation was 3â€¯× 21.5 Gy normalized to the mean ITV dose. Additional dose objectives for target volumes and OAR were provided. RESULTS: In all, 52 plans from 25 institutions were included in this analysis: 8 robotic radiosurgery (RRS), 34 intensity-modulated (MOD), and 10 3D-conformal (3D) radiation therapy plans. The distribution of the mean dose in the PTV did not differ significantly between the two patients (median 56.9 Gy vs 56.6 Gy). There was only a small difference between the techniques, with RRS having the lowest mean PTV dose with a median of 55.9 Gy followed by MOD plans with 56.7 Gy and 3D plans with 57.4 Gy having the highest. For the different organs at risk no significant difference between the techniques could be found. CONCLUSIONS: This planning study pointed out that multiparameter dose prescription including normalization on the mean ITV dose in combination with detailed objectives for the PTV and ITV achieve consistent dose distributions for peripheral lung tumors in combination with an ITV concept between different delivery techniques and across institutions.

[Evaluation of the electron density phantom CIRS Model 62]. / Evaluation des Elektronendichte-Phantoms CIRS Model 62.

The properties of the electron density phantom CIRS M62 were studied. Physical and electron densities were calculated for all tissue substitutes, and compared to the values given in the data sheet and to the recommendations of the ICRU Report 44. The phantom was scanned in standard-CT and spiral-CT mode with different acquisition parameters. Especially manufactured tissue substitutes for dense bone were analysed for artifacts. Hounsfield values of all tissues substitutes were determined with the treatment planning system CadPlan. Calibration curves were calculated and compared to a stoichiometric calibration for real tissue. There was good agreement between the values calculated and the values given by the manufacturer. The tissue substitutes were only approximately tissue equivalent. The tissue substitutes for dense bone showed small artifacts. The calculated calibration curves were in good agreement with the stoichiometric calibration curve.

On small angle multiple coulomb scattering of protons in the gaussian approximation.

Experimental data from the literature on small-angle multiple Coulomb scattering of protons in various materials were analysed in order to device an equation for the scattering angle in the Gaussian approximation. In comparison to Highland's well-known formula, the present approximation can be integrated to take into account energy loss in the scattering media. In addition, it is more precise than Highland's formulation for thin and thick scatterers consisting of elements with low atomic number. The simple equation obtained in this study can be used to obtain prompt answers for scattering problems which can occur, for example, in proton therapy or proton radiography.

CT based lung density correction verification with in vivo dosimetry using diodes.

In vivo dose measurements with diodes are easy to perform. The first aim of our study was to show whether diode measurements of the patient exit doses are precise enough for verifying inhomogeneity corrections used for treatment planning. The second aim was to assess the precision of the modified Batho Law inhomogeneity correction of the CadPlan treatment planning system. For this purpose, entrance and lait doses were measured in the thoracic region of 115 patients. Diode measurements were sufficiently precise to verify the density corrections predicted by the treatment planning system (< 0.5% of ICRU dose). The measured doses were compared with calculations of the CadPlan treatment planning system. The mean deviation of the exit dose calculations within the measurements error was zero. The present results show that measurements of exit dose even in a small number of patients are sufficient to identify systematic errors in the dose calculation.

Influence of respiration-induced organ motion on dose distributions in treatments using enhanced dynamic wedges.

The mean velocity of respiration-induced organ motion in cranio-caudal direction is of the same magnitude as the velocity of the moving jaw during a treatment with an enhanced dynamic wedge. Therefore, if organ motion is present during collimator movement, the resulting dose distribution in wedge direction may differ from that obtained for the static case, i.e., without organ motion. The position as a function of time of the moving jaw has been derived from a log-file generated during each treatment. Parameters for the respiratory cycle and information about respiration-induced motion for organs in the upper abdomen were taken from the literature. Both movements were superimposed and the resulting monitor unit distribution has been calculated in the intrinsic coordinate system of the organ. The deviations from the static case have been studied as a function of wedge angle, amplitude of organ motion, respiratory rate, asymmetry of the respiratory cycle, beam energy, and the dose rate. If an amplitude of 30 mm and a respiratory rate of 10 min(-1) are assumed, the maximum deviation in monitor units is 2.5% for a 10 degees wedge, 7% for a 30 degrees wedge, and 16% for a 60 degrees wedge. Furthermore, a dose distribution for an organ undergoing respiration-induced motion has been generated and we found dose deviations of the same magnitude as calculated with the monitor unit distribution.