Interfractional Errors Research Articles

Inter-fractional error and intra-fractional motion of prostate and dosimetry comparisons of patient position registrations with versus without fiducial markers during treatment with carbon-ion radiotherapy.

A few reports have discussed the influence of inter-fractional position error and intra-fractional motion on dose distribution, particularly regarding a spread-out Bragg peak. We investigated inter-fractional and intra-fractional prostate position error by monitoring fiducial marker positions. In 2020, data from 15 patients with prostate cancer who received carbon-ion beam radiotherapy (CIRT) with gold markers were investigated. We checked marker positions before and during irradiation to calculate the inter-fractional positioning and intra-fractional movement and evaluated the CIRT dose distribution by adjusting the planning beam isocenter and clinical target volume (CTV) position. We compared the CTV dose coverages (CTV receiving 95% [V95%] or 98% [V98%] of the prescribed dose) between skeletal and fiducial matching irradiation on the treatment planning system. For inter-fractional error, the mean distance between the marker position in the planning images and that in a patient starting irradiation with skeletal matching was 1.49 ± 1.11mm (95th percentile = 1.85mm). The 95th percentile (maximum) values of the intra-fractional movement were 0.79mm (2.31mm), 1.17mm (2.48mm), 1.88mm (4.01mm), 1.23mm (3.00mm), and 2.09mm (8.46mm) along the lateral, inferior, superior, dorsal, and ventral axes, respectively. The mean V95% and V98% were 98.2% and 96.2% for the skeletal matching plan and 99.5% and 96.8% for the fiducial matching plan, respectively. Fiducial matching irradiation improved the CTV dose coverage compared with skeletal matching irradiation for CIRT for prostate cancer.

Development of Magnetic Resonance-Compatible Head Immobilization Device and Initial Experience of Magnetic Resonance-Guided Radiation Therapy for Central Nervous System Tumors

PurposeWe aimed to develop and investigate positional reproducibility using a fixation device (Unity Brain Tumor Immobilization Device, UBID) in brain tumor patients undergoing magnetic resonance (MR)-guided radiotherapy (RT) with a 1.5 Tesla (T) MR-linear accelerator (LINAC) to evaluate its feasibility in clinical practice and report representative cases of central nervous system (CNS) tumor patients. Materials and MethodsQuantitative analysis was performed by comparing images obtained by placing only the MR phantom on the couch with those obtained by placing UBID next to the MR phantom. Twenty patients who underwent RT for CNS tumors using 1.5T MR-LINAC between June 2022 and October 2022 were retrospectively analyzed. Among them, five did not use UBID, whereas 15 used UBID. The positional reproducibility of UBID was evaluated using the median interfractional and intrafractional errors in the first 10 fractions. ResultsEach MR quality factor of the MR phantom with UBID satisfied the criteria presented by Elekta. Median values of median shifts in the mediolateral, anteroposterior, and craniocaudal axes for interfractional errors were 2.98-mm, 2.35-mm, and 1.40-mm, respectively. For intrafractional errors, the median values were 0.05-mm, 0.03-mm, and 0.06-mm, respectively. The median values of the median rotations in pitch, roll, and yaw for both interfractional and intrafractional rotations were 0.00°. One patient diagnosed with an optic nerve sheath meningioma received RT with motion monitoring during irradiation. In two patients, changes in the tumor cavity and residual lesions were observed in the MR images obtained using 1.5T MR-LINAC on the day of the first treatment and immediately before the 21st fraction, respectively; therefore, offline/online adaptation was performed. ConclusionThe reproducible and immobile UBID is clinically feasible in patients with CNS tumors receiving RT with 1.5T MR-LINAC. Based on our initial experience, we developed a workflow for 1.5T MR-LINAC treatment of CNS tumors.

Two-dimensional and Three-dimensional Image-guided Evaluation of Patient Positioning for Intensity-modulated Radiation Therapy of Head and Neck Cancer

Patient positioning accuracy is critical in radiation therapy, especially in intensity-modulated radiation therapy (IMRT) for head and neck cancer (HNC), as it can affect treatment effectiveness and safety. This retrospective study aimed to evaluate the accuracy of patient positioning techniques and compare the effectiveness of using multiple image-guided (IG) methods for IG-IMRT of HNC. Cone-beam computed tomography (CBCT) and kV-planar imaging (OBI) collected 3240 treatment couch coordinates in three translational directions from 60 HNC patients undergoing IMRT. Inter-fraction errors were assessed by registering the scans to the planning CT, and the population systematic set-up error (Σ), random error (σ), and planning target volume (PTV) margin were calculated. The results between OBI and CBCT were analyzed and compared using one-way ANOVA. The findings demonstrated that more than 80% of the image-guided patient positioning set-ups were acceptable. The mean couch displacement for image-guided techniques was negligible in all translational directions using OBI and CBCT. However, the PTV margin for both methods was more than 0.5 cm, except for the CBCTA-P direction. This study highlights the effectiveness of OBI and CBCT as modalities for evaluating and improving the accuracy of IMRT in HNC patients. Determination of the systematic and random errors and calculating the optimal PTV margin without rematching images can help improve the precision of patient positioning and ultimately lead to better treatment outcomes.

Surface guided radiation therapy with an innovative open-face mask and mouth bite: patient motion management in brain stereotactic radiotherapy.

To guarantee treatment reproducibility and stability, immobilization devices are essential. Additionally, surface-guided radiation therapy (SGRT) serves as an accurate complement to frameless stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) by aiding patient positioning and real-time monitoring, especially when non-coplanar fields are in use. At our institute, we have developed a surface-guided SRS (SG-SRS) workflow that incorporates our innovative open-face mask (OM) and mouth bite (MB) to guarantee a precise and accurate dose delivery. This study included 40 patients, and all patients were divided into closed mask (CM) and open-face mask (OM) groups according to different positioning flow. Cone beam computed tomography (CBCT) scans were performed, and the registration results were recorded before and after the treatment. Then Bland-Altman method was used to analyze the consistency of AlignRT-guided positioning errors and CBCT scanning results in the OM group. The error changes between 31 fractions in one patient were recorded to evaluate the feasibility of monitoring during treatment. The median of translation error between stages of the AlignRT positioning process was (0.03-0.07) cm, and the median of rotation error was (0.20-0.40)°, which were significantly better than those of the Fraxion positioning process (0.09-0.11) cm and (0.60-0.75)°. The mean bias values between the AlignRT guided positioning errors and CBCT were 0.01cm, -0.07cm, 0.03cm, -0.30°, -0.08° and 0.00°. The 31 inter-fractional errors of a single patient monitored by SGRT were within 0.10cm and 0.50°. The application of the SGRT with an innovative open-face mask and mouth bite device could achieve precision positioning accuracy and stability, and the accuracy of the AlignRT system exhibits excellent constancy with the CBCT gold standard. The non-coplanar radiation field monitoring can provide reliable support for motion management in fractional treatment.

Impact of planning organ at risk volume margins and matching method on late gastrointestinal toxicity in moderately hypofractionated IMRT for locally advanced pancreatic ductal adenocarcinoma

BackgroundThis study examined the differences in late gastrointestinal (GI) toxicities in moderately hypofractionated intensity-modulated radiation therapy (IMRT) for locally advanced pancreatic ductal adenocarcinoma (LA-PDAC) by changing the planning organs at risk volume (PRV) margin and the target matching method and assessed the causes of adverse events.MethodsWe examined 37 patients with LA-PDAC who underwent moderately hypofractionated IMRT between 2016 and 2020 at our institution; 23 patients were treated with wide PRV margins and soft tissue matching (Protocol A) and 14 with narrow PRV margins and fiducial marker matching (Protocol B). The GI toxicities, local control (LC) rate, and overall survival (OS) were assessed for each protocol. The initially planned and daily doses to the gross tumor volume (GTV), stomach, and duodenum, reproduced from cone-beam computed tomography, were evaluated.ResultsThe late GI toxicity rate of grades 3–4 was higher in Protocol B (42.9%) than in Protocol A (4.3%). Although the 2-year LC rates were significantly higher in Protocol B (90.0%) than in Protocol A (33.3%), no significant difference was observed in OS rates. In the initial plan, no deviations were found for the stomach and duodenum from the dose constraints in either protocol. In contrast, daily dose evaluation for the stomach to duodenal bulb revealed that the frequency of deviation of V3 Gy per session was 44.8% in Protocol B, which was significantly higher than the 24.3% in Protocol A.ConclusionsReducing PRV margins with fiducial marker matching increased GI toxicities in exchange for improved LC. Daily dose analysis indicated the trade-off between the GTV dose coverage and the irradiated doses to the GI. This study showed that even with strict matching methods, the PRV margin could not be reduced safely because of GI inter-fractional error, which is expected to be resolved with online adaptive radiotherapy.

Impact of Interfractional Error on Dosiomic Features.

ObjectivesThe purpose of this study was to investigate the stability of dosiomic features under random interfractional error. We investigated the differences in the values of features with different fractions and the error in the values of dosiomic features under interfractional error.Material and MethodsThe isocenters of the treatment plans of 15 lung cancer patients were translated by a maximum of ±3 mm in each axis with a mean of (0, 0, 0) and a standard deviation of (1.2, 1.2, 1.2) mm in the x, y, and z directions for each fraction. A total of 81 dose distributions for each patient were then calculated considering four fraction number groups (2, 10, 20, and 30). A total of 93 dosiomic features were extracted from each dose distribution in four different regions of interest (ROIs): gross tumor volume (GTV), planning target volume (PTV), heart, and both lungs. The stability of dosiomic features was analyzed for each fraction number group by the coefficient of variation (CV) and intraclass correlation coefficient (ICC). The agreements in the means of dosiomic features among the four fraction number groups were tested by ICC. The percent differences (PD) between the dosiomic features extracted from the original dose distribution and the dosiomic features extracted from the dose distribution with interfractional error were calculated.ResultsEleven out of 93 dosiomic features demonstrated a large CV (CV ≥ 20%). Overall CV values were highest in GTV ROIs and lowest in lung ROIs. The stability of dosiomic features decreased as the total number of fractions decreased. The ICC results showed that five out of 93 dosiomic features had an ICC lower than 0.75, which indicates intermediate or poor stability under interfractional error. The mean dosiomic feature values were shown to be consistent with different numbers of fractions (ICC ≥ 0.9). Some of the dosiomic features had PD greater than 50% and showed different PD values with different numbers of fractions.ConclusionSome dosiomic features have low stability under interfractional error. The stability and values of the dosiomic features were affected by the total number of fractions. The effect of interfractional error on dosiomic features should be considered in further studies regarding dosiomics for reproducible results.

PO-1415 Inter-fraction error analyses and rectal and bladder volume variations using CBCT in prostate cancer

PO-1415 Inter-fraction error analyses and rectal and bladder volume variations using CBCT in prostate cancer

Original Knee Fixation Device as a Useful Fixation Method during Prostate Intensity-Modulated Radiation Therapy.

Purpose:The purpose of he study was to reduce setup errors during intensity-modulated radiation therapy (IMRT) with an original knee fixation device (KFD) and evaluate the clinical target volume (CTV) coverage.Methods:Participants were classified into two groups: knee flexion (KF) group (n = 16), wherein participants' knees were fixed in a flexed position using the proposed KFD during planning computed tomography, and knee extension group (KE; n = 15), wherein no KFD was used. We investigated the residual rotational errors and inter-fractional setup errors with or without KFD. Furthermore, inter-fractional margins were calculated using logistic regression analysis, and CTV coverage was evaluated.Results:The residual rotational errors in the yaw and roll directions (P < 0.02) and the inter-fractional error in the anterior-posterior (A-P) direction (P < 0.02) improved significantly in the KF group compared with the KE group. Repeatability was improved for the pitch direction. The inter-fractional margins were 6.68 mm and 4.87 mm in the A-P and superior-inferior (S-I) directions, respectively, in the KF group, representing reductions (mm) of 20.8% and 12.6% compared with the KE group, respectively. The odds ratios for CTV coverage in the KF group compared to the KE group were 2.76 (P < 0.001) and 1.74 (P < 0.05) in the A-P and S-I directions, respectively.Conclusions:The IMRT fixation method using an original KFD improved the residual rotational error in the three directions and the inter-fractional error in the A-P direction, reduced the interfractional margins in the A-P, and S-I directions and improved CTV coverage. Our original KFD may be a useful fixation method during prostate IMRT.

Organ motion in linac-based SBRT for glottic cancer

PurposeThe purpose of this study is to evaluate inter- and intra-fraction organ motion as well as to quantify clinical target volume (CTV) to planning target volume (PTV) margins to be adopted in the stereotactic treatment of early stage glottic cancer.Methods and materialsStereotactic body radiotherapy (SBRT) to 36 Gy in 3 fractions was administered to 23 patients with early glottic cancer T1N0M0. Patients were irradiated with a volumetric intensity modulated arc technique delivered with 6 MV FFF energy. Each patient underwent a pre-treatment cone beam computed tomography (CBCT) to correct the setup based on the thyroid cartilage position. Imaging was repeated if displacement exceeded 2 mm in any direction. CBCT imaging was also performed after each treatment arc as well as at the end of the delivery. Swallowing was allowed only during the beam-off time between arcs. CBCT images were reviewed to evaluate inter- and intra-fraction organ motion. The relationships between selected treatment characteristics, both beam-on and delivery times as well as organ motion were investigated.ResultsFor the population systematic (Ʃ) and random (σ) inter-fraction errors were 0.9, 1.3 and 0.6 mm and 1.1, 1.3 and 0.7 mm in the left-right (X), cranio-caudal (Y) and antero-posterior (Z) directions, respectively. From the analysis of CBCT images acquired after treatment, systematic (Ʃ) and random (σ) intra-fraction errors resulted 0.7, 1.6 and 0.7 mm and 1.0, 1.5 and 0.6 mm in the X, Y and Z directions, respectively. Margins calculated from the intra-fraction errors were 2.4, 5.1 and 2.2 mm in the X, Y and Z directions respectively. A statistically significant difference was found for the displacement in the Z direction between patients irradiated with > 2 arcs versus ≤ 2 arcs, (MW test, p = 0.038). When analyzing mean data from CBCT images for the whole treatment, a significant correlation was found between the time of delivery and the three dimensional displacement vector (r = 0.489, p = 0.055), the displacement in the Y direction (r = 0.553, p = 0.026) and the subsequent margins to be adopted (r = 0.626, p = 0.009). Finally, displacements and the subsequent margins to be adopted in Y direction were significantly greater for treatments with more than 2 arcs (MW test p = 0.037 and p = 0.019, respectively).ConclusionsIn the setting of controlled swallowing during treatment delivery, intra-fraction motion still needs to be taken into account when planning with estimated CTV to PTV margins of 3, 5 and 3 mm in the X, Y and Z directions, respectively. Selected treatments may require additional margins.

Comparison of Setup Variations and Defining a CTV-PTV Margin in Fractionated Radiotherapy of Brain Tumor Using Novalis Tx Orthogonal OBI (kv) Versus Brain Lab ExacTrac (kv) Imaging Systems with Brain Lab Mask

The objective of this study is to evaluate the setup discrepancy between Brain Lab ExacTrac (kv) and Orthogonal Novalis Tx (kv) imaging systems and to obtain an optimal CTV-PTV margin. We recruited 35 consecutive brain tumor patients, immobilized with non-invasive thermoplastic BrainLab mask, between August 2016 and August 2017. The position verification done by ExacTrac, Infrared Positioning System was followed by a set of images by OBI at 0° and 90° position. The images were autofused and compared with DRR generated by TPS. The extent of translational and rotational shift was then applied and documented. The translational inter-fractional setup error (mean ± SD) in X-axis detected by OBI and ET was 0.002 ± 0.227 and −0.042 ± 0.084, respectively. In Y-axis, the error detected was −0.01 ± 0.176 and 0.075 ± 0.079, respectively, whereas in Z-axis the error with OBI and ET was 0.329 ± 0.521 and 0.051 ± 0.091, respectively. The P-value of inter-fractional errors in X, Y, and Z axes was 0.294, 0.013, and 0.004, which is statistically significant in Y- and Z-axes. Rotational inter-fractional setup errors (mean ± SD) for OBI and ET were −0.101 ± 0.257 and −0.076 ± 0.102, respectively. The rotational setup errors detected by ET in pitch and roll dimension are −0.003 ± 0.084 and −0.085 ± 0.073, respectively. The data obtained were put in Van Herk’s formula (PTV margin = 2.5 ∑ + 0.74 σ), and CTV-PTV margin was calculated. CTV-PTV margin for ET was 2.70, 2.72, and 2.94 mm and 3.38o, 2.84o, and 2.51o in lateral, longitudinal, vertical and pitch, roll, yaw dimension, respectively. The CTV-PTV margin for OBI was 7.40, 6.20, and 5.30 mm and 7.27o in lateral, longitudinal, vertical, and yaw dimension, respectively. Daily setup verifications that use the Brain LAB Exac Trac 6D image-guided system are very useful for brain tumor treatment; nevertheless, OBI kv imaging can be used for FRT of brain tumor with extended margin.

Statistical control of processes applied to geometric uncertainties for CTV expansion margins determination in prostate cancer patients treated with VMAT: a prospective study in 57 patients.

To study the control graphs applicability for the geometric uncertainties of VMAT treatments in prostate cancer patients, and their use to verify the hypothesis of the data obtained randomness, to apply the margins of Van Herk expression. During the first 5days of treatment, and then once a week, a Kv CBCT was performed, compared with the simulation CT and adjusted the displacements, to determine the inter-fraction errors. Immediately after radiation therapy, another CBCT was performed (for intra-fraction errors). With these data, the X, R position control charts have been made. The patients, not maintained the deviations within the charts control limits, were called "anomalies". Then, we compared the deviations and margins calculated with the van Herk expression for all patients and for those without anomalies. The margins determined show appreciable differences if there were calculated for the total set of patients or for the set of them without anomalies in the control charts. For the overall set of patients, the lateral, longitudinal, and vertical margins were 0.45cm, 0.52cm, 0.56cm, while for the set of patients without anomalies were 0.29cm, 0.35cm, and 0.38cm. The use of control charts allows tracking geometric deviations both inter and intra-fraction, variability real-time control and to detect situations in which it can change for non-random reasons, and require immediate investigation. Maintaining geometric deviations in the control state decreases the margins needed to administer a high dose to CTV in a high percentage of cancer prostate patients.

Assessment of set up accuracy using an evacuated cushion for proton beam therapy

Keywords – Proton Beam Therapy, VacBag, Setup Introduction: Effective, reproducible patient positioning is paramount for radiotherapy1,2. With the introduction of a new proton beam therapy (PBT) service, evacuated cushions (vacbags) have been implemented for patients receiving treatment to the limbs, abdomen, thorax and pelvis. This work reports on an audit reviewing set up times, image review times and positional reproducibility to confirm the suitability of the vacbag as an immobilisation tool in PBT. Method and Materials: The first ten patients treated on vacbags were included in this study (n=10). Patients were initially imaged with kilovoltage (kV) 2-dimensional (2D) image pairs as a gross error check (GEC), followed by a conebeam CT (CBCT) from which positional errors were corrected and verified with a final 2DkV image before proceeding to treatment. Images were compared to the planning CT to determine translational and rotational displacements. Treatment and image review times were also reported to audit the new service. Results: Compared to the planning CT, translational displacements were satisfactory with 95% <5mm and 100% <10mm. Greater variation was seen in rotational displacements, however, on average only 3% of all exceeded the 2⁰ tolerance. Agreement between 2D and 3D imaging were within the institutions defined GEC tolerance of <1cm and <2⁰. This agreement was reached in 99% of translational and 91% of rotational displacements. Treatment delivery times in minutes (mins) were reduced from a mean of 44 mins for the first patient to <30 mins for the most recent patient. Average Image review time for the GEC 2D orthogonal images also reduced from 6 mins to 3 mins, however the trend for review time of CBCT remained an average of <10 mins (range 7 mins – 12mins). Conclusion and Discussion: Initial results show that vacbags are suitable for accurate patient positioning and minimising translational displacements. Rotational displacements appeared more random. As such it has been determined, the use of a 6⁰ of freedom bed, combined with daily imaging, is paramount for accurate set up. Further investigation is ongoing to validate these results including the dosimetric impacts of changes in patient contours on the accuracy of intended treatment delivery. Numerical References (1) Aran Kim, BSc, MRT(T)a*, Valerie Kelly, MSc, MRT(T)ab, Colleen Dickie, MSc, MRT(T) (MR)ab, Charles Catton, MD, FRCPCab and Winnie Li, MSc, MRT(T)ab. Impact of Immobilization on Interfractional Errors for Upper Extremity SoftTissue Sarcoma Radiation Therapy, Journal of Medical Imaging and Radiation Sciences, 50, 308-316 (2) Winnie Li, B.Sc., R.T.T.,Arjun Sahgal, M.D.,Matthew Foote, M.D.,Barbara-Ann Millar, M.D.,David A. Jaffray, Ph.D.,and Daniel Letourneau, Ph.D. Impact of Immobilization on Intrafraction Motion for Spine Stereotactic Body Radiotherapy Using Cone Beam Computed Tomography, Int J Radiat Oncol Biol Phys, 2011, 84, 520-525

Estimating PTV Margins in Head and Neck Stereotactic Ablative Radiation Therapy (SABR) Through Target Site Analysis of Positioning and Intrafractional Accuracy

Recurrent or previously irradiated head and neck cancers (HNC) are therapeutically challenging and may benefit from high-dose, highly accurate radiation techniques, such as stereotactic ablative radiation therapy (SABR). Here, we compare set-up and positioning accuracy across HNC subsites to further optimize the treatment process and planning target volume (PTV) margin recommendations for head and neck SABR. We prospectively collected data on 405 treatment fractions across 79 patients treated with SABR for recurrent/previously irradiated HNC. First, interfractional error was determined by comparing ExacTrac x-ray to the treatment plan. Patients were then shifted and residual error was measured with repeat x-ray. Next, cone beam computed tomography (CBCT) was compared with ExacTrac for positioning agreement, and final shifts were applied. Lastly, intrafractional error was measured with x-ray before each arc. Results were stratified by treatment site into skull base, neck/parotid, and mucosal. Most patients (66.7%) were treated to 45 Gy in 5 fractions (range, 21-47.5 Gy in 3-5 fractions). The initial mean ± standard deviation interfractional errors were -0.2 ± 1.4 mm (anteroposterior), 0.2 ± 1.8 mm (craniocaudal), and -0.1 ± 1.7 mm (left-right). Interfractional 3-dimensional vector error was 2.48 ± 1.44, with skull base significantly lower than other sites (2.22 vs 2.77; P = .0016). All interfractional errors were corrected to within 1.3 mm and 1.8°. CBCT agreed with ExacTrac to within 3.6 mm and 3.4°. CBCT disagreements and intrafractional errors of >1 mm or >1° occurred at significantly lower rates in skull base sites (CBCT: 16.4% vs 50.0% neck, 52.0% mucosal, P < .0001; intrafractional: 22.0% vs 48.7% all others, P < .0001). Final PTVs were 1.5 mm (skull base), 2.0 mm (neck/parotid), and 1.8 mm (mucosal). Head and neck SABR PTV margins should be optimized by target site. PTV margins of 1.5 to 2 mm may be sufficient in the skull base, whereas 2 to 2.5 mm may be necessary for neck and mucosal targets. When using ExacTrac, skull base sites show significantly fewer uncertainties throughout the treatment process, but neck/mucosal targets may require the addition of CBCT to account for positioning errors and internal organ motion.

Feasibility of Gold Fiducial Markers as a Surrogate for Gross Tumor Volume Position in Image-Guided Radiation Therapy of Rectal Cancer

To evaluate the feasibility of fiducial markers as a surrogate for gross tumor volume (GTV) position in image-guided radiation therapy of rectal cancer. We analyzed 35 fiducials in 19 patients with rectal cancer who received short-course radiation therapy or long-course chemoradiation therapy. Magnetic resonance imaging examinations were performed before and after the first week of radiation therapy, and daily pre- and postirradiation cone beam computed tomography scans were acquired in the first week of radiation therapy. Between the 2 magnetic resonance imaging examinations, the fiducial displacement relative to the center of gravity of the GTV (COGGTV) and the COGGTV displacement relative to bony anatomy were determined. Using the cone beam computed tomography scans, inter- and intrafraction fiducial displacement relative to bony anatomy were determined. The systematic error of the fiducial displacement relative to the COGGTV was 2.8, 2.4, and 4.2 mm in the left-right, anterior-posterior (AP), and craniocaudal (CC) directions, respectively. Large interfraction systematic errors of up to 8.0 mm and random errors up to 4.7 mm were found for COGGTV and fiducial displacements relative to bony anatomy, mostly in the AP and CC directions. For tumors located in the mid and upper rectum, these errors were up to 9.4 mm (systematic) and 5.6 mm (random) compared with 4.9 mm and 2.9 mm for tumors in the lower rectum. Systematic and random errors of the intrafraction fiducial displacement relative to bony anatomy were ≤2.1 mm in all directions. Large interfraction errors of the COGGTV and the fiducials relative to bony anatomy were found. Therefore, despite the observed fiducial displacement relative to the COGGTV, the use of fiducials as a surrogate for GTV position reduces the required margins in the AP and CC directions for a GTV boost using image-guided radiation therapy of rectal cancer. This reduction in margin may be larger in patients with tumors located in the mid and upper rectum compared with the lower rectum.

Evaluation of an in-house self-held respiration monitoring device for deep inspiration breath hold techniques for radiotherapy of patients with cancer of the left breast

AbstractBackground:An in-house self-held respiration monitoring device (SHRMD) was developed for providing deep inspiration breath hold (DIBH) radiotherapy. The use of SHRMD is evaluated in terms of reproducibility, stability and heart dose reduction.Methods and materials:Sixteen patients receiving radiotherapy of left breast cancer were planned for treatment with both a free breathing (FB) scan and a DIBH scan. Both FB and DIBH plans were generated for comparison of the heart, left anterior descending (LAD) artery and lung dose. All patients received their treatments with DIBH using SHRMD. Megavoltage cine images were acquired during treatments for evaluating the reproducibility and stability of treatment position using SHRMD.Results:Compared with FB plans, the maximum dose to the heart by DIBH technique with SHRMD was reduced by 29·9 ± 15·6%; and the maximum dose of the LAD artery was reduced by 41·6 ± 18·3%. The inter-fractional overall mean error was 0·01 cm and the intra-fractional overall mean error was 0·04 cm.Conclusion:This study demonstrated the potential benefits of using the SHRMD for DIBH to reduce the heart and LAD dose. The patients were able to perform stable and reproducible DIBHs.

Assessment of the therapeutic accuracy of cone beam computed tomography-guided nasopharyngeal carcinoma radiotherapy.

The aim of the present study was to determine the ability of cone beam computed tomography (CBCT) to improve the accuracy of nasopharyngeal carcinoma (NPC) radiotherapy by analyzing the setup and inter-fraction errors at different levels and directions of the target volumes. A total of 113 patients with NPC who were undergoing intensity-modulated radiotherapy were recruited for the present study. Each patient had at least three CBCT exams prior to the start of radiation therapy. Three anatomic bony landmarks, including the upper neck, lower neck and head, were used to represent the different levels of assessment. The positioning errors were registered in three planes throughout the course of radiotherapy: The right-left (RL), superior-inferior (SI) and anterior-posterior (AP) directions. The planning CT images were matched with the CBCT images to determine the naso-pharynx shifts. A receiver operating characteristic curve was plotted to establish the specificity and sensitivity of CBCT. The planning target volume margin (MPTV) for the head was 0.9 mm, 1.4 mm for the upper neck and 2.0 mm for the lower neck. MPTVs of 1.5, 0.6 and 2.2 mm in the RL, SI and AP directions, respectively, were detected. In addition, there was evidence of setup errors in the three planes (RL, SI and AP) with the greatest error observed in the AP direction. Furthermore, the setup uncertainties in the neck region were greater than those of the head. In conclusion, CBCT could greatly improve the accuracy of radiotherapy by minimizing the setup errors and MPTV.

Setup strategies and uncertainties in esophageal radiotherapy based on detailed intra- and interfractional tumor motion mapping

Background and purposeDetailed knowledge of target motion is important for improved accuracy and decreased toxicity of esophageal cancer radiotherapy. This study uses the 3D trajectories of implanted markers during setup CBCT scans to investigate the intra- and interfractional tumor motion in esophageal cancer radiotherapy. Material and methodsFor 21 esophageal cancer patients with implanted fiducial markers, 60-s 3D marker trajectories were estimated from the 2D marker positions in the projections of daily setup CBCT scans by a probability-based method. The motion was separated into respiratory and cardiac components by frequency analysis and motion magnitude (2nd–98th percentile) was extracted for each marker. The mean motion was calculated over all markers. The daily mean setup interfraction error for bony-anatomy and soft-tissue setup was used to estimate the margin accounting for interfractional motion. ResultsA total of 1036 marker trajectories were extracted using 427 CBCT scans and 63 markers. The mean motion magnitude over all markers was 2.9 mm (left–right (LR)), 8.8 mm (cranio-caudal (CC)) and 4.1 mm (anterior–posterior (AP)) for the full motion during CBCT acquisition with mean magnitudes of 2.7 mm (LR), 8.4 mm (CC) and 3.5 mm (AP) for respiratory motion and 1.0 mm (LR), 1.5 mm (CC) and 1.4 mm (AP) for cardiac motion. Substantial daily marker shifts relative to bones resulted in margins of 8.9 mm (LR), 9.5 mm (CC), and 7.3 mm (AP). Soft-tissue based setup in and near the CTV combined with rescanning of patients with anatomical changes reduced the margins to 6.9 mm (LR), 6.8 mm (CC), and 5.6 mm (AP). ConclusionsEsophageal tumor motion was mapped with unprecedented detail throughout the radiotherapy course. Respiratory motion dominated and was largest in the CC direction. Soft-tissue matching and an adaptive strategy reduced interfractional margins by 2–3 mm compared to bony-anatomy matching.

Impact of Immobilization on Interfractional Errors for Upper Extremity Soft Tissue Sarcoma Radiation Therapy

Introduction/BackgroundOwing to the rare nature and presentation of upper extremity soft tissue sarcomas (STSs) and the high mobility of associated anatomy, various patient positioning strategies are used for radiation therapy. The purpose of this study is to measure the interfractional setup errors associated with upper extremity sarcomas using different immobilization methods through cone-beam computed tomography (CBCT) images. MethodsAll patients treated with daily CBCT guidance for upper extremity STSs during 2014–2015 were identified and triaged based on type of immobilization. After defining an optimal region of interest for image registration, daily CBCT images were automatically registered to reference CT images to quantify positional discrepancies. Means and standard deviations were calculated, and one-way analysis of variance was calculated to determine significance of data. ResultsSeventeen patients with upper extremity sarcoma met inclusion criteria: 13 were treated to the shoulder/axilla/upper arm and 4 to the arm/elbow/forearm. Three main types of immobilization were identified: vacuum cradle with custom thermoplastic shell, vacuum cradle alone, and no immobilization accessory used. Patient repositioning occurred if translational and rotational displacements were larger than 1 mm and 5°, respectively, as per institutional guidelines. Patient repositioning rates were 18% for vacuum cradle with thermoplastic shells, 15% for vacuum cradles only, and 6% for no immobilization accessories. Mean translational displacements in right/left (R/L), superior/inferior (S/I), and anterior/posterior (A/P) directions were −0.04 ± 0.33 cm, 0.32 ± 0.33 cm, and 0.12 ± 0.25 cm for vacuum cradle with thermoplastic shell; 0.25 ± 0.10 cm, −0.07 ± 0.22 cm, and 0.00 ± 0.17 cm for vacuum cradle alone; and 0.14 ± 0.15 cm, 0.08 ± 0.45 cm, and −0.01 ± 0.24 cm for no immobilization. For all patients, rotational displacements in the pitch, roll, and yaw were 0.15 ± 1.99°, 0.31 ± 2.11°, and −0.21 ± 1.76°, respectively. There were significant (P < .05) differences in systematic error values for all translational and rotational axes between immobilization methods. ConclusionLarge interfractional errors, especially in the rotational axes, were observed, regardless of immobilization strategy. Small study population and unequal representation of different parts of the upper extremity are identified limitations. The need for better immobilization techniques for upper extremity STS treatments is clearly demonstrated.

Image-guided study of inter-fraction and intra-fraction set-up variability and margins in reverse semi-decubitus breast radiotherapy

BackgroundThis study aimed to evaluate the inter-fraction set-up error and intra-fraction motion during reverse semi-decubitus (RSD) breast radiotherapy, and to determine a planning target volume (PTV) margin.Material and methodsPre- and post-treatment cone-beam computed tomography (CBCT) scans were prospectively acquired at fractions 1, 4, 7, 8, 11, and 14 for 30 patients who underwent RSD breast radiotherapy. Online correction for initial set-up error greater than 5 mm or 2° was performed and post-correction CBCT was acquired. An off-line analysis was performed to quantify initial and residual inter-fraction set-up errors and intra-fraction motion in three-dimensions. Patient inter-fraction errors were analysed for time trends during the course of radiotherapy. PTV margins were calculated from the systematic and random errors.ResultsThe initial inter-fraction population systematic errors were 1.8–3.3 mm (translation) and 0.5° (rotation); random errors were 1.8–2.1 mm (translation) and 0.3–0.5° (rotation). After online correction, the residual inter-fraction population systematic errors were 1.2–1.8 mm (translation) and 0.3–0.4° (rotation); random errors were 1.4–1.6 mm (translation) and 0.3–0.4° (rotation). Intra-fraction population systematic and random errors were ≤ 1.3 mm (translation) and ≤ 0.2° (rotation). The magnitude of inter-fraction set-up errors in the anterior-posterior direction, roll, and yaw were significantly correlated with higher body weight and body mass index (BMI). The inter-fraction set-up error did not change significantly as a function of time during the course of radiotherapy. The magnitude of intra-fraction motion was not correlated with patient characteristics and treatment time. The total PTV margins accounting for pre-correction and intra-fraction errors were 6.5–10.2 mm; those accounting for post-correction and intra-fraction errors were 4.7–6.3 mm.ConclusionsCBCT is an effective modality to evaluate and improve the inter-fraction set-up reproducibility in RSD breast radiotherapy, particularly for patients with higher BMI. Intra-fraction motion was minimal during RSD breast radiotherapy.

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

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