Intrafractional Variations Research Articles

Treatment Time and Dosimetric Advantage in Cone Beam Computed Tomography-Guided Online Adaptive Radiation Therapy Considering Interfractional and Intrafractional Changes in Patients With Gastric Mucosa-Associated Lymphoid Tissue Lymphoma

Radiation therapy is the standard treatment for localized gastric mucosa-associated lymphoid tissue (MALT) lymphoma. The ETHOS system (Varian Medical System) has enabled us to perform cone beam computed tomography (CBCT)-guided online adaptive radiation therapy (oART). This study presents a retrospective dosimetric analysis for interfractional and intrafractional change and treatment time in oART for gastric MALT lymphoma. We included 3 male patients with gastric MALT lymphoma who underwent exhalation breath-hold fasting oART using the SpiroDynr'X system. Treatment details and plans (3 reference [REF] plans, 51 scheduled [SCH] plans, and adapted [ADP] plans) were retrospectively analyzed. Doses to the clinical target volume in planning CT (CTV_REF), CTV1, and CTV2 (representing the stomach in planning and preirradiation CBCT, respectively) and planning target volume (PTV) in the planning CBCT were estimated. D2%, D98%, D50%, and Dmean for these volumes, along with organ-at-risk doses, were examined across the 3 plans. The PTV dose coverage of CTV2 on preirradiation CBCT was calculated. CBCT-guided oART was completed within the scheduled period, using the ADP plans instead of the SCH plans on each treatment day in all cases. The average treatment time was approximately 45 minutes. CTV1 and CTV2 exhibited intrafractional and interfractional variations, fluctuating above and below CTV_REF. Some ADP plans resulted in incomplete PTV coverage of CTV2, but the unincluded volume was <1% of CTV2. D50%, D98%, and Dmean of CTV1, CTV2, and PTV were significantly improved in the ADP plans than in the SCH plans. Moreover, the Dmean to the liver and kidneys was reduced in the ADP plans. CBCT-guided oART in patients with gastric MALT lymphoma demonstrated that ADP plans improved CTV1, CTV2, and PTV doses and reduced the mean bilateral kidney and liver doses, suggesting that it may offer enhanced treatment precision for gastric MALT lymphoma.

Randomized self-controlled study comparing open-face vs. closed immobilization masks in fractionated cranial radiotherapy

PurposeTo compare patient discomfort and immobilisation performance of open-face and closed immobilization masks in cranial radiotherapy. Material and methodsThis was a single-center randomized self-controlled clinical trial. At CT simulation, an open-face and closed mask was made for each patient and treatment plans with identical dose prescription were generated for each mask. Patients were randomised to start treatment with an open-face or closed mask. Masks were switched halfway through the treatment course; every patient was their own control. Patients self-reported discomfort, anxiety and pain using the visual analogue scale (VAS). Inter- and intrafraction set-up variability was measured with planar kV imaging and a surface guided radiotherapy (SGRT) system for the open-face masks. Results30 patients with primary or metastatic brain tumors were randomized − 29 completed radiotherapy to a median total dose of 54 Gy (range 30–60 Gy). Mean discomfort VAS score was significantly lower with open-face masks (0.5, standard deviation 1.0) vs. closed masks (3.3, standard deviation 2.9), P < 0.0001. Anxiety and pain VAS scores were significantly lower with open-face masks (P < 0.0001). Closed masks caused more discomfort in infraorbital (P < 0.001) and maxillary (P = 0.02) areas. Two patients and 27 patients preferred closed or open-face masks, respectively. Interfraction longitudinal shifts and roll and yaw rotations were significantly smaller and lateral shifts were significantly larger with closed masks in combination with the laser system (P < 0.05) compared to open masks in combination with a SGRT system. Intrafraction variability did not differ between the masks. ConclusionsOpen-face masks are associated with decreased patient discomfort without compromising patient positioning and immobilisation accuracy.

Intra-fractional geometric and dose/volume metric variations of magnetic resonance imaging-guided stereotactic radiotherapy of prostate bed after radical prostatectomy

Background and purposeMagnetic Resonance Imaging (MRI)-guided Stereotactic body radiotherapy (SBRT) treatment to prostate bed after radical prostatectomy has garnered growing interests. The aim of this study is to evaluate intra-fractional anatomic and dose/volume metric variations for patients receiving this treatment. Materials and methodsNineteen patients who received 30–34 Gy in 5 fractions on a 0.35T MR-Linac were included. Pre- and post-treatment MRIs were acquired for each fraction (total of 75 fractions). The Clinical Target Volume (CTV), bladder, rectum, and rectal wall were contoured on all images. Volumetric changes, Hausdorff distance, Mean Distance to Agreement (MDA), and Dice similarity coefficient (DSC) for each structure were calculated. Median value and Interquartile range (IQR) were recorded. Changes in target coverage and Organ at Risk (OAR) constraints were compared and evaluated using Wilcoxon rank sum tests at a significant level of 0.05. ResultsBladder had the largest volumetric changes, with a median volume increase of 48.9 % (IQR 28.9–76.8 %) and a median MDA of 5.1 mm (IQR 3.4–7.1 mm). Intra-fractional CTV volume remained stable with a median volume change of 1.2 % (0.0–4.8 %). DSC was 0.97 (IQR 0.94–0.99). For the dose/volume metrics, there were no statistically significant changes observed except for an increase in bladder hotspot and a decrease of bladder V32.5 Gy and mean dose. The CTV V95% changed from 99.9 % (IQR 98.8–100 %) to 99.6 % (IQR 93.9–100 %). ConclusionDespite intra-fractional variations of OARs, CTV coverage remained stable during MRI-guided SBRT treatments for the prostate bed.

Intra- and inter-fractional variations of tumors with fiducial markers measured using respiratory-correlated computed tomography images for respiratory gated lung stereotactic body radiation therapy.

This study evaluated the intra- and inter-fractional variation of tumors with fiducial markers (FMs), relative to the tumor-FM distance, to establish how close an FM should be inserted for respiratory-gated stereotactic body radiation therapy (RG-SBRT). Forty-five lung tumors treated with RG-SBRT were enrolled. End-expiratory computed tomography (CT) (CTplan) and four-dimensional-CT (4D-CT) scans were obtained for planning. End-expiratory CT (CTfr) scanning was performed before each fraction. The FMs were divided into two groups based on the median tumor-FM distance in the CTplan (Dp). For the intra-fractional variation, the correlations between the corresponding tumor and FM intra-fractional motions, defined as the centroid coordinates of those in each 0-90% phase, with the 50% phase of 4D-CT as the origin, were calculated in the left-right, anterior-posterior, and superior-inferior directions. Furthermore, the maximum difference in the tumor-FM distance in each phase of 4D-CT scan, based on those in the 50% phase of 4D-CT scan (Dmax), was obtained. Inter-fractional variation was defined as the maximum distance between the tumors in CTplan and CTfr, when the CT scans were fused based on each FM or vertebra. The median Dp was 26.1mm. While FM intra-fractional motions were significantly and strongly correlated with the tumor intra-fractional motions in only anterior-posterior and superior-inferior directions for the Dp>26mm group, they were significantly and strongly correlated in all directions for the Dp ≤ 26mm group. In all directions, Dmax values of the Dp ≤ 26mm group were lower than those of the Dp>26mm group. The inter-fractional variations based on the Dp ≤ 26mm were smaller than those on the Dp>26mm and on the vertebra in all directions. Regarding intra- and inter-fractional variation, FMs for Dp ≤ 26mm can increase the accuracy for RG-SBRT.

Optimal planning target margin for prostate radiotherapy based on interfractional and intrafractional variability assessment during 1.5T MRI-guided radiotherapy.

We analyzed daily pre-treatment- (PRE) and real-time motion monitoring- (MM) MRI scans of patients receiving definitive prostate radiotherapy (RT) with 1.5 T MRI guidance to assess interfractional and intrafractional variability of the prostate and suggest optimal planning target volume (PTV) margin. Rigid registration between PRE-MRI and planning CT images based on the pelvic bone and prostate anatomy were performed. Interfractional setup margin (SM) and interobserver variability (IO) were assessed by comparing the centroid values of prostate contours delineated on PRE-MRIs. MM-MRIs were used for internal margin (IM) assessment, and PTV margin was calculated using the van Herk formula. We delineated 400 prostate contours on PRE-MRI images. SM was 0.57 ± 0.42, 2.45 ± 1.98, and 2.28 ± 2.08 mm in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively, after bone localization and 0.76 ± 0.57, 1.89 ± 1.60, and 2.02 ± 1.79 mm in the LR, AP, and SI directions, respectively, after prostate localization. IO was 1.06 ± 0.58, 2.32 ± 1.08, and 3.30 ± 1.85 mm in the LR, AP, and SI directions, respectively, after bone localization and 1.11 ± 0.55, 2.13 ± 1.07, and 3.53 ± 1.65 mm in the LR, AP, and SI directions, respectively, after prostate localization. Average IM was 2.12 ± 0.86, 2.24 ± 1.07, and 2.84 ± 0.88 mm in the LR, AP, and SI directions, respectively. Calculated PTV margin was 2.21, 5.16, and 5.40 mm in the LR, AP, and SI directions, respectively. Movements in the SI direction were the largest source of variability in definitive prostate RT, and interobserver variability was a non-negligible source of margin. The optimal PTV margin should also consider the internal margin.

Optimal Planning Target Margin for Prostate Radiotherapy Based on Interfractional and Intrafractional Variability Assessment during 1.5T MR-Guided Radiotherapy.

MR-guided radiotherapy (MRgRT) provides superior soft-tissue contrast over CT-based image guidance. We collected and analyzed daily pre-treatment (PRE) and real-time motion-monitoring (MM) MR images of patients receiving prostate radiotherapy to assess interfractional and intrafractional variability of prostate using two localization methods: pelvic bony anatomy (bone) and prostate during online adaptive radiotherapy (ART). PRE and MM MRIs for the first five fractions of twenty prostate cancer patients who received definitive MRgRT with 1.5T MRI were collected. Using MIM software, rigid registration between PRE MRI and planning CT images based on pelvic bony anatomy and prostate reproduced bone localization and online ART, respectively. To determine interfractional setup margin (SM), prostate was delineated on all PRE MRIs registered after bone and prostate localizations by a radiation oncologist, and centroid values of prostate contours between planning CT and PRE MRIs were compared. To determine interobserver variability, another radiation oncologist, a medical physicist, and a radiotherapist contoured prostate for both localization methods. For internal margin (IM) assessment, we used MM MRIs of the five patients who had all three sets of coronal, sagittal, and axial cine images and determined the maximum contour displacement using in-house MATLAB-based software converting binary image files to 2D cine images with a superimposed grid of 1 mm spacing. A total of 100 PRE and 25 MM MRIs were analyzed. Four hundred prostate contours were delineated on MR images registered with planning CT based on both bony anatomy and prostate. After bone localization, SM was 0.57±0.42 mm in left-right (LR), 2.45±1.98 mm in anterior-posterior (AP), and 2.28±2.08 mm in superior-inferior (SI) directions, and IO was 1.06±0.58 mm in LR, 2.32±1.08 mm in AP, and 3.30±1.85 mm in SI directions. After prostate localization, SM was 0.76±0.57 mm in LR, 1.89±1.60 mm in AP, and 2.2±1.79 mm in SI directions, and IO was 1.11±0.55 mm in LR, 2.13±1.07 mm in AP, and 3.53±1.65 mm in SI directions. Average IM was 2.12±0.86 mm, 2.24±1.07 mm, and 2.84±0.88 mm in LR, AP, and SI directions, respectively. Using daily MRIs from MRgRT, we showed that movements in the SI direction were the largest source of variability in prostate definitive RT. In addition, interobserver variability was a non-negligible source of margin. Optimal PTV margin should also consider internal margin, especially in the SI direction.

Early Clinical Experience of Utilizing Online Adaptive Radiotherapy for Accelerated Partial Breast Irradiation with Stereotactic Body Radiation Therapy for Early-Stage Breast Cancer Patients

Accelerated partial breast irradiation (APBI) with stereotactic body radiation therapy (SBRT) is becoming popular for early-stage breast cancer (ESBC) due to decreased treatment time and dose to healthy breast tissue. With variability in lumpectomy cavity size post-surgery, CBCT-guided online adaptive RT (oART) may improve target coverage and reduce normal tissue exposure by accounting for inter- and intrafractional cavity variations. In this study, we report our initial clinical experience utilizing APBI oART for patients with ESBC. A total of 15 patients with 17 ESBC cavities (2 with bilateral disease) were treated with SBRT using oART over one year. Patients were immobilized using standard breast simulation setup with free breathing and breath hold (if left sided). The clinical target volume consisted of an isometric 1 cm expansion off the lumpectomy cavity, which was expanded another 3 mm (cropped 3mm from surface) to create the planning target volume (PTV). 95% of the PTV received prescription dose (30 Gy in 5 fractions). The oART delivery process (supervised by physician and physicist) initially coregisters the daily CBCT to the treatment planning CT with alignment to lumpectomy cavity. Target volumes and organs-at-risk were modified at each treatment and the plan re-optimized. Either the re-optimized oART plan or scheduled plan was chosen for each fraction pending which plan best met prespecified clinical goals. Acute toxicity was assessed at one-month follow-up using CTCAE. Patients were age 56 or older, ECOG 0-1, with no known genetic abnormality, and with anatomic stage 0-1A ESBC. Most tumors were located in the upper outer quadrant (n = 12, 70.6%) at middle or posterior depth (n = 15, 88.2%). Final margins were negative. 88% had tantalum clips placed. One patient had an Oncotype DS score of 34 and received systemic therapy. 82.3% met suitable APBI criteria and 3 met cautionary criteria due to high grade, of which 2 had ipsilateral trimodality treatment 10+ years prior and denied mastectomy at recurrence. Mean time to starting RT after lumpectomy was 2.5 months. Mean PTV to breast ratio was 0.12. From simulation to end of treatment, lumpectomy cavity size decreased a median of 27%. oART plans were chosen 89% of the time. VMAT was utilized 52.9% of the time compared to static-field IMRT (47.1%). Three patients were treated with breath hold. 93% of patients were treated on non-consecutive days with average treatment lasting 9.1 ± 2.2 days. CTCAE grade 1 toxicities were hyperpigmentation (n = 6, 40%), breast pain (n = 2, 13.3%), persistent seroma (n = 1, 6.7%), and fatigue (n = 7, 46.7%). No patients experienced fat necrosis, telangiectasias, breast shrinkage, lymphedema, or CTCAE grade 2+ toxicities. oART for APBI using SBRT for patients with ESBC is clinically feasible and allows for variations in lumpectomy cavity size. Nearly half of patients had no complaints or breast changes at one-month follow-up and the remaining had CTCAE grade 1 toxicities alone.

Influence of Eyes Movement on Lens Dose during MR-Guided Radiotherapy for Brain Tumor

Modern radiotherapy techniques achieve highly conformal target doses while avoiding organs at risk such as lens. The magnitude of eyes movement and its influence on lens dose is unclear. The lens dose is relevant for cataract formation. This study aims to evaluate the movement-range of lens and its influence on dose distribution during MR-guided radiotherapy for brain cancer patients. Fifty patients with brain cancer who were treated with MR-guided radiotherapy on 1.5 T MR-Linac were included in this study. All patients underwent a pre-treatment MRI and post-treatment MRI for each fraction. The eyes and lens were delineated on each MR image by a radiation oncologist. The reference treatment plans based original CT were recalculated on each MRI by "adapt to position" workflow. Then, we created planning risk volume (PRV) for lens by adding population margin and reoptimized the reference plans to evaluate the dose changes for lens. Inter-fraction and intrafraction variability for eyes and lens were evaluated with the Dice similarity coefficient (DSC), the Hausdorff distance (HD) and mean distance to agreement (MDA). Differences in daily dose (Dmax and Dmean) for eyes and lens were assessed. A total of 300 MR images were analyzed in this study. The eyes motion was observed (minimum DSC: 0.57; maximum HD: 10.53; maximum MDA: 3.13). And the obvious lens motion also was found (minimum DSC: 0.01; maximum HD: 10.78; maximum MDA: 5.68). The maximum dose changes for eyes were up to 34.1% and the mean dose changes were up to 18.1%. The maximum dose changes for lens were up to 97.9% and the mean dose changes for eyes were up to 89%. When the reoptimized plans were generated by PRV lens, the dose changes were decreased nearly to 0. Eyes movements in all radiotherapy fractions result in higher lens doses and potentially increase cataract formation rate. Adding margin expansions in treatment planning, to account for eyes motion, is the feasible approach to limit lens dose.

4D Monte Carlo dose reconstructions using surface motion measurements

PurposeTo investigate the feasibility of a 4D Monte Carlo based dose reconstruction method to study the dosimetric impact of respiratory motion using surface motion measurements for patients undergoing VMAT treatments for Non-Small Cell Lung Cancer. MethodsThe 4Ddefdosxyznrc/EGSnrc algorithm was used to reconstruct VMAT doses delivered to the patients using machine log files and respiratory traces measured with the RADPOS 4D dosimetry system. The RADPOS sensor was adhered to the patient’s abdomen prior to each treatment fraction and its position was used as a surrogate for tumour motion. Treatment log files were synchronized with the patient respiratory traces. Patient specific respiratory models were generated from deformable registration of the inhale and exhale 4DCT images and the respiratory traces.The reconstructed doses were compared to planned doses calculated with DOSXYZnrc/EGSnrc on the average-intensity and the exhale phase CT images. ResultsRespiratory motion measurements and log files were acquired for 2 patients over 5 treatment fractions each. The motion was predominantly along the anterior/posterior direction (A/P). The average respiratory amplitudes were 8.7 ± 2.7 mm and 10.0 ± 1.2 mm for Patient 1 and 2, respectively. Both patients displayed inter- and intra-fractional variations in the baseline position. Small inter-fractional differences were observed in the reconstructed doses for each patient. Differences between the reconstructed and planned doses were attributed to differences in organ volumes. ConclusionThe 4D reconstruction method was successfully implemented for the two patients studied. Small differences between the planned and reconstructed doses were observed due to the small tumour motion of these patients.

Repeated deep-inspiration breath-hold CT scans at planning underestimate the actual motion between breath-holds at treatment for lung cancer and lymphoma patients

Purpose/objectiveDeep-inspiration breath-hold (DIBH) during radiotherapy may reduce dose to the lungs and heart compared to treatment in free breathing. However, intra-fractional target shifts between several breath-holds may decrease target coverage. We compared target shifts between four DIBHs at the planning-CT session with those measured on CBCT-scans obtained pre- and post-DIBH treatments. Material/methodsTwenty-nine lung cancer and nine lymphoma patients were treated in DIBH. An external gating block was used as surrogate for the DIBH-level with a window of 2 mm. Four DIBH CT-scans were acquired: one for planning (CTDIBH3) and three additional (CTDIBH1,2,4) to assess the intra-DIBH target shifts at scanning by registration to CTDIBH3. During treatment, pre-treatment (CBCTpre) and post-treatment (CBCTpost) scans were acquired. For each pair of CBCTpre/post, the target intra-DIBH shift was determined. For lung cancer, tumour (GTV-Tlung) and lymph nodes (GTV-Nlung) were analysed separately. Group mean (GM), systematic and random errors, and GM for the absolute maximum shifts (GMmax) were calculated for the shifts between CTDIBH1,2,3,4 and between CBCTpre/post. ResultsFor GTV-Tlung, GMmax was larger at CBCT than CT in all directions. GMmax in cranio-caudal direction was 3.3 mm (CT)and 6.1 mm (CBCT). The standard deviations of the shifts in the left–right and cranio-caudal directions were larger at CBCT than CT. For GTV-Nlung and CTVlymphoma, no difference was found in GMmax or SD. ConclusionIntra-DIBH shifts at planning-CT session are generally smaller than intra-DIBH shifts observed at CBCTpre/post and therefore underestimate the intra-fractional DIBH uncertainty during treatment. Lung tumours show larger intra-fractional variations than lymph nodes and lymphoma targets.

Assessing target localization accuracy across different soft-tissue matching protocols using end-exhalation breath-hold cone-beam computed tomography in patients with pancreatic cancer.

The purpose of this study was to retrospectively assess target localization accuracy across different soft-tissue matching protocols using cone-beam computed tomography (CBCT) in a large sample of patients with pancreatic cancer and to estimate the optimal margin size for each protocol. Fifty-four consecutive patients with pancreatic cancer who underwent 15-fraction volumetric modulated arc therapy under the end-exhalation breath-hold condition were enrolled. Two soft-tissue matching protocols were used according to the resectability classification, including gross tumor volume (GTV) matching for potentially resectable tumors and planning target volume (PTV) matching for borderline resectable or unresectable tumors. The tolerance of the target localization error in both matching protocols was set to 5mm in any direction. The optimal margin size for each soft-tissue matching protocol was calculated from the systematic and random errors of the inter- and intrafraction positional variations using the van Herk formula. The inter- and intrafraction positional variations of PTV matching were smaller than those of GTV matching. The percentage of target localization errors exceeding 5mm in the first CBCT scan of each fraction in the superior-inferior direction was 12.6 and 4.8% for GTV and PTV matching, respectively. The optimal margin sizes for GTV and PTV matching were 3.7 and 2.7, 5.4 and 4.1 and 3.9 and 3.0mm in the anterior-posterior, superior-inferior and left-right directions, respectively. Target localization accuracy in PTV matching was higher than that in GTV matching. By setting the tolerance of the target localization error, treatment can be successful within the planned margin size.

PO-1899 Intra-fraction variation of volume and position of stomach, gastric gas and gastric contents on MRI

PO-1899 Intra-fraction variation of volume and position of stomach, gastric gas and gastric contents on MRI

Development of an abdominal dose accumulation tool and assessments of accumulated dose in gastrointestinal organs

Objective. Online adaptive radiotherapy has demonstrated improved dose conformality in response to inter-fraction geometric variations in the abdomen. The dosimetric impact of intra-fractional variations in anatomic configuration resulting from breathing, gastric contraction and slow configuration motion, however, have been largely ignored, leading to differences between delivered and planned. To investigate the impact of intra-fractional abdominal motions on delivered dose, anatomical deformations due to these three motion modes were extracted from dynamic MRI data using a previously developed hierarchical motion modeling methodology. Approach. Motion magnitudes were extracted from deformation fields between a reference state and all other motion states of the patient. Delivered dose estimates to various gastrointestinal organs (stomach, duodenum, small bowel and colon) were calculated on each motion state of the patient and accumulated to estimate the delivered dose to each organ for the entire treatment fraction. Main results. Across a sample of 10 patients, maximal motions of 33.6, 33.4, 47.6 and 49.2 mm were observed over 20 min for the stomach, duodenum, small bowel and colon respectively. Dose accumulation results showed that motions could lead to average increases of 2.0, 2.1, 1.1, 0.7 Gy to the maximum dose to 0.5cc (D0.5cc) and 3.0, 2.5, 1.3, 0.9 Gy to the maximum dose to 0.1cc (D0.1cc) for these organs at risk. From the 40 dose accumulations performed (10 for each organ at risk), 27 showed increases of modeled delivered dose compared to planned doses, 4 of which exceeded planned dose constraints. Significance. The use of intra-fraction motion measurements to accumulate delivered doses is feasible, and supports retrospective estimation of dose delivery to improve estimates of delivered doses, and further guide strategies for both plan adaptation as well as advances in intra-fraction motion management.

Inter- and intrafraction dose variations in robotic stereotactic body radiation therapy (SBRT) for perihilar cholangiocarcinoma in the prospective phase I STRONG trial.

Using fiducial-marker-based robotic respiratory tumor tracking, we treated perihilar cholangiocarcinoma patients in the STRONG trial with 15 daily fractions of 4 Gy. For each of the included patients, in-room diagnostic-quality repeat CTs (rCT) were acquired pre- and post-dose delivery in 6 treatment fractions to analyze inter- and intrafraction dose variations. Planning CTs (pCTs) and rCTs were acquired in expiration breath-hold. Analogous to treatment, spine and fiducials were used to register rCTs with pCTs. In each rCT, all OARs were contoured, and the target was rigidly copied from the pCT based on grey values. The rCTs acquired were used to calculate the doses to be delivered through the treatment-unit settings. On average, target doses in rCTs and pCTs were similar. However, due to target displacements relative to the fiducials in rCTs, 10% of the rCTs showed PTV coverage losses of >10%. Although target coverages had been planned below desired values in order to protect OARs, many pre-rCTs contained OAR constraint violations: 44.4% for the 6 major constraints. Most OAR dose differences between pre- and post-rCTs were not statistically significant. The dose deviations observed in repeat CTs represent opportunities for more advanced adaptive approaches to enhancing SBRT treatment quality.

Real-time prediction of stomach motions based upon gastric contraction and breathing models

Objective. Precision radiation therapy requires managing motions of organs at risk that occur during treatment. While methods have been developed for real-time respiratory motion tracking, non-breathing intra-fractional variations (including gastric contractile motion) have seen little attention to date. The purpose of this study is to develop a cyclic gastric contractile motion prediction model to support real-time management during radiotherapy. Approach. The observed short-term reproducibility of gastric contractile motion permitted development of a prediction model that (1) extracts gastric contraction motion phases from few minutes of golden angle stack of stars scanning (at patient positioning), (2) estimate gastric phase of real-time sampled data acquired during treatment delivery to these reconstructed phases and (3) predicting future gastric phase by linear extrapolation using estimation results from step 2 to account for processing and system latency times. Model was evaluated on three parameters including training time window for step 1, number of spokes for real-time sampling data in step 2 and future prediction time. Main results. The model was tested on a population of 20 min data samples from 25 scans from 15 patients. The mean prediction error with 10 spokes and 2 min training was 0.3 ± 0.1 mm (0.1–0.7 mm) with 5.1 s future time, slowly rising to 0.6 ± 0.2 mm (0.2–1.1 mm) for 6.8 s future time and then increasing rapidly for longer forward predictions, for an average 3.6 ± 0.5 mm (2.8–4.7 mm) HD95 of gastric motion. Results showed that reducing of train time window (5–2 min) does not influence the prediction performance, while using 5 spokes increased prediction errors. Significance. The proposed gastric motion prediction model has sufficiently accurate prediction performance to allow for sub-millimeter accuracy while allowing sufficient time for data processing and machine interaction and shows the potential for clinical implementation to support stomach motion tracking during radiotherapy.

Simulation of dosimetric consequences of intrafraction variation of tumor drift in lung cancer stereotactic body radiotherapy.

The purpose of this study was to investigate the target dose discrepancy caused by intrafraction variation during stereotactic body radiotherapy (SBRT) for lung cancer. Intensity-modulated radiation therapy (IMRT) plans were designed based on average computed tomography (AVG CT) utilizing the planning target volume (PTV) surrounding the 65% and 85% prescription isodoses in both phantom and patient cases. Variation was simulated by shifting the nominal plan isocenter along six directions from 0.5mm to 4.5mm with a 1-mm step size to produce a series of perturbed plans. The dose discrepancy between the initial plan and the perturbed plans was calculated as the percentage of the initial plan. Dose indices, including ΔD99 for internal target volume (ITV) and gross tumor volume (GTV), were adopted as endpoint samples. The mean dose discrepancy was calculated under the 3-dimensional space distribution. We found that motion can lead to serious dose degradation of the target and ITV in lung SBRT, especially during SBRT with PTV surrounding the lower isodose line. Lower isodose line may lead to larger dose discrepancy, while make steeper dose fall-off gradient. This phenomenon was compromised when 3-dimensional space distribution was considered. This result may provide a prospective reference for target dose degradation due to motion during lung SBRT treatment.

Dosimetry Study of Radiation Toxicity in Young Patients with Mediastinal Lymphoma by the Use of an Inclined Board and Deep Inspiration Breath-Holding, under the Guidance of an Optical Surface Management System

<h3>Purpose/Objective(s)</h3> Survivors of mediastinal lymphoma are at increased risk of lung cancer, breast cancer, and cardiovascular disease after radiotherapy. We compared treatment plans for patients in one of two positions while using one of two breathing techniques: supine on an inclined board (IB) or flat; and using deep inspiration breath-hold (DIBH) or free-breathing. <h3>Materials/Methods</h3> Ten young patients with mediastinal lymphoma were eligible for the study. Sets of intensity-modulated radiation therapy (IMRT) treatment plans were created for each of 10 patients with mediastinal lymphoma (6 women, 4 men): DIBH-IB (for women); FB-IB (for women); DIBH-flat (for men); and FB-flat (for men and women). During DIBH, an optical surface management system (OSMS) was used to monitor breathing rate and magnitude, track respiratory status in real time. Tumor target volumes and organs at risk (OARs; lungs, breasts [in women], thyroid, and heart and its substructures) were compared in all four conditions. <h3>Results</h3> A total of 26 plans were created for all 10 patients. The DIBH-IB plan (for women) and the DIBH-flat plan (for men) was used for actual patient treatment. Daily pre-RT cone beam CT (CBCT) was used before treatment to adjust for set-up errors. Daily post-RT CBCT was used after treatment to assess intra-fractional variability. Registration of post-RT CBCT with the planning CT showed that both tumor and OARs matched well, with rather small intra-fractional variability. The planning target volume (PTV) was considerably smaller in the DIBH plans (380.0 cm³ for DIBH vs 527 cm³ for FB, <i>P</i><0.001), because of the smaller clinical target volume-to-PTV margins in the DIBH plans and the compression of the CTV due to the deep inspiration and breath hold. Plans for DIBH-IB for women and DIBH-flat for men produced lower mean lung dose (6.1 vs 8.1 Gy; <i>P</i><0.001), lung V5 (36.1% vs 47.9%; <i>P</i>=0.001), and lung V20 (7.9% vs 11.7%; <i>P</i><0.001) compared with FB-IB and FB-flat plans. Plans for FB-IB also resulted in smaller mean breast dose and breast V5 than FB-flat (3.4 vs 4.0 Gy, 16.3% vs 20.4%, <i>P</i>=0.018 and 0.008). Use of DIBH-IB reduced both measures still further relative to FB-flat (2.6 vs 4.0 Gy, 13.8% vs 20.4%, <i>P</i>=0.015 and 0.013). Plans for DIBH produced lower mean heart dose (8.7 vs 10.4Gy; <i>P</i>=0.048) and heart V5 (40.6% vs 55.4%; <i>P</i>=0.023) compared with FB plans. Use of DIBH brought more dosimetric benefits of mean dose, V5 and V20 to the right heart substructures including right ventricle, right atrium and right coronary artery. <h3>Conclusion</h3> In this small group of patients with mediastinal lymphoma, use of DIBH (with an OSMS) and an IB resulted in lower estimated doses to lungs, breasts and heart, especially the right heart, than did FB; these reduced doses may reduce the probability of pulmonary, breast and cardiac radiation toxicity.

Treatment Time Optimization in Single Fraction Stereotactic Ablative Radiation Therapy: A 10-Year Institutional Experience

PurposeStereotactic ablative radiation therapy (SABR) delivered in a single fraction (SF) can be considered to have higher uncertainty given that the error probability is concentrated in a single session. This study aims to report the variation in technology and technique used and its effect on intrafraction motion based on a 10 years of experience in SF SABR. Methods and MaterialsRecords of patients receiving SF SABR delivered at our instruction between 2010 and 2019 were included. Treatment parameters were extracted from the patient management database by using an in-house script. Treatment time was defined as the time difference between the first image acquisition to the last beam off of a single session. The intrafraction variation was measured from the 3-dimensional couch displacement measured after the first cone beam computed tomography (CBCT) acquired during a treatment. ResultsThe number of SF SABR increased continuously from 2010 to 2019 and were mainly lung treatments. Treatment time was minimized by using volumetric modulated arc therapy, flattening filter-free dose rate, and coplanar field (24 ± 9 min). Treatment time increased as the number of CBCTs per session increased. The most common scenario involved both 2 and 3 CBCTs per session. On the average, a CBCT acquisition added 6 minutes to the treatment time. All treatments considered, the average intrafraction variation was 1.7 ± 1.6 mm. ConclusionsSF SABR usage increased with time in our institution. The intrafraction motion was acceptable and therefore a single fraction is an efficacious treatment option when considering SABR.

PD-0232 Large intra-fractional tumor position variations in deep-inspiration breath-hold lung SBRT

PD-0232 Large intra-fractional tumor position variations in deep-inspiration breath-hold lung SBRT

PO-1720 Inter- and intrafraction dose variations in perihilar cholangiocarcinoma SBRT in the STRONG trial

PO-1720 Inter- and intrafraction dose variations in perihilar cholangiocarcinoma SBRT in the STRONG trial