Retrospective assessment of a single fiducial marker tracking regimen with robotic stereotactic body radiation therapy for liver tumours

AimTo investigate tumour motion tracking uncertainties in the CyberKnife Synchrony system with single fiducial marker in liver tumours.BackgroundIn the fiducial-based CyberKnife real-time tumour motion tracking system, multiple fiducial markers are generally used to enable translation and rotation corrections during tracking. However, sometimes a single fiducial marker is employed when rotation corrections are not estimated during treatment.Materials and methodsData were analysed for 32 patients with liver tumours where one fiducial marker was implanted. Four-dimensional computed tomography (CT) scans were performed to determine the internal target volume (ITV). Before the first treatment fraction, the CT scans were repeated and the marker migration was determined. Log files generated by the Synchrony system were obtained after each treatment and the correlation model errors were calculated. Intra-fractional spine rotations were examined on the spine alignment images before and after each treatment.ResultsThe mean (standard deviation) ITV margin was 4.1 (2.3) mm, which correlated weakly with the distance between the fiducial marker and the tumour. The mean migration distance of the marker was 1.5 (0.7) mm. The overall mean correlation model error was 1.03 (0.37) mm in the radial direction. The overall mean spine rotations were 0.27° (0.31), 0.25° (0.22), and 0.23° (0.26) for roll, pitch, and yaw, respectively. The treatment time was moderately associated with the correlation model errors and weakly related to spine rotation in the roll and yaw planes.ConclusionsMore caution and an additional safety margins are required when tracking a single fiducial marker.     

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

PurposeTo 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.MethodsA 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 (TL_AXB) and between XVMC and AAA (TL_AAA) were calculated as the mean ± 1.96 standard deviations.ResultsAXB 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 TL_AXB and TL_AAA 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.ConclusionsThis 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.     

Impact of patient setup error in the treatment of head and neck cancer with intensity modulated radiation therapy

PurposeTo study the impact of setup errors on the dose to the target volume and critical structures in the treatment of cancer of nasopharynx with intensity modulated radiation therapy (IMRT).Methods and materialsTwelve patients of carcinoma of nasopharynx treated by IMRT with simultaneous integrated boost technique were enrolled. The gross tumor volume, clinical target volume and low-risk nodal region were planned for 70, 59.4 and 54 Gy, respectively, in 33 fractions. Based on the constraints, treatment plans were generated. Keeping it as the base plan, the patient setup error was simulated for 3, 5 and 10 mm by shifting the isocenter in all three directions viz. anterior, posterior, superior, inferior, right and left lateral. The plans were evaluated for mean dose, maximum dose, volume of PTV receiving >110% and <93% of the prescribed dose. For both the parotids, the mean dose and the dose received by >50% of the parotid were evaluated. The maximum dose and dose received by 2 cc of spinal cord were also analyzed.ResultsThe dose to the target volume decreases gradually with increase in setup error. The superior and inferior shifts play major role in tumor under-dosage. A setup error of 3 mm along the posterior and lateral directions significantly affects the dose to the spinal cord. Similarly, setup error along lateral and anterior directions affects the dose to both parotids.ConclusionsThe isocenter position should be verified regularly to ensure that the goal of IMRT is achieved.     

Dosimetric impact of uncorrected systematic yaw rotation in VMAT for peripheral lung SABR

AimThis study aimed to evaluate the dosimetric impact of uncorrected yaw rotational error on both target coverage and OAR dose metrics in this patient population.BackgroundRotational set up errors can be difficult to correct in lung VMAT SABR treatments, and may lead to a change in planned dose distributions.Materials and methodsWe retrospectively applied systematic yaw rotational errors in 1° degree increments up to −5° and +5° degrees in 16 VMAT SABR plans. The impact on PTV and OARs (oesophagus, spinal canal, heart, airway, chest wall, brachial plexus, lung) was evaluated using a variety of dose metrics. Changes were assessed in relation to percentage deviation from approved planned dose at 0 degrees.ResultsTarget coverage was largely unaffected with the largest mean and maximum percentage difference being 1.4% and 6% respectively to PTV D98% at +5 degrees yaw.Impact on OARs was varied. Minimal impact was observed in oesophagus, spinal canal, chest wall or lung dose metrics. Larger variations were observed in the heart, airway and brachial plexus. The largest mean and maximum percentage differences being 20.77% and 311% respectively at −5 degrees yaw to airway D0.1cc, however, the clinical impact was negligible as these variations were observed in metrics with minimal initial doses.ConclusionsNo clinically unacceptable changes to dose metrics were observed in this patient cohort but large percentage deviations from approved dose metrics in OARs were noted. OARs with associated PRV structures appear more robust to uncorrected rotational error.     

Characterization of robust optimization for VMAT plan for liver cancer

PurposeWe investigated the feasibility of robust optimization for volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) for liver cancer in comparison with planning target volume (PTV)-based optimized plans. Treatment plan quality, robustness, complexity, and accuracy of dose delivery were assessed.MethodsTen liver cancer patients were selected for this study. PTV-based optimized plans with an 8-mm PTV margin and robust optimized plans with an 8-mm setup uncertainty were generated. Plan perturbed doses were evaluated using a setup error of 8 mm in all directions from the isocenter. The dosimetric comparison parameters were clinical target volume (CTV) doses (D_98%, D_50%, and D_2%), liver doses, and monitor unit (MU). Plan complexity was evaluated using the modulation complexity score for VMAT (MCSv).ResultsThere was no significant difference between the two optimizations with respect to CTV doses and MUs. Robust optimized plans had a higher liver dose than did PTV-based optimized plans. Plan perturbed dose evaluations showed that doses to the CTV for the robust optimized plans had small variations. Robust optimized plans were less complex than PTV-based optimized plans. Robust optimized plans had statistically significant fewer leaf position errors than did PTV-based optimized plans.ConclusionsComparison of treatment plan quality, robustness, and plan complexity of both optimizations showed that robust optimization could be feasibile for VMAT of liver cancer.     

Clinical impact of dosimetric changes for volumetric modulated arc therapy in log file-based patient dose calculations

PurposeA log file-based method cannot detect dosimetric changes due to linac component miscalibration because log files are insensitive to miscalibration. Herein, clinical impacts of dosimetric changes on a log file-based method were determined.Methods and materialsFive head-and-neck and five prostate plans were applied. Miscalibration-simulated log files were generated by inducing a linac component miscalibration into the log file. Miscalibration magnitudes for leaf, gantry, and collimator at the general tolerance level were ±0.5 mm, ±1°, and ±1°, respectively, and at a tighter tolerance level achievable on current linac were ±0.3 mm, ±0.5°, and ±0.5°, respectively. Re-calculations were performed on patient anatomy using log file data.ResultsChanges in tumor control probability/normal tissue complication probability from treatment planning system dose to re-calculated dose at the general tolerance level was 1.8% on planning target volume (PTV) and 2.4% on organs at risk (OARs) in both plans. These changes at the tighter tolerance level were improved to 1.0% on PTV and to 1.5% on OARs, with a statistically significant difference.ConclusionsWe determined the clinical impacts of dosimetric changes on a log file-based method using a general tolerance level and a tighter tolerance level for linac miscalibration and found that a tighter tolerance level significantly improved the accuracy of the log file-based method.     

A study on conventional IMRT and RapidArc treatment planning techniques for head and neck cancers

AimTo evaluate the performance of volumetric arc modulation with RapidArc against conventional IMRT for head and neck cancers.BackgroundRapidArc is a novel technique that has recently been made available for clinical use. Planning study was done for volumetric arc modulation with RapidArc against conventional IMRT for head and neck cancers.Materials and methodsTen patients with advanced tumors of the nasopharynx, oropharynx, and hypopharynx were selected for the planning comparison study. PTV was delineated for two different dose levels and planning was done by means of simultaneously integrated boost technique. A total dose of 70 Gy was delivered to the boost volume (PTV boost) and 57.7 Gy to the elective PTV (PTV elective) in 35 equal treatment fractions. PTV boost consisted of the gross tumor volume and lymph nodes containing visible macroscopic tumor or biopsy-proven positive lymph nodes, whereas the PTV elective consisted of elective nodal regions. Planning was done for IMRT using 9 fields and RapidArc with single arc, double arc. Beam was equally placed for IMRT plans. Single arc RapidArc plan utilizes full 360° gantry rotation and double arc consists of 2 co-planar arcs of 360° in clockwise and counter clockwise direction. Collimator was rotated from 35 to 45° to cover the entire tumor, which reduced the tongue and groove effect during gantry rotation. All plans were generated with 6 MV X-rays for CLINAC 2100 Linear Accelerator. Calculations were done in the Eclipse treatment planning system (version 8.6) using the AAA algorithm.ResultsDouble arc plans show superior dose homogeneity in PTV compared to a single arc and IMRT 9 field technique. Target coverage was almost similar in all the techniques. The sparing of spinal cord in terms of the maximum dose was better in the double arc technique by 4.5% when compared to the IMRT 9 field and single arc techniques. For healthy tissue, no significant changes were observed between the plans in terms of the mean dose and integral dose. But RapidArc plans showed a reduction in the volume of the healthy tissue irradiated at V_15 Gy (5.81% for single arc and 4.69% for double arc) and V_20 Gy (7.55% for single arc and 5.89% for double arc) dose levels when compared to the 9-Field IMRT technique. For brain stem, maximum dose was similar in all the techniques. The average MU (±SD) needed to deliver the dose of 200 cGy per fraction was 474 ± 80 MU and 447 ± 45 MU for double arc and single arc as against 948 ± 162 MU for the 9-Field IMRT plan. A considerable reduction in maximum dose to the mandible by 6.05% was observed with double arc plan. Double arc shows a reduction in the parotid mean dose when compared with single arc and IMRT plans.ConclusionRapidArc using double arc provided a significant sparing of OARs and healthy tissue without compromising target coverage compared to IMRT. The main disadvantage with IMRT observed was higher monitor units and longer treatment time.     

Effect of patient positioning on carbon-ion therapy planned dose distribution to pancreatic tumors and organs at risk

PurposePancreatic tumor treatment dose distribution variations associated with supine and prone patient positioning were evaluated.MethodsA total of 33 patients with pancreatic tumors who underwent CT in the supine and prone positions were analyzed retrospectively. Gross tumor volume (GTV), planning target volume (PTV), and organs at risk (OARs) (duodenum and stomach) were contoured. The prescribed dose of 55.2 Gy (RBE) was planned from four beam angles (0°, 90°, 180°, and 270°). Patient collimator and compensating boli were designed for each field. Dose distributions were calculated for each field in the supine and prone positions. To improve dose distribution, patient positioning was selected from supine or prone for each beam field.ResultsCompared with conventional beam angle and patient positioning, D2cc of 1st-2nd portion of duodenum (D1-D2), 3rd-4th portion of duodenum (D3-D4), and stomach could be reduced to a maximum of 6.4 Gy (RBE), 3.5 Gy (RBE), and 4.5 Gy (RBE) by selection of patient positioning. V10 of D1-D2, D3-D4, and stomach could be reduced to a maximum of 7.2 cc, 11.3 cc, and 11.5 cc, respectively. D95 of GTV and PTV were improved to a maximum of 6.9% and 3.7% of the prescribed dose, respectively.ConclusionsOptimization of patient positioning for each beam angle in treatment planning has the potential to reduce OARs dose maintaining tumor dose in pancreatic treatment.     

Dosimetric impact on changes in target volumes during intensity-modulated radiotherapy for nasopharyngeal carcinoma

Background and purposeTo assess anatomic changes during intensity modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC) and to determine its dosimetric impact.Patients and methodsTwenty patients treated with IMRT for NPC were enrolled in this study. A second CT was performed at 38 Gy. Manual contouring of the macroscopic tumor volumes (GTV) and the planning target volumes (PTV) were done on the second CT. We recorded the volumes of the different structures, D98 %, the conformity, and the homogeneity indexes for each PTV. Volume percent changes were calculated.ResultsWe observed a significant reduction in tumor volumes (58.56 % for the GTV N and 29.52 % for the GTV T). It was accompanied by a significant decrease in the D98 % for the 3 PTV (1.4 Gy for PTV H, p = 0.007; 0.3 Gy for PTV I, p = 0.03 and 1.15 Gy for PTV L, p = 0 0.0066). In addition, we observed a significant reduction in the conformity index in the order of 0.02 (p = 0.001) and 0.01 (p = 0.007) for PTV H and PTV I, respectively. The conformity variation was not significant for PTV L. Moreover, results showed a significant increase of the homogeneity index for PTV H (+ 0.03, p = 0.04) and PTV L (+ 0.04, p = 0.01).ConclusionTumor volume reduction during the IMRT of NPC was accompanied by deterioration of the dosimetric coverage for the different target volumes. It is essential that a careful adaptation of the treatment plan be considered during therapy for selected patients.     

Assessment of intra-fraction motion during automated linac-based SRS treatment delivery with an open face mask system

Purpose/objectiveTo evaluate intra-fraction target shift during automated mono-isocentric linac-based stereotactic radiosurgery with open-face mask system and optical real-time tracking.Materials/methodsNinety-five patients were treated using automated linac-based stereotactic radiosurgery in 1–5 fractions with single isocenter for a total of 195 fractions. During treatment, patient positioning was tracked real-time with optical surface guidance and immobilized with a rigid open-face mask. Patients were re-positioned if optical surface guidance error exceeded 1 mm magnitude or 1°. Translational and rotational intra-fractional changes were determined by post-treatment CBCT matched to the planning CT. Target specific error was calculated by translation and rotation matrices applied to isocenter and target spatial coordinates.ResultsFor 132 fractions with isocenter within a single target, the median shift magnitude was 0.40 mm with a maximum shift of 1.17 mm. A total of 398 targets treated for plans having multiple or single targets that lied outside isocenter, resulted in a median shift magnitude of 0.46 mm, with median translational shifts of 0.20 mm and 0.20° rotational shifts. A 1 mm PTV margin was insufficient in 18% of targets at a distance greater than 6 cm away from isocenter, but sufficient for 96% of targets within 6 cm.ConclusionsThe findings of this study support 1 mm PTV expansion due to intra-fraction motion to ensure target coverage for plans with isocenter placement less than 6 cm away from the targets.     

Balloon Catheter Position and its Relationship with Esophageal Temperature during Pulmonary Vein Isolation using High-Intensity Focused Ultrasound

BackgroundHIFU can achieve PVI, but severe esophageal complications have happened. We analyzed relative position of HIFU balloon catheter (BC) to esophageal temperature (ET) probe and correlated it to ET changes.Methods and ResultsBefore each ablation relative position of HIFU BC to ET probe was recorded in RAO 30° and LAO 40°. We compared ablations where ET at end of ablation was < 38.5°C or ≥ 38.5°C and < 40.0°C or ≥ 40.0°C.A total of 600 images from 311 ablations in 28 patients (18 male, age 63 ± 7 years), were analyzed. ET ≥ 38.5°C was reached when distance from BC to ET probe was: < 20 mm in LAO for RSPV and < 29 mm in LAO for RIPV. For RIPV ET ≥ 38.5°C was reached when angle between BC and ET probe was significantly smaller in LAO and RAO. ET ≥ 40.0°C was reached when distance of BC to ET probe was: < 20 mm in LAO for RIPV, < 14 mm in RAO for RIPV, < 18 mm in RAO for LIPV. ET increased to ≥ 40.0°C when distance from BC to ET probe was significantly longer in LAO for LIPV. For RIPV ET ≥ 40.0°C was reached when angle between BC and ET probe was significantly smaller in LAO.ConclusionsThere is a relationship between distance/angle of HIFU BC to ET probe and ET: shorter distances and smaller angles can cause higher ET.     

Quantification of residual dose estimation error on log file-based patient dose calculation

PurposeThe log file-based patient dose estimation includes a residual dose estimation error caused by leaf miscalibration, which cannot be reflected on the estimated dose. The purpose of this study is to determine this residual dose estimation error.Methods and materialsModified log files for seven head-and-neck and prostate volumetric modulated arc therapy (VMAT) plans simulating leaf miscalibration were generated by shifting both leaf banks (systematic leaf gap errors: ±2.0, ±1.0, and ±0.5 mm in opposite directions and systematic leaf shifts: ±1.0 mm in the same direction) using MATLAB-based (MathWorks, Natick, MA) in-house software. The generated modified and non-modified log files were imported back into the treatment planning system and recalculated. Subsequently, the generalized equivalent uniform dose (gEUD) was quantified for the definition of the planning target volume (PTV) and organs at risks.ResultsFor MLC leaves calibrated within ±0.5 mm, the quantified residual dose estimation errors that obtained from the slope of the linear regression of gEUD changes between non- and modified log file doses per leaf gap are in head-and-neck plans 1.32 ± 0.27% and 0.82 ± 0.17 Gy for PTV and spinal cord, respectively, and in prostate plans 1.22 ± 0.36%, 0.95 ± 0.14 Gy, and 0.45 ± 0.08 Gy for PTV, rectum, and bladder, respectively.ConclusionsIn this work, we determine the residual dose estimation errors for VMAT delivery using the log file-based patient dose calculation according to the MLC calibration accuracy.     

A dosimetric evaluation of IGART strategies for cervix cancer treatment

PurposeImage guided adaptive radiotherapy (IGART) strategies can be used to include the temporal aspects of radiotherapy treatment. A dosimetric evaluation of on- and off-line adaptive strategies are done in this study.MethodsA library of equivalent uniform dose (EUD)-based Intensity Modulated Radiotherapy Treatment plans with incrementally increasing clinical target volume (CTV)-to-planning target volume (PTV) margins were developed for 10 patients. Utilizing daily computed tomography (CT) images an on-line strategy using a margin-of-the-day (MOD) concept that selects the best plan from the library was employed. This was compared to an off-line strategy with full analysis of accumulated dose between fractions where dosimetric deviations from the treatment intent triggered plan adaptation. A fixed margin treatment approach was used as benchmark.ResultsUsing fixed margins of <15 mm lead to under-dosages of more than 5 Gy in total delivered dose. The average CTV EUD for the off-line and on-line strategy was 50.0 ± 5.0 Gy and 50.4 ± 2.0 Gy respectively and OAR doses were comparable.ConclusionA fixed margin treatment approach yields a significant probability of CTV under-dosage. Using EUD dose metrics CTV coverage can be restored in both the off-line and on-line adaptive strategies at acceptable OAR dose levels. Considering the workload and time on the treatment machine, the off-line strategy proves to be sufficient and more practical.     

Dosimetric effect of limited aperture multileaf collimator on VMAT plan quality: A study of prostate and head-and-neck cancers

Aim

The aim of study was to evaluate the dosimetric effect of collimator-rotation on VMAT plan quality, when using limited aperture multileaf collimator of Elekta Beam Modulator? providing a maximum aperture of 21 cm × 16 cm.

On-site audits to investigate the quality of radiation physics of radiation therapy institutions in the Republic of Korea

PurposeTo investigate and improve the domestic standard of radiation therapy in the Republic of Korea.MethodsOn-site audits were performed for 13 institutions in the Republic of Korea. Six items were investigated by on-site visits of each radiation therapy institution, including collimator, gantry, and couch rotation isocenter check; coincidence between light and radiation fields; photon beam flatness and symmetry; electron beam flatness and symmetry; physical wedge transmission factors; and photon beam and electron beam outputs.ResultsThe average deviations of mechanical collimator, gantry, and couch rotation isocenter were less than 1 mm. Those of radiation isocenter were also less than 1 mm. The average difference between light and radiation fields was 0.9 ± 0.6 mm for the field size of 20 cm × 20 cm. The average values of flatness and symmetry of the photon beams were 2.9% ± 0.6% and 1.1% ± 0.7%, respectively. Those of electron beams were 2.5% ± 0.7% and 0.6% ± 1.0%, respectively. Every institutions showed wedge transmission factor deviations less than 2% except one institution. The output deviations of both photon and electron beams were less than ±3% for every institution.ConclusionsThrough the on-site audit program, we could effectively detect an inappropriately operating linacs and provide some recommendations. The standard of radiation therapy in Korea is expected to improve through such on-site audits.     

Translational and rotational localization errors in cone-beam CT based image-guided lung stereotactic radiotherapy

PurposeAccurate localization is crucial in delivering safe and effective stereotactic body radiation therapy (SBRT). The aim of this study was to analyse the accuracy of image-guidance using the cone-beam computed tomography (CBCT) of the VERO system in 57 patients treated for lung SBRT and to calculate the treatment margins.Materials and methodsThe internal target volume (ITV) was obtained by contouring the tumor on maximum and mean intensity projection CT images reconstructed from a respiration correlated 4D-CT. Translational and rotational tumor localization errors were identified by comparing the manual registration of the ITV to the motion-blurred tumor on the CBCT and they were corrected by means of the robotic couch and the ring rotation. A verification CBCT was acquired after correction in order to evaluate residual errors.ResultsThe mean 3D vector at initial set-up was 6.6 ± 2.3 mm, which was significantly reduced to 1.6 ± 0.8 mm after 6D automatic correction. 94% of the rotational errors were within 3°. The PTV margins used to compensate for residual tumor localization errors were 3.1, 3.5 and 3.3 mm in the LR, SI and AP directions, respectively.ConclusionsOn-line image guidance with the ITV–CBCT matching technique and automatic 6D correction of the VERO system allowed a very accurate tumor localization in lung SBRT.     

The Italian multicentre dosimetric study for lesion dosimetry in 223Ra therapy of bone metastases: Calibration protocol of gamma cameras and patient eligibility criteria

PurposeThe aims of this work were to explore patient eligibility criteria for dosimetric studies in ²²³Ra therapy and evaluate the effects of differences in gamma camera calibration procedures into activity quantification.MethodsCalibrations with ²²³Ra were performed with four gamma cameras (3/8-inch crystal) acquiring planar static images with double-peak (82 and 154 keV, 20% wide) and MEGP collimator. The sensitivity was measured in air by varying activity, source-detector distance, and source diameter. Transmission curves were measured for attenuation/scatter correction with the pseudo-extrapolation number method, varying the experimental setup. ²²³Ra images of twenty-five patients (69 lesions) were acquired to study the lesions visibility. Univariate ROC analysis was performed considering visible/non visible lesions on ²²³Ra images as true positive/true negative group, and using as score value the lesion/soft tissue contrast ratio (CR) derived from ^99mTc-MDP WB scan.ResultsSensitivity was nearly constant varying activity and distance (maximum s.d. = 2%). Partial volume effects were negligible for object area ⩾960 mm². Transmission curve measurements are affected by experimental setup and source size, leading to activity quantification errors up to 20%. The ROC analysis yielded an AUC of 0.972 and an optimal threshold of CR of 10, corresponding to an accuracy of 92%.ConclusionThe minimum calibration protocol requires sensitivity and transmission curve measurements varying the object size, performing a careful procedure standardisation. Lesions with ^99mTc-MDP CR higher than 10, not overlapping the GI tract, are generally visible on ²²³Ra images acquired at 24 h after the administration, and possibly eligible for dosimetric studies.     

The effect of rectal gas on dose distribution during prostate cancer treatment using full arc and partial arc Volumetric Modulated Arc Therapy (VMAT) treatment plans

Background/AimIn this study, we investigated the effect of rectal gas on the dose distribution of prostate cancer using a volumetric modulated arc therapy (VMAT) treatment planning.Materials and MethodsThe first is the original structure set, clinical target volume (CTV), the rectum, and the bladder used clinically. The second is a structure set (simulated gas structure set) in which the overlapping part of the rectum and PTV is overwritten with Hounsfield Unit −950 as gas. Full arc and limited gantry rotation angle with VMAT were the two arcs. The VMAT of the full arc was 181°–179° in the clockwise (CW) direction and 179°–181° in the counterclockwise (CCW) direction. Three partial arcs with a limited gantry rotation angle were created: 200°–160 °CW and 160°–200 °CCW; 220°–140 °CW and 140°–220 °CCW; and finally, 240°–120 °CW and 120°–240 °CCW. The evaluation items were dose difference, distance to agreement, and gamma analysis.ResultIn the CTV, the full arc was the treatment planning technique with the least effect of rectal gas. In the rectum, when the gantry rotation angle range was short, the pass rate tended to reduce for all evaluation indices. The bladder showed no characteristic change between the treatment planning techniques in any of the evaluation indices.ConclusionsThe VMAT treatment planning with the least effect on dose distribution caused by rectal gas was shown to be a full arc.