CC Directions Research Articles

Assessment of Setup Errors in Gynecological Malignancies Treated With Radiotherapy Using Onboard Imaging.

Introduction Radiotherapy plays a vital role in the management of gynecological malignancies. However, maintaining patient position poses a challenge during daily radiotherapy treatment of these patients. This study identifies and calculates setup errors in interfraction radiotherapy and optimum clinical target volume-planning target volume (CTV-PTV) margins in patients with gynecological malignancies. Material and methods A total of 38 patients with gynecological malignancies were included in the study. They were treated with a dose of 50 Gy in 25 fractions for five weeks, followed by brachytherapy. All patients were immobilized using a 4-point thermoplastic cast. Anteroposterior and lateral images were taken thrice weekly for five weeks. Setup verification was done using kilovoltage images obtained using Varian On-board Imager (Varian Medical System, Inc., Palo Alto, CA). Manual matching was done utilizing bony landmarks such as the widest portion of the pelvic brim, anterior border of S1 vertebrae, and pubic symphysis in the X, Y, and Z axes, respectively. Results A total of 1140 images were taken. The individual systematic errors ranged from -0.24 to 0.17 cm (LR), -0.15 to 0.19 cm (AP) and -0.36 to 0.29 cm (CC) while the individual random errors ranged from 0.04 to 0.36 cm (LR), 0.06 to 0.33 cm (AP) and 0.10 to 0.29 cm (CC). The calculated CTV-PTV margins in LR, AP and CC directions were 0.17, 0.18, and 0.25 cm (ICRU-62); 0.28, 0.31 and 0.47 cm in LR, AP and CC directions (Stroom's), and 0.32, 0.36 and 0.55 cm (Van Herk) respectively. Conclusion Based on this study, the calculated CTV-PTV margin is 6 mm in gynecological malignancies, and the present protocol of 7 mm of PTV margin is optimum.

Just a matter of indexes: symmetry in walking

Gait symmetry is related to differences between the left and right side of the body and provides information about functional limitation due to pathology/injury [1]. Many metrics exist to quantify gait symmetry, differing for the considered data source and mathematical approach [1]. Although the body center of mass (BCoM) is the point that globally represents an individual’s movement, very few studies have focused on the quantification of gait symmetry from its trajectory (Simmetry Index, SI) [2]. Conversely, the use of the Harmonic Ratio and of its improved version (iHR) has gained great popularity in recent years [3], being extracted from trunk 3D accelerations easily measured by wearable inertial devices. The relationship between the SI and iHR, however, has not been quantified yet. The aim of this study is thus to investigate this relationship in healthy adults during walking at different speeds. Five healthy adults (2 women, 1.69±0.08 m, 29.8±2.8 y) participates to this study (CAR 101/2021). They walked for 60 s on a treadmill at six different speeds (from 0.55 to 1.95 m/s with step of 0.28 m/s). The trajectories of 18 markers located on the main joint centers [4] were collected by an optoelectronic system (Vicon, 200 Hz). Simultaneously, lower trunk 3D linear accelerations were measured by an inertial sensor (APDM Opal, 200 Hz). The 3D trajectory of the BCoM was estimated from marker trajectories as the weighted mean of an 11–segment model [4]. Gait symmetry was then quantified for each stride and each direction (antero-posterior AP, medio-lateral ML, cranio-caudal CC) using two indices: the Symmetry Index (SI) from the BCoM trajectory [2] and the iHR from the trunk accelerations [3]. Both iHR and SI are based on the harmonic content of the relevant signal and range from 100% (perfect symmetry) to 0% (total asymmetry). To assess the effect of walking speed on SI and iHR values, a two-way repeated measure ANOVA was performed after verifying for normal data distribution. A total number of 1572 strides were analyzed, with a minimum of 288 strides for each participant. SI and iHR significantly differed in all three directions for each velocity (p<0.01), except at 1.39, 1.67 and 1.95 m/s in the ML direction (Fig. 1). When considering the effect of gait speed on symmetry values, the latter significantly changed only for the AP and CC components (p<0.0001) ( Fig. 1 ). SI and iHR may provide substantial different information about the level of gait symmetry, with iHR characterized by greater values in AP and CC directions and smaller values in ML direction. Interestingly, gait symmetry increases with speed, at least up to 1.67 m/s, in AP and CC directions. This is not the case for the ML component: whereas SI slightly decreases with speed (p<0.05), the high variability of iHR, already discussed in the literature [4], does not allow to identify any clear trend. It is suggested to carefully describe the methodology used to assess gait symmetry, as the type of data and the method used may strongly influence the final outcome and, thus, its clinical interpretation.

Thulium-doped fiber laser with bidirectional output in a ring laser cavity

A thulium-doped fiber laser (TDFL) with bidirectional output was proposed and demonstrated herein. Clockwise (CW) and counter-clockwise (CCW) lasing output with different lasing wavelengths was realized by two uniform fiber Bragg gratings (UFBGs) and two optical circulators. The UFBGs located at 1941.238 and 2049.325 nm were incorporated in the laser cavity to act as the wavelength-selective element. Switchable operation was realized through tuning the gain and loss of the circulating light by applying curvature on a section of the single-mode fiber. When the TDFL was operated in the CW direction, single-wavelength output with a center wavelength, optical signal-to-noise ratio (OSNR), and maximal output power of 2049.335 nm, 33.175 dB, and 280.6 mW was obtained, and the fluctuation of the center wavelength and the output power were less than 0.041 nm and 1.400 dB within an observation time of 60 min. When the TDFL was operated in the CCW direction, single-wavelength output with a center wavelength, OSNR, and maximal output power of 1941.312 nm, 37.108 dB, and 486 mW could be obtained, and the maximal fluctuation of the center wavelength and the output power were 0.128 nm and 0.832 dB within an observation period of 60 min. In addition, the proposed TDFL can operate in both CC and CCW directions simultaneously with different output wavelengths. The potential application of a TDFL strain sensor with sensitivities of 1.33 and 1.22 pm/με in the CW and CCW directions was also demonstrated and the proposed TDFL may also well suited for use in medical and free-space optical communication systems.

An Evaluation of Total Internal Motions of Locally Advanced Pancreatic Cancer during SABR Using Calypso® Extracranial Tracking, and Its Possible Clinical Impact on Motion Management.

(1) Background: the aims of this study were to determine the total extent of pancreatic cancer’s internal motions, using Calypso® extracranial tracking, and to indicate possible clinical advantages of continuous intrafractional fiducial-based tumor motion tracking during SABR. (2) Methods: thirty-four patients were treated with SABR for LAPC using Calypso® for motion management. Planning MSCTs in FB and DBH, and 4D-CTs were performed. Using data from Calypso® and 4D-CTs, the movements of the lesions in the CC, AP and LR directions, as well as the volumes of the 4D-CT-based ITV and the volumes of the Calypso®-based ITV were compared. (3) Results: significantly larger medians of tumor excursions were found with Calypso® than with 4D-CT: CC: 29 mm (p < 0.001); AP: 14 mm (p < 0.001) and LR: 11 mm (p < 0.039). The median volume of the Calypso®-based ITV was significantly larger than that of the 4D-CT based ITV (p < 0.001). (4) Conclusion: beside known respiratory-induced internal motions, pancreatic cancer seems to have significant additional motions which should be considered during respiratory motion management. Only direct and continuous intrafractional fiducial-based motion tracking seems to provide complete coverage of the target lesion with the prescribed isodose, which could allow for safe tumor dose escalation.

Patient-specific tumor and respiratory monitoring phantom design for quality controls of stereotactic ablative body radiotherapy in lung cancer cases.

The design, production and adaptability to clinical routine of a patient-specific tumor and respiratory monitoring phantom (TRMP) was investigated using 3D printer technology. TRMP and GTV modelling were done using 4D-CT images of the inhalation phase. The model was converted to STL (Stereolithography) format and printed with STH (Strong Herbal) biopolymer with HU (Hounsfield Unit) suitable for imaging purposes. The assembly of TRMP motorized parts and mechanical equipment has been completed and made suitable for clinical use. In the first part of the study, the deviations of radio-opaque markers attached to the TRMP sternum to perform mechanical quality control tests and T1-7 costal vertebrae in CC, AP, and LAT directions were evaluated. In the second part of the study, in order to evaluate the usability of the TRMP in quality assurance (QA), point dose measurements with BeO OSL dosimetry and EBT3 gafchromic film measurements were taken in Trilogy® radiotherapy accelerator and CyberKnife® robotic radiosurgery accelerator. In this study, we present a highly flexible TMRP capable of performing independent internal and external motions. TRMP was successfully tested in different treatment accelerators, both mechanically and dosimetrically.

Margin Reduction by Virtual Couch Shift in Prostate SBRT on a 1.5T MR-Linac

We treat localized prostate cancer on a 1.5T MR-Linac using plan re-optimization to the anatomy of the day in each fraction. We investigated the efficacy of a 2-stage, clinically available workflow to reduce intrafraction motion during the 40-minute patient on-couch time and potential margin reduction. Patients with intermediate risk prostate cancer (NCNN classification) were treated with 5 x 7.25 Gy on an MRL. Each fraction, an initial T2 weighed MR scan (‘INI’, 2 min scan time) was acquired, and pre-treatment contours were propagated and manually adapted to this scan, thus correcting for prostate rotation and organ deformation. Next, dose re-optimization (7 beam IMRT) was applied (Monaco treatment planning system), with a 5 mm CTV-PTV margin. Because the average time between the initial scan and beam on is approximately 30 minutes, prostate intrafraction motion can be considerable. Therefore, during the last minutes of this process, a position verification (‘PV’) MR scan was obtained to determine this motion. In our initial treatments (group 1, 26 patients) a virtual couch shift (VCS) was applied if part of the prostate was outside the PTV, (i.e., a prostate shift > 5 mm). The VCS involved a translation of the planned dose according to the PV to INI scan registration of the prostate, using a fast re-optimization method. In the subsequent 25 patients (group 2), the VCS was always applied, also for shifts below 5 mm, to further reduce the impact of pre-delivery intrafraction motion. Next, the plan was delivered in typically 7 minutes, while 3D cine-MR dynamics were acquired simultaneously (balanced turbo field echo). Each 3D dynamic spanned 9.4 seconds, and consisted of 448x448x45 voxels of 1.0x1.0x2.2mm. The prostate in all dynamics was automatically registered to INI and PV scans using a previously published method (D M de Muinck Keizer et al 2019 Phys. Med. Biol. 64 235008). In group 1, 24% of the patients required a VCS. In this group, intrafraction motion shifts relative to the treatment plan anatomy during dose delivery were 0.9±0.9 mm, 1.8±1.5 mm and 1.6±1.4 mm for respectively LR, AP and CC directions (mean ± SD of absolute values). In group 2, the VCS reduced the corresponding values to 0.4±0.4 mm, 1.4±1.2 mm and 1.3±1.3 mm. The average time between PV scan and beam on was 5 min, of which the VCS calculation time was below 1 min. An analysis of the required PTV margins in the corresponding directions showed that in group 2 margins could be safely reduced to 2, 4 and 4 mm in the LR, CC and AP directions. Daily VCS shortly before dose delivery allowed for a PTV margin reduction below conventionally applied 5 mm margins. However, due to the continuous and stochastical nature of intrafraction motion, margin reduction below 4 mm would require intrafraction plan adaptation. Presently, we develop such methods based on the high quality of the 1.5T 3D cineMR.

Evaluation of interbreath-hold lung tumor position reproducibility with vector volume histogram using the breath-hold technique

Tumor geometric reproducibility for lung stereotactic body radiotherapy (SBRT) is an important issue in the breath-hold (BH) technique. We investigated the inter-BH reproducibility of the tumor position in expiratory BH using our proposed vector volume histogram (VVH) method. Subjects comprising 14 patients with lung cancer who were treated with lung SBRT under expiratory BH conditions were monitored by the Abches system. Multiple computed tomography (CT) scans were performed to evaluate the inter-BH reproducibility of the tumor position at the expiratory BH in the simulation session. Gross tumor volume was delineated by a physician. Deformable image registration was used to deform the images from the 3 expiratory BH-CTs to the treatment planning expiratory BH-CT. To evaluate the inter-BH reproducibility of the tumor positions, we measured the largest motion extent within the organ of 3 dimensions (left-right, LR; anterior-posterior, AP; cranio-caudal, CC) and a 3D vector using the VVH method. The average and standard deviations of the inter-BH reproducibility of the tumor position in the LR, AP, and CC directions, and the 3D vector were 1.7 ± 0.5, 2.0 ± 0.7, 2.1 ± 0.7, and 2.7 ± 0.7 mm, respectively. Ten patients exhibited inter-BH displacements of the lung tumor >3 mm in the 3D vector. No displacement >5 mm was observed in any direction for all patients. Our study indicated that the inter-BH variation of the tumor position was small for lung cancer patients, using the Abches system and the VVH method.

Accuracy of liver ablation zone prediction in a single 2450 MHz 100 Watt generator model microwave ablation system: An in human study

PurposeTo compare manufacturer provided predictions and realized ablation dimensions in the liver using one 2450MHz 100 Watt generator model microwave ablation (MWA) system. Materials and methodBetween 1/1/2015 and 2/1/2018, MWAs were performed in 86 patients who underwent a total of 103 MWAs with a single MWA system. There were 64 men and 22 women with a mean age of 63.9±9.9 (SD) years (range: 30–88 years). Demographic, procedural, and outcomes data was recorded. The manufacturer predicted ablation zone sizes in three dimensions (anterior-posterior [AP], transverse [TR], and cranial caudal [CC]) were recorded and then compared to the actual ablation zone sizes at one month follow-up imaging. ResultsMWAs were most commonly performed to treat hepatocellular carcinoma (92/103, 89.3%). Dividing the actual ablation size by the manufacturer prediction in the AP, TR, and CC directions resulted in a mean of 88.3±20.6 (SD) % (range: 33.3–156.4%), 80.2±26.5 (SD) % (range: 29.6–182.9%), and 86.7±25.1 (SD) % (range: 37–186.1%), respectively. The realized AP direction was statistically closer to the manufacturer prediction than the TR (P<0.01). Ablation Watt setting of 100 Watts resulted in more accurate predictions than the 75 or 45 Watt settings in the AP direction (P=0.03). ConclusionsThis 2450MHz 100 Watt generator MWA system manufacturer provided model fairly accurately predicts ablation zone dimensions, but tends to over predict realized dimensions in this mainly hepatocellular carcinoma, and therefore cirrhotic, cohort. The TR is the most inaccurately predicted dimension and manufacturer predictions appear to be best in the 100W setting, important aspects for interventionalists to consider during ablation planning and execution.

Target Site Analysis of Set-up and Positioning Accuracy for Head and Neck Stereotactic Ablative Radiation Therapy (SABR)

Stereotactic ablative radiotherapy (SABR) is utilized to address recurrent and/or previously irradiated head and neck cancers (HNC) and requires high set-up accuracy. We previously demonstrated excellent results with our custom cushion/mask/bite-block (CMB) immobilization system. Here, we compare set-up and positioning accuracy across different head and neck tumor locations to further optimize PTV margins for SABR treatment planning and delivery. We analyzed 375 treatment sessions across 73 patients with recurrent HNC treated with SABR using our custom CMB immobilization system. Initial set-up was performed using ExacTrac infrared tracking, and interfractional error was determined by comparing initial X-ray to the treatment plan. Patients were then shifted using ExacTrac and the residual set-up error was measured with repeat X-ray verification. Next, cone-beam CT (CBCT) was obtained and compared to ExacTrac for 3-dimensional positioning agreement. Finally, intrafractional positioning errors were measured by taking pre-beam X-ray. The data was stratified based on treatment site: paranasal sinus, nasopharynx, skull base (SB), cervical and parotid nodes (Neck), and oropharynx, oral cavity, larynx (Mucosal). The median dose and volume were 45Gy in 5 fractions (range: 27-47.5Gy, 3-5 fractions) and 17.12cm3 (range: 1.47-69.20cm3). There were no significant differences in the mean age, treatment volume, or monitor units (MU) between groups. The initial translational set-up errors were -0.3 ± 1.1mm, 0.3 ± 1.5mm, and -0.2 ± 1.4mm in the AP, CC, and LR directions, respectively. Site-specific mean 3D interfractional vectors were 2.2 ± 1.2mm (SB; P<0.00001 vs others), 2.9 ± 1.4mm (Neck), and 2.5 ± 1.7mm (Mucosal). Overall, the mean ExacTrac-CBCT agreement was between -0.1 and 0.1mm in translational and rotation directions, and overall residual errors were -0.1 ± 0.2mm, 0.0 ± 0.2mm, and 0.0 ± 0.2mm in the AP, CC, and LR directions, respectively. Site-specific mean 3D residual vectors were 0.4 ± 0.3mm (SB), 0.7 ± 0.7mm (Neck), and 0.5 ± 0.3mm (Mucosal), with a residual 3D vector >1.0mm occurring in 5.4% of SB patients compared to 26.1% (Neck) and 26.7% (Mucosal)(P=0.013). Intrafractional errors showed a mean 3D vector of 0.7 ± 0.5mm (SB; P<0.05 vs others), 1.3 ± 0.9mm (Neck), and 1.4 ± 1.5mm (Mucosal). An intrafractional 3D vector >1.0mm occurred in 16.2% of SB patients compared to 52.2% (Neck) and 46.7% (Mucosal) (P<0.0001). The calculated PTV margin for each site was ≤1.3mm (Skull Base), ≤2.2mm (Neck), and ≤3.4mm (Mucosal). In a large comparison of set-up and positioning accuracy for head and neck SABR, PTV treatment margins can be optimized depending on the treatment site. The error magnitude is significantly smaller for skull base targets and a 2-mm PTV margin may be sufficient. For mucosal targets, a 4-mm PTV margin may be necessary to account for internal organ motion.

Feasibility of MR-Generated Synthetic CT Images for Planning Stereotactic Radiosurgery of Brain Metastases

Frameless Linac-based stereotactic radiosurgery (SRS) treatment planning typically involves spatial registration of MR and CT images. A solely MR-guided workflow would eliminate the burdens of CT scanning and associated time as well as alignment errors, but would require dose calculations and patient positioning based on MR-derived information. The feasibility of MR-guided SRS of brain metastases was investigated. Specifically, it was hypothesized that synthetic CT images (MRCTs) could be used to generate plans and reference images for positioning that would not differ from SRS using a traditional CT-guided technique. MR and CT images were acquired for 19 patients undergoing SRS for brain metastases. Following rigid image alignment, PTV and OAR contours were delineated. MRCT image volumes were generated using a voxel-based probabilistic classification technique and aligned rigidly to their respective CT scans. The CT structures were transferred onto the MRCT volumes. CT-based plans were created with the intent to deliver 99.5% of the prescription dose to the PTV and avoid excess doses to the OARs. The fluences from the CT plans then were used for dose calculations on a “maskless” CT plan (in which the attenuation of the immobilization device was removed) and MRCT images. PTV D99.5 and OAR D0.1 cc values were compared. OARs (brainstem, optic nerves/chiasm, eyes, lenses, cochleae) typically defined on CT were drawn on the source MR (T1- and T2-weighted image volumes) images and compared to the original CT volumes. Cone beam CT (CBCT) volumes used for positioning at treatment were aligned to the MRCT and CT volumes, and resulting transforms compared. A total of 43 PTVs were studied with a mean prescription dose of 21.1 Gy (range 12 to 30 Gy). The mean difference in PTV D99.5 between the CT versus maskless CT was 0.0 % (range -0.0 to 0.1 %) and CT versus MRCT was 0.0 % (range -0.1 to 0.0 %) of the prescription doses. Dose metrics for a total of 185 OARs were calculated. OAR D0.1 cc difference between the CT versus maskless CT was 0.0 Gy (range -0.1 to 0.0 Gy) and CT versus MRCT was 0.0 Gy (range -0.2 to 0.7 Gy). For OAR volumes larger than 1 cc (n=64), the mean Dice coefficient between CT and MR structures was 0.82 (range 0.17 to 0.93). Mean differences in alignment between the CBCT volumes and MRCT and CT volumes resulted in isocenter positioning differences of 0.0 mm (range -0.2 to 0.1 mm), 0.0 mm (range -0.5 to 0.4 mm), and 0.1 mm (range -0.2 to 0.5 mm) in the LR, AP, and CC directions. The accuracy of delineation, calculated doses, and patient positioning indicate that MR-guided frameless SRS is feasible. Future investigations will explore generation of reference image volumes for alignment that combine MR and synthesized CT contrasts. Supported by NIH R01EB016079.

SU‐G‐BRA‐12: Development of An Intra‐Fractional Motion Tracking and Dose Reconstruction System for Adaptive Stereotactic Body Radiation Therapy in High‐Risk Prostate Cancer

Purpose:A clinical trial on stereotactic body radiation therapy (SBRT) for high‐risk prostate cancer is undergoing at our institution. In addition to escalating dose to the prostate, we have increased dose to intra‐prostatic lesions. Intra‐fractional prostate motion deteriorates well planned radiation dose, especially for the small intra‐prostatic lesions. To solve this problem, we have developed a motion tracking and 4D dose‐reconstruction system to facilitate adaptive re‐planning.Methods:Patients in the clinical trial were treated with VMAT using four arcs and 10 FFF beam. KV triggered x‐ray projections were taken every 3 sec during delivery to acquire 2D projections of 3D anatomy at the direction orthogonal to the therapeutic beam. Each patient had three implanted prostate markers. Our developed system first determined 2D projection locations of these markers and then 3D prostate translation and rotation via 2D/3D registration of the markers. Using delivery log files, our GPU‐based Monte Carlo tool (goMC) reconstructed dose corresponding to each triggered image. The calculated 4D dose distributions were further aggregated to yield the delivered dose.Results:We first tested each module in our system. MC dose engine were commissioned to our treatment planning system with dose difference of &lt;0.5%. For motion tracking, 1789 kV projections from 7 patients were acquired. The 2D marker location error was &lt;1 mm. For 3D motion tracking, root mean square (RMS) errors along LR, AP, and CC directions were 0.26mm, 0.36mm, and 0.01mm respectively in simulation studies and 1.99mm, 1.37mm, and 0.22mm in phantom studies. We also tested the entire system workflow. Our system was able to reconstruct delivered dose.Conclusion:We have developed a functional intra‐fractional motion tracking and 4D dose re‐construction system to support our clinical trial on adaptive high‐risk prostate cancer SBRT. Comprehensive evaluations have shown the capability and accuracy of our system.

Improved setup and positioning accuracy using a three-point customized cushion/mask/bite-block immobilization system for stereotactic reirradiation of head and neck cancer.

The purpose of this study was to investigate the setup and positioning uncertainty of a custom cushion/mask/bite‐block (CMB) immobilization system and determine PTV margin for image‐guided head and neck stereotactic ablative radiotherapy (HN‐SABR). We analyzed 105 treatment sessions among 21 patients treated with HN‐SABR for recurrent head and neck cancers using a custom CMB immobilization system. Initial patient setup was performed using the ExacTrac infrared (IR) tracking system and initial setup errors were based on comparison of ExacTrac IR tracking system to corrected online ExacTrac X‐rays images registered to treatment plans. Residual setup errors were determined using repeat verification X‐ray. The online ExacTrac corrections were compared to cone‐beam CT (CBCT) before treatment to assess agreement. Intrafractional positioning errors were determined using prebeam X‐rays. The systematic and random errors were analyzed. The initial translational setup errors were −0.8±1.3 mm, −0.8±1.6 mm, and 0.3±1.9 mm in AP, CC, and LR directions, respectively, with a three‐dimensional (3D) vector of 2.7±1.4 mm. The initial rotational errors were up to 2.4° if 6D couch is not available. CBCT agreed with ExacTrac X‐ray images to within 2 mm and 2.5°. The intrafractional uncertainties were 0.1±0.6 mm, 0.1±0.6 mm, and 0.2±0.5 mm in AP, CC, and LR directions, respectively, and 0.0∘±0.5°, 0.0∘±0.6°, and −0.1∘±0.4∘ in yaw, roll, and pitch direction, respectively. The translational vector was 0.9±0.6 mm. The calculated PTV margins mPTV(90,95) were within 1.6 mm when using image guidance for online setup correction. The use of image guidance for online setup correction, in combination with our customized CMB device, highly restricted target motion during treatments and provided robust immobilization to ensure minimum dose of 95% to target volume with 2.0 mm PTV margin for HN‐SABR.PACS number(s): 87.55.ne

Evaluation of the motion of lung tumors during stereotactic body radiation therapy (SBRT) with four-dimensional computed tomography (4DCT) using real-time tumor-tracking radiotherapy system (RTRT)

PurposeWe investigated the usefulness of four-dimensional computed tomography (4DCT) performed before stereotactic body radiation therapy (SBRT) in determining the internal margins for peripheral lung tumors. Methods and MaterialsThe amplitude of the movement of a fiducial marker near a lung tumor measured using the maximum intensity projection (MIP) method in 4DCT imaging was acquired before the SBRT (AmpCT) and compared with the mean amplitude of the marker movement during SBRT (Ampmean) and with the maximum amplitude of the marker movement during SBRT (Ampmax) using a real-time tumor-tracking radiotherapy (RTRT) system with 22 patients. ResultsThere were no significant differences between the means of the Ampmean and the means of the AmpCT in all directions (LR, P = 0.45; CC, P = 0.80; AP, P = 0.65). The means of the Ampmax were significantly larger than the means of the AmpCT in all directions (LR, P < 0.01; CC, P = 0.03; AP, P < 0.01). In the lower lobe, the mean difference of the AmpCT from the mean of the Ampmax was 5.7 ± 8.0 mm, 12.5 ± 16.7 mm, and 6.8 ± 8.5 mm in the LR, CC, and AP directions, respectively. ConclusionsAcquiring 4DCT MIP images before the SBRT treatment is useful to establish the mean amplitude for a patient during SBRT but it underestimates the maximum amplitude during actual SBRT. Caution must be paid to determine the margin with the 4DCT especially for tumors at the lower lobe where it is of the potentially greatest benefit.

A comprehensive evaluation of treatment accuracy, including end-to-end tests and clinical data, applied to intracranial stereotactic radiotherapy

Background and purposeA methodology is presented to quantify the uncertainty associated with linear accelerator-based frameless intracranial stereotactic radiotherapy (SRT) combining end-to-end phantom tests and clinical data. Methods and materialsThe following steps of the SRT chain were analysed: planning computed tomography (CT) and magnetic resonance (MR) scans registration, target volume delineation, CT and cone beam CT (CBCT) registration and intrafraction-patient displacement. The overall accuracy was established with an end-to-end test. The measured uncertainties were combined, deriving the total systematic (ΣT) and random (σT) error components, to estimate the GTV-PTV margin. ResultsThe uncertainty in the MR-CT registration was on average 0.40mm (averaged over AP, CC and LR directions). Rotational variations were smaller than 0.5° in all directions.Interobser variation in GTV delineation was on average 0.29mm.The uncertainty in the CBCT-CT registration was on average 0.15mm. Again, rotational variations were smaller than 0.5° in all directions.The systematic and random intrafraction displacement errors were on average 0.55mm and 0.45mm, respectively.The systematic and random positional errors from the end-to-end test were on average 0.49mm and 0.53mm, respectively.Combining these uncertainties resulted in an average ΣT=0.9mm and σT=0.7mm and an average GTV-PTV margin of 2.8mm. ConclusionThis comprehensive methodology including end-to-end tests enabled a GTV-PTV margin calculation considering all sources of uncertainties. This generic method can also be used for other treatment sites.

SU‐C‐141‐06: Registration Accuracy with Four‐Dimensional Cone‐Beam CT for Lung Cancer Treatment

Purpose: A four‐dimensional (4D) cone‐beam CT (CBCT) is widely implemented in the image‐guided radiation therapy (IGRT) for lung cancer treatment. Recently, we established a verifying system using in‐treatment 4D CBCT acquired during Volumetric Modulated Arc Therapy (VMAT). With this system, we assess the registration accuracy with pre‐treatment 4D CBCT for IGRT. Methods: Fifteen lung cancer patients treated with single‐arc VMAT were selected in this study. The breathing condition of all patients was constrained with a stereotactic body frame (ELEKTA) accompanied by abdominal compression plate. The gross tumor volumes were contoured on 10 CT datasets from 4D CT (TOSHIBA and ANZAI) and merged into internal target volumes (ITVs). The planning target volume (PTV) margin was isotropically taken as 5 mm. The patient registration was performed by an Elekta x‐ray volume imaging functionality version 4.5 (XVI ver.4.5), which allows to use 4D CBCT respiratory sorting for registration. The VMAT treatment was performed with simultaneous acquisition of the kV projection images. The in‐treatment 4D CBCT was reconstructed by in‐house software. The reconstructed images were exported to a treatment planning system. To assess set‐up errors and respiratory motion, the displacement of tumors between the planning ITV and the shadow of the tumor in in‐treatment 4D CBCT images was measured in CC, left‐right (LR), and anterior‐posterior (AP) directions. Results: The average displacements between the ITV and the actual tumor location during treatment were 0.24 +/− 0.45, 0.19 +/− 0.46, and 0.42 +/− 0.43 mm for the CC, LR, and AP directions, respectively. The discrepancy in each phase did not exceed 3 mm in any direction. Conclusion: With in‐treatment 4D CBCT, which is a direct method for quantitatively assessing the intrafractional location, we confirmed that the required PTV margins were 3 mm when the registration was performed using pre‐treatment 4D CBCT. This work was supported by JSPS KAKENHI 22791176.

SU-E-J-128: Liver Motion during CBCT-Guided SBRT.

Purpose of this study was to investigate the motion characteristics of liver during SBRT, based on fiducial markers tracked with X-ray projections of CBCT scans, taken immediately prior to each treatment. Purpose of this study was to investigate the motion characteristics of liver during SBRT, based on fiducial markers tracked with X-ray projections of CBCT scans, taken immediately prior to each treatment. The range of motion for twenty patients were 3.00 ± 2.04 mm, 5.08 ± 3.12 mm and 17.93 ± 5.11 mm in the planning 4DCT, and 2.77 ± 1.6 mm, 5.29 ± 3.10 mm and 16.46 ± 5.69 mm in the treatment CBCT, for LR, AP, and CC directions, respectively. The range of respiratory period was 3.94 ± 0.65 and 4.18 ± 0.75 seconds during 4DCT simulation and CBCT scans, respectively. We found that breathing-induced AP and CC motions are highly correlated. The absolute difference in motion magnitude between the markers, had a positive correlation with absolute distance between the markers (R2=0.69, linear-fit). The inter-fractional gating window varied significantly for some patients, with the largest having 29.5-56.4% range between fractions. This study analyzed the liver motion characteristics of 20 patients undergoing SBRT. A large variation in motion was observed, inter- and intra-fractionally, and that as the distance between the markers increased, the difference in the absolute range of motion also increased. This suggests that marker(s) in closest proximity to the target be used. This project was supported by Varian Medical Systems.

Comparison of the planning target volume based on three-dimensional CT and four-dimensional CT images of non-small-cell lung cancer

Background and purpose To compare positional and volumetric differences of planning target volumes (PTVs) based on axial three-dimensional CT (3DCT) and four-dimensional CT (4DCT) for the primary tumor of non-small cell lung cancer (NSCLC). Materials and methods Twenty-eight patients with NSCLC underwent 3DCT and 4DCT scans of the thorax during normal free breathing. PTV vector was defined on 3DCT using the individual tumor motion vector measured by 4DCT accounting for tumor motion; PTV 4D was defined on all phases of 4DCT images. In addition, a 7 mm margin for microscopic disease and a 3 mm setup margin were used for above PTVs, respectively. The differences in target position, volume and coverage between PTV vector and PTV 4D were evaluated for tumors in different lobes, respectively. Results The median motion vector for tumors located in the upper lobe (group A) and in the middle lower lobe (group B) was 2.8 and 7 mm, respectively. The mean centroid shifts between PTV vector and PTV 4D in the LR, AP and CC directions for group A and B were close to zero. The median size ratio of PTV 4D to PTV vector was 0.75 and 0.52 for group A and B. The motion vector showed a significant correlation to the ratio of PTV 4D to PTV vector for group A and B ( p = 0.008 and 0.003). The median DI of PTV vector in PTV 4D was 69.19% for group A and 51.60% for group B. The median DI of PTV 4D in PTV vector was 98.99% for group A and 99.94% for group B. Conclusion It is necessary to expand the internal margin isotropically in a single direction for 3DCT treatment planning due to the uncertainty of the 3DCT-based target position. The 3DCT-based PTV using individual margins provides a good coverage of the 4DCT-based PTV, meanwhile encompasses relatively large normal tissues, especially for middle and lower lobe tumors. We should be cautious about the use of the individual PTV derived from 3DCT in treatment planning.

Geometric Uncertainties in High Precision Radiotherapy for Brain Tumors: A Prospective Study

62 ( P + 0.7 s) Stroom’s (2 P + 0.7 s) and Van Herk’s (2.5 P + 0.7 s) formula, ( P = systematic error and s = random error). Results:With GTC frame, mean displacement in AP axis is 0.15 mm (± 0.38), CC axis -0.17 mm (± 0.42) and ML axis 0.38 mm (± 0.56). The mean vector displacement for the population is 1.03 mm (± 0.34). The errors were within ±2 mm in ML direction in 99.6% cases, in 99.2% cases in AP and 97.6% cases in CC direction. The random error was 0.73 mm in CC, 1.86 mm in AP and 1.41 in ML axes and it was normally distributed. The margins (in mm) in ML CC and AP directions were 1.55, 0.93, 1.68 (ICRU 62), 2.1, 1.35, 2.06 (Stroom’s) and 2.39, 1.56 and 2.25 (van Herk’s). In BrainLAB, mean errors in ML, CC and AP directions was 0.14 cm (SD 0.27), -0.044cm (SD 0.151) and 0.059 cm (SD 0.150), respectively. The systematic errors in ML, SI and AP directions were 0.03 cm, 0.052 cm and 0.124 cm, respectively. The random errors in ML, SI and AP directions were 0.24 cm, 0.143 cm and 0.106 cm, respectively. The margins (in cm) in ML CC and AP directions were 0.19, 0.15, 0.19 (ICRU 62), 0.23, 0.20, 0.32 (Stroom’s) and 0.24, 0.23, and 0.38 (van Herk’s). In ray cast, the mean error in ML, SI, and AP directions was 0.15 cm (SD 0.14), -0.015 cm (SD 0.326), and 0.15 cm (SD 0.1668). The systematic errors in ML, SI and AP directions were 0.133 cm, 0.2831 cm and 0.0887 cm while random errors in the same directions were 0.160 cm, 0.1784 cm and 0.1429 cm respectively. The margins (in cm) in ML CC and AP directions were 0.24, 0.40, 0.18 (ICRU 62), 0.37, 0.69, 0.27 (Stroom’s) and 0.44, 0.83 and 0.32 (van Herk’s). Conclusions: Immobilization with GTC is superior to BrainLab device for brain. In 97-99% cases the error lies between ± 2 mm. Therefore, a margin of 2 mm around the tumor seems to be adequate. BrainLAB immobilization was better than thermoplastic ray cast in reducing set up errors in radiotherapy for brain tumors. Author Disclosure: S. Das, None; R. Isiah, None; R. B, None; P. S, None; S. John, None.

A Daily Cone Beam CT Pre and Post Treatment Study for Prostate Cancer RT: Relation between Treatment Time and Dose Uncertainty

Results: All patients consented to the Phase II study had respiratory motion evaluated for 10-20 seconds on lateral and anterior fluoroscopy using soft-tissue or a biliary stent as a surrogate for the pancreas tumor. Five hundred ninety-one at-treatment frames were assessed for 6 treatment beams over 73 fractions (mean 24 fractions/patient) acquired at 2 frames/second. Beams were evaluated if it was possible to (1) visualize the surrogate and (2) acquire at minimum 4 image frames. In 12% of fractions, degraded image quality, due to abdominal gas, prevented clear visualization of surrogates on $1 beam. The mean (range) surrogate motion seen at planning was 1.2 cm (0.9, 1.8), and 0.7 cm (0.5, 0.8) in the CC and AP directions. RL motion of 0.3cm was evaluable in only one patient. The mean (range) at-treatment motion was 0.4 cm (0.2, 0.8), 0.2 cm (0.1, 0.3) and 0.2 cm (0.1, 0.4) in the CC , AP and RL directions, respectively, based on MV cine exit images. Intra-patient inter-fraction motion varied minimally with SDs \2 mm in all patients for all directions. This strategy continues to be used clinically. Conclusions: In this study, fluoroscopic soft-tissue and stent motion seen at treatment planning over-estimated at-treatment respiratory motion in all patients seen on MV exit beams. This preliminary work corresponds well with reported observations from fiducial marker studies. However further studies are required comparing surrogate motion to volumetric pancreas motion from 4DCT or fast MRI and respiratory correlated cone beam CT in larger patient cohorts. In the absence of volumetric motion imaging, fluoroscopy and cine-MV exit imaging, is an inexpensive, non-invasive method that may suffice to estimate/confirm respiratory motion, and may be suitable for quality control in multi-centre trials.

SU‐FF‐T‐252: Improvement of Localization Accuracy by Using 3D Cone Beam CT for Stereotactic Body Radiation Therapy of Liver, Lung and Spine Lesions

Purpose: To compare treatment isocenter placement based on OBI 2D kV imaging and OBI 3D‐CBCT imaging for patients undergoing SBRT, and compare the CBCT based isocenter shifts among liver, lung and spine lesions. Material and Methods: 119 SBRT fractions were delivered to 40 lesions of 32 patients. The patients were initially localized with 2D orthogonal on‐board kV images. 3D on‐board CBCT images were then acquired to localize the isocenter for treatment. The shifts from 2D to 3D localization were recorded. Histograms, mean values and standard deviation of the isocenter shifts were calculated. Results: The isocenter shifts based on the 3D CBCT for all 119 SBRT fractions were 0.27 ± 0.25 cm, 0.19 ± 0.23 cm, and 0.23 ± 0.27 cm in the A‐P, C‐C, and M‐L direction, respectively. The mean values and standard deviations of the magnitudes of the shifts along AP, CC and ML directions were 0.30 ± 0.26 cm, 0.21 ± 0.20 cm, and 0.31 ± 0.35 cm for liver patients, 0.31 ± 0.29 cm, 0.30 ± 0.31 cm, and 0.28 ± 0.33 cm for lung patients, and 0.22 ± 0.21 cm, 0.13 ± 0.16 cm, and 0.18 ± 0.18 cm for spine patients. Conclusion: CBCT guidance enhances the setup accuracy of SBRT treatment by using 3D anatomical information.Partly supported by Varian Research Grant.