Impact Of Anatomical Changes Research Articles

Automated dose evaluation on daily cone-beam computed tomography for breast cancer patients

Background and purposeOur goal was to develop a workflow to automatically evaluate delivered dose on daily cone beam computed tomography (CBCT) in all breast cancer patients to assess dosimetric impact of anatomical changes and guide decision-making for offline plan adaptation. Materials and methodsThe workflow automatically processes the daily CBCTs of all breast cancer patients receiving local and locoregional radiotherapy. The planning-CT is registered to the CBCT to create a synthetic CT and propagate contours. A forward dose calculation is performed, and DVH parameters are extracted and printed in a report. We evaluated the workflow on a group level and in a subset of 30 patients on a patient-specific level, including comparison to clinical evaluation on additional planning-CT in 10 patients. Results7454 fractions in 647 patients were analyzed over a period of seven months. Median breast clinical target volume V95% was ≥ 95 % for 97 % of the patients. The workflow would have provided useful additional insights for decision-making for the requirement of plan adaptation, based on debatable disagreement with the clinical decision in half of the cases with an additional planning-CT. The workflow also identified cases with suboptimal coverage not identified in the clinical procedure. ConclusionWe developed a fully automated workflow for dose evaluation on daily CBCT for local and locoregional breast radiotherapy. We have demonstrated its potential for aiding decision-making for plan adaptation in patients with changing anatomy and its capability to highlight patients that may receive suboptimal treatment and require closer clinical evaluation of treatment quality.

Evaluating the impact of anatomical changes on dose distributions in head and neck cancer

Abstract Introduction: Thanks to modern IGRT procedures, it is possible to track changes in the patient's anatomy and thus calculate the dose distribution for the current anatomical conditions of the patient. This allows the scheduled dose to be compared with the delivered dose. In the case of large discrepancies, it is possible to improve the treatment plan. Radiotherapy, during which the treatment plan is modified, resulting from changes in anatomy, is referred to as adaptive radiotherapy. Material and methods: This study was performed for 30 patients with H&N cancer at the University Hospital in Zielona Góra. All patients were treated with VMAT. The Simultaneous Integrated Technique was used. In each treatment session, set-up verification was performed. Alternating every other day, the CBCT and two orthogonal portal images were made, and position correction prior to each session was performed. For all patients, new planning CT was made after the 11th and 22nd treatment sessions. Dose distributions with the initial plan on CT11 and CT22 were calculated. The initial dose-volume histograms DVH0 were compared with dose-volume histograms DVH11 and DVH22 calculated on CT11 and CT22. Results: We compared the dose distribution in the CTVs and in the most important organs at risk obtained for initial anatomy and dose distributions calculated with the initial plan on the CTs performed after the second and the fourth week of irradiation. The differences between mean doses and V95% to GTV obtained for the initial CT and two other CTs were small. For a few CTs, the values of V95% were smaller by more than 5% points. In most patients, the mean dose in salivary glands increased during treatment. Conclusions: Anatomical changes occurring during radiotherapy in patients with head and neck cancers have little influence on the dose deposited in the Clinical Target Volume. Adaptive therapy may be of particular importance if relapse occurs and re-irradiation.

Automatic dose verification system for breast radiotherapy: Method validation, contour propagation and DVH parameters evaluation

PurposeImage guided radiotherapy (IGRT) strategies allow detecting and monitoring anatomical changes during external beam radiotherapy (EBRT). However, assessing the dosimetric impact of anatomical changes is not straightforward. In current IGRT strategies dose volume histograms (DVH) are not available due to lack of contours and dose recalculations on the cone-beam CT (CBCT) scan. This study investigates the feasibility of using automatically calculated DVH parameters in CBCTs using an independent dose calculation engine and propagated contours. MethodA prospective study (NCT03385031) of thirty-one breast cancer patients who received additional CBCT imaging (N = 70) was performed. Manual and automatically propagated contours were generated for all CBCTs and an automatic dose recalculation was performed. Differences between planned and CBCT-derived DVH parameters (mean and maximum dose to targets, 95% volume coverage to targets and mean heart dose (MHD)) were calculated using the dose verification system with manual and propagated contours and, in both cases, benchmarked against DVH differences quantified in the TPS using manually contoured CBCTs. ResultsDifferences in DVH parameters between the TPS and dose verification system with propagated contours were −1.3% to 0.7% (95% CI) for mean dose to the target volume, −0.3 to 0.2 Gy (95% CI) in MHD and −3.9% to 2.9% (95% CI) in target volume coverage. ConclusionThe use of an independent fully automatic dose verification system with contour propagation showed to be feasible and sufficiently reliable to recalculate CBCT based DVHs during breast EBRT. Volume coverage parameters, i.e. V95%, proved to be especially sensitive to contouring differences.

Changes in Organs at Risk Dose Estimates Due to Anatomic Changes and Registration Method During Head and Neck Radiotherapy and its Relationship to Percent Weight Loss

<h3>Purpose/Objective(s)</h3> Weight loss and anatomic changes are common during radiation therapy for head and neck cancers. The best method for assessing the impact of anatomic changes on dose distribution is unknown. We compared planned doses to organs at risk (OARs) to delivered dose estimates using a cone beam computed tomography (CBCT) from the final week of treatment. <h3>Materials/Methods</h3> Ten patients who underwent bilateral neck radiation and had weekly extended CBCT's available to capture critical OARs were identified. CBCT from the final week was reconstructed to 2mm slices to match the resolution of the CT simulation (CTsim). The CBCT's were registered to the sim using both rigid and deformable registration techniques. The parotid glands, spinal cord, and skin (defined as 3mm from body contour) were contoured on the CBCT's. Mean dose to the parotids, and maximum dose (D0.035cc) to the spinal cord and skin were extracted from the treatment plan based on CTsim, rigid CBCT and deformed CBCT respectively. Independent t-tests were used to determine and compare mean percent (%) dose changes in OAR's from initially planned to delivered dose estimates using the rigid and deformed CBCT's. Linear regression was then performed to identify which set of OAR doses were the best predictors of the variation in % mean weight change during the treatment course. Statistical software was used for the statistical analyses. <h3>Results</h3> Among the 10 patients analyzed, mean % change in mean right parotid dose was -6.99% between planned and rigid CBCT estimated dose and +12.02% between planned and deformed CBCT estimated dose, for a mean total difference of 19.01% (SE 5.61%, p=0.004). These differences were statistically significant. A similar trend was observed for the left parotid gland, with mean % changes of -1.67%, +6% and 7.67% respectively (SE 5.62%, p=0.19). No differences were observed for cord (mean difference 0.19%, SE 1.27%, p = 0.89) or skin (mean difference 0.26%, SE 2.83%, p = 0.92). On regression analysis the delivered dose estimates from the rigid CBCT model were a better predictor of % weight change (R²=0.543) compared to the deformed CBCT (R²=0.463) or CT sim planned doses (R²=0.460) <h3>Conclusion</h3> The data shows a significant difference in estimating dose delivered to the right parotid gland using final week CBCT between using rigid and deformable registration techniques. Change in dose to left parotid gland also trended towards significance and may prove to be meaningful with a larger sample size. There were no significant differences in cord or skin % dose changes. We found that doses collected from the Rigid CBCT were the best predictors of % weight change during the whole course of treatment on regression analysis. Future work will focus on mid treatment dose estimates using a larger sample size, and correlation with clinical outcomes.

The impact of anatomical changes during photon or proton based radiation treatment on tumor dose in glioblastoma dose escalation trials

Purpose/ObjectiveMost dose-escalation trials in glioblastoma patients integrate the escalated dose throughout the standard course by targeting a specific subvolume. We hypothesize that anatomical changes during irradiation may affect the dose coverage of this subvolume for both proton- and photon-based radiotherapy. Material and MethodsFor 24 glioblastoma patients a photon- and proton-based dose escalation treatment plan (of 75 Gy/30 fr) was simulated on the dedicated radiotherapy planning MRI obtained before treatment. The escalated dose was planned to cover the resection cavity and/or contrast enhancing lesion on the T1w post-gadolinium MRI sequence. To analyze the effect of anatomical changes during treatment, we evaluated on an additional MRI that was obtained during treatment the changes of the dose distribution on this specific high dose region. ResultsThe median time between the planning MRI and additional MRI was 26 days (range 16–37 days). The median time between the planning MRI and start of radiotherapy was relatively short (7 days, range 3–11 days). In 3 patients (12.5%) changes were observed which resulted in a substantial deterioration of both the photon and proton treatment plans. All these patients underwent a subtotal resection, and a decrease in dose coverage of more than 5% and 10% was observed for the photon- and proton-based treatment plans, respectively. ConclusionOur study showed that only for a limited number of patients anatomical changes during photon or proton based radiotherapy resulted in a potentially clinically relevant underdosage in the subvolume. Therefore, volume changes during treatment are unlikely to be responsible for the negative outcome of dose-escalation studies.

Trend analysis of the dosimetric impact of anatomical changes during proton therapy for maxillary sinus carcinoma.

PurposeAnatomical changes, such as shrinkage and aeration, can affect dose distribution in proton therapy (PT) for maxillary sinus carcinoma (MSC). These changes can affect the dose to the target and organs at risk (OARs); however, when these changes occur during PT is unclear. This study aimed to investigate the dosimetric impact of anatomical changes during PT.Materials and MethodsFifteen patients with MSC were enrolled in this study. Initial PT plans were generated based on initial computed tomography (CT) images. Several repeat CT images were obtained to confirm anatomical changes during PT. Evaluation PT plans were generated by copying initial PT plans to repeat CT images. The dose differences of the target and OARs were evaluated by comparing both the plans.ResultsAt 3–4 weeks after the initiation of PT, the target volume reduced by approximately 10% as compared with the initial volume. Consequently, the target volumes gradually varied until the end of treatment. The value of V95 (volume that received 95% of the prescription dose) in the clinical target volume of the evaluation PT plan was similar to that of the initial PT plan. However, the dose to OARs, such as the contralateral optic nerve, contralateral eyeball, brainstem, and optic chiasm, increased significantly from the middle to the later phases of the treatment course. In contrast, there was a slight dose difference in the ipsilateral optic apparatus.ConclusionThe trend analysis in this study showed that anatomical changes appeared 3–4 weeks after the start of PT, and the dose to the OARs tended to increase. Therefore, it is recommended to check the status of tumor 3–4 weeks after the start of treatment to avoid the deterioration of dose distribution due to these changes.

Impact of anatomical changes on radiation absorbed dose of prostate and bladder in a potential scenario of magnetic resonance imaging (MRI)-guided carbon-ion radiotherapy (MRgCT) of prostate cancer

In this study, the impact of anatomical changes on radiation dose variations of the prostate and bladder (as an organ at risk) was assessed for a potential scenario of magnetic resonance imaging-guided carbon-ion radiotherapy (MRgCT) of prostate cancer. To achieve this goal, carbon-ion beams perpendicular to a pelvis phantom affected by 1.5 T perpendicular magnetic field were simulated using the FLUKA code. Prostate diameter change from 6 to 1 cm leads to a prostate dose reduction of 88%. Bladder diameter change from 10 to 5 cm leads to a reduction of 92% in bladder dose. The set-up error of 5 mm leads to a decrease of 5% in the prostate dose. Rectum diameter change from 5 to 3 cm leads to reductions of 20% and 26% in prostate and bladder dose, respectively. In conclusion, anatomical changes impact on dose variations in MRgCT of prostate cancer.

Dosimetric variations for high-risk prostate cancer by VMAT plans due to patient’s weight changes

AbstractPurpose:The aim of this study is to investigate the impact of anatomical changes in prostate cancer patients on the target coverage when using 6 MV beams-VMAT therapy and to propose strategies that allow us to evaluate the dose or correct it by normalization without having to perform a new simulation.Methods and materials:Ten patients of high-risk prostate cancer were chosen for the study. All test plans were delivered using the same isocenter and monitor units as the original plan and compared against the original unedited plan. The expansion and contraction of body contours due to size changes was mimicked by increasing and decreasing the body contour with depths of −2, −1·5, …, 1·5, 2 cm, in the anterior, and both lateral directions of the patient. A total of 90 plans were evaluated, 9 for each patient. Dose-volume histogram statistics were extracted from each plan and normalized to prescription dose.Results:Weight changes resulted in considerable dose modifications to the target and critical structures. Plans were found to be varied with 2·9% ± 0·3% per cm SSD change for VMAT treatment with a correlation index close to one. Therefore, doses variations were linear to the changes of depth. Gamma index evaluation was performed for the 10 renormalized plans. All of them passed criteria of 3%/3 mm in at least 98.2% of points. Eight of them passed criteria in 99% points. Gamma index 4%/4 mm passed 100% points in all patients for the chosen region of interest.Conclusions:The dosimetry estimation presented in this study shows important data for the radiation oncology staff to justify whether a CT rescan is necessary or not when a patient experiences weight changes during treatment. Based on the results of our study, discrepancies between real dose and planned dose were &gt;5% for 1·7 cm of difference in external contour in the anterior and both lateral directions of the patient.

EP-2196 Dosimetric impact of anatomical changes during IMRT for prostate cancer

EP-2196 Dosimetric impact of anatomical changes during IMRT for prostate cancer

Geometric and dosimetric impact of anatomical changes for MR-only radiation therapy for the prostate.

PurposeWith the move towards magnetic resonance imaging (MRI) as a primary treatment planning modality option for men with prostate cancer, it becomes critical to quantify the potential uncertainties introduced for MR‐only planning. This work characterized geometric and dosimetric intra‐fractional changes between the prostate, seminal vesicles (SVs), and organs at risk (OARs) in response to bladder filling conditions.Materials and methodsT2‐weighted and mDixon sequences (3–4 time points/subject, at 1, 1.5 and 3.0 T with totally 34 evaluable time points) were acquired in nine subjects using a fixed bladder filling protocol (bladder void, 20 oz water consumed pre‐imaging, 10 oz mid‐session). Using mDixon images, Magnetic Resonance for Calculating Attenuation (MR‐CAT) synthetic computed tomography (CT) images were generated by classifying voxels as muscle, adipose, spongy, and compact bone and by assignment of bulk Hounsfield Unit values. Organs including the prostate, SVs, bladder, and rectum were delineated on the T2 images at each time point by one physician. The displacement of the prostate and SVs was assessed based on the shift of the center of mass of the delineated organs from the reference state (fullest bladder). Changes in dose plans at different bladder states were assessed based on volumetric modulated arc radiotherapy (VMAT) plans generated for the reference state.ResultsBladder volume reduction of 70 ± 14% from the final to initial time point (relative to the final volume) was observed in the subject population. In the empty bladder condition, the dose delivered to 95% of the planning target volume (PTV) (D95%) reduced significantly for all cases (11.53 ± 6.00%) likely due to anterior shifts of prostate/SVs relative to full bladder conditions. D15% to the bladder increased consistently in all subjects (42.27 ± 40.52%). Changes in D15% to the rectum were patient‐specific, ranging from −23.93% to 22.28% (−0.76 ± 15.30%).ConclusionsVariations in the bladder and rectal volume can significantly dislocate the prostate and OARs, which can negatively impact the dose delivered to these organs. This warrants proper preparation of patients during treatment and imaging sessions, especially when imaging required longer scan times such as MR protocols.

Toward adaptive radiotherapy for lung patients: feasibility study on deforming planning CT to CBCT to assess the impact of anatomical changes on dosimetry

Changes in lung architecture during a course of radiotherapy can alter the planned dose distribution to the extent that it becomes clinically unacceptable. This study aims to validate a quantitative method of determining whether a replan is required during the course of conformal radiotherapy. The proposed method uses deformable image registration (DIR) to flexibly map planning CT (pCT) data to the anatomy of online CBCT images. The resulting deformed CT (dCT) images are used as a basis for assessing the effect of anatomical change on dose distributions. The study used retrospective data from a sample of seven replanned lung patients. The settings of an in-house, open-source DIR algorithm were first optimised for CT-to-CBCT registrations of the anatomy of the thorax. Using these optimised parameters, each patient’s pCT was deformed to the CBCT acquired immediately before the replan. Registration accuracy was rigorously validated both geometrically and dosimetrically to confirm that the dCTs could reliably be used to inform replan decisions. A retrospective evaluation of the changes in dose delivered over time was then carried out for a single patient to demonstrate the clinical application of the proposed method. The geometric analysis showed good agreement between deformed structures and those same structures manually outlined on the CBCT images. Results were consistently better than those achieved with rigid-only registration. In the dosimetric analysis, dose distributions derived from the dCTs were found to match closely to the ‘gold standard’ replan CT (rCT) distributions across dose volume histogram and absolute dose difference measures. The retrospective analysis of serial CBCTs of a single patient produced reliable quantitative assessment of the dose delivery. Had the proposed method been available at the time of treatment, it would have enabled a more objective replan decision. DIR is a valuable clinical tool for dose recalculation in adaptive radiotherapy protocols for lung cancer patients.

The impact of childbirth on pelvic floor morphology in primiparous Black South African women: a prospective longitudinal observational study.

There is a lack of prospective studies evaluating the impact of childbirth on the pelvic floor in non-white populations. We intended to study delivery-related changes in pelvic floor morphology in Black South African primiparae. We also intended to determine the impact of anatomical changes on symptoms in the postpartum period. A total of 153 nulliparous women between 35 and 39weeks gestation were recruited from a district antenatal clinic. All women had a standardized interview, completed the International Consultation on Incontinence Vaginal Symptoms questionnaire followed by three/four dimensional transperineal ultrasonography. This was repeated at 3-6months postpartum. Of the 153 women, 84 (54.9%) returned at a mean of 4.8months postpartum. Of these women, 60 (71.4%) had a vaginal delivery and the remainder a caesarean section (20 emergency and 4 elective). Overall, there were statistically significant increases in bladder neck descent (P = 0.003), pelvic organ descent and levator hiatal distensibility (all P < 0001) at the postpartum assessment. Levator avulsion was diagnosed in nine (15%) of those delivered vaginally. Postpartum vaginal laxity was the commonest bothersome vaginal symptom, reported by 51 women (60.7%). There is significant alteration in pelvic organ support and levator hiatal distensibility postpartum, with more marked effects in women after vaginal delivery. Of Black primiparous women, 15% sustained levator trauma after their first vaginal delivery.

17. Evaluation of a b-spline deformable image registration algorithm using numerical phantoms and physical phantom (“Phydeform”) for dose warping

Introduction Adaptive Radiotherapy (ART) is a technique which minimizes the dosimetric impact of anatomical changes that may occur during treatment. One of the milestones of ART for dose warping is deformable image registration (DIR) between images acquired during treatment and original planning images. The aim of this study is to propose a method to evaluate the performances and characterize the use of a b-spline DIR algorithm implemented in Velocity® v.3.2.0 (Varian Medical Systems, USA). Methods Two types of dataset were used to perform the evaluation, numerical phantoms and physical phantoms (called “Phydeform”). IMSIMQA™ software (OSL, UK) provides a numerical phantoms library and the opportunity to generate deformation. For algorithm characterization, deformation scenarios on simple volumes were created and two sets of images were systematically obtained for each induced deformation: initial CT and deformed CT . The evaluation related to the comparison of deformation vectors field via error histograms and contours with metrics like DICE and MDC (Mean Distance Conformity). The contour comparison was done between distorted contours obtained by Velocity® and expert contours on deformed CT . Voxel’s size influence with DIR was also evaluated. Physical phantom “Phydeform” with cubic shape of a homogenous material was used to evaluate the spatial deformation with markers and dose deformation with detectors like TLD’s (IRSN, France) and MOSFET’s (Best Medical, Canada). Two images series were compared: no-deformed CT and deformed CT obtained following the application of uniform compression on phantom. Marker’s position was evaluated between non deformed and deformed reference. Three fixed beams were delivered. The conditions were selected for highlighting dose variation between reference and deformed phantom state. Dose “deformed” value obtained after DIR application was compared with detectors measurement. Results For numerical phantoms, average Dice values are 0.83 (±2σ = 0.1) and 1.8 mm for MDC for tested deformation scenarios. For vector fields comparison, the criterion 95% voxels not exceeding 2 mm is respected. A reduced voxel size improved results. The markers position comparison between reference frame “no deformed – deformed” and “no deformed – deformed after DIR” gives us an uncertainty associated with voxels mapping of 10.3% on average. In terms of dose, uncertainty associated with voxels mapping is 4.3% for MOSFET’s and 10.4% for TLD’s. Conclusions ART software commissioning requires images deformation and dose warping validation. Results with numerical and physical phantoms make it possible to consider implementation of the first clinical applications using Velocity®.

A Monte-Carlo study to assess the effect of 1.5 T magnetic fields on the overall robustness of pencil-beam scanning proton radiotherapy plans for prostate cancer

Combining magnetic-resonance imaging (MRI) and proton therapy (PT) using pencil-beam scanning (PBS) may improve image-guided radiotherapy. We aimed at assessing the impact of a magnetic field on PBS-PT plan quality and robustness. Specifically, the robustness against anatomical changes and positioning errors in an MRI-guided scenario with a 30 cm radius 1.5 T magnetic field was studied for prostate PT.Five prostate cancer patients with three consecutive CT images (CT1-3) were considered. Single-field uniform dose PBS-PT plans were generated on the segmented CT1 with Monte-Carlo-based treatment planning software for inverse optimization. Plans were optimized at 90° gantry angle without B-field (no B), with ±1.5 T B-field (B and minus B), as well as at 81° gantry angle and +1.5 T (B G81). Plans were re-calculated on aligned CT2 and CT3 to study the impact of anatomical changes. Dose distributions were compared in terms of changes in DVH parameters, proton range and gamma-index pass-rates. To assess the impact of positioning errors, DVH parameters were compared for ±5 mm CT1 patient shifts in anterior–posterior (AP) and left–right (LR) direction.Proton beam deflection considerably reduced robustness against inter-fractional changes for the B scenario. Range agreement, gamma-index pass-rates and PTV V95% were significantly lower compared to no B. Improved robustness was obtained for minus B and B G81, the latter showing only minor differences to no B. The magnetic field introduced slight dosimetric changes under LR shifts. The impact of AP shifts was considerably larger, and equivalent for scenarios with and without B-field.Results suggest that robustness equivalent to PT without magnetic field can be achieved by adaptation of the treatment parameters, such as B-field orientation (minus B) with respect to the patient and/or gantry angle (B G81). MRI-guided PT for prostate cancer might thus be implemented without compromising robustness compared to state-of-the-art CT-guided PT.

The NARLAL2 dose escalation trial: dosimetric implications of inter-fractional changes in organs at risk

Background: Phase II trials suggested that survival rates for locally advanced lung cancer could be increased by radiotherapy dose escalation. However, results of the phase III RTOG 0617 trial illustrated an imminent risk of treatment-related death. This could be thwarted with strict constraints to organs at risk (OARs) and control of the delivered dose. This study investigates the impact of anatomical changes during radiotherapy on escalated dose distributions used in the Danish NARLAL2 dose escalation trial.Material and methods: The phase III NARLAL2 trial randomizes patients between a standard and an escalated treatment plan. In the escalated arm, mean doses up to 95 Gy/33 fractions (tumour) and 74 Gy/33 fractions (lymph nodes) are delivered to the most 18fluorodeoxyglucose-positron emission tomography (18FDG PET) active regions. The dose distributions are limited by strict constraints to OARs. For a group of 27 patients, a surveillance scan (sCT) was acquired at fraction 11. The original-escalated treatment plans were recalculated on the sCTs and the impact of inter-fractional changes evaluated.Results: A total of 13 patients (48%) had overdosage of least one OAR. Constraints for the oesophagus, trachea and aorta were violated in 26% of the patients. No overdosage was seen for heart or bronchi. For the connective tissue (all tissue in the mediastinum not identified as OAR or tumour) overdosage was seen in 41% of the patients and for the chest wall in 30% of the patients. The main reason for overdosage was tumour shrinkage.Conclusions: Anatomical changes during radiotherapy caused one or more OAR constraint violations for approximately half of the patient cohort. The main cause was tumour shrinkage. For lung cancer radiotherapy dose escalation trials, we recommend incorporation of adaptive radiotherapy strategies.

OC-0354: Dosimetric impact of anatomical changes in photon and particle therapy for pancreatic cancer

OC-0354: Dosimetric impact of anatomical changes in photon and particle therapy for pancreatic cancer

8. Validation of a “Demons” intensity-based deformable image registration algorithm using numerical phantoms and CT-patients

Introduction Adaptive Radiotherapy (ART) is a technique which minimizes the dosimetric impact of anatomical changes that may occur during treatment. One of the milestones of ART is deformable image registration (DIR) between original planning images and images acquired during treatment. The aim of this study is to propose a method to evaluate the performances and validate the clinical use of a “Demons” intensity-based DIR algorithm implemented in SmartAdapt® v.13.5 (Varian Medical Systems, USA). Methods Two types of dataset were used to perform the evaluation: numerical phantoms and CT-CBCT images of 5 patients with head and neck (HN max = 0.92) and 0.73 (min = 0.65; max = 0.9) for registrations CTD0-CTre-CT and CTD0-CBCTD16 respectively. The dispersion of the values is mainly due to organs shape and algorithm choice. No significant variation is obtained between DIR for single-modality (CT–CT) and multi-modality (CT–CBCT). Moreover, contours propagation reduces significantly physicians required time for contouring patient images. Conclusions Given the results obtained with numerical phantoms and CT-patients, a clinical use appears possible. SmartAdapt® enables significant time-saving compared to manual re-contouring for re-planning strategies. However, a careful review by a specialist for DIR-generated contours is essential for clinical cases.

Effect of Anatomic Changes on Pencil Beam Scanned Proton Dose Distributions for Cranial and Extracranial Tumors

Toestimate the frequency and impact of anatomic changes on the delivered dose in pencil beam scanning proton therapy, to assess the need for repeat CT scanning and adaptive replanning. A total of 730 patients treated at Paul Scherrer Institut between 2007 and 2014 were included in this study, for which the number of patients who had control CT scans and who were replanned as a result of anatomic changes was analyzed. For those that were replanned, the nominal dose distributions (originally optimized on the planning CT scan) were recalculated on the replanning CT scan and differences evaluated using standard dose metrics for planning target volumes and clinical target volumes and organs at risk (OARs). Control CT studies were acquired for 244 patients (33.5%), and replanning was deemed clinically necessary for 40 (16%) of these (5.5% of the total cohort). The OARs and target dose differences between the nominal and recalculated dose distributions were found to be strongly dependent on the subgroup of patients. Nevertheless, dose differences were found to be ≤ 5% for 88% of all analyzed OARs, and planning target volume/clinical target volume V95% was reduced by≤5% in 87%/90% of cases. Despite anatomic variations, clinically delivered plans have been found to be robust to anatomic changes, with replanning being deemed necessary in only a small number of cases. However, because the dosimetric effect of such changes can be quite large for some cases, they have to be monitored and evaluated on an individual basis.

MO-FG-CAMPUS-JeP1-05: Water Equivalent Path Length Calculations Using Scatter-Corrected Head and Neck CBCT Images to Evaluate Patients for Adaptive Proton Therapy

Purpose: To establish a method to evaluate the dosimetric impact of anatomic changes in head and neck patients during proton therapy by using scatter-corrected cone-beam CT (CBCT) images. Methods: The water equivalent path length (WEPL) was calculated to the distal edge of PTV contours by using tomographic images available for six head and neck patients received photon therapy. The proton range variation was measured by calculating the difference between the distal WEPLs calculated with the planning CT and weekly treatment CBCT images. By performing an automatic rigid registration, six degrees-of-freedom (DOF) correction was made to the CBCT images to account for the patient setup uncertainty. For accurate WEPL calculations, an existing CBCT scatter correction algorithm, whose performance was already proven for phantom images, was calibrated for head and neck patient images. Specifically, two different image similarity measures, mutual information (MI) and mean square error (MSE), were tested for the deformable image registration (DIR) in the CBCT scatter correction algorithm. Results: The impact of weight loss was reflected in the distal WEPL differences with the aid of the automatic rigid registration reducing the influence of patient setup uncertainty on the WEPL calculation results. The WEPL difference averaged over distal area was 2.9 ± 2.9 (mm) across all fractions of six patients and its maximum, mostly found at the last available fraction, was 6.2 ± 3.4 (mm). The MSE-based DIR successfully registered each treatment CBCT image to the planning CT image. On the other hand, the MI-based DIR deformed the skin voxels in the planning CT image to the immobilization mask in the treatment CBCT image, most of which was cropped out of the planning CT image. Conclusion: The dosimetric impact of anatomic changes was evaluated by calculating the distal WEPL difference with the existing scatter-correction algorithm appropriately calibrated. Jihun Kim, Yang-Kyun Park, Gregory Sharp, and Brian Winey have received grant support from the NCI Federal Share of program income earned by Massachusetts General Hospital on C06 CA059267, Proton Therapy Research and Treatment Center.

PO-0846: The impact of anatomical changes on the accumulated carbon ion dose in pancreatic cancer patients

PO-0846: The impact of anatomical changes on the accumulated carbon ion dose in pancreatic cancer patients