![]() Bowel and bladder filling can have an effect on internal anatomical changes, but can be partially controlled by drinking, and/or voiding schemes, rectal balloons, or the use of laxatives or enemas to control rectal filling 7, 8, 9, 10. The prostate as an example is generally influenced by intrafractional motion but does not undergo interfractional changes in shape 5, 6. However, recently, several reports using medical imaging have identified and quantified internal anatomical changes, and found changes in target shape occurring over time periods varying from hours to days (interfractional change) 2, 3, 4. Current treatment planning assumes a patient’s internal anatomy to be unchanged throughout the treatment course. Especially in carbon-ion pencil-beam scanning therapy, it has been possible to decrease the number of treatment fractions while increasing the dose (hypofractionated treatment) owing to this method’s unique physical and biological characteristics. This has enabled the performance of dose escalation studies to increase the prescribed dose to the tumor 1. Particle beam treatment techniques have improved in the past few years, allowing a higher dose to the tumor with reduced doses to surrounding normal tissues. The WEPL-based image registration algorithm improved the target coverage from the other algorithms and reduced rectal dose from the target-based image registration, even though the magnitude of the interfractional variation was increased. The WEPL-based image registration was CTV-D95 to 99.0 ± 0.4% and rectal Dmax to 51.9 ± 1.9 Gy (RBE) compared to 49.4 ± 9.1 Gy (RBE) with intensity-based image registration and 52.2 ± 1.8 Gy (RBE) with target-based image registration. The mean CTV-D95 values were 95.8 ± 11.5% and 98.8 ± 1.7% with the intensity-based image registration and target-based image registration, respectively. WEPL differences between the planning CT and the treatment CT were 1.2 ± 0.6 mm-H 2O ( 95% of the prescribed dose in all cases. Interfractional CTV displacement over all patients was 6.0 ± 2.7 mm (19.3 mm maximum standard amount). Statistical significance was assessed using the Wilcoxon signed-rank test. Dosimetry, including the dose received by 95% of the clinical target volume (CTV-D95), rectal volumes receiving > 20 Gy (RBE) (V20), > 30 Gy (RBE) (V30), and > 40 Gy (RBE) (V40), were calculated. Weekly dose distributions using the three different algorithms were calculated by applying the treatment plan parameters to the weekly CT data. ![]() The treatment plan parameters were optimized to administer the prescribed dose to the PTV on the planning CT. Initial dose distributions were calculated using the planning CT with the lateral beam angles. ![]() WEPL-based image registration registers the treatment CTs to the planning CT using WEPL values. Target-based image registration uses target position on the treatment CTs to register it to that on the planning CT. Intensity-based image registration uses CT voxel intensity information. ![]() Three CT–CT registration algorithms were used to register the treatment CTs to the planning CT. We used the data of the carbon ion therapy planning CT and the four-weekly treatment CTs of 19 prostate cancer cases. To perform setup procedures including both positional and dosimetric information, we developed a CT–CT rigid image registration algorithm utilizing water equivalent pathlength (WEPL)-based image registration and compared the resulting dose distribution with those of two other algorithms, intensity-based image registration and target-based image registration, in prostate cancer radiotherapy using the carbon-ion pencil beam scanning technique. ![]()
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