In carbon-ion radiotherapy for pancreatic cancer, altered dose distributions due to changes in the gastrointestinal gas volume and anatomy during irradiation are an unresolved therapeutic issue. We developed and investigated an adaptive strategy involving beam angle selection to improve dose distributions in pancreatic cancer. In the adaptive strategy, multiple beams were prepared with angles similar to those of the conventional strategy, and the beam that best reproduces the dose distribution of the treatment plan was used. The dose distributions of the adaptive strategy were compared with those of the conventional strategy for five patients. Patients underwent computed tomography (CT) before every irradiation. The adaptive strategy was evaluated using the same irradiation schedule as that of the conventional method and an adjusted method based on anatomical changes per fraction. Dose distributions on the pre-treatment CT and accumulated dose distributions on the treatment planning CT were evaluated using the volume receiving ≥95% of the prescription dose (V95) from the clinical target volume (CTV) between strategies. There were significant differences in the CTV V95 values for the pre-treatment CT between all strategies. The median (range) CTV V95 for the conventional strategy was 92.7% (87.1–96.1%), for the proposed adaptive strategy without adjusted schedules was 96.9% (95.1–97.8%), and for the proposed strategy with adjusted schedules was 97.8% (96.5–99.2%). The adaptive strategy can improve target coverage for the pre-treatment CT and accumulated dose distributions for the treatment planning CT without increasing the dose to critical organs.

Background and purpose: The standard dosimetry system of medical accelerators in radiotherapy consists of an ionization chamber, an electrometer, and cables. Guidance for TG‐51 reference dosimetry reported that the electrometer correction factor (P_elec) should be checked every few years. Therefore, continuous Pelec measurements have not been reported. The purpose of this study is to measure the Pelec with a charge generator at our institution and to evaluate variations over time. The measurements are compared with calibration data given by an Accredited Dosimetry Calibration Laboratory (ADCL).Materials and methods: We used four reference‐class electrometers: RT521R (RTQM system/EMF Japan), Model 35040 (FLUKE), RAMTEC Duo (Toyo medic), and UNIDOS‐E (PTW). Each electrometer was connected to the charge generator, and the required charge was applied. The measurement points used were the same as those used for calibration by the ADCL. From the measured charges at each point, the P_elec was obtained from the slope of the linear regression function. The measurements were repeated over a 3‐month period to evaluate variations over time for each electrometer. Additionally, error budgets for the P_elec measurements were estimated, and the overall uncertainty was determined.Results: The measured P_elec values were 1.0000, 0.9995, 1.0009/0.9999, and 0.9995/0.9998 for RT521R, Model 35040, the low/medium (L/M) ranges of RAMTEC Duo, and the L/M ranges of UNIDOS‐E, respectively. The measured P_elec values agreed within 0.1% with those given by the ADCL. We found a small drift in the measurements for one electrometer. Additionally, the uncertainty considered was 0.26% for k = 2 (k, coverage factor).Conclusion: In this study, stable P_elec values were obtained for four electrometers using a charge generator over a three‐month period. The measured P_elec values were within the overall uncertainty stated in the electrometer guidelines. However, performing periodic measurements for the P_elec was able to help in detecting small errors.

Upright patient positioning is an emerging approach in radiotherapy that may overcome current limitations in carbon ion radiotherapy (CIRT). This study evaluated upright immobilization techniques for abdominal and head-and-neck (HN) cancers in a CIRT setting. Ten volunteers were positioned on the Chair, a demo upright positioning system (Leo Cancer Care, USA). Three setups were evaluated: no immobilization devices, vacuum bags alone (ESFORM; Engineering System Co., Japan), and vacuum bags with a thermoplastic shell (Shellfitter; Kuraray Trading Co., Japan), called the “shell” setup. Interfractional and intrafractional errors were assessed using skin markers drawn on the abdominal and HN areas. Interfractional shifts per direction and Euclidean distances were calculated by comparing reference and repeated images. Intrafractional shifts per direction and Euclidean distances were calculated from images acquired over a 15-min period. Also, volunteers rated their comfort for each setup on a scale of 1–5. Differences in the interfractional and intrafractional motion, and comfort ratings between the three setups were investigated. Significant differences (p < 0.05) in the interfractional Euclidean distances were observed between the three setups in both anatomical areas, while significant differences were observed in more shifts per direction in the abdominal case. The “shell” setup minimized intrafractional distances. No significant differences were observed in the comfort ratings between the setups. Upright positioning in CIRT appears feasible. Further research is needed to refine immobilization techniques to support clinical implementation of upright positioning in CIRT.

Discretization errors due to the digitization of computed tomography images and the calculation grid are a significant issue in radiation therapy. Such errors have been quantitatively reported for a fixed multifield intensity-modulated radiation therapy using traditional linear accelerators. The aim of this study is to quantify the influence of the calculation grid size on the dose distribution in TomoTherapy. This study used ten treatment plans for prostate cancer. The final dose calculation was performed with “fine” (2.73 mm) and “normal” (5.46 mm) grid sizes. The dose distributions were compared from different points of view: the dose-volume histogram (DVH) parameters for planning target volume (PTV) and organ at risk (OAR), the various indices, and dose differences. The DVH parameters were used Dmax, D2%, D2cc, Dmean, D95%, D98%, and Dmin for PTV and Dmax, D2%, and D2cc for OARs. The various indices used were homogeneity index and equivalent uniform dose for plan evaluation. Almost all of DVH parameters for the “fine” calculations tended to be higher than those for the “normal” calculations. The largest difference of DVH parameters for PTV was Dmax and that for OARs was rectal D2cc. The mean difference of Dmax was 3.5%, and the rectal D2cc was increased up to 6% at the maximum and 2.9% on average. The mean difference of D95% for PTV was the smallest among the differences of the other DVH parameters. For each index, whether there was a significant difference between the two grid sizes was determined through a paired t-test. There were significant differences for most of the indices. The dose difference between the “fine” and “normal” calculations was evaluated. Some points around high-dose regions had differences exceeding 5% of the prescription dose. The influence of the calculation grid size in TomoTherapy is smaller than traditional linear accelerators. However, there was a significant difference. We recommend calculating the final dose using the “fine” grid size.

Electrometers are important devices that are part of the standard dosimetry system. Therefore, we evaluated the variation of electrometer calibration coefficients (k_elec) over 1 year in this study. We investigated two types of electrometers: a rate mode and an integrate mode. Each electrometer was connected to a charge generator, a constant charge was applied, and k_elec was determined by measuring the current. The current measurements were repeated once a month. For electrometers with multiple ranges, measurements were taken at low and medium ranges. Almost all k_elec measurements agreed within 0.2% of the initial measurements. However, the low range of the electrometer with an integrate mode showed seasonal variation, with a variation greater than 0.2%. This study shows that electrometers may exhibit errors that cannot be detected through annual inspections. The importance of quality assurance using a charge generator at one’s own institution was demonstrated.