Inferior Direction Research Articles

Tripping Elicits Earlier and Larger Deviations in Linear Head Acceleration Compared to Slipping.

Slipping and tripping contribute to a large number of falls and fall-related injuries. While the vestibular system is known to contribute to balance and fall prevention, it is unclear whether it contributes to detecting slip or trip onset. Therefore, the purpose of this study was to investigate the effects of slipping and tripping on head acceleration during walking. This information would help determine whether individuals with vestibular dysfunction are likely to be at a greater risk of falls due to slipping or tripping, and would inform the potential development of assistive devices providing augmented sensory feedback for vestibular dysfunction. Twelve young men were exposed to an unexpected slip or trip. Head acceleration was measured and transformed to an approximate location of the vestibular system. Peak linear acceleration in anterior, posterior, rightward, leftward, superior, and inferior directions were compared between slipping, tripping, and walking. Compared to walking, peak accelerations were up to 4.68 m/s2 higher after slipping, and up to 10.64 m/s2 higher after tripping. Head acceleration first deviated from walking 100-150ms after slip onset and 0-50ms after trip onset. The temporal characteristics of head acceleration support a possible contribution of the vestibular system to detecting trip onset, but not slip onset. Head acceleration after slipping and tripping also appeared to be sufficiently large to contribute to the balance recovery response.

Estimating the gravity induced three dimensional deformation of the breast

As human breast tissue is continuously deformed by gravity, it is difficult to identify the non-loaded neutral breast position from which to take measurements. To estimate the neutral nipple position, this study proposed a simple novel method to counteract the three dimensional effect of gravity on the breast using the buoyant forces from water and soybean oil (ρWATER = 994kgm−3; ρOIL = 909kgm−3). Fourteen female participants with breast sizes ranging from 30 to 34in. under band and B to E cup size took part in this study. Each participant had their static gravity-loaded nipple position measured and their neutral nipple position estimated (as the midpoint between the nipple position during water and soybean oil immersion). Participants were asked to sit in each fluid and fully submerge their torso and breasts. The mean gravity-induced nipple displacements from the neutral nipple position were 15.3mm in the posterior direction, 7.4mm in the lateral direction, and 25.7mm in the inferior direction. Gravity had a significant (p < 0.05, r > 0.82) measurable effect on the static nipple position, particularly in the inferior and posterior directions. Furthermore the density difference between water and soybean oil produced a significant difference (p < 0.05, r = 0.72) in superior–inferior nipple position (5.6mm). These findings suggest that neglect of gravity-induced breast deformations may lead to errors when assessing breast position and its relationship to possible breast pain, and that water alone may not be sufficient to estimate the neutral nipple position.

Hyoid Movement and Laryngeal Penetration During Sequential Swallowing

This study examined hyoid movement during sequential swallowing and evaluated the relationship among trajectory patterns, swallowing types based on laryngeal movement and laryngeal penetration. Twelve healthy adults underwent videofluorography during sequential swallowing. Each swallow was classified into two types: opened laryngeal vestibule after swallow (i-Segmental type) and closed laryngeal vestibule after swallow (i-Continuous type). Each participant's swallowing type was classified by the same method. We analyzed two-dimensional movements of the hyoid, and the trajectories were classified into three patterns: hyoid moved in the anterosuperior direction after the onset of swallow (pattern L); hyoid moved in the anterosuperior direction after the onset of swallow, beyond the mandibular plane (pattern X); and the hyoid was above the mandibular plane at the onset of swallow, and then it moved in the inferior direction (pattern U). Two-way ANOVA revealed that there was a significant interaction between the swallowing type and trajectory pattern, and laryngeal penetration was the highest for the combination of individual swallow of i-Continuous type and pattern L. In sequential swallowing, the existence of a penetration-prone swallowing pattern combination was an interesting phenomenon relevant to eating management for dysphagic patients; however further study in patient groups is required.

Assessment of changes in pharyngeal airway size and hyoid bone position following orthodontic treatment of Class I bimaxillary dentoalveolar protrusion

Objectives: To investigate the influence of orthodontic treatment on pharyngeal airway size and position of hyoid bone in patients treated with extraction of all first premolars. Materials and Methods: Pre- and post-treatment lateral cephalograms of twenty patients were obtained, and cephalometric analysis was done to assess skeletal and dental changes, airway dimensions, and hyoid bone position using 17 linear and 4 angular measurements. Results: All the readings were statistically analyzed using paired t -test. Change in nasopharyngeal airway size was not statistically significant. However, airway size was reduced significantly at soft palate (SPP-SPPW) by 7.4%, at uvula (U-MPW) by 19.2%, at tongue (TB-TPPW) by 20.8%, and at the base of tongue (V-LPW) by 13.2%. The analyzed linear measurements showed relocation of hyoid bone in an inferior direction. Simple linear correlation analysis revealed positive correlation between incisor retraction and airway size. Conclusions: There was a significant narrowing of pharyngeal airway behind soft palate, uvula, and at the base of the tongue following retraction. There is a change in the position of the hyoid bone in an inferior direction as an adaptation preventing an encroachment of the tongue into the pharyngeal airway.

Topographic profile of choroid in eyes after acute primary angle-closure

ObjectiveTo characterize and compare the topographic profile of choroid in eyes after acute primary angle-closure (APAC) and in normal controls. Design, Setting, and ParticipantsA prospective observational study was conducted. Forty-four consecutive patients who had experienced unilateral APAC that was resolved by treatment were recruited. Seventy age- and sex-matched normal individuals were recruited as controls. MethodsEnhanced depth imaging–optical coherence tomography was used to measure choroidal thickness (CT) in 9 diffuse locations of macular region. The average CT was compared among each location in the APAC and control groups and between the same locations in the 2 groups. ResultsVariants of CT at different macular locations were found in both groups; it was greatest at subfoveal locations and spread thinner around them, to reach a significant decrease (all p ≤ 0.05) 3 mm away in all directions from the fovea except for the superior direction in normal controls (p = 0.472). Among the 4 directions, the superior had the thickest CT in both groups, followed by the temporal, inferior, and nasal directions in both groups, whereas the APAC had a thicker choroid profile at each location (all p < 0.05). Univariate analysis showed that age, sex, anterior chamber depth, axial length, and spherical equivalent were all associated with the subfoveal choroidal thickness (SFCT) (all p < 0.1). Multivariable linear regression analysis showed that the SFCT was significantly thicker in association with an APAC diagnosis, shorter axial length, greater spherical equivalent refractive error, and younger female subjects (all p ≤ 0.05). ConclusionsAPAC eyes have a thicker choroid than normal eyes in the macular region, and the increase in CT in APAC eyes follows topographic distribution as in normal control eyes.

Intra-fraction respiratory motion and baseline drift during breast Helical Tomotherapy

Background and purposeTo investigate the intra-fraction breast motion during long-lasting treatments of breast cancer with Helical Tomotherapy by means of an optical tracking system. Materials and methodsA set of seven radio-transparent passive markers was placed on the thoraco-abdominal surface of twenty breast cancer patients and tracked by an infrared tracking system. A continuous non-invasive monitoring of intra-fraction motion from patient setup verification and correction to the end of radiation delivery was thus obtained. The measured displacements were analysed in terms of cyclic respiratory motion and slow baseline drift. ResultsThe average monitoring time per patient was 15.57min. The breathing amplitude of the chest was less than 2mm, on average, along all anatomical directions. The baseline drift of the body led to more significant setup uncertainties than the respiratory motion. The main intra-fraction baseline drifts were in posterior and inferior directions and occurred within the first eight minutes of monitoring. Considering the intra-fraction motion only, the resultant clinical-to-planning target volume safety margins are highly patient-specific and largely anisotropic. ConclusionThe non-respiratory motion occurring during prolonged treatments induces notable uncertainties. Non-invasive continuous monitoring of patient setup variations including baseline drifts is recommended in order to minimize dosimetric deviations, which might jeopardize the therapeutic ratio between target coverage and the sparing of organs at risk.

Considerable pancreatic tumor motion during breath-holding

Background: Breath-holding (BH) is often used to reduce abdominal organ motion during radiotherapy. However, for inhale BH, abdominal tumor motion during BH has not yet been investigated. The aim of this study was to quantify tumor motion during inhale BH and tumor position variations between consecutive inhale BHs in pancreatic cancer patients.Material and methods: Twelve patients with intratumoral fiducials were included and asked to perform three consecutive 30-second inhale BHs on each of three measurement days. During BH, lateral fluoroscopic movies were obtained and a two-dimensional (2D) image correlation algorithm was used to track the fiducials and the diaphragm, yielding the tumor and diaphragm motion during each BH. The tumor position variation between consecutive BHs was obtained from the difference in initial tumor position between consecutive BHs on a single measurement day.Results: We observed tumor motion during BH with a mean absolute maximum displacement over all BHs of 4.2 mm (range 1.0–11.0 mm) in inferior-superior (IS) direction and 2.7 mm (range 0.5–8.0 mm) in anterior-posterior (AP) direction. We found only a moderate correlation between tumor and diaphragm motion in the IS direction (Pearson’s correlation coefficient |r|>0.6 in 45 of 76 BHs). The mean tumor position variation between consecutive BHs was 0.2 [standard deviation (SD) 1.7] mm in the inferior direction and 0.5 (SD 0.8) mm in the anterior direction.Conclusion: We observed substantial pancreatic tumor motion during BH as well as considerable position variation between consecutive BHs on a single day. We recommend further quantifying these uncertainties before introducing breath-hold during radiation treatment of pancreatic cancer patients. Also, the diaphragm cannot be used as a surrogate for pancreatic tumor motion.

Post-Segmentation Swallowing Accelerometry Signal Trimming and False Positive Reduction

Dual-axis swallowing accelerometry has been proposed as a method for quantifying swallowing function. Acceleration signals in anterior–posterior and superior–inferior anatomical directions are processed and automatically segmented. However, the latter is often too liberal, admitting pre- and post-swallowing activity while also giving rise to non-swallow segments. These segmentation shortcomings adversely affect feature extraction and ultimately classification of swallowing function. In this paper, we propose a kernel density estimation-based algorithm to adaptively trim the swallow segments, and energy and noise-floor algorithms to reduce the number of false positive swallow segments. The balance between false positive reduction and loss of true positives can be adjusted according to algorithmic thresholds. Dramatic reductions ( $-$ 85.4%) in false positives can be achieved with a moderate loss of true positives ( $-$ 15.1%).

MRI quantification of pancreas motion as a function of patient setup for particle therapy -a preliminary study.

Particle therapy (PT) has shown positive therapeutic results in local control of locally advanced pancreatic lesions. PT effectiveness is highly influenced by target localization accuracy both in space, since the pancreas is located in proximity to radiosensitive vital organs, and in time as it is subject to substantial breathing‐related motion. The purpose of this preliminary study was to quantify pancreas range of motion under typical PT treatment conditions. Three common immobilization devices (vacuum cushion, thermoplastic mask, and compressor belt) were evaluated on five male patients in prone and supine positions. Retrospective four‐dimensional magnetic resonance imaging data were reconstructed for each condition and the pancreas was manually segmented on each of six breathing phases. A k‐means algorithm was then applied on the manually segmented map in order to obtain clusters representative of the three pancreas segments: head, body, and tail. Centers of mass (COM) for the pancreas and its segments were computed, as well as their displacements with respect to a reference breathing phase (beginning exhalation). The median three‐dimensional COM displacements were in the range of 3 mm. Latero–lateral and superior–inferior directions had a higher range of motion than the anterior–posterior direction. Motion analysis of the pancreas segments showed slightly lower COM displacements for the head cluster compared to the tail cluster, especially in prone position. Statistically significant differences were found within patients among the investigated setups. Hence a patient‐specific approach, rather than a general strategy, is suggested to define the optimal treatment setup in the frame of a millimeter positioning accuracy.PACS number(s): 87.55.‐x, 87.57.nm, 87.61

The effect of the arthroscopic augmentation of the subscapularis tendon on shoulder instability and range of motion: A biomechanical study

BackgroundAnterior shoulder dislocation is common. The treatment of recurrence with glenoid bone defect is still considered controversial. A new arthroscopic subscapularis augmentation has recently been described that functions to decrease the anterior translation of the humeral head. The purpose of the presented study was to examine the biomechanical effect on glenohumeral joint motion and stability. MethodsEight fresh frozen cadaver shoulders were studied by use of a force guided industrial robot fitted with a six-component force-moment sensor to which the humerus was attached. The testing protocol includes measurement of glenohumeral translation in the anterior, anterior-inferior and inferior directions at 0°, 30° and 60° of glenohumeral abduction, respectively, with a passive humerus load of 30N in the testing direction. The maximum possible external rotation was measured at each abduction angle applying a moment of 1Nm. Each specimen was measured in a physiologic state, as well as after Bankart lesion with an anterior bone defect of 15–20% of the glenoid, after arthroscopic subscapularis augmentation and after Bankart repair. FindingsThe arthroscopic subscapularis augmentation decreased the anterior and anterior-inferior translation. The Bankart repair did not restore the mechanical stability compared to the physiologic shoulder group. External rotation was decreased after arthroscopic subscapularis augmentation compared to the physiologic state, however, the limitation of external rotation was decreased at 60° abduction. InterpretationThe arthroscopic subscapularis augmentation investigated herein was observed to restore shoulder stability in an experimental model.

The accuracy of extracted target motion trajectories in four-dimensional cone-beam computed tomography for lung cancer patients

PurposeTo quantify the accuracy of extracted target motion trajectories in dual-source four-dimensional cone-beam computed tomography (4D-CBCT) by comparison with the actual three-dimensional (3D) target motion acquired simultaneously during 4D-CBCT scan. Materials and methods4D-CBCT scans were performed for 19 different sinusoidal-like patterns and 13 lung cancer patients with implanted markers. Internal (In) or external (Ex) surrogates with amplitude (Amp)- or phase (Ph)-based sorting were used for the reconstructions. The targets were a pseudo-tumor and implanted marker for the phantom and clinical studies, respectively. The accuracy was evaluated by determining the maximum error (MaxEi) between the 3D target position extracted from 4D-CBCT and the actual 3D target position detected by fluoroscopy in each ith phase (0⩽i⩽7). ResultsMedian peak-to-peak target displacements in the superior–inferior (SI) direction were 20.6 and 20.6mm in the phantom and clinical studies, respectively. In the phantom and clinical studies, the maximum of median MaxEis in the SI direction was 4.6 and 9.2mm in the In_Ph reconstruction. In the clinical study, the maximum of median MaxEis was observed during the end-inhalation phase among all reconstruction approaches. ConclusionsThis study showed the magnitude of underestimation toward the inferior direction of target motion in clinical 4D-CBCT.

Clinical validation and benchmarking of knowledge-based IMRT and VMAT treatment planning in pelvic anatomy

PurposeThe aim of this work was to determine whether a commercial knowledge-based treatment planning (KBP) module can efficiently produce IMRT and VMAT plans in the pelvic region (prostate & cervical cancer), and to assess sensitivity of plan quality to training data and model parameters. MethodsInitial benchmarking of KBP was performed using prostate cancer cases. Structures and dose distributions from 40 patients previously treated using a 5-field IMRT technique were used for model training. Two types of model were created: one excluded statistical outliers (as identified by RapidPlan guidelines) and the other had no exclusions. A separate model for cervix uteri cancer cases was subsequently developed using 37 clinical patients treated for cervical cancer using RapidArc™ VMAT, with no exclusions. The resulting models were then used to generate plans for ten patients from each patient group who had not been included in the modelling process. Comparisons of generated RapidPlans with the corresponding clinical plans were carried out to indicate the required modifications to the models. Model parameters were then iteratively adjusted until plan quality converged with that obtained by experienced planners without KBP. ResultsInitial automated model generation settings led to poor conformity, coverage and efficiency compared to clinical plans. Therefore a number of changes to the initial KBP models were required. Before model optimisation, it was found that the PTV coverage was slightly reduced in the superior and inferior directions for RapidPlan compared with clinical plans and therefore PTV parameters were adjusted to improve coverage. OAR doses were similar for both RapidPlan and clinical plans (p>0.05). Excluding outliers had little effect on plan quality (p≫0.05). Manually fixing key optimisation objectives enabled production of clinically acceptable treatment plans without further planner intervention for 9 of 10 prostate test patients and all 10 cervix test patients. ConclusionsThe Varian RapidPlan™ system was able to produce IMRT & VMAT treatment plans in the pelvis, in a single optimisation, that had comparable sparing and comparable or better conformity than the original clinically acceptable plans. The system allows for better consistency and efficiency in the treatment planning process and has therefore been adopted clinically within our institute with over 100 patients treated.

Robotic Technique for Accessing Left Hepatic Vein Through Ligamentum Venosum in Left Hepatectomy: How I Do It?

Blood loss is associated with postoperative morbidity and mortality. Outflow control could be used concomitantly with inflow control for maximum reduction in blood loss during parenchymal transection. However, in left hepatectomy (LH) and left lateral sectionectomy (LLS), extrahepatic control of the left hepatic vein (LHV) is still less commonly used. Some authors reported extrahepatic LHV control technique using ligamentum venosum (LV) in open or laparoscopic surgery, but no reports of this technique in robotic surgery have been found. The dissection of LV from the liver was performed, followed by clipping and transecting LV. The cephalad LV stump was retracted to the left inferior direction and then the dissection of the posterior part of LHV was performed until the opening space was connected to the anterior part of LHV. The LHV was encircled with the tape and vascular stapler was inserted to manage LHV stump. From 2013 to 2015, LHV was successfully encircled by using this technique in 7 out of 11 cases. The mean operative time was 244 minutes, the mean estimated blood loss was 300 mL, and the mean length of hospital stay was 4.8 days. Neither injury to LHV, nor complications related to the liver were found. From our experiences, LHV was routinely controlled in robotic LH or LLS and the success rate was 60%. However, this technique is not recommended for the tumor that compresses or abuts LHV/LV, and in cirrhotic liver with associated hypertrophic left lateral segment.

Analysis of 3D strain in the human medial meniscus

This study presents a method to evaluate three-dimensional strain in meniscal tissue using medical imaging. Strain is calculated by tracking small teflon markers implanted within the meniscal tissue using computed tomography imaging. The results are presented for strains in the middle and posterior third of the medial menisci of 10 human cadaveric knees, under simulated physiologically relevant loading. In the middle position, an average compressive strain of 3.4% was found in the medial–lateral direction, and average tensile strains of 1.4% and 3.5% were found in the anterior–posterior and superior–inferior directions respectively at 5° of knee flexion with an applied load of 1× body weight. In the posterior position, under the same conditions, average compressive strains of 2.2% and 6.3% were found in the medial–lateral and superior–inferior directions respectively, and an average tensile strain of 3.8% was found in the anterior–posterior direction. No statistically significant difference between strain in the middle or posterior of the meniscus or between the global strains is uncovered.

Time Course of Pupil Center Location after Ocular Drug Application.

To investigate the time course of pupil centration after application of common topical ocular drugs. Single drops of 2.5% phenylephrine hydrochloride, 1% tropicamide, and 2% pilocarpine hydrochloride were applied on different days to the right eyes of 12 participants. Anterior eye images were captured, at 5-min intervals for an hour, using an infrared-sensitive camera. The images were analyzed to determine pupil diameter and pupil center, the latter with respect to the limbal center. As a control, natural pupil size and pupil center were determined under different illuminances. Pupil centers of natural pupils shifted temporally as pupils dilated. At common pupil sizes, drug-induced pupil centers were different from natural pupil centers. Phenylephrine produced a center shift in the nasal and inferior directions that peaked after a mean of 30 min, whereas dilation was continuing up to 60 min. Tropicamide produced transient center shifts in the nasal and inferior directions that peaked at about 10 min before reducing toward baseline values, whereas dilation reached a peak at about 25 min. Pilocarpine produced a small sustained superior shift that, like constriction, reached a peak after about 25 min. Application of topical ophthalmic drugs cause shifts in pupil center that do not match those produced by natural changes in pupil size and that, in the cases of phenylephrine and tropicamide, follow a different time course than the pupil size changes.

SU-G-TeP1-01: A Simulation Study to Investigate Maximum Allowable Deformations of Implant Geometry Before Plan Objectives Are Violated in Prostate HDR Brachytherapy

Purpose: In prostate HDR brachytherapy dose distributions are highly sensitive to changes in prostate volume and catheter displacements. We investigate the maximum deformations in implant geometry before planning objectives are violated. Methods: A typical prostate Ir-192 HDR brachytherapy reference plan was calculated on the Oncentra planning system, which used CT images from a tissue equivalent prostate phantom (CIRS Model 053S) embedded inside a pelvis wax phantom. The prostate was deformed and catheters were displaced in simulations using a code written in MATLAB. For each deformation dose distributions were calculated, based on TG43 methods, using the MATLAB code. The calculations were validated through comparison with Oncentra calculations for the reference plan, and agreed within 0.12%SD and 0.3%SD for dose and volume, respectively. Isotropic prostate volume deformations of up to +34% to −27% relative to its original volume, and longitudinal catheter displacements of 7.5 mm in superior and inferior directions were simulated. Planning objectives were based on American Brachytherapy Society guidelines for prostate and urethra volumes. A plan violated the planning objectives when less than 90% of the prostate volume received the prescribed dose or higher (V100), or the urethral volume receiving 125% of prescribed dose or higher was more than 1 cc (U125). Lastly, the dose homogeneity index (DHI=1-V150/V100) was evaluated; a plan was considered sub-optimal when the DHI fell below 0.62. Results and Conclusion: Planning objectives were violated when the prostate expanded by 10.7±0.5% or contracted by 11.0±0.2%; objectives were also violated when catheters were displaced by 4.15±0.15 mm and 3.70±0.15 mm in the superior and inferior directions, respectively. The DHI changes did not affect the plan optimality, except in the case of prostate compression. In general, catheter displacements have a significantly larger impact on plan optimality than prostate volume changes.

SU‐F‐T‐396: Impact of Shoulder Deformation for Head and Neck VMAT

Purpose:For head and neck VMAT (HN‐VMAT), variations of position and deformation of patient's shoulders is a concern to affect inaccuracy of dose distribution. It has been reported that the setup error of the shoulders was variable from 5 mm – 1 cm. The beams of the HN‐VMAT pass through the shoulders. We assessed the impact of shoulder deformation to dose distribution for HN‐VMAT.Methods:One HN‐VMAT plan was generated using a patient's CT. The patient's CT was deformed using ImSimQA (Oncology Systems Limited, Shrewsbury, Shropshire, UK) to generate several patterns of the shoulders’ deformations when the right and left humeral heads were shifted with 3, 6, and 15 mm in the superior and inferior directions (SI), 3, 5, and 15 mm in the anterior and posterior directions (AP), and 5 and 15 mm in the right or left direction (LR). DVH comparison was performed in the different deformation patterns. The dosimetric parameters of D95% for CTV70Gy, CTV60Gy and CTV54Gy and dmax for Spinal cord were also measured. Gamma index evaluation (Criteria: 3%/2mm) was performed to exhibit clinically tolerable area in the comparison.Results:DVH comparison shows similar for all structures. As the comparison for the dosimetric parameters, the variations of D95% in the LR and AP were within 1%. There were larger variations in the SI than those in the other directions, however were within 1.5%. In gamma index evaluation, the small spots with higher gamma index values were appeared when the shift was 6 mm, however the pass ratio was 99.13%.Conclusion:HN‐VMAT should be robust for shoulder deformation and geometric accuracy within 6 mm from patient's setup and image‐guided radiotherapy may be clinically acceptable for target dose coverage or normal tissue dose sparing.

SU‐F‐P‐32: A Phantom Study of Accuracy of Four‐Dimensional Cone‐Beam CT (4D‐CBCT) Vs. Three‐Dimensional Cone Beam CT (3D‐CBCT) in Image Guided Radiotherapy

Purpose:SymmetryTM 4D IGRT system of Elekta has been installed at our institution, which offers the 4D CBCT registration option. This study is to evaluate the accuracy of 4D CBCT system by using the CIRS 4D motion phantom and to perform a feasibility study on the implementation of 4D‐CBCT as image guidance for SBRT treatment.Methods:The 3D and 4D CT image data sets are acquired using the CIRS motion phantom on a Philips large bore CT simulator. The motion was set as 0.5 cm superior and inferior directions with 6 seconds recycle time. The 4D CT data were sorted as 10 phases. One identifiable part of the 4D CT QA insert from CIRS phantom was used as the target. The ITV MIP was drawn based on maximum intensity projection (MIP) and transferred as a planning structure into 4D CBCT system. Then the 3D CBCT and 4D CBCT images were taken and registered with the free breath (3D), MIP (4D) and average intensity projection (AIP)(4D) reference data sets. The couch shifts (X, Y, Z) are recorded and compared.Results:Table 1 listed the twelve couch shifts based on the registration of MIP, AIP and free breath CT data sets with 3D CBCT and 4D CBCT for both whole body and local registration. X, Y and Z represent couch shifts in the direction of the right‐left, superior‐inferior and anterior‐posterior. The biggest differences of 0.73 cm and 0.57 cm are noted in the free breath CT data with 4D CBCT and 3D CBCT data registration. Fig. 1 and Fig. 2 are the shift analysis in diagram. Fig. 3 shows the registration.Conclusion:Significant differences exist in the shifts corresponding with the direction of target motion. Further investigations are ongoing.

TH-CD-202-12: Online Inter-Beam Replanning Based On Real-Time Dose Reconstruction

Purpose: This work provides a proof-of-concept study for online replanning during treatment delivery for step-and-shoot prostate SBRT, based on real-time dose reconstruction. Online replanning is expected to improve the trade-off between target coverage and organ-at-risk dose in the presence of intra-fractional motion. Methods: We have implemented an online replanning workflow on top of our previously reported real-time dose reconstruction software which connects to an Elekta research linac. The treatment planning system DynaPlan was extended to (1) re-optimize and sequence treatment plans (in clockwise beam order) before each beam, based on actual delivered dose, in a timeframe limited by the gantry rotation between subsequent beams, and (2) send the respective segments to the delivery control software DynaTrack which starts/continues treatment immediately.To investigate the impact of a reduced safety margin, we have created and delivered (on a linac emulator) a conventional CTV+5/3mm (I) and a reduced CTV+1mm margin (II) treatment plan for a prostate patient. We have assessed CTV coverage with and without inter-beam replanning, all exposed to a gradual target shift of 0–5mm in posterior and inferior direction from start until the end of delivery. Results: For the reconstructed conventional plan (I), D98 for CTV was 100% of D98 of the planned dose. For the reconstructed margin-reduced plan (II), D98 for CTV was 95% of the planned D98 without replanning, but could be recovered to 99% by replanning for each beam. Plan (II) with replanning resulted in a decrease for bladder V90% by 88% and an increase to rectum V90% by 9% compared to the conventional plan (I). Dose calculation/accumulation was performed in <15ms per MLC aperture, replanning in <15s per beam. Conclusion: We have shown that online inter-beam replanning is technically feasible and potentially allows for a margin reduction. Future investigation considering motion-robust replanning optimization parameters is in progress. We acknowledge support of the MLC research from Elekta AB. This work is supported by Cancer Research UK under Programme C33589/A19908. Research at ICR is also supported by Cancer Research UK under Programme C33589/A19727 and NHS funding to the NIHR Biomedical Research Centre at RMH and ICR.

WE‐AB‐207B‐03: A Computational Methodology for Determination of CTV‐To‐PTV Margins with Inter Fractional Shape Variations Based On a Statistical Point Distribution Model for Prostate Cancer Radiation Therapy

Purpose:Our assumption was that interfractional shape variations of target volumes could not be negligible for determination of clinical target volume (CTV)‐to‐planning target volume (PTV) margins. The aim of this study was to investigate this assumption as a simulation study by developing a computational framework of CTV‐to‐PTV margins with taking the interfractional shape variations into account based on point distribution model (PDM)Methods:The systematic and random errors for interfractional shape variations and translations of target volumes were evaluated for four types of CTV regions (only a prostate, a prostate plus proximal 1‐cm seminal vesicles, a prostate plus proximal 2‐cm seminal vesicles, and a prostate plus whole seminal vesicles). The CTV regions were delineated depending on prostate cancer risk groups on planning computed tomography (CT) and cone beam CT (CBCT) images of 73 fractions of 10 patients. The random and systematic errors for shape variations of CTV regions were derived from PDMs of CTV surfaces for all fractions of each patient. Systematic errors of shape variations of CTV regions were derived by comparing PDMs between planning CTV surfaces and average CTV surfaces. Finally, anisotropic CTV‐to‐PTV margins with shape variations in 6 directions (anterior, posterior, superior, inferior, right, and left) were computed by using a van Herk margin formula.Results:Differences between CTV‐to‐PTV margins with and without shape variations ranged from 0.7 to 1.7 mm in anterior direction, 1.0 to 2.8 mm in posterior direction, 0.8 to 2.8 mm in superior direction, 0.6 to 1.6 mm in inferior direction, 1.4 to 4.4 mm in right direction, and 1.3 to 5.2 mm in left direction.Conclusion:More than 1.0 mm additional margins were needed at least in 3 directions to guarantee CTV coverage due to shape variations. Therefore, shape variations should be taken into account for the determination of CTV‐to‐PTV margins.