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.

Increased malignant behavior in neuroblastoma cells with acquired multi-drug resistance does not depend on P-gp expression.

Acquisition of P-gp-mediated multidrug-resistance does not always correlate with observed malignant behavior of NB. To characterize alterations accompanying development of multidrug-resistance in NB we established two neuroblastoma cell sublines resistant to vincristine (UKF-NB-3rVCR10) and doxorubicin (UKF-NB-3rDOX20). UKF-NB-3rVCR10 and UKF-NB-3rDOX20 overexpressed functional P-gp and developed an increased malignant phenotype: presented constitutive phosphorylation of AKT, resistance to gamma-irradiation, and had increased survival in serum-free medium. Inhibition of P-gp restored chemosensitivity but did not affect increased survival in serum-free medium and sensitivity to gamma-irradiation. Inhibition of AKT had no influence on chemoresistance but restored sensitivity to serum starvation. Both resistant cell lines acquired additional chromosomal changes. UKF-NB-3rVCR10 cells acquired a missense P53 mutation in exon 5, an increased MYCN amplification, an enhanced adhesion to endothelium, a decreased NCAM expression, a distinctly higher clonogenicity, and an increased in vivo tumorigenicity. We conclude that acquisition of increased malignant behavior in neuroblastoma occurs concomitantly with multidrug-resistance and is P-gp-independent.

Investigations of dose response of BANG polymer-gels to carbon ion irradiation.

In this presentation BANG polymer-gel dosimetry using magnetic resonance imaging (MRI) is applied to densely ionizing radiation such as carbon ion beams. BANG polymer-gels were irradiated with monoenergetic 12C ions at an energy of 205 MeVu(-1). The irradiation of the gels with doses up to 100 Gy were performed at the radiotherapy facility of the GSI, Darmstadt, Germany. For comparison with sparsely ionizing radiation data were obtained for 6 MV photon radiation, too. It was the object to examine the saturation effect for densely ionizing radiation that occurs at high values of linear energy transfer (LET). Up to now the dose response is unknown for mixed radiation fields of primary carbon ions. Therefore, to facilitate such conversions of measured MR signals into dose model calculations are proposed. This model relies only on heavy ion track structure and the experimentally determined photon response.