CT Slice Thickness Research Articles

Feasibility of MR-alone-based Brachytherapy Treatment Planning Using a Titanium Tandem and Ring Applicator for Cervical Cancer

Materials and Methods 3D-fGRE MR was found to have the least image artifacts of the TR p=0.007) and Sigmoid D2cc (3.45±0.59Gy EQD2 and 3.31±0.58Gy EQD2 in Plan1 and Plan2 respectively, p=0.001). No significant differences were found in other metrics (p-value range: 0.13 to 0.44).

Depiction of the parotid duct on axial CT images

Objectives This study aimed to clarify the anatomical and CT imaging features of the parotid duct at the anterior portion near the orifice together with its surrounding structures and to determine appropriate CT slice thicknesses for depicting the duct.

Impact of different CBCT imaging monitor units, reconstruction slice thicknesses, and planning CT slice thicknesses on the positioning accuracy of a MV‐CBCT system in head‐and‐neck patients

The purpose of this study was to investigate the impact of different CBCT imaging monitor units (MUs), reconstruction slice thicknesses, and planning CT slice thicknesses on the positioning accuracy of a megavoltage cone‐beam computed tomography (MV‐CBCT) system in image‐guided radiation therapy (IGRT) in head‐and‐neck patients. The MV‐CBCT system was a Siemens MVision, a commercial system integrated into the Siemens ONCOR linear accelerator. The positioning accuracy of the MV‐CBCT system was determined using an anthropomorphic phantom while varying the MV‐CBCT imaging MU, reconstruction slice thickness, and planning CT slice thickness. A total of 240 CBCT images from six head‐and‐neck patients who underwent intensity‐modulated radiotherapy (IMRT) treatment were acquired and reconstructed using different MV‐CBCT scanning protocols. The interfractional setup errors of the patients were retrospectively analyzed for different imaging MUs, reconstruction slice thicknesses, and planning CT slice thicknesses. Using the anthropomorphic phantom, the largest measured mean deviation component and standard deviation of the MVision in 3D directions were 1.3 and 1.0 mm, respectively, for different CBCT imaging MUs, reconstruction slice thicknesses, and planning CT slice thicknesses. The largest setup group system error (M), system error (∑), and random error (σ) from six head‐and‐neck patients were 0.6, 1.2, and 1.7 mm, respectively. No significant difference was found in the positioning accuracy of the MV‐CBCT system between the 5 and 8 MUs, and between the 1 and 3 mm reconstruction slice thicknesses. A thin planning CT slice thickness may achieve higher positioning precision using the phantom measurement, but no significant difference was found in clinical setup precision between the 1 and 3 mm planning CT slice thicknesses.PACS number: 87.55 ne

PACS에서 JPEG2000을 이용한 AAPM CT Performance Phantom영상의 압축에 따른 화질변화

본 연구는 PACS에서 JPEG200을 이용하여 AAPM CT Performance Phantom 영상을 다양한 비율로 압축 한 후 영상의 질 변화를 알아보았다. 원 영상을 기준으로 압축된 영상을 비교했을 때 물의 CT 계수 측정에서는 압축률 15:1에서 1.93%의 변화, 절편 두께 측정에서는 15:1에서 0.81% 의 변화를 보였다. 균일도는 규칙적인 변화나 통계적으로 유의한 차이가 나타나지는 않았다. 노이즈 측정의 경우 10:1에서 1.47%의 변화를 보이나 15:1에서 10.99%로 증가한 후 변화율 증가 폭이 확대되어 40:1에서 81.68%의 변화율을 보였다. CT 계수, 균일도, 절편 두께, 공간 분해능, 대조도 분해능의 경우 압축률 증가에 따라 영상의 화질 변화율도 증가하나 영향은 크지 않은 것으로 나타났다. 상대적으로 노이즈는 압축에 따른 영향을 많이 받는 것으로 나타났다. JPEG2000 압축 기법으로 AAPM CT Performance Phantom 영상을 평가한 결과 CT영상의 압축을 시행하는 경우 10:1정도의 압축률이 적정한 것으로 판단된다. This study examines image quality of medical image after compression using JPEG2000 for AAPM CT Performance Phantom in PACS. The compressed images of 15:1 showed change of 1.93% and 0.81% in the CT number of water and the slice thickness, respectively, compared to the original images. The variation of the uniformity did not give a correlation for each measured area. In noise measurements at compressions of 10:1 and 15:1, changes of 1.47% to 10.99% were observed, respectively. The noise showed incremation tendency as increasing over the compression ratio 15:1, and the noise of 81.68% was measured at a compression of 40:1. CT number, uniformity, slice thickness, spatial resolution and contrast resolution for the compressed images were slightly changed by increasing the compression ratio. However, the noise was seriously changed relatively at the compressed images. Thus the noise was a important factor to determine the compression ration. A compression ratio of 10:1 for the AAPM CT Performance Phantom image was appropriate and could be applied to diagnostic images.

Comparison of planning target volume for esophageal cancer target delineation with different ways.

e14735 Background: To compare the planning target volume (PTV) of esophageal cancer target delineation with different ways. Methods: There were 15 patients with middle chest esophageal cancer, involving 12 males and 3 females and aged 45-72 years. The median age was 59 years. Positioning in the CT slice thickness of 3mm on the outline of esophageal lesions as GTV.The CTV0 was created by manually contouring the esophagus for 30mm superiorly and inferior to GTV then adding 5mm circumferentially to the created volume to account for microscopic spread. For the PTV0, a 10mm margin in all direction was added to CTV0. The CTV6 was created by manually contouring the esophagus for 24mm superiorly and inferior to GTV then adding 5mm For circumferentially. The PTV6, a 16mm margin in superiorly and inferiorall direction and a 10mm margin in other direction was added to CTV6. The CTV15 was created by manually contouring the esophagus for 15mm superiorly and inferior to GTV then adding 5mm For circumferentially. The PTV15, a 25mm margin in superiorly and inferiorall direction and a 10mm margin in other direction was added to CTV15.The CTV30 was created by manually contouring the esophagus for 0mm superiorly and inferior to GTV then adding 5mm For circumferentially. The PTV30, a 40mm margin in superiorly and inferiorall direction and a 10mm margin in other direction was added to CTV15. Paired t-test using SPSS17.0 to p <0.05 was considered significant difference, compared PTV30, PTV15, PTV6, PTV0 volume. Results: The volumes of PTV30, PTV15, PTV6 and PTV0 showing with the mean ± standard deviation, were 392.51 ± 123.32cm3, 366.07 ± 112.78cm3, 346.55 ± 103.70cm3 and 342.36 ± 108.65cm3, respectively. The results were significant differences (p ≤ 0.001) except PTV6 vs PTV0. Conclusions: Different ways of esophageal cancer target delineation make significant effects on the PTVs. Choosing the delineation ways of CTV6 or CTV0 are better. It may benefit patients.

Investigating the relationship between virtual cystoscopy image quality and CT slice thickness

To investigate the effect of reconstruction slice thickness on image quality at CT virtual cystoscopy (VC). Pelvic CT examinations in bladder cancer patients were reconstructed at different slice thicknesses (0.6-5 mm) and intervals, and resulting VC images assessed. Quality indicators were ridging, holes, floaters and dimpling artefacts, tumour definition, and an overall score, ranked 1 (best) to 7 (worst). CT number and standard deviation (SD) for bladder contents and bladder wall were recorded. The mean SD was used as a measure of noise, and the contrast-to-noise ratio (CNR) was calculated as the CT number difference between them divided by the average image noise. The mean CNR across the three levels was used for analysis. Each qualitative image quality measure was compared with CT number, noise and CNR measurements. Dimpling artefacts increased with thinner slice reconstruction and correlated with increased noise, often resulting in poor tumour definition. The best overall image quality score was seen for VC images reconstructed at 1.2 mm slice thickness, probably because of the competing effects of spatial resolution and CNR. A slice thickness reconstruction <1.2 mm does not provide for better image quality at VC owing to the presence of increased noise.

Tomotherapy planning and image registration for planning CT image slice thickness for stereotactic radiosurgery

Tomotherapy (TomoTherapy, Inc., Madison, WI, USA) is a helical intensity modulated radiation therapy (IMRT) modality using rotational delivery of a fan beam like a CT scanner to deliver highly conformal radiation dose distributions in an intensity-modulated fashion. Tomotherapy can image guided radiation therapy (IGRT) using megavoltage computed tomography (MVCT). In this study, different CT slice thicknesses were acquired in order to evaluate the effect of CT slice thickness on tomotherapy planning and image registration for stereotactic body radio therapy (SBRT) or stereotactic radiosurgery (SRS) by using end-to-end (EtoE) phantom (Accuray, USA). EtoE phantom kilovoltage CT (KVCT) and MVCT images were acquired using CT (LightSpeed 16, GE, USA), and tomotherapy (Tomotherapy, USA), respectively. Reference planning target volumes (RPTV) in the EtoE phantom were contoured. The volume of the RPTV was 14.13 cc, and the CT image thicknesses were 2, 4, and 6 mm. CT images were planned based on the tomotherapy planning system (TPS), and all plans had the same constraints and iterations. The PTV volumes were 16.58 cc, 18.88 cc, and 20.28 cc in 2-, 4- and 6 mm planning KVCTs, respectively. The PTV volumes were 16.32, 18.00, and 22.04 cc in 2-, 4-, and 6 mm using MVCT, respectively. Heavy thickness planning of the CT slice overestimated the RPTV volume in TPS. Also, the heavy thickness MVCT image skipped target slice images. The target coverage of the treatment plans for different slice thicknesses was correlated with the tumor volume. At 10-Gy prescription dose of PTV, dose coverages of the reference planning target volume were 10.14, 10.25, and 10.27 Gy for 2-, 4-, and 6-mm slice thicknesses. Therefore, when radiosurgery is supplemented with tomotherapy, thin slice (<2 mm) planning should be done and MVCT images should be acquired

Detection of pulmonary nodules by C-arm CT using a phantom lung: comparison with CT

Evaluation of detection of lung nodules by C-arm CT (CACT) is important before this procedure can be used to guide percutaneous lung interventions. To compare the efficacy of CACT with CT in the detection of pulmonary nodules using a phantom lung. A phantom lung containing 12 phantom nodules in four sizes (5 mm/8 mm/10 mm/12 mm) and three CT values (one solid nodule, +100 HU; two ground glass nodules, -630 and -800 HU) was used. Six sessions of CACT (slice thickness 4.5 mm) and CT (slice thickness 5 mm) were performed. In each session, the locations of nodules were arbitrarily changed in the phantom. Three radiologists assessed the detection of a total of 72 nodules. Statistical analysis was performed for the sensitivity and positive predictive value of lung nodules between CACT and CT by the McNemar test and paired t-test (P < 0.05). Sensitivity did not differ between CACT and CT, respectively (reader 1, 82% vs. 88%, P = 0.22; reader 2, 82% vs. 78%, P = 0.37; reader 3, 79% vs. 83%, P = 0.48). For nodules of 8 mm or larger, the sensitivity increased for each reader and showed no significant difference between CACT vs. CT. The positive predictive value did not differ between CACT and CT. In this phantom study, CT and CACT show similar sensitivity for the detection of pulmonary nodules. CACT could be used in percutaneous interventional procedures in the lungs.

Clinical accuracy of ExacTrac intracranial frameless stereotactic system

In this paper, the authors assess the accuracy of the Brainlab ExacTrac system for frameless intracranial stereotactic treatments in clinical practice. They recorded couch angle and image fusion results (comprising lateral, longitudinal, and vertical shifts, and rotation corrections about these axes) for 109 stereotactic radiosurgery and 166 stereotactic radiotherapy patient treatments. Frameless stereotactic treatments involve iterative 6D image fusion corrections applied until the results conform to customizable pass criteria, theirs being 0.7 mm and 0.5° for each axis. The planning CT slice thickness was 1.25 mm. It has been reported in the literature that the CT slices' thickness impacts the accuracy of localization to bony anatomy. The principle of invariance with respect to patient orientation was used to determine spatial accuracy. The data for radiosurgery comprised 927 image pairs, of which 532 passed (pass ratio of 57.4%). The data for radiotherapy comprised 15983 image pairs, of which 10 050 passed (pass ratio of 62.9%). For stereotactic radiotherapy, the combined uncertainty of ExacTrac calibration, image fusion, and intrafraction motion was (95% confidence interval) 0.290-0.302 and 0.306-0.319 mm in the longitudinal and lateral axes, respectively. The combined uncertainty of image fusion and intrafraction motion in the anterior-posterior coordinates was 0.174-0.182 mm. For stereotactic radiosurgery, the equivalent ranges are 0.323-0.393, 0.337-0.409, and 0.231-0.281 mm. The overall spatial accuracy was 1.24 mm for stereotactic radiotherapy (SRT) and 1.35 mm for stereotactic radiosurgery (SRS). The ExacTrac intracranial frameless stereotactic system spatial accuracy is adequate for clinical practice, and with the same pass criteria, SRT is more accurate than SRS. They now use frameless stereotaxy exclusively at their center.

Impact of different CT slice thickness on clinicaltarget volume for intensity modulated radiationtherapy : A phantom study

Impact of different CT slice thickness on clinicaltarget volume for intensity modulated radiationtherapy : A phantom study

SU‐E‐J‐149: Validation of the Spatial Accuracy of the ExacTrac Adaptive Gating System

Purpose: To evaluate the positioning accuracy of the BrainLAB ExacTrac Image Guidance System under gating conditions. Methods: Two types of phantoms were used in measurements: an anthropomorphic RANDO head phantom and a BrainLAB ExacTrac Gating Phantom. Our setup included a Varian Novalis Tx radiosurgery system equipped with the ExacTrac 6D IGRT. This system consists of an infrared positioning system for the initial patient positioning and patient tracking, and a stereoscopic kV X‐ray imaging system for final localization using internal markers or anatomy. Uncertainties were broken down into individual components, and the different BrainLAB fusion modalities (internal markers and bony fusion) were used to compare the effect of slice thickness on positional accuracy. Gating uncertainties were deduced with varying tumor motion amplitudes and window sizes in conjunction with a hidden target test. Results: Our results of CT slice thickness dependence for both fusion algorithms with the hidden target test gave similar deviation (&lt;0.7mm), and were reasonably consistent up to a 5 mm slice width. Tumor motion and gating window size yielded an uncertainty of up to 1 mm for the parameters tested. Combining a non‐gating uncertainty of 0.9 mm with the gating uncertainty resulted in a geometrical accuracy of 1.6 ± 0.7 mm for 2.25 cm tumor amplitude and a 30% window size. For tumor motions up to 3 cm and gating window sizes up to 30%, the localization accuracy remained within 2 mm. Conclusions: We have tested the gating window and tumor amplitude effects on the spatial accuracy of the ExacTrac System equipped Novalis Tx linac for stereotactic body radiation therapy. While the CT slice thickness, mechanical deviation of the linac and gating window size contribute to uncertainty, the system provides an external modality that allows for localization accuracy of less than 2 mm for gated delivery.

SU‐E‐I‐59: CT DOSE REDUCTION WITHOUT SACRIFICING IMAGE QUALITY: A Collaborative Intradepartmental Effort

Purpose: Recent headlines have put radiation dose and its potential deleterious effects in the limelight, and have caused some anxiety amongst patients and radiation safety personnel. Thus, it is even more important to minimize the stochastic effects associated with radiation. We demonstrate how an integrated departmental approach to reducing CT dose without sacrificing image quality can be achieved. Methods: A cohort of subspecialized radiologists, radiology residents, a radiation safety officer, a medical physicist, and radiology technologists was assembled. Using, but also significantly modifying, the ACR (American College of Radiology) CT Chest dose reduction Practice Quality Improvement (PQI) project as a template, multiple factors were manipulated to evaluate their effect on dose reduction. Initially, a retrospective review of patient dose data from head, chest and body CT examinations was performed. Next, selected changes were made to select body parts. For example, the kVp of a chest CT was reduced from 120 to 100, and image quality before and after the alteration was assessed and compared. Similarly, abdominal CT slice thickness was increased from 1.25mm to 2.5mm. Results: After the two interventions of decreasing the kVp and increasing the slice thickness, dose reduction of 23% and 20% was achieved, respectively. Conclusions: : A multidisciplinary approach to CT dose reduction with dedicated involvement from the entire Radiology department can result in significant dose reduction to patients. Using easily modifiable parameters, an intuitive flow chart was developed to help facilitate data tracking. While data has only been evaluated on some of the variables to date, we look forward to seeing the impact of other interventions as the study continues. Additionally, we believe this process can be replicated at other institutions with similar success, to ultimately reduce dose dramatically without significantly impacting image quality.

Comparative effectiveness of using computed tomography alone to exclude cervical spine injuries in obtunded or intubated patients: meta-analysis of 14,327 patients with blunt trauma

The current standard of practice for clearance of the cervical spine in obtunded patients suffering blunt trauma is to use CT and an adjuvant imaging modality (such as MR imaging). The objective of this study was to determine the comparative effectiveness of multislice helical CT alone to diagnose acute unstable cervical spine injury following blunt trauma. The authors performed a meta-analysis of studies comparing modern CT with adjunctive imaging modalities and required that studies present acute traumatic findings as well as treatment for unstable injuries. Study quality, population characteristics, diagnostic protocols, and outcome data were extracted. Positive disease status included all injuries necessitating surgical or orthotic stabilization identified on imaging and/or clinical follow-up. Seventeen studies encompassing 14,327 patients met the inclusion criteria. Overall, the sensitivity and specificity for modern CT were both > 99.9% (95% CI 0.99-1.00 and 0.99-1.00, respectively). The negative likelihood ratio of an unstable cervical injury after a CT scan negative for acute injury was < 0.001 (95% CI 0.00-0.01), while the negative predictive value of a normal CT scan was 100% (95% CI 0.96-1.00). Global severity of injury, CT slice thickness, and study quality did not significantly affect accuracy estimates. Modern CT alone is sufficient to detect unstable cervical spine injuries in trauma patients. Adjuvant imaging is unnecessary when the CT scan is negative for acute injury. Results of this meta-analysis strongly show that the cervical collar may be removed from obtunded or intubated trauma patients if a modern CT scan is negative for acute injury.

Effect of CT slice thickness on accuracy of implant positioning in navigated total hip arthroplasty

There are many published reports demonstrating the accuracy of CT-based navigation systems. However, the use of such systems often subjects patients to a high level of radiation exposure. CT scans acquired using thinner slices are considered to lead to more accurate results, but also increase radiation exposure. We took the postoperative CT scans for 56 cases of total hip arthroplasty performed using a CT-based navigation system and analyzed the accuracy of the cup and stem positioning. Of these cases, 41 were performed using 3-mm CT slices and 15 were performed using 1-mm slices, enabling us to compare the accuracy of the system and the radiation exposure using the different slice thicknesses.CT-based navigation appears to be very accurate with regard to cup anteversion and leg length, but inaccurate with regard to stem anteversion. As for the varus/valgus angle of the stem, the navigated approach seems to be very accurate in terms of the numerical value, but this does not satisfy us: Stem anteversion is still inaccurate with this system, while cup inclination is sufficiently accurate with both navigation and manual methods.Use of 1-mm CT slices results in twice the radiation exposure associated with 3-mm CT slices, but there is little difference with respect to accuracy. It is therefore recommended to use a CT-based navigation system with 3-mm CT slices for accurate and safe total hip arthroplasty.

Optimal slice thickness for cone-beam CT with on-board imager.

Purpose:To find the optimal slice thickness (Δτ) setting for patient registration with kilovoltage cone-beam CT (kVCBCT) on the Varian On Board Imager (OBI) system by investigating the relationship of slice thickness to automatic registration accuracy and contrast-to-noise ratio.Materials and method:Automatic registration was performed on kVCBCT studies of the head and pelvis of a RANDO anthropomorphic phantom. Images were reconstructed with 1.0 ≤ Δτ (mm) ≤ 5.0 at 1.0 mm increments. The phantoms were offset by a known amount, and the suggested shifts were compared to the known shifts by calculating the residual error. A uniform cylindrical phantom with cylindrical inserts of various known CT numbers was scanned with kVCBCT at 1.0 ≤ Δτ (mm) ≤ 5.0 at increments of 0.5 mm. The contrast-to-noise ratios for the inserts were measured at each Δτ.Results:For the planning CT slice thickness used in this study, there was no significant difference in residual error below a threshold equal to the planning CT slice thickness. For Δτ > 3.0 mm, residual error increased for both the head and pelvis phantom studies. The contrast-to-noise ratio is proportional to slice thickness until Δτ = 2.5 mm. Beyond this point, the contrast-to-noise ratio was not affected by Δτ.Conclusion:Automatic registration accuracy is greatest when 1.0 ≤ Δτ (mm) ≤ 3.0 is used. Contrast-to-noise ratio is optimal for the 2.5 ≤ Δτ (mm) ≤ 5.0 range. Therefore 2.5 ≤ Δτ (mm) ≤ 3.0 is recommended for kVCBCT patient registration where the planning CT is 3.0 mm.

SU-GG-T-75: Phantom Studies of Difficulties with Seed Detection Software Following I-125 Prostate Implants

Purpose: In our experience, both recent versions (7.1 and 8.0) of the Variseed software have shown variability in seed detection capabilities—wrong identification, missing seeds. A phantom study was begun to improve our understanding of the effectiveness of seed identification in CT scans of patients following prostate implantation. Materials and Methods: A simple planar phantom was constructed for investigation of the effects of seed spacing, orientation, and CT slice thickness on the ability of commercial software to detect the positions and numbers of seeds accurately. Single seed spacings were varied from 4 mm to 11 mm in the longitudinal direction and from 1.9 to 8 mm in the axial plane; stranded seeds (10 mm spacing) were used to test the effect of plane orientations of 90, 75, and 60 deg (relative to the axial CT plane); CT slice thicknesses were varied from 1 to 5 mm; two versions of Varian's seed finder software (7.1 and 8.0) were compared for loose seed detection. V8.0 was used for the angular/slice thickness study. Non‐radioactive STM 1251 seeds were supplied by Bard Brachytherapy, Inc. Results: V 7.1 was slightly superior to V8.0 for loose seed identification in the longitudinal (100% vs 93%), but inferior (13% vs 98%) in the cross direction. No effect of slice thickness was observed. Increasing the angle between the seed plane and CT cross plane tended to increase both mean and maximum errors in seed spacing (up to 5 mm). Slice thicknesses of 2 or 3 mm seemed optimal. Conclusions: The seed phantom revealed differences between the two versions of the Variseed software for seed detection, a dependence on seed plane angle and confirmed the recommended CT slice thickness of 2–3 mm (TG137).

SU-GG-T-581: Volume Variability in Treatment Planning Systems

Purpose:Dose‐volume histogram has become an important tool for evaluation of radiation outcome and is required in most clinical protocols. While accuracy of dosimetry in treatment planning system (TPS) is well quantified, the variability in volume estimation is uncertain which is investigated in this study. Material & Methods: Structures varying from 0.6 cm3 to 270 cm3 were contoured on a CT of a patient with head and neck cancer.CT data with contoured structures were sent to 5 centers with different TPS for volume estimation. Additionally, a water phantom with accurately known volume objects was scanned with 1, 2, 3, 5, and 10 mm slice thickness. These CTdata sets were also sent to various institutions for the estimation of the volume from their TPS. The data were analyzed and compared for error in each volume of structure. Results: The variability of volumes among TPS may be significant (−20% to 300%), especially for small volume structures. The minimum and maximum errors is the range of −13% to 300%, −12%–183%,−24% to 92%, −20% to 30% for CMS‐XiO, Pinnacle, Eclipse, and Oncentra, respectively. These differences become smaller for larger volumes with nearly a cutoff at about 20 cm3. The calculated volume of the objects is a function of algorithm and slice thickness. For small objects like optic nerve, cochlea, lens etc, the estimation of the volume could have up to 300% error in some TPS. Based on this study, CMS‐XiO has poor and Oncentra has superior contouring algorithm for volume estimates used in DVH. Conclusions: Estimation of a volume is dependent on CT slice thickness and the quality of contouring algorithm in TPS. During commissioning of TPS and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH from TPS.

SU‐GG‐T‐515: The Influence of Longitudinal CT Resolution on Target Delineation and Treatment Planning for Stereotactic Radiosurgery

Purpose: To investigate the effect of longitudinal CT resolution on target delineation and treatment planning in stereotactic radiosurgery (SRS) to determine optimal acquisition parameters for SRS simulation. Method and Materials: CT images for 6 SRS brain patients were acquired and image sets were retrospectively reconstructed with 1, 2 and 3mm slice thicknesses. In total, 9 lesions were contoured 3 times each to quantify intra‐observer variability. The volumes contoured on the 2mm scan were the designated reference consistent with current clinical protocol. Variation in GTV with different CT slice thicknesses was evaluated. Additionally, treatment plans were created and optimized using the 2mm scans. Superimposing these plans on the 1mm and 3mm image sets provided a basis for dosimetric comparison—specifically in terms of target coverage. Results: All of the 3mm GTVs were larger than the 2mm GTVs. However, this consistent overestimation in the GTV was not observed at 1mm, suggesting that the 1mm and 2mm scans produced comparable GTVs. The average % difference in GTV volume when compared to the 2mm scans was 4.6% higher for the 3mm versus 1mm scans. Increasing CT slice thickness from 1 to 3mm caused an increase in the intra‐observer variation; volume standard deviations increased up to 51%. The intra‐observer variation also increased at larger tumor volumes. Average standard deviations (SD) in GTV extent were 0.14, 0.11 and 0.14cm in the sup‐inf, ant‐post, and lat dimensions, respectively, implying no directional dependence in target delineation. Dosimetrically, variations in GTV resulted up to a 19% loss in PTV dose coverage. Conclusion: This study indicates that longitudinal CT resolution causes considerable variation in GTV definition which can potentially compromise SRS treatment plan quality. Therefore, the SRS simulation process may benefit from improvement in scanning parameters in order to yield the most accurate information on the CT dataset.

SU-FF-T-586: Evaluation of Dose Distribution According to Various CT Slice Thickness

Purpose: We investigated the variation of dose distributions in the treatment plans based on CTimage sets of different slice thicknesses. Method and Materials: An anthropomorphic phantom was scanned with a slice thickness of 1 mm. The acquired images were reconstructed at the slice thicknesses of 2 mm, 3 mm, 5 mm, and 10 mm. For each image set of slice thicknesses, the treatment plan was generated by using a commercial RTP system. Treatment sites were on the breast and H&N (Head & Neck). The dose distributions of 3‐axes (sagittal, transverse, and coronal) were exported at the iso‐center. The image set of 2 mm slice thickness was used as the reference. Dose differences were evaluated in terms of gamma‐index at various criteria. Results: Passing ratio of evaluating gamma‐index decreases with increasing slice thickness of images. The ratios of 3 mm, 5 mm, and 10 mm thicknesses were 96.3%, 95.9%, and 95.3%, respectively on coronal images of H&N and were 100%, 99.9%, and 93.8% on those of breast. On transversal images of both H&N and breast, the ratios were less than 90%. Conclusion: The slice thickness of CTimages affects dose distributions on treatment plans. There might be little dose difference in treatment plans based on CTimage sets of 2 to 5 mm slice thickness. However, such difference becomes considerable in the organ having a steep slope and/or high dose gradients.

Dosimetric variation due to CT inter-slice spacing in four-dimensional carbon beam lung therapy

When CT data with thick slice thickness are used in treatment planning, geometrical uncertainty may induce dosimetric errors. We evaluated carbon ion dose variations due to different CT slice thicknesses using a four-dimensional (4D) carbon ion beam dose calculation, and compared results between ungated and gated respiratory strategies. Seven lung patients were scanned in 4D mode with a 0.5 mm slice thickness using a 256-multi-slice CT scanner. CT images were averaged with various numbers of images to simulate reconstructed images with various slice thicknesses (0.5–5.0 mm). Two scenarios were studied (respiratory-ungated and -gated strategies). Range compensators were designed for each of the CT volumes with coarse inter-slice spacing to cover the internal target volume (ITV), as defined from 4DCT. Carbon ion dose distribution was computed for each resulting ITV on the 0.5 mm slice 4DCT data. The accumulated dose distribution was then calculated using deformable registration for 4D dose assessment. The magnitude of over- and under-dosage was found to be larger with the use of range compensators designed with a coarser inter-slice spacing than those obtained with a 0.5 mm slice thickness. Although no under-dosage was observed within the clinical target volume (CTV) region, D95 remained at over 97% of the prescribed dose for the ungated strategy and 95% for the gated strategy for all slice thicknesses. An inter-slice spacing of less than 3 mm may be able to minimize dose variation between the ungated and gated strategies. Although volumes with increased inter-slice spacing may reduce geometrical accuracy at a certain respiratory phase, this does not significantly affect delivery of the accumulated dose to the target during the treatment course.