Evaluation of a novel CBCT conversion method implemented in a treatment planning system.

BACKGROUND: To evaluate a novel CBCT conversion algorithm for dose calculation implemented in a research version of a treatment planning system (TPS). METHODS: The algorithm was implemented in a research version of RayStation (v. 11B-DTK, RaySearch, Stockholm, Sweden). CBCTs acquired for each ten head and neck (HN), gynecology (GYN) and lung cancer (LNG) patients were collected and converted using the new algorithm (CBCTc). A bulk density overriding technique implemented in the same version of the TPS was used for comparison (CBCTb). A deformed CT (dCT) was created by using deformable image registration of the planning CT (pCT) to the CBCT to reduce anatomical changes. All treatment plans were recalculated on the pCT, dCT, CBCTc and the CBCTb. The resulting dose distributions were analyzed using the MICE toolkit (NONPIMedical AB Sweden, Umeå) with local gamma analysis, with 1% dose difference and 1 mm distance to agreement criteria. A Wilcoxon paired rank sum test was applied to test the differences in gamma pass rates (GPRs). A p value smaller than 0.05 considered statistically significant. RESULTS: The GPRs for the CBCTb method were systematically lower compared to the CBCTc method. Using the 10% dose threshold and the dCT as reference the median GPRs were for the CBCTc method were 100% and 99.8% for the HN and GYN cases, respectively. Compared to that the GPRs of the CBCTb method were lower with values of 99.8% and 98.0%, for the HN and GYN cases, respectively. The GPRs of the LNG cases were 99.9% and 97.5% for the CBCTc and CBCTb method, respectively. These differences were statistically significant. The main differences between the dose calculated on the CBCTs and the pCTs were found in regions near air/tissue interfaces, which are also subject to anatomical variations. CONCLUSION: The dose distribution calculated using the new CBCTc method showed excellent agreement with the dose calculated using dCT and pCT and was superior to the CBCTb method. The main reasons for deviations of the calculated dose distribution were caused by anatomical variations between the pCT and the corrected CBCT.

Characterization of the PTW-34089 type 147 mm diameter large-area ionization chamber for use in light-ion beams.

A newly-designed large-area plane-parallel ionization chamber (of type PTW 34089), denoted BPC150, with a nominal active volume diameter of 147 mm is characterized in this study. Such chambers exhibit benefits compared to smaller chambers in the field of scanned light-ion beam dosimetry because they capture a larger fraction of the laterally-spread beam fragments and ease positioning with respect to small fields. The chamber was characterized in 60Co, 200 kV x-ray, proton and carbon ion beams. The chamber-specific beam-quality correction factor kQ,Q0 was determined. To investigate the homogeneity of the chamber's response, a radial response map was acquired. An edge correction was applied when the proton beam only partly impinged on the chamber's active surface. The measured response map showed that the response in the chamber's center is 3% lower than the average response over the total active area. Furthermore, percentage depth dose (PDD) curves in carbon ions were acquired and compared to those obtained with smaller-diameter chambers (i.e. 81.6 mm and 39.6 mm) as well as with results from Monte Carlo simulations. The measured absorbed dose to water cross calibration coefficients resulted in a kQ,Q0 of 0.981 ± 0.020. Regarding carbon ion PDD curves, relative differences between the BPC150 and smaller chambers were observed, especially for higher energies and in the fragmentation tail. These differences reached 10%-22% in the fragmentation tail (compared to the 81.6 mm diameter chamber). Differences increased when comparing to a chamber with 39.6 mm diameter. The provided results characterize the BPC150 thoroughly for usage in scanned light-ion beam dosimetry and demonstrate its advantage of capturing a larger fraction of the laterally-integrated dose in the fragmentation tail.