Interfractional Variations Research Articles

Anatomical robust optimization to account for nasal cavity filling variation during intensity-modulated proton therapy: a comparison with conventional and adaptive planning strategies

The aim of this study is to develop an anatomical robust optimization method for intensity-modulated proton therapy (IMPT) that accounts for interfraction variations in nasal cavity filling, and to compare it with conventional single-field uniform dose (SFUD) optimization and online plan adaptation. We included CT data of five patients with tumors in the sinonasal region. Using the planning CT, we generated for each patient 25 ‘synthetic’ CTs with varying nasal cavity filling. The robust optimization method available in our treatment planning system ‘Erasmus-iCycle’ was extended to also account for anatomical uncertainties by including (synthetic) CTs with varying patient anatomy as error scenarios in the inverse optimization. For each patient, we generated treatment plans using anatomical robust optimization and, for benchmarking, using SFUD optimization and online plan adaptation. Clinical target volume (CTV) and organ-at-risk (OAR) doses were assessed by recalculating the treatment plans on the synthetic CTs, evaluating dose distributions individually and accumulated over an entire fractionated 50 GyRBE treatment, assuming each synthetic CT to correspond to a 2 GyRBE fraction. Treatment plans were also evaluated using actual repeat CTs. Anatomical robust optimization resulted in adequate CTV doses (V95% ⩾ 98% and V107% ⩽ 2%) if at least three synthetic CTs were included in addition to the planning CT. These CTV requirements were also fulfilled for online plan adaptation, but not for the SFUD approach, even when applying a margin of 5 mm. Compared with anatomical robust optimization, OAR dose parameters for the accumulated dose distributions were on average 5.9 GyRBE (20%) higher when using SFUD optimization and on average 3.6 GyRBE (18%) lower for online plan adaptation. In conclusion, anatomical robust optimization effectively accounted for changes in nasal cavity filling during IMPT, providing substantially improved CTV and OAR doses compared with conventional SFUD optimization. OAR doses can be further reduced by using online plan adaptation.

8. Monitoring of prostate cancer with a transperineal ultrasound probe: Impact on treatment margins

Introduction To achieve a better accuracy in dose delivering in prostate cancer treatment a real-time prostate monitoring system for correcting interand intrafraction motion, such as the intramodality transperineal ultrasound device (TP-US) (Clarity®, Elekta, Sweden) is required. Previous studies had shown a good correlation between Cone beam CT (CBCT) without fiducial markers (FM) and TP-US [1] . However important differences remained in the antero-posterior direction (shift agreement at 5 mm: 82.6%) probably because of the low soft-tissue contrast of CBCT acquisitions. The first goal of this study was to compare pre-treatment registration results obtained with CBCT imaging with FM (CBCT + FM) to TP-US for prostate cancer treatment. Finally treatment margins were calculated for 4 treatment protocols: CT/CBCT + FM (1), CT/CBCT + FM followed by intra-fraction monitoring (IFM) (2), TP-US/TP-US + IFM (3) and TP-US/TP-US registration corrected by CT/CBCT + FM for the first 5 sessions + IFM (4). Methods 16 prostate patients were involved in this study. Pre-treatment positioning was first performed with TP-US/TP-US registration then corrected by CT/CBCT + FM registration as it was considered as the “gold standard”. The shifts detected during CBCT imaging and treatment were collected. After 5 sessions the mean of the differences between CBCT + FM and TP-US was calculated and applied to subsequent TP-US registrations to correct for the systematic differences observed between the 2 modalities. The differences between TP-US/TP-US and CBCT/CT registrations were analyzed on 255 sessions. Finally treatment margins considering inter-fraction motion, IFM and inter-operator variability as sources of uncertainties were calculated using van Herk formula [2] . Results CBCT + FM and TP-US shift agreements at ±5 mm were 91.8%, 92.2%, 95.3% in the left-right (LR), anterior-posterior (AP) and superior-inferior (SI) direction, respectively. Treatment margins were smaller using CT/CBCT + FM as a reference modality for pretreatment registration ( Table 1 ). The main differences between CBCT and TP-US protocols came from the uncertainties of interfraction motion, which were not considered in CBCT protocols, as it was the “gold standard”. The use of the IFM (2) as well as correction of systematic shifts for TP-US registrations (4) enabled to considerably reduce margins compared to (1) and (3), respectively. Conclusions A strong correlation was found between TP-US and CBCT + FM registrations. The use of IFM enabled to considerably reduce treatment margins. Furthers investigations are in progress to assess the robustness of CBCT + FM registrations notably in case of prostate rotations.

Evaluation of interfractional variation of organs and displacement of catheters during high-dose-rate interstitial brachytherapy for gynecologic malignancies

PurposeTo investigate the dosimetric effects due to interfractional changes in catheter position and variation in patient's anatomy during the course of interstitial high-dose-rate (HDR) brachytherapy. Methods and MaterialsA total of 15 patients with either cervical or vaginal cancer underwent interstitial HDR brachytherapy. Interstitial catheters and fiducials were placed under fluoroscopy and intraoperative 3T MRI to confirm the desired catheter placement for adequate target volume coverage. Single plan was generated from first-fraction CT fused with the MRI and used for all fractions of treatment. CT image was acquired before each treatment and registered to the first-fraction CT. Displacement of fiducials and catheters was calculated for each fraction and its effects on dosimetric parameters such as dose covering 90% for high-risk clinical target volume and intermediate-risk clinical target volume and dose to the 2 cm3 of the volume for bladder, rectum, sigmoid, and bowel were studied. ResultsAverage movements of fiducials and catheters were 1.6 mm (range: 0.1–7.1 mm) and 1.7 mm (range: 0.1–4.5 mm), respectively. Overall, deviation of the delivered dose to the target in each fraction was insignificant for all patients (p-value: 0.66 for high-risk clinical target volume and 0.87 for intermediate-risk clinical target volume). The mean dose to organs at risk showed maximum difference up to 0.9, 2.7, 1.6, and 2.1 Gy for bladder, rectum, sigmoid, and bowel, respectively (p-value: 0.88, 0.34, 0.68, and 0.85 for bladder, rectum, sigmoid, and bowel, respectively). ConclusionsThe interfractional dosimetric variation for both target and organs at risk was within clinically acceptable limit throughout the entire course of interstitial HDR-Syed brachytherapy. Only 6% of cases performed replanning, which could be readily identified using CT imaging.

The long- and short-term variability of breathing induced tumor motion in lung and liver over the course of a radiotherapy treatment

PurposeTo evaluate the short and long-term variability of breathing induced tumor motion. Materials and methods3D tumor motion of 19 lung and 18 liver lesions captured over the course of an SBRT treatment were evaluated and compared to the motion on 4D-CT. An implanted fiducial could be used for unambiguous motion information. Fast orthogonal fluoroscopy (FF) sequences, included in the treatment workflow, were used to evaluate motion during treatment.Several motion parameters were compared between different FF sequences from the same fraction to evaluate the intrafraction variability. To assess interfraction variability, amplitude and hysteresis were compared between fractions and with the 3D tumor motion registered by 4D-CT. Population based margins, necessary on top of the ITV to capture all motion variability, were calculated based on the motion captured during treatment. ResultsBaseline drift in the cranio-caudal (CC) or anterior-poster (AP) direction is significant (ie. >5 mm) for a large group of patients, in contrary to intrafraction amplitude and hysteresis variability. However, a correlation between intrafraction amplitude variability and mean motion amplitude was found (Pearson's correlation coefficient, r = 0.72, p < 10−4). Interfraction variability in amplitude is significant for 46% of all lesions. As such, 4D-CT accurately captures the motion during treatment for some fractions but not for all. Accounting for motion variability during treatment increases the PTV margins in all directions, most significantly in CC from 5 mm to 13.7 mm for lung and 8.0 mm for liver. ConclusionBoth short-term and day-to-day tumor motion variability can be significant, especially for lesions moving with amplitudes above 7 mm. Abandoning passive motion management strategies in favor of more active ones is advised.

Dosimetric effects of anatomical changes during fractionated photon radiation therapy in pancreatic cancer patients.

Pancreatic tumors show large interfractional position variation. In addition, changes in gastrointestinal gas volumes and body contour take place over the course of radiation therapy. We aimed to quantify the effect of these anatomical changes on target dose coverage, for the clinically used fiducial marker‐based patient position verification and, for comparison, also for simulated bony anatomy‐based position verification. Nine consecutive patients were included in this retrospective study. To enable fraction dose calculations on cone‐beam CT (CBCT), the planning CT was deformably registered to each CBCT (13–15 per patient); gas volumes visible on CBCT were copied to the deformed CT. Fraction doses were calculated for the clinically used 10 MV VMAT treatment plan (with for the planning target volume (PTV): D98% = 95%), according to fiducial marker‐based and bony anatomy‐based image registrations. Dose distributions were rigidly summed to yield the accumulated dose. To evaluate target dose coverage, we defined an iCTV +5 mm volume, i.e., the internal clinical target volume (iCTV) expanded with a 5 mm margin to account for remaining uncertainties including delineation uncertainties. We analyzed D98%, Dmean, and D2% for iCTV +5 mm and PTV (i.e., iCTV plus 10 mm margin). We found that for fiducial marker‐based registration, differences between fraction doses and planned dose were minimal. For bony anatomy‐based registration, fraction doses differed considerably, resulting in large differences between planned and accumulated dose for some patients, up to a decrease in D98% of the iCTV +5 mm from 95.9% to 85.8%. Our study shows that fractionated photon irradiation of pancreatic tumors is robust against variations in body contour and gastrointestinal gas, with dose coverage only mildly affected. However, as a result of interfractional tumor position variations, target dose coverage can severely decline when using bony anatomy for patient position verification. Therefore, the use of intratumoral fiducial marker‐based daily position verification is essential in pancreatic cancer patients.

SBRT in the Post-Prostatectomy Setting: Analysis of Interfraction Variability

Our institution has a phase 1 clinical trial evaluating acute toxicity in the setting of SBRT to the prostate fossa following prostatectomy. Patients enrolled on this trial are simulated and treated with fiducial markers in the prostate fossa, a rectal balloon and a daily ultrasound (US) of the bladder for immobilization and target visualization. This study reports the interfraction motion analysis based on the described immobilization technique. At this point, the study has enrolled 12 patients onto dose levels 1 and 2 (3.6Gy/fraction x 15 fractions and 4.7Gy/fraction x 10 fractions, respectively). All patients had a rectal balloon for consistency in rectal volume and immobilization of the prostate bed. Patients were treated with a full bladder that was within 20% of the bladder volume on the day of CT simulation as verified by daily US. All patients were implanted with three gold fiducial markers into the prostate bed. KV and CBCT were performed on day 1 to verify fiducial placement and bladder and rectal volume. Daily kV and weekly CBCT were used for daily alignment based on fiducial markers. For 10 out of 12 patients, the patients’ shift data were gathered and a shift vector was calculated. The mean shifts for this cohort of the patients was -0.2 mm, 0.4 mm, -1.1 mm in anterior and posterior, superior and inferior, and left and right, respectively. The standard deviation was 4.0 mm, 3.7 mm, -3.4 mm in anterior and posterior, superior and inferior and left and right, respectively. It was demonstrated that the current bladder filling and IGRT protocol resulted in minimal systematic shift (small mean shifts). Most shifts were random which were reflected in the standard deviation. The mean and standard deviation of the shift vector was 5.3 mm and 3.9 mm, respectively. The calculated percentage bladder volume compared to bladder volume during the CT simulation had a mean of 3.6% and standard deviation of 8.9%. No correlation was found between magnitudes of patient shifts and bladder filling for this cohort of patients. With the current rectal/bladder immobilization and IGRT protocol, this cohort of the patients had a mean shift of no more than 1.1 mm but a relatively large standard deviation of 4.0 mm in each direction. Setup variation did not follow a specific pattern based on bladder or rectal filling. For the subset of the patients who have reasonable CBCT volumetric information, dosimetric analysis will be performed by deforming planning CT and dose to CBCT using VelocityTM.

The influence of inter-fractional anatomy variation on secondary cancer risk estimates following radiotherapy

PurposeIn silico studies comparing estimated risks of radiation-induced secondary cancer (SC) are frequently performed in assessment of radiotherapy techniques. Since inter-patient anatomy variations can result in considerable differences in estimated risk we aimed to explore the influence of inter-fractional organ motion patterns on SC risk. MethodsVolumetric modulated arc therapy (VMAT) and intensity-modulated proton therapy (IMPT) plans were generated on the planning CT (pCT) scans of eight prostate cancer patients. In addition, the treatment plans were re-calculated on 8–9 repeat CTs (rCTs) of each patient acquired throughout the treatment course. Relative risk (RR) of SC (VMAT/IMPT) was calculated for the planned and the re-calculated dose distributions using the organ equivalent dose concept adapted to a linear and a bell-shaped competition dose–response model. ResultsDay-to-day variations in anatomy lead to fluctuations in SC risk estimates of the same order of magnitude as those caused by inter-patient variations. Using the competition model, the RR range for bladder cancer based on the pCTs was 0.4–3.4, while a considerably wider range was found when including all rCTs (0.2–6.7). There was nevertheless a correlation in RR based on repeat CTs for individual patients, indicating that patient-specific SC risks could be estimated. ConclusionsThe estimated relative risks varied considerably across rCTs and could change the risk in favour of VMAT/IMPT depending on the anatomy of the day. The results demonstrate the importance of performing in silico studies of SC risk on a cohort of patients or multiple CTs when structures subject to organ motion are involved.

Robust Planning for a Patient Treated in Decubitus Position with Proton Pencil Beam Scanning Radiotherapy

A challenging case was reported for a patient treated in decubitus position with proton pencil beam scanning. A regular robust plan with the consideration of the uncertainties of translational alignment and range accuracy cannot ensure the target coverage as revealed in two verification computed tomography (CT) scans during the first week of the treatment. The irreproducibility of daily alignment and anatomical variations in such a position is mainly due to patient’s roll. To mitigate the interfractional effect on the target coverage, a novel robust optimization against the patient’s angular setup uncertainties was implemented to improve the plan quality by introducing two artificial CT image sets by rolling the planning CT three degrees in both clockwise and counter-clockwise directions and adding them into robust optimization scenarios, which was shown to be an effective and simple way to mitigate target dose degradation with respect to interfractional variations. This method can be easily generalized and applied to other situations where angular variations in patient’s setup can introduce large dosimetric effects. It is recommended that angularly robust optimization method should be integrated into the treatment planning system as an option particularly for patient’s treatment subject to large angular variations, such as the one in the decubitus position reported here.

Sensitivity of post treatment positron emission tomography/computed tomography to detect inter-fractional range variations in scanned ion beam therapy

Background: Ion therapy, especially with modern scanning beam delivery, offers very sharp dose gradients for highly conformal cancer treatment. However, it is very sensitive to uncertainties of tissue stopping properties as well as to anatomical changes and setup errors, making range verification highly desirable. To this end, positron emission tomography (PET) can be used to measure decay products of β+-emitters created in interactions inside the patient. This work investigates the sensitivity of post treatment PET/CT (computed tomography) to detect inter-fractional range variations.Material and methods: Fourteen patients of different indication underwent PET/CT monitoring after selected treatment fractions with scanned proton or carbon ion beams. In addition to PET/CT measurements, PET and dose distributions were simulated on different co-registered CT data. Pairs of PET data were then analyzed in terms of longitudinal shifts along the beam path, as surrogate of inter-fractional range deviations. These findings were compared to changes of dose-volume-histogram indexes and corresponding dose as well as CT shifts to disentangle the origin of possible PET shifts.Results: Biological washout modeling (PET simulations) and low (<55 Bq/ml) activity concentrations (offline PET measurements, especially for 12C ions) were the main limitations for clinical treatment verification. For two selected cases, the benefit of improved washout modeling based on organ segmentation could be demonstrated. Overall, inter-fractional range shifts up to ±3 mm could be deduced from both PET measurements and simulations, and found well correlated (typically within 1.8 mm) to anatomical changes derived from CT scans, in agreement with dose data.Conclusions: Despite known limitations of post treatment PET/CT imaging, this work indicates its potential for assessing inter-fractional changes and points to future developments for improved PET-based treatment verification.

Interfractional dosimetric study of target volume and organs at risk following intracavitary brachytherapy for cervical cancer

Objective To examine the interfractional dosimetric variations among inverse three-dimensional (3D) plan, forward 3D plan, and two-dimensional (2D) plan of intracavitary brachytherapy for cervical cancer, and to discuss the risk of implementing the interval plan on different implantation applicators at short time intervals. Methods Twenty-five groups of CT-guided intracavitary brachytherapy (two consecutive radiations at ≤4 d apart) plans from 11 cervical cancer patients who received radical radiation therapy in our hospital were reviewed and compared. The dwelling location and time of the first intracavitary brachytherapy plan (Plan-1) were simulated on the CT image of the second intracavitary brachytherapy to form Plan-1-S. The target coverage indices and D 2 cc of organs at risk (OARs) of Plan-1-S and Plan 2(actual plan of the second intracavitary brachytherapy) under the three planning modes were recorded and compared using the paired t-test, Wilcoxon signed rank test, and ANOVA. Results The D90, D100, and V100 of high-risk CTV were significantly lower in Plan-1-S created under the inverse mode in the actual plan (-9.11±13.46%, -13.16±18.79%, and -7.80±13.34%, P=0.002, 0.002, and 0.005, respectively). D90, D100, and V100 of the interval plan had the greatest reduction under the inverse mode (76%, 80%, and 76%, respectively). The maximum reductions in D90, D100, and V100 were 332.14 cGy (2D), 244.12 cGy (forward), and 41.76%(inverse). OAR overdose occurred most frequently under the forward mode; the rates of D90, D100, and V100 reductions accompanied by one OAR overdose were 29.41%, 37.50%, and 25.00%, and the rates of D90, D100, and V100 reductions by two OAR overdoses were 5.88%, 12.50%, and 6.25%, respectively. Overdose occurred most frequently in the small intestine (36%). Comparison of the three planning modes showed that the inverse plan had a greater reduction in each target coverage index than the 2D plan. Conclusions The simulated interval plan can significantly reduce target coverage and increase the risk of OAR overdose regardless of the planning mode and the short time intervals, and is therefore not recommended for clinical application. Key words: Cervical neoplasms/brachytherapy; Interfractional dosimetric; Interval model plan

Motion monitoring during a\xa0course of lung radiotherapy with anchored electromagnetic transponders

PurposeAnchored electromagnetic transponders for tumor motion monitoring during lung radiotherapy were clinically evaluated. First, intrafractional motion patterns were analyzed as well as their interfractional variations. Second, intra- and interfractional changes of the geometric transponder positions were investigated.Materials and methodsIntrafractional motion data from 7 patients with an upper or middle lobe tumor and three implanted transponders each was used to calculate breathing amplitudes, overall motion amount and motion midlines in three mutual perpendicular directions and three-dimensionally (3D) for 162 fractions. For 6 patients intra- and interfractional variations in transponder distances and in the size of the triangle defined by the transponder locations over the treatment course were determined.ResultsMean 3D values of all fractions were up to 4.0, 4.6 and 3.4 mm per patient for amplitude, overall motion amount and midline deviation, respectively. Intrafractional transponder distances varied with standard deviations up to 3.2 mm, while a maximal triangle shrinkage of 36.5% over 39 days was observed.ConclusionsElectromagnetic real-time motion monitoring was feasible for all patients. Detected respiratory motion was on average modest in this small cohort without lower lobe tumors, but changes in motion midline were of the same size as the amplitudes and greater midline motion can be observed in some fractions. Intra- and interfractional variations of the geometric transponder positions can be large, so for reliable motion management correlation between transponder and tumor motion needs to be evaluated per patient.

Interfraction Variation and Dosimetric Changes in Patients With Cervical Cancer Treated With Intracavitary Brachytherapy.

PurposeIntracavitary brachytherapy is integral in the treatment of cervical cancer. Because of interfraction variation, the current standard is replanning with every fraction. This study aimed to determine whether there was a difference in relative dosimetry if the source position and dwell time of the first fraction were applied to subsequent fractions.Materials and MethodsThe authors performed a retrospective review of charts and films from 2007 to 2012. Eligible cases were patients with cervical cancer treated with brachytherapy with the same dose prescription to point A. Replanning was done on the first set of orthogonal plates. Source position and dwell time were subsequently applied to the remaining fractions using actual films.ResultsTwenty-nine patients were included in this study. The results showed that cervical, rectal, and bladder dose between the actual plan and the hypothetical plan were not statistically different. In the hypothetical plan, the source activity and dwell time of the first plan were applied to the orthogonal films of the subsequent fractions and showed no significant difference in all dose points.ConclusionThe results of this study showed proof of concept of the safety of using the source position and dwell time of the first plan for subsequent fractions. Until further studies are performed (also using three-dimensional planning software), the concept should be considered investigational because of the small sample size of the study. Until such research is performed, it is still strongly recommended that replanning be performed with every fraction whenever it is feasible.

Subpopulation-based correspondence modelling for improved respiratory motion estimation in the presence of inter-fraction motion variations

Correspondence modelling between low-dimensional breathing signals and internal organ motion is a prerequisite for application of advanced techniques in radiotherapy of moving targets. Patient-specific correspondence models can, for example, be built prior to treatment based on a planning 4D CT and simultaneously acquired breathing signals. Reliability of pre-treatment-built models depends, however, on the degree of patient-specific inter-fraction motion variations. This study investigates whether motion estimation accuracy in the presence of inter-fraction motion variations can be improved using correspondence models that incorporate motion information from different patients. The underlying assumption is that inter-patient motion variations resemble patient-specific inter-fraction motion variations for subpopulations of patients with similar breathing characteristics. The hypothesis is tested by integrating a sparse manifold clustering approach into a regression-based correspondence modelling framework that allows for automated identification of patient subpopulations. The evaluation is based on a total of 73 lung 4D CT data sets, including two cohorts of patients with repeat 4D CT scans (cohort 1: 14 patients; cohort 2: ten patients). The results are consistent for both cohorts: The subpopulation-based modelling approach outperforms general population modelling (models built on all data sets available) as well as pre-treatment-built models trained on only the patient-specific motion information. The results thereby support the hypothesis and illustrate the potential of subpopulation-based correspondence modelling.

A feasibility study for the introduction of micro-enema to improve organ consistency in patients receiving radiotherapy for urinary bladder cancer

AbstractAimsThe aim of the study was to assess the effect on rectal consistency, of introducing a micro-enema in the preparation of patients receiving radiotherapy treatment of urinary bladder cancer.Materials and methodsThe treatment cone beam computed tomography (CBCT) images from patients receiving radiotherapy for bladder cancer were retrospectively assessed. CBCT datasets from nine patients treated without rectal preparation (97 CBCT), and 13 patients (134 CBCT) treated following micro-enema use before planning and treatment were evaluated. CBCT were compared with the planning computed tomography for rectal status, rectal diameter and presence of gas.ResultsReproducibility of an empty rectum was achieved in 70% of treatment fractions delivered using an enema protocol compared with 33% of fractions delivered without preparation. In total, 10% of fractions were delivered with the presence of faeces or faeces and gas, compared with 46% of fractions for the non-intervention group. Enemas did not affect the proportion of fractions delivered with gas, however, where gas was present, 65% of CBCT fractions had &lt;5% gas for patients using enemas compared with 32% without.FindingsThe use of a micro-enema before planning scan and each fraction was well tolerated and proved effective in managing and reducing inter-fraction variations in rectal volume and contents.

A Dosimetric Evaluation of Threshold Bladder Volumes for Prostate Cancer Radiotherapy

BackgroundAn interfraction variation in bladder filling results in uncertainties of dose received and also has workflow implications for busy departments. This study aims to examine the dosimetric impact of a reduced bladder volume while determining a suitable threshold for treatment. Materials and MethodsA total of 15 definitive prostate patients were included for this retrospective dosimetry study. Each patient was planned to receive 80 Gy in 40 fractions using intensity-modulated radiation therapy. For each patient, a series of shrunken bladder volumes were created in 50-mL increments. The volume of bladder receiving 65 Gy (V65), 70 Gy, 75 Gy, and 80 Gy for each “shrunken” bladder volume were analyzed with paired samples t-tests. The effect of the shrunken volume relative to the established dose-volume constraint (DVC) was then assessed using single sample t-tests. ResultsThe mean planning bladder volume was 345.01 ± 138.51 mL. Under maximum bladder shrinkage, mean difference between the percentage dose received and each DVC was seen to be statistically significant (P < .05). However, for the majority of patients, DVCs were only violated once the bladder volume shrunk to less than 150 mL. On average, the DVCs were violated once the bladder volume fell below 150 mL for the V75 and V80 constraints, with no violations noted for V65 and V70. ConclusionEven under exacerbated bladder shrinkage, bladder DVC violations were found to be rare. A bladder threshold of 150 mL would prove sufficient to meet bladder DVCs in over 90% of patients; however, case-by-case assessment is required to ensure patient suitability.

Interfractional variability of respiration-induced esophageal tumor motion quantified using fiducial markers and four-dimensional cone-beam computed tomography

PurposeTo investigate the interfractional variability of respiration-induced esophageal tumor motion using fiducial markers and four-dimensional cone-beam computed tomography (4D-CBCT) and assess if a 4D-CT is sufficient for predicting the motion during the treatment. Materials and methodsTwenty-four patients with 63 markers visible in the retrospectively reconstructed 4D-CBCTs were included. For each marker, we calculated the amplitude and trajectory of the respiration-induced motion. Possible time trends of the amplitude over the treatment course and the interfractional variability of amplitudes and trajectory shapes were assessed. Further, the amplitudes measured in the 4D-CT were compared to those in the 4D-CBCTs. ResultsThe amplitude was largest in the cranial–caudal direction of the distal esophagus (mean: 7.1mm) and proximal stomach (mean: 7.8mm). No time trend was observed in the amplitude over the treatment course. The interfractional variability of amplitudes and trajectory shapes was limited (mean: ≤1.4mm). Moreover, small and insignificant deviation was found between the amplitudes quantified in the 4D-CT and in the 4D-CBCT (mean absolute difference: ≤1.0mm). ConclusionsThe limited interfractional variability of amplitudes and trajectory shapes and small amplitude difference between 4D-CT-based and 4D-CBCT-based measurements imply that a single 4D-CT would be sufficient for predicting the respiration-induced esophageal tumor motion during the treatment course.

Examination of a deformable motion model for respiratory movements and 4D dose calculations using different driving surrogates.

The aim of this study was to evaluate a surrogate-driven motion model based on four-dimensional computed tomography that is able to predict CT volumes corresponding to arbitrary respiratory phases. Furthermore, the comparison of three different driving surrogates is examined and the feasibility of using the model for 4D dose re-calculation will be discussed. The study is based on repeated 4DCTs of twenty patients treated for bronchial carcinoma and metastasis. The motion model was estimated from the planning 4DCT through deformable image registration. To predict a certain phase of a follow-up 4DCT, the model considers inter-fractional variations (baseline correction) and intra-fractional respiratory parameters (amplitude and phase) derived from surrogates. The estimated volumes resulting from the model were compared to ground-truth clinical 4DCTs using absolute HU differences in the lung region and landmarks localized using the Scale Invariant Feature Transform. Finally, the γ-index was used to evaluate the dosimetric effects of the intensity differences measured between the estimated and the ground-truth CT volumes. The results show absolute HU differences between estimated and ground-truth images with median value (± standard deviation) of (61.3 ± 16.7) HU. Median 3D distances, measured on about 400 matching landmarks in each volume, were (2.9 ± 3.0) mm. 3D errors up to 28.2 mm were found for CT images with artifacts or reduced quality. Pass rates for all surrogate approaches were above 98.9% with a γ-criterion of 2%/2 mm. The results depend mainly on the image quality of the initial 4DCT and the deformable image registration. All investigated surrogates can be used to estimate follow-up 4DCT phases, however, uncertainties decrease for volumetric approaches. Application of the model for 4D dose calculations is feasible.

EP-1656: The inter-fraction variation of the supraclavicular- and the axilla-area in breast cancer patients

EP-1656: The inter-fraction variation of the supraclavicular- and the axilla-area in breast cancer patients

OC-0377: Limited interfractional variability of respiration-induced tumor motion in esophageal cancer RT

OC-0377: Limited interfractional variability of respiration-induced tumor motion in esophageal cancer RT

Investigation of interfractional shape variations based on statistical point distribution model for prostate cancer radiation therapy.

The setup errors and organ motion errors pertaining to clinical target volume (CTV) have been considered as two major causes of uncertainties in the determination of the CTV-to-planning target volume (PTV) margins for prostate cancer radiation treatment planning. We based our study on the assumption that interfractional target shape variations are not negligible as another source of uncertainty for the determination of precise CTV-to-PTV margins. Thus, we investigated the interfractional shape variations of CTVs based on a point distribution model (PDM) for prostate cancer radiation therapy. To quantitate the shape variations of CTVs, the PDM was applied for the contours of 4 types of CTV regions (low-risk, intermediate- risk, high-risk CTVs, and prostate plus entire seminal vesicles), which were delineated by considering prostate cancer risk groups on planning computed tomography (CT) and cone beam CT (CBCT) images of 73 fractions of 10 patients. The standard deviations (SDs) of the interfractional random errors for shape variations were obtained from covariance matrices based on the PDMs, which were generated from vertices of triangulated CTV surfaces. The correspondences between CTV surface vertices were determined based on a thin-plate spline robust point matching algorithm. The systematic error for shape variations was defined as the average deviation between surfaces of an average CTV and planning CTVs, and the random error as the average deviation of CTV surface vertices for fractions from an average CTV surface. The means of the SDs of the systematic errors for the four types of CTVs ranged from 1.0 to 2.0 mm along the anterior direction, 1.2 to 2.6 mm along the posterior direction, 1.0 to 2.5 mm along the superior direction, 0.9 to 1.9 mm along the inferior direction, 0.9 to 2.6 mm along the right direction, and 1.0 to 3.0 mm along the left direction. Concerning the random errors, the means of the SDs ranged from 0.9 to 1.2 mm along the anterior direction, 1.0 to 1.4 mm along the posterior direction, 0.9 to 1.3 mm along the superior direction, 0.8 to 1.0 mm along the inferior direction, 0.8 to 0.9 mm along the right direction, and 0.8 to 1.0 mm along the left direction. Since the shape variations were not negligible for intermediate and high-risk CTVs, they should be taken into account for the determination of the CTV-to-PTV margins in radiation treatment planning of prostate cancer.