Efficacy of gonadal shielding in dose reduction for female pelvic radiography.

This study aimed to evaluate the dose reduction using gonad shielding (GS) during pelvic imaging. Three types of pelvic images (radiography, magnetic resonance and computed tomography) were fused to elucidate the three-dimensional relationship between the position of ovaries and GS. To estimate the dose received by the ovaries, the off-axis dose at any given depth was measured under two different imaging conditions using thermoluminescence dosemeters and a polymethyl methacrylate phantom. The mean ovarian depth was 8.4 cm. The mean estimated ovarian dose without an additional filter was 0.36 mGy without GS and 0.14 mGy with GS. The mean estimated ovarian dose with an additional filter was 0.24 mGy without GS and 0.10 mGy with GS. The efficacy of ovarian dose reduction should be evaluated based on the achieved ovarian dose, considering the ovarian depth and use of additional filtration, rather than the ovarian protection rate of GS.

Applicability evaluation of the TRS-483 protocol for the determination of small-field output factors using different multi-leaf collimator and field-shaping types.

PURPOSE: To evaluate the applicability of TRS-483 output correction factors (CFs) for small-field output factors (OFs) using different multi-leaf collimators (MLC) and field-shaping types. METHODS: All measurements were performed on TrueBeam, TrueBeam STx, and Halcyon using 6 MV flattening filter-free energy. Four detectors, including CC01, CC04, microDiamond, and EDGE, were used. Nominal field sizes ranging from 1 × 1 to 4 × 4, and 10 × 10 cm2 were used to measure small-field OFs at source-to-axis distance of 100 cm with a 0° gantry angle in a 3D water phantom. Further, the field-shaping types were defined using jaw collimator or MLC (five different configurations). A field size of 10 × 10 cm2 was used as the reference for calculation of OFs obtained as ratio of detector readings (OFdet). The percentage difference and coefficient of variation of OFdet and OFdet corrected by applying CF were compared for each field size and configuration. RESULTS: For OFdet corrected by applying CF, the ranges of percentage difference and coefficient of variation in all configurations for ≥ 2 × 2 cm2 fields were reduced from 1.2-2.2 to 0.8-1.3 percentage points (%pt) and from 0.5-1.0 to 0.4-0.7%, respectively. For 1 × 1 cm2 field, the ranges of percentage difference and coefficient of variation were reduced from 3.3-5.7 to 1.2-2.2 %pt and from 2.2-3.7 to 0.8-1.1%, respectively. CONCLUSIONS: The CFs described in TRS-483 dosimetry protocol have broad applicability in reducing OF variations between detectors under different MLC and field-shaping types.

New patient setup procedure using surface-guided imaging to reduce body touch and skin marks in whole-breast irradiation during the COVID-19 pandemic.

This study aimed to assess the effectiveness of a new patient-setup procedure using surface-guided imaging during the coronavirus disease 2019 (COVID-19) pandemic for left-sided whole-breast irradiation with deep inspiration breath-hold. Two setup procedures were compared regarding patient positioning accuracy for the first 22 patients. The first was a traditional setup (T-setup) procedure that used a surface-guided system after patient setup with traditional skin marks and lasers. The second procedure involved a new setup (N-setup) that used only a surface-guided system. The positioning accuracy of the remaining 23 patients was assessed using a setup that combined marker reduction and the N-setup procedure. No significant difference was observed in positioning accuracy between the two setups. The positioning accuracy of the marker-reduction setup was within 3 mm in all directions. The N-setup procedure may be a useful strategy for preventing infection during or after the COVID-19 pandemic.

Assessing target localization accuracy across different soft-tissue matching protocols using end-exhalation breath-hold cone-beam computed tomography in patients with pancreatic cancer.

The purpose of this study was to retrospectively assess target localization accuracy across different soft-tissue matching protocols using cone-beam computed tomography (CBCT) in a large sample of patients with pancreatic cancer and to estimate the optimal margin size for each protocol. Fifty-four consecutive patients with pancreatic cancer who underwent 15-fraction volumetric modulated arc therapy under the end-exhalation breath-hold condition were enrolled. Two soft-tissue matching protocols were used according to the resectability classification, including gross tumor volume (GTV) matching for potentially resectable tumors and planning target volume (PTV) matching for borderline resectable or unresectable tumors. The tolerance of the target localization error in both matching protocols was set to 5 mm in any direction. The optimal margin size for each soft-tissue matching protocol was calculated from the systematic and random errors of the inter- and intrafraction positional variations using the van Herk formula. The inter- and intrafraction positional variations of PTV matching were smaller than those of GTV matching. The percentage of target localization errors exceeding 5 mm in the first CBCT scan of each fraction in the superior-inferior direction was 12.6 and 4.8% for GTV and PTV matching, respectively. The optimal margin sizes for GTV and PTV matching were 3.7 and 2.7, 5.4 and 4.1 and 3.9 and 3.0 mm in the anterior-posterior, superior-inferior and left-right directions, respectively. Target localization accuracy in PTV matching was higher than that in GTV matching. By setting the tolerance of the target localization error, treatment can be successful within the planned margin size.

Effect of angular dependence for small-field dosimetry using seven different detectors.

BACKGROUND: Small-field dosimetry is challenging for radiotherapy dosimetry because of the loss of lateral charged equilibrium, partial occlusion of the primary photon source by the collimating devices, perturbation effects caused by the detector materials and their design, and the detector size relative to the radiation field size, which leads to a volume averaging effect. Therefore, a suitable tool for small-field dosimetry requires high spatial resolution, tissue equivalence, angular independence, and energy and dose rate independence to achieve sufficient accuracy. Recently, with the increasing use of combinations of coplanar and non-coplanar beams for small-field dosimetry, there is a need to clarify angular dependence for dosimetry where the detector is oriented at various angles to the incident beam. However, the effect of angular dependence on small-field dosimetry with coplanar and non-coplanar beams has not been fully clarified. PURPOSE: This study clarified the effect of angular dependence on small-field dosimetry with coplanar and non-coplanar beams using various detectors. METHODS: Seven different detectors were used: CC01, RAZOR, RAZOR Nano, Pinpoint 3D, stereotactic field diode (SFD), microSilicon, and microDiamond. All measurements were taken using a TrueBeam STx with 6 MV and 10 MV flattening filter-free (FFF) energies using a water-equivalent spherical phantom with a source-to-axis distance of 100 cm. The detector was inserted in a perpendicular orientation, and the gantry was rotated at 15° increments from the incidence beam angle. A multi-leaf collimator (MLC) with four field sizes of 0.5 × 0.5, 1 × 1, 2 × 2, and 3 × 3 cm2 , and four couch angles from 0°, 30°, 60°, and 90° (coplanar and non-coplanar) were adopted. The angular dependence response (AR) was defined as the ratio of the detector response at a given irradiation gantry angle normalized to the detector response at 0°. The maximum AR differences were calculated between the maximum and minimum AR values for each detector, field size, energy, and couch angle. RESULTS: The maximum AR difference for the coplanar beam was within 3.3% for all conditions, excluding the maximum AR differences in 0.5 × 0.5 cm2 field for CC01 and RAZOR. The maximum AR difference for non-coplanar beams was within 2.5% for fields larger than 1 × 1 cm2 , excluding the maximum AR differences for RAZOR Nano, SFD, and microSilicon. The Pinpoint 3D demonstrated stable AR tendencies compared to other detectors. The maximum difference was within 2.0%, except for the 0.5 × 0.5 cm2 field and couch angle at 90°. The tendencies of AR values for each detector were similar when using different energies. CONCLUSION: This study clarified the inherent angular dependence of seven detectors that were suitable for small-field dosimetry. The Pinpoint 3D chamber had the smallest angular dependence of all detectors for the coplanar and non-coplanar beams. The findings of this study can contribute to the calculation of the AR correction factor, and it may be possible to adapt detectors with a large angular dependence on coplanar and non-coplanar beams. However, note that the gantry sag and detector-specific uncertainties increase as the field size decreases.

[Verification of Effectiveness of Gonad Shielding in Female Hip Joint Radiography].

PURPOSE: It has been reported that the placement of protective equipment in female pelvic radiography is predominantly inadequate compared with that of male. We analyzed the actual situation of ovarian shielding by protective devices using X-ray, CT, and MR images obtained in the past, and evaluated the effectiveness of gonadal protection in female hip radiography. METHODS: The ovaries were contoured in MR images and the pelvic bone was extracted by CT images. The MR/CT fusion images were created using a 3D workstation. The amount of physiological fluctuation in the ovarian location was measured. The fusion images in the ray-summation display were manually superimposed with the X-ray image, and the percentage of ovaries that could be shielded by the protective device was classified into four categories: (a) complete protection, (b) partial protection, (c) failure of protection, and (d) image retaking. RESULTS: The mean and maximum ovarian fluctuations were 1.1 cm and 3.9 cm in the superior and inferior directions, respectively, and 0.7 cm and 2.0 cm in the left and right directions, respectively. The percentage of ovaries shielded was 18.9% for complete protection, 58.5% for partial protection, 15.1% for inadequate protection, and 7.5% for image retaking. CONCLUSION: The effectiveness of gonadal protection is low because the protective device could not cover the entire ovary in about 80% of the female hip radiographs.

A case of a pregnant woman with locally advanced cervical esophageal cancer treated with definitive chemoradiotherapy.

The information of definitive radiotherapy for a pregnant woman with malignancy was limited; however, it was reported to be potentially feasible with minimal risks. We performed definitive chemoradiotherapy for a pregnant woman with locally advanced cervical esophageal cancer. Feasibility of radiotherapy and safety of fetus were confirmed by the phantom study estimating fetal dose, and monitoring it in each radiotherapy session. The planned chemoradiotherapy completely eradicated esophageal cancer while preserving her laryngopharyngeal function. A female infant was delivered by cesarian section after planned chemoradiotherapy, and she grew without any apparent disorders 2 years after chemoradiotherapy. Chemoradiotherapy might be one of the treatment options for a pregnant woman with cervical esophageal cancer especially wishing the preservation of laryngopharyngeal function.

Evaluation of correlation between intrafractional residual setup errors and accumulation of delivered dose distributions in single isocenter volumetric modulated arc therapy for multiple brain metastases.

PURPOSE: To evaluate the displacement of gross tumor volume (GTV) positions caused by intrafractional residual setup errors (RSEs) and to accumulate delivered dose distributions considering intrafraction RSEs in fractionated-stereotactic radiotherapy (f-SRT) with single isocenter volumetric modulated arc therapy (SI-VMAT) for multiple brain metastases. METHODS: Overall, 72 consecutive patients who underwent f-SRT with SI-VMAT for multiple brain metastases were included. For all patients, 6D correction was performed using the ExacTrac X-ray (ETX) system. GTV displacement (ΔD) was calculated considering the intrafractional RSEs measured by the ETX system during irradiation. The correlation between ΔD and the distance from the isocenter to each GTV (d) was analyzed. Computed tomography (CT) images considering the intrafractional RSEs were generated for five patients with ΔD > 1 mm. The delivered dose distributions for all fractions were reconstructed on the corresponding CT, followed by their accumulation. RESULTS: The 95th percentile of ΔD from 7,270 resultant center positions of 417 GTVs was 0.92 mm. No correlation was observed between ΔD and d. For 53 GTVs from five patients with ΔD > 1 mm, the difference of GTV D99.5% and D0.5% between the planned and accumulated values was -0.4 ± 2.5% and -1.0 ± 0.8%, respectively. There was no correlation between d and the difference of GTV D99.5% and D0.5%. CONCLUSIONS: We found no significant difference in GTV D99.5% and D0.5%, despite the location of GTVs far from the isocenter. However, it should be noted that this result was because the intrafractional RSEs were reduced to a clinically acceptable level.

Evaluation of intrafractional head motion for intracranial stereotactic radiosurgery with a thermoplastic frameless mask and ceiling-floor-mounted image guidance device.

PURPOSE: To evaluate intrafractional head motion (IFM) in patients who underwent intracranial stereotactic radiosurgery with the ExacTrac X-ray system (ETX) and a frameless mask. METHODS: A total of 143 patients who completed a pre-treatment examination for IFM were eligible for this study. The frameless mask type B R408 (Klarity Medical & Equipment Co., Ltd., Guangzhou, China), which covers the back of the head, and the entire face, was used for patient immobilization. After the initial 6D correction and first X-ray verification (IFM1), X-ray verification was performed every 3 min for a duration of 15 min. The IFMp (2 ≤ p ≤ 6) was calculated as the positional difference from IFM1. In addition, the inter-phase IFM (IP-IFM) and IFMm were calculated. The IP-IFM was defined as |IFMp - IFMp-1|, and IFMm as the difference between the values after all patients were asked to move their heads intentionally with the frameless mask on. RESULTS: Both translational IFMp and IP-IFM exceeded 1 mm for a single patient, whereas, for all patients, the translational IFMm values were kept to within 1 mm in all directions. The proportions of the rotational IFMp, IP-IFM, and IFMm values within 0.5° were greater than 94.4%, 98.6%, and 90.2% for all of the rotational axes, respectively. CONCLUSIONS: A frameless mask achieved highly accurate patient positioning in combination with ETX and a 6°-of-freedom robotic couch; however, a deviation over 1 mm and 0.5° was observed with low frequency. Therefore, X-ray verification and correction are required during treatment.

Early gastric cancer with splenic artery aneurysm successfully treated with laparoscopic distal gastrectomy after endovascular coil embolization: A case report.

Splenic artery aneurysm (SAA) is a relatively rare disease. Most patients with SAA have no symptoms, and detection is incidental detection. The incidence of rupture is not particularly high, but the mortality rate of ruptured SAAs is high. The main treatment for gastric cancer is gastrectomy with lymph node dissection, with dissection around the celiac artery suggested to be the most important. A 68-year-old woman with early gastric cancer in the lesser curvature of the lower gastric corpus was referred to our hospital. CT showed no remarkable findings except for a saccular SAA (diameter, 1.5 cm). We planned laparoscopic distal gastrectomy. However, because the SAA was close to the surgical field and its saccular shape created a rupture risk, we performed interventional radiology for SAA before surgery. One month later, laparoscopic distal gastrectomy with D1+ was performed successfully. The patient has remained disease-free in the 51 months since the operation.

EVALUATION OF ORGAN DOSE BASED ON PATIENT DATA IN EAST ASIAN DESCENT PEDIATRIC HEAD CT EXAMINATIONS.

The purpose of this study was to investigate other indices estimating absorbed dose for eye lens and brain, using clinical images of East Asian pediatric patients. We simulated head computed tomography (CT ) examinations in 104 pediatric patients. Effective diameter (deff) and water equivalent diameter (dw) were measured on clinical images. Various size metrics and age were compared with absorbed dose normalised by CTDIvol (nD). The nD was estimated for eye and brain. The nD tended to decrease with advancing age. R2 between age and nD were 0.38 and 0.31 for eye and brain, respectively. Increasing head diameters decreased each nD. R2 between deff and dw, and nD were 0.20-0.24 and 0.51-0.53 for eye and brain, respectively. Head sizes allowed us to estimate absorbed dose in brain CT on East Asian pediatric patients. Scanning parameters for pediatric head CT may need to be based on individual patient information.

Positional repeatability and variation in internal and external markers during volumetric-modulated arc therapy under end-exhalation breath-hold conditions for pancreatic cancer patients.

The purpose of this study was to assess the positional repeatability of internal and external markers among multiple breath-hold (BH) sessions and evaluate the positional variation of these markers within BH sessions for volumetric-modulated arc therapy (VMAT) for pancreatic cancer patients. A total of 13 consecutive pancreatic cancer patients with an internal marker were enrolled. Single full-arc coplanar VMAT was delivered under end-exhalation BH conditions while monitoring the internal marker with kilovoltage (kV) X-ray fluoroscopy. Positional repeatability of the internal and external markers was determined by the difference between the reference and zero position in all BH sessions, and positional variation was defined by the displacement from the reference position in each BH session during megavolt beam delivery. The overall positional repeatability was 0.6 ± 1.5 mm in the X-axis for the centroid of the internal marker (CoIM), -0.1 ± 2.2 mm in the Y-axis for the CoIM, and 0.8 ± 2.2 mm for the external marker. The frequency of an internal marker position appearing > 2 mm from the reference position in the Y-axis, despite the external marker position being ≤2 mm from the reference position, ranged from 0.0 to 39.9% for each patient. Meanwhile, the proportion of sessions with positional variation ≤2 mm was 93.2 and 98.7% for the CoIM and external marker, respectively. External marker motion can be used as a surrogate for pancreatic tumor motion during BH-VMAT delivery; however, margins of ~5 mm were required to ensure positional repeatability.

Variation in accumulated dose of volumetric-modulated arc therapy for pancreatic cancer due to different beam starting phases.

PURPOSE: To assess the effects of different beam starting phases on dosimetric variations in the clinical target volume (CTV) and organs at risk (OARs), and to identify the relationship between plan complexity and the dosimetric impact of interplay effects in volumetric-modulated arc therapy (VMAT) plans for pancreatic cancer. METHODS: Single and double full-arc VMAT plans were generated for 11 patients. A dose of 50.4 Gy in 28 fractions was prescribed to cover 50% of the planning target volume. Patient-specific Digital Imaging and Communications in Medicine-Radiation Therapy plan files were divided into 10 files based on the respiratory phases in four-dimensional computed tomography (4DCT) simulations. The phase-divided VMAT plans were calculated in consideration of the beam starting phase for each arc and were then combined in the mid-ventilation phase of 4DCT (4D plans). The dose-volumetric parameters were compared with the calculated dose distributions without consideration of the interplay effects (3D plans). Additionally, relationships among plan parameters such as modulation complexity scores, monitor units (MUs), and dose-volumetric parameters were evaluated. RESULTS: Dosimetric differences in the median values associated with different beam starting phases were within ± 1.0% and ± 0.2% for the CTV and ± 0.5% and ± 0.9% for the OARs during single and double full-arc VMAT, respectively. Significant differences caused by variations in the beam starting phases were observed only for the dose-volumetric parameters of the CTV during single full-arc VMAT (P < 0.05), associated with moderate or strong correlations between the MUs and the dosimetric differences between the 4D and 3D plans. CONCLUSIONS: The beam starting phase affected CTV dosimetric variations of single full-arc VMAT. The use of double full-arc VMAT mitigated this problem. However, variation in the dose delivered to OARs was not dependent on the beam starting phase, even for single full-arc VMAT.

[Comparison of Monitor Unit between Electron Monte Carlo Algorithm by Different Dose Normalization Methods and Conventional Manual Calculation].

The purpose of this study was to evaluate the discrepancy between the monitor unit (MU) calculated by different dose normalization methods in the electron Monte Carlo (eMC) algorithm and the conventional manual MU. In the water phantom condition, the manual MU obtained from the measured output factor was compared with the calculated MU by the eMC algorithm, using 24 different irradiation field shapes and several different energies of electron beam. In the breast boost condition, calculated MUs by both calculation methods were evaluated for 45 cases. As a result, the MUs computed by the eMC algorithm in the water phantom varied according to the dose normalization methods, and the mean±standard deviation of the difference between the manual and calculated MU were 1.1±1.4%, 0.0±1.0% and 0.4±1.2% in peak depth normalization (PN), no plan normalization (NPN) and 100% at body maximum (100%BM), respectively. In breast-boost cases, the MU difference between the manual and the calculated MU were 6.1±3.7%, 3.4±2.8% and 1.1±2.9% in PN, NPN and 100%BM, respectively. We revealed that the resultant MU calculated by eMC algorithm was dependent on the dose normalization method and the averaged differences exceeded 6% in PN, especially in breast boost condition. When using the eMC in the breast boost condition, it is desirable to select an appropriate dose normalization method according to dose prescription policies at each facility.

Effects of Batting Practice and Visual Training Focused on Pitch Type and Speed on Batting Ability and Visual Function.

This study aimed to examine the effects of batting practice and visual training focused on the pitch type and speed on batting ability and visual function. A total of 46 participants took part in 12 training sessions for 4 weeks. The participants were divided into six groups according to the training type as follows: Group 1, batting practice with a fastball at 100 km/h; Group 2, tracking (watching) a fastball at 100 km/h; Group 3, batting practice with a fastball at 115 km/h; Group 4, tracking a fastball at 115 km/h; Group 5, batting practice with a curve ball at 100 km/h; and Group 6, tracking a curve ball at 100 km/h. Dynamic visual acuity, depth perception, hand-eye coordination, and batting ability were measured before and after training. Group 1 showed significant improvement in batting ability in the tests with 100 km/h fastballs and curve balls, while Groups 3 and 5 showed significant improvement in batting ability with 100 km/h fastballs and curve balls, respectively. Group 6 also showed significant improvement in batting ability with 100 km/h fastballs. Moreover, Groups 2 and 4 showed significant improvement in Dynamic visual acuity and hand-eye coordination, respectively. The results of the present study suggest that batting practice and visual training improve batting ability for the same pitch types and speeds as those encountered in practice. Therefore, visual training may be an effective method for improving batting ability and visual function in coaching settings.

Dosimetric impact of translational and rotational setup errors for spine stereotactic body radiotherapy: A phantom study.

This study aimed to investigate experimentally the effect of translational and rotational setup errors on 3-dimensional dose distributions by using the gamma index and dose volumetric indices for spine stereotactic body radiotherapy. Treatment plans were designed in accordance with the Radiation Therapy Oncology Group (RTOG) 0631 protocol. Measurements were taken using a Delta4 phantom (ScandiDos, Uppsala, Sweden). Setup errors were generated using the HexaMotion 6D moving platform (ScandiDos). Dose distributions in the presence of setup errors were evaluated, according to the γ passing rate with the 3% and 2 mm criteria (γ3%/2 mm) and dose volumetric indices (D90 for the target volume and D2 for the spinal cord), using the Delta4 device (ScandiDos). The sensitivity coefficient, which represented the correlation between the γ3%/2 mm passing rate and dose volumetric indices, was determined to assess robustness against setup errors. Rotational setup errors of 2° were equivalent to translational setup errors of 2 mm for the γ3%/2 mm passing rate, D90 for the target, and D2 for the spinal cord. D90 for the target had low robustness against a translational setup error in the vertical direction and a rotational setup error in the pitch direction. D2 for the spinal cord was sensitive to a translational setup error in the lateral direction and a rotational setup error in the roll direction. The positioning accuracy of the rotational setup error, corresponding to the tolerance level of image-guided radiotherapy in the RTOG 0631 protocol, was required to be ≤ 2°.

Use of a second-dose calculation algorithm to check dosimetric parameters for the dose distribution of a first-dose calculation algorithm for lung SBRT plans.

PURPOSE: To verify lung stereotactic body radiotherapy (SBRT) plans using a secondary treatment planning system (TPS) as an independent method of verification and to define tolerance levels (TLs) in lung SBRT between the primary and secondary TPSs. METHODS: A total of 147 lung SBRT plans calculated using X-ray voxel Monte Carlo (XVMC) were exported from iPlan to Eclipse in DICOM format. Dose distributions were recalculated using the Acuros XB (AXB) and the anisotropic analytical algorithm (AAA), while maintaining monitor units (MUs) and the beam arrangement. Dose to isocenter and dose-volumetric parameters, such as D2, D50, D95 and D98, were evaluated for each patient. The TLs of all parameters between XVMC and AXB (TLAXB) and between XVMC and AAA (TLAAA) were calculated as the mean±1.96 standard deviations. RESULTS: AXB values agreed with XVMC values within 3.5% for all dosimetric parameters in all patients. By contrast, AAA sometimes calculated a 10% higher dose in PTV D95 and D98 than XVMC. The TLAXB and TLAAA of the dose to isocenter were -0.3±1.4% and 0.6±2.9%, respectively. Those of D95 were 1.3±1.8% and 1.7±3.6%, respectively. CONCLUSIONS: This study quantitatively demonstrated that the dosimetric performance of AXB is almost equal to that of XVMC, compared with that of AAA. Therefore, AXB is a more appropriate algorithm for an independent verification method for XVMC.

Three-dimensional intrafractional internal target motions in accelerated partial breast irradiation using three-dimensional conformal external beam radiotherapy.

PURPOSE: We evaluated three-dimensional intrafractional target motion, divided into respiratory-induced motion and baseline drift, in accelerated partial breast irradiation (APBI). METHODS: Paired fluoroscopic images were acquired simultaneously using orthogonal kV X-ray imaging systems at pre- and post-treatment for 23 patients who underwent APBI with external beam radiotherapy. The internal target motion was calculated from the surgical clips placed around the tumour cavity. RESULTS: The peak-to-peak respiratory-induced motions ranged from 0.6 to 1.5mm in all directions. A systematic baseline drift of 1.5mm towards the posterior direction and a random baseline drift of 0.3mm in the lateral-medial and cranial-caudal directions were observed. The baseline for an outer tumour cavity drifted towards the lateral and posterior directions, and that for an upper tumour cavity drifted towards the cranial direction. Moderate correlations were observed between the posterior baseline drift and the patients' physical characteristics. The posterior margin for intrafractional uncertainties was larger than 5mm in patients with greater fat thickness due to the baseline drift. CONCLUSIONS: The magnitude of the intrafractional motion was not uniform according to the direction, patients' physical characteristics, or tumour cavity location due to the baseline drift. Therefore, the intrafractional systematic movement should be properly managed.

Optimization of Couch Modeling in the Change of Dose Calculation Methods and Their Versions.

In external radiotherapy, the X-ray beam passes through the treatment couch, leading to the dose reduction by the attenuation of the couch. As a method to compensate for the reduction, radiation treatment planning systems (RTPS) support virtual couch function, namely "couch modeling method". In the couch modeling method, the computed tomography (CT) numbers assigned to each structure should be optimized by comparing calculations to measurements for accurate dose calculation. Thus, re-optimization of CT numbers will be required when the dose calculation algorithm or their version changes. The purpose of this study is to evaluate the calculation accuracy of the couch modeling method in different calculation algorithms and their versions. The optimal CT numbers were determined by minimizing the difference between measured transmission factors and calculated ones. When CT numbers optimized by Anisotropic Analytical Algorithm (AAA) Ver. 8.6 were used, the maximum and the mean difference of transmission factor were 5.8% and 1.5%, respectively, for Acuros XB (AXB) Ver. 11.0. However, when CT numbers optimized by AXB Ver. 11.0 were used, they were 2.6% and 0.6%, respectively. The CT numbers for couch structures should be optimized when changing dose calculation algorithms and their versions. From the comparison of the measured transmission to calculation, it was found that the CT numbers had high accuracy.