Multi-institutional retrospective analysis of the inhomogeneity correction for radiation therapy of lung cancer.

The purpose of this work is to retrospectively analyze the effect of the inhomogeneity correction using clinically treated plan of stage III non-small-cell lung cancer within multiple institutions in Japan. Twenty-five patients among five facilities of radiation therapy were registered for this study. The isocenter dose or D(95) of PTV or other important values were compared with and without an inhomogeneity correction using model-based algorithm. The differences in isocenter dose were 4% average and 10% maximum for the first Anterior-Posterior opposed field plan to 40 Gy and 6% average and 11% maximum for the off-cord boost oblique field plan of 20 Gy. The differences in D(95) dose were 1% average and 9% maximum for the first plan and 1% average and 6% maximum for the boost plan. D(95) prescription seemed to be a superior method; however, its reliability depends on each clinical case. Additionally, maximum dose, minimum dose and mean dose for both the primary tumor and the metastatic lymph node were analyzed, and the minimum dose had the most impressive results. In some cases, the target volume had unintended underdose of more than 10%. Finally, an analysis of the organ at risk was added, and this showed no meaningful differences for the V(20) of the lung and the maximum dose of the spinal cord. These results provide a standard for the effects of the inhomogeneity correction.

Monte Carlo calculations of correction factors for plastic phantoms in clinical photon and electron beam dosimetry.

The purpose of this study is to calculate correction factors for plastic water (PW) and plastic water diagnostic-therapy (PWDT) phantoms in clinical photon and electron beam dosimetry using the EGSnrc Monte Carlo code system. A water-to-plastic ionization conversion factor k(pl) for PW and PWDT was computed for several commonly used Farmer-type ionization chambers with different wall materials in the range of 4-18 MV photon beams. For electron beams, a depth-scaling factor c(pl) and a chamber-dependent fluence correction factor h(pl) for both phantoms were also calculated in combination with NACP-02 and Roos plane-parallel ionization chambers in the range of 4-18 MeV. The h(pl) values for the plane-parallel chambers were evaluated from the electron fluence correction factor phi(pl)w and wall correction factors P(wall,w) and P(wall,pl) for a combination of water or plastic materials. The calculated k(pl) and h(pl) values were verified by comparison with the measured values. A set of k(pl) values computed for the Farmer-type chambers was equal to unity within 0.5% for PW and PWDT in photon beams. The k(pl) values also agreed within their combined uncertainty with the measured data. For electron beams, the c(pl) values computed for PW and PWDT were from 0.998 to 1.000 and from 0.992 to 0.997, respectively, in the range of 4-18 MeV. The phi(pl)w values for PW and PWDT were from 0.998 to 1.001 and from 1.004 to 1.001, respectively, at a reference depth in the range of 4-18 MeV. The difference in P(wall) between water and plastic materials for the plane-parallel chambers was 0.8% at a maximum. Finally, h(pl) values evaluated for plastic materials were equal to unity within 0.6% for NACP-02 and Roos chambers. The h(pl) values also agreed within their combined uncertainty with the measured data. The absorbed dose to water from ionization chamber measurements in PW and PWDT plastic materials corresponds to that in water within 1%. Both phantoms can thus be used as a substitute for water for photon and electron dosimetry.