Planning Target Volume Margins Research Articles

Effects of Expansion Size from Gross Tumor Volume to Planning Target Volume on Control Rates and Patterns of Recurrence in Conventionally Fractionated Radiotherapy for WHO Grade I Meningiomas

<h3>Purpose/Objective(s)</h3> In single fraction stereotactic radiosurgery (SRS) for WHO grade I meningiomas, no- or minimal GTV to PTV margin is accepted practice with good outcomes. For complex tumors not eligible for SRS due to size or proximity to critical structures, there has been historical heterogeneity in target volume expansions practices, ranging from margins of a few mm to up to 2 cm. Additional margin is typically added for fractionated treatments to account for setup variability and less precise imaging and setup technique. Larger expansions result in treatment of uninvolved brain parenchyma and organs at risk, which is a concern in patients with expected long survivals. We sought to evaluate whether there is a control difference based on GTV to PTV expansion size in fractionated radiotherapy with either stereotactic or conventional conformal technique. <h3>Materials/Methods</h3> With IRB approval, we identified 87 patients from an institutional database with imaging-defined or WHO grade I meningioma treated with either 3D or IMRT radiation with 5 to 15 mm expansions from GTV (with or without intermediate CTV) or fractionated stereotactic radiotherapy (fSRT) using a system with ≤ 3mm GTV to PTV expansions, treated between 1999 and 2018 with at least 2 years of follow-up. All treatments were delivered using 1.8 Gray (Gy) fractions. Kaplan-Meier estimators were used for local failure-free survival (LFFS), marginal-failure free survival (MFFS) and distant failure-free survival (DFFS) analysis, with censoring occurring at failure event or most recent MRI. Local failure was defined as within the treatment field, marginal within 2 cm, and distant as beyond 2 cm. Patient clinical and demographic characteristics analyzed with χ² testing. <h3>Results</h3> With a median follow-up of 9.0 years, 25 patients (29%) received 3D or IMRT and 62 patients (71%) received fSRT. Doses delivered varied from 50.4-54 Gy, with the majority (71%) receiving 54 Gy. Thirty-two (37%) underwent surgery; 24 were sub-total resections. Total events were low, with 4 local failures (5%), 1 marginal failure (1%) and 1 distant failure (1%). The fSRT and 3D/IMRT groups each had two local failures; 3/4 local failures occurred in areas near critical organs at risk (two in suprasellar region, one in cavernous sinus). Extent of surgical resection was not correlated with risk for local failure. For 3D/IMRT vs fSRT, 5- and 10-year LFFS were 100% vs 98% (p=0.46) and 94% vs 96% (p=0.34), 5- and 10-year MFFS were 100% vs 100% and 100% vs 92% (p=0.004), and 5- and 10-year DFFS were 100% vs 98% at both time points (p=0.65 and p=0.67, respectively). <h3>Conclusion</h3> In this patient cohort, there was no local control benefit to larger GTV to PTV expansions. For patients with tumors not eligible for SRS, fractionated treatment using a rigorous stereotactic workflow with ≤ 3 mm PTV expansions is an effective approach for WHO grade I meningiomas.

Five-Fraction Radiotherapy for Breast Cancer: Which are the Most Suitable Patients?

<h3>Purpose/Objective(s)</h3> FAST-Forward trial reported that five-fraction radiotherapy (5fx-RT) schedule with 26 Gy in adjuvant setting after breast-conserving surgery for early-breast cancer (BC) was non-inferior to the standard hypo-fx (SHfx) schedule with 40 Gy in 15fx in terms of local tumor control. Since the COVID-19 pandemic started RT expert groups encouraged professionals to implement hypo-fx schedules in order to decrease visits of patients to hospitals. The aim of this study is to help radiation oncologists choose the most suitable patients for 5fx-RT schedule according to anatomical features. <h3>Materials/Methods</h3> Between March 2020 and December 2021, 239 patients suitable for 5fx-RT schedule were referred to our department. Prescribed RT dose was 26 Gy in 5fx to the whole breast plus a simultaneously integrated boost (SIB) up to 29 Gy to tumor bed if indicated. Patients were divided into 3 groups: Group A: Patients treated with 5fx-RT schedule that met all the FAST-Forward constraints for normal tissues; Group B: Patients treated with 5fx-RT schedule that slightly did not meet all constraints, Group C: Patients switched to SHfx schedule as they did not meet constraints in an acceptable way. For each patient, we draw an imaginary straight line connecting the medial and lateral borders of PTV in the axial slice of simulation CT where the longest anteroposterior diameter was found. From midpoint of this line, we measured the tangent distance to PTV margin and collected the data as medial-to-lateral tangent (MELT). Data was analyzed using statistical software. <h3>Results</h3> 150 patients were included in group A, 75 in group B and 14 in group C. The median MELT distance was: 1.91cm, 2.48cm and 3cm respectively. We found that the increase in MELT distance was significantly associated with a poorer compliance of normal tissue constraints (p<0.0001). Patient´s distribution among 3 groups for MELT distance intervals are shown in table 1. Median V8 for ipsilateral lung was: 13.1, 15.46 and 20.49% for groups A, B and C respectively. For the heart, median mean dose was: 1.06, 1.8 and 2.25 Gy for left breast cancer patients and 0.28, 0.33 and 0.48 Gy for right breast cancer patients for groups A, B and C respectively. We found a moderate positive correlation between MELT distance and dosimetric parameters assessed above (r=0.545, 0.475 and 0.418 respectively). According to laterality, for a higher MELT distance the % of left BC patients increased significantly (p=0.039). <h3>Conclusion</h3> MELT distance is an easy tool that helps radiation oncologists predict which BC patients are the most suitable for 5fx-RT before RT planning begins. This could avoid delays in starting RT for patients with a high MELT distance directly planning them with the SHfx schedule.

Dosimetric Impact of Intrafraction Prostate Motion in Dose-Escalated Linac-Based SBRT

<h3>Purpose/Objective(s)</h3> The aim of this study was to investigate the impact of observed intrafraction prostate motion on dose metrics and the effect of beam gating and motion correction using a novel electromagnetic (EM) tracking device in dose-escalated Linac-based SBRT. <h3>Materials/Methods</h3> Thirteen patients (56 fractions) with organ-confined prostate cancer received 40 Gy in 5 fractions or 38 Gy in 4 fractions using VMAT technique with flattening filter-free (FFF) arcs. The EM tracking device consisted of an integrated Foley catheter with a transmitter and provided real-time 3D prostate motion data. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and, unless the offset was transient, target position was re-defined by repeating a ConeBeam-CT (CBCT). Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed. Both actually delivered treatments (case A) and non-gated treatments (case B) were simulated by incorporating the observed prostate motion for each fraction into the patient original treatment plan with an isocenter shift method. Target and organs at risk (OARs) dosimetric parameters of the two motion-inclusive plans were compared to planned values and protocol dose constraints. <h3>Results</h3> Treatment interruptions were needed in 25 fractions (45%) because of target motion beyond the predefined threshold. The prostate was found within 2 mm from its initial position in 96% of the treatment time, i.e., in 94% of the time during the setup, and in 98% during the delivery. In non-gated treatments, the prostate would have been found outside the tolerance in 8% of the total session time, in 4% of the time during the setup, and in 14% during beam-on. The predominant motion pattern was posterior and its probability increased with time, with a mean motion ≤ 2 mm occurring within 10 minutes. Mean relative dose differences respect to original plans were -0.1% [-1.8 – 1.0] for CTV D99% and -0.2% [-1.6 – 0.7] for PTV D95% in case A. The same values were -1.2% [-8.8 – 0.8] and -1.2% [-5.9 – 0.7] in case B. In both cases, urethra planning organ at risk volume was slightly degraded after taking motion into account, and the rectum and bladder dose metrics showed a favorable underexposition of rectum and an undesirable overdose to bladder. Nevertheless, no protocol dose constraints violations were observed. <h3>Conclusion</h3> Our findings showed that the majority of the observed prostate motion was modest; however, there were a non-insignificant number of fractions with motion exceeding the predetermined threshold. The current CTV to PTV margins, the robustness of original treatment plans, and the fast FFF beams delivery led to minimal degradations of dose metrics for the target and OARs in both cases. Anyway, real-time monitoring with beam gating and motion correction ensured superior results and are recommended in dose-escalated prostate SBRT.

Surface Imaging-Guided Adaptive Radiotherapy (ART): Imaging Protocol and Verification for Inter- and Intra-Fractional Motion Managements

<h3>Purpose/Objective(s)</h3> CBCT-guided adaptive radiotherapy (CT-ART) has proven to account for inter-fractional anatomic changes. However, no commercial solution for intra-fractional respiratory motion management existed in CT-ART. Therefore, we investigated intra-fractional motion-managed treatment strategies in CT-ART (or Image-guided RT: IGRT) using optical surface imaging combined with <i>in-situ</i> CBCTs. <h3>Materials/Methods</h3> Eight of ten cancer patients with mobile lower lung or upper abdominal malignancies have completed CT-ART (or IGRT) using Ethos radiotherapy in an IRB-approved clinical trial (phase I). Four of the eight patients underwent additional MR simulations, including cine MRIs, to estimate the tumor/abdominal skin motion correlation. In CT-ART, a pre-configured gating window (±2 mm in vertical position) on surface imaging was used for manually triggering intra-fractional CBCT and treatment beam irradiation in breath-hold (six patients for exhalation and two patients for inhalation). A donut-shaped imaging-surrogate on patient's abdomen was used to localize Region of Interest (ROI) of the abdomen on CBCTs. Two inter-fractional CBCTs at exhalation and inhalation in each fraction were acquired to verify the tumor/imaging-surrogate motions range. Intra-fractional CBCTs were used to quantify the residual motion of the tumor/imaging-surrogate within the pre-configured gating window. Tumor motion was measured by tracking the position of a tumor centroid. <h3>Results</h3> The pre-configured gating window on surface imaging was confirmed based on the correlation of the tumor and abdominal skin motions using cine MRIs. Thirty-six pairs of inter-fractional CBCTs and 100 intra-fractional CBCTs from 36 fractions were analyzed (13 fractions for IGRT and 23 for ART). In the motion range study from the inter-fractional CBCTs, the values of mean ± std were 17.3 ± 12.0 mm in the SI direction for the tumors and 16.6 ± 7.9 mm in the AP direction for the imaging-surrogate, respectively. The residual motion of the imaging-surrogate from intra-fractional CBCTs was 2.5 ± 2.1 mm for SI, 2.3 ± 2.0 mm for RL, and 2.1 ± 2.3 mm for AP directions. The residual motion of the tumors was 2.4 ± 3.1 mm for SI, 2.3 ± 2.3 mm for RL, and 2.2 ± 2.0 mm for AP directions. Trends in the residual motion during the course of treatment fractions and over time through delivery were stable. Inter-fractional residual motions of the tumors were 2.3 ± 0.8 mm for SI, 2.3 ± 0.6 mm for RL, and 2.2 ± 0.7 mm for AP directions. Intra-fractional residual motions of the tumors were 2.4 ± 0.9 mm for SI, 2.4 ± 0.7 mm for RL, and 3.2 ± 1.3 mm for AP. <h3>Conclusion</h3> We demonstrated the inter- and intra-fractional motion-managed treatment strategies in CBCT-guided ART using surface imaging. Since the residual motion of the tumors was comparable to that of the imaging-surrogate within clinical PTV margins (5 mm) but a bit larger than the pre-configured gating window, additional caution is needed with intra-fractional motion management in CT-ART using the optical surface imaging.

Real-World Dose Delivery Accuracy of Prostate SBRT with Tomotherapy Based Target Tracking

<h3>Purpose/Objective(s)</h3> The frameless robotic radiosurgery system target tracking system was recently adapted to Tomotherapy. While a previous dosimetric study has demonstrated comparable prostate SBRT plans between the two systems, we report here our real-world dose delivery accuracy of synchronization technology motion compensation for prostate SBRT and the initial clinical outcomes. <h3>Materials/Methods</h3> Patients undergo insertion of 3 fiducials and hydrogel before planning CT and MRI. Prostate and proximal seminal vesicles were included in the CTV. A PTV margin of 3 to 5 mm was utilized. Prescription was 40 Gy in 5 fractions. Patient motion files were analyzed and the maximum shift and gradient in each direction calculated to allow for motions to be categorized. Eight patients were enrolled in the TPS which uses daily imaging to deform VOI's and accumulate delivered doses based on organ volume and position changes. <h3>Results</h3> 14 patients with Gleason 6/7 prostate cancer were included with median prostate volume of 32.3 cm³ (17.7 - 99.6 cm³). Patient motion files showed 20 deliveries had motions <2 mm, 26 with slow position changes ≥ 2 mm. 10 deliveries had rapid position changes <5 mm and 14 had rapid changes ≥5 mm. For most OARs delivered D_max was within 2% of the planned dose. Larger differences were found for OARs located superiorly or inferiorly to the target, due to the rapid dose fall off in this direction. Large reductions in the D_max and D50 to the bladder and sigmoid are seen in patients who were asked to increase their bladder fill to reduce bowel dose. Mean dose to the CTV and PTV was found to agree to within 1.5% and 2.0% respectively. From 12 patients with median follow up time of 1 month (1 – 9 months), 3 reported grade 1 GU toxicities. 2 patients reported grade 1 GI toxicities and grade 1 fatigue. <h3>Conclusion</h3> The range of motions measured highlight the need for compensation techniques for precise dose deposition with Tomotherapy. Daily OAR variations resulted in dose delivery accuracy within 2% of planned dose.

A prospective study to assess and quantify the setup errors with cone-beam computed tomography in head-and-neck cancer image-guided radiotherapy treatment.

This study was done to quantify the translational setup errors with cone-beam computed tomography (CBCT) in the image-guided radiation therapy (IGRT) treatment of head-and-neck cancer (HNC) patients. The objective was to quantify the setup errors by CBCT. One hundred patients of HNC were enrolled from March 2020 to March 2021 for IGRT treatment. Pretreatment kV-CBCT images were obtained at the first 3 days of irradiations, and setup error corrections were done in the mediolateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions. Subsequently, a weekly kV-CBCT was repeated for whole duration of radiotherapy for the next 6-7 weeks. Adequacy of planning target volume (PTV) margins was assessed by van Herk's formula. Total 630 CBCT scans of 100 patients were analyzed. Setup errors greater than 3 mm and 5 mm were seen in 11.4% and 0.31% of the patients, respectively. Systematic errors and random errors before correction in ML, SI, and AP directions were 0.10 cm, 0.11 cm, and 0.12 cm and 0.24 cm, 0.20 cm, and 0.21 cm, respectively. Systematic errors and random errors after correction in ML, SI, and AP directions were 0.06 cm, 0.07 cm, and 0.07 cm and 0.13 cm, 0.10 cm, and 0.12 cm, respectively. CBCT at the first 3 fractions and then weekly during radiotherapy is effective to detect the setup errors. An isotropic PTV margin of 5 mm over clinical target volume is safe to account for setup errors, however, in the case of close organ at risk, or with IGRT, a PTV margin of 3 mm can be considered.

Dosimetric and feasibility evaluation of a CBCT-based daily adaptive radiotherapy protocol for locally advanced cervical cancer.

Evaluate a cone-beam computed tomography (CBCT)-based daily adaptive platform in cervical cancer for multiple endpoints: (1) physics contouring accuracy of daily CTVs, (2) CTV coverage with adapted plans and reduced PTV margins versus non-adapted plans with standard-of-care (SOC) margins, (3) dosimetric improvements to CTV and organs-at-risk (OARs), and (4) on-couch time. Using a Varian Ethos™ emulator and KV-CBCT scans, we simulated the doses 15 retrospective cervical cancer patients would have received with/without online adaptation for five fractions. We compared contours and doses from SOC plans (5-15mm CTV-to-PTV margins) to adapted plans (3mm margins). Auto-segmented CTVs and OARs were reviewed and edited by trained physicists. Physics-edited targets were evaluated by an oncologist. Time spent reviewing and editing auto-segmented structures was recorded. Metrics from the CTV (D99%), bowel (V45Gy, V40Gy), bladder (D50%), and rectum (D50%) were compared. The physician approved the physics-edited CTVs for 55/75 fractions; 16/75 required reductions, and 4/75 required CTV expansions. CTVs were encapsulated by unadapted, SOC PTVs for 56/75 (72%) fractions-representative of current clinical practice. CTVs were completely covered by adapted 3mm PTVs for 71/75 (94.6%) fractions. CTV D99% values for adapted plans were comparable to non-adapted SOC plans (average difference of -0.9%), while all OAR metrics improved with adaptation. Specifically, bowel V45Gy and V40Gy decreased on average by 87.6 and 109.4 cc, while bladder and rectum D50% decreased by 37.7% and 35.8%, respectively. The time required for contouring and calculating an adaptive plan for 65/75 fractions was less than 20 min (range: 1-29 min). Improved dose metrics with daily adaption could translate to reduced toxicity while maintaining tumor control. Training physicists to perform contouring edits could minimize the time physicians are required at adaptive sessions improving clinical efficiency. All emulated adaptive sessions were completed within 30 min however extra time will be required for patient setup, image acquisition, and treatment delivery.

Same-day consultation, simulation and lung Stereotactic Ablative Radiotherapy delivery on a Magnetic Resonance-linac.

Magnetic resonance-guided radiotherapy (MRgRT) with real-time intra-fraction tumor motion monitoring allows for high precision Stereotactic Ablative Radiotherapy (SABR). This study aimed to investigate the clinical feasibility, patient satisfaction and delivery accuracy of single-fraction MR-guided SABR in a single day (one-stop-shop, OSS). Ten patients with small lung tumors eligible for single fraction treatments were included. The OSS procedure consisted of consultation, treatment simulation, treatment planning and delivery. Following SABR delivery, patients completed a reported experience measure (PREM) questionnaire. Prescribed doses ranged 28-34Gy. Median GTV was 2.2cm3 (range 1.3-22.9cm3). A gating boundary of 3mm, and PTV margin of 5mm around the GTV, were used with auto-beam delivery control. Accuracy of SABR delivery was studied by analyzing delivered MR-cines reconstructed from machine log files. All 10 patients completed the OSS procedure in a single day, and all reported satisfaction with the process. Median time for the treatment planning step and the whole procedure were 2.8h and 6.6h, respectively. With optimization of the procedure, treatment could be completed in half a day. During beam-on, the 3mm tracking boundary encompassed between 78.0 and 100% of the GTV across all patients, with corresponding PTV values being 94.4-100% (5th-95th percentiles). On average, system-latency for triggering a beam-off event comprised 5.3% of the delivery time. Latency reduced GTV coverage by an average of -0.3 %. Duty-cycles during treatment delivery ranged from 26.1 to 64.7%. An OSS procedure with MR-guided SABR for lung cancer led to good patient satisfaction. Gated treatment delivery was highly accurate with little impact of system-latency.

Adaptive margins for online adaptive radiotherapy

Objective. In online adaptive radiotherapy a new plan is generated every fraction based on the organ and clinical target volume (CTV) delineations of that fraction. This allows for a planning target volume margin that does not need to be constant over the whole course of treatment, as is the case in conventional radiotherapy. This work aims to introduce an approach to update the margins each fraction based on the per-patient treatment history and explore the potential benefits of such adaptive margins. Approach. We introduce a novel methodology to implement adaptive margins, isotropic and anisotropic, during a treatment course based on the accumulated dose to the CTV. We then simulate treatment histories for treatments delivered in up to 20 fractions using various choices for the standard deviations of the systematic and random errors and homogeneous and inhomogeneous dose distributions. The treatment-averaged adaptive margin was compared to standard constant margins. The change in the minimum dose delivered to the CTV was compared on a patient and a population level. All simulations were performed within the van Herk approach and its known limitations. Main results. The population mean treatment-averaged margins are down to 70% and 55% of the corresponding necessary constant margins for the isotropic and anisotropic approach. The reduction increases with longer fractionation schemes and an inhomogeneous target dose distribution. Most of the benefit can be attributed to the elimination of the effective systematic error over the course of treatment. Interpatient differences in treatment-averaged margins were largest for the isotropic margins. For the 10% of patients that would receive a lower than prescribed dose to the CTV this minimum dose to the CTV is increased using the adaptive margin approaches. Significance. Adaptive margins can allow to reduce margins in most patients without compromising patients with greater than average target motion.

Investigation of the Constancy of the Dosimetric Parameters of the Proton Therapy System

Purpose: Determination of fluctuation ranges of dosimetry parameters of the proton therapy system&#x0D; (PTS) to clarify the permissible limit deviations.&#x0D; Methods and materials: The basis for the quality assurance program (QA) of the PTS is the report of the&#x0D; American Association of Physicists in Medicine AAPM TG-224. According to this report, the QA program is a living, adaptive document that needs to be refined, corrected and optimized taking into account the features of the PTS being in use and the available dosimetry equipment. After setting-up&#x0D; checks within the framework of the daily QA program, it became possible to automatically collect data&#x0D; containing dosimetry parameters of the PTS for subsequent analysis. It was decided to investigate the&#x0D; stability over time of some dosimetry parameters of the PTS in order to determine the ranges of their&#x0D; fluctuations and, as a result, to clarify the permissible deviations of these parameters.&#x0D; Results: The stability of the parameters associated with the constancy of the proton beam energy selection was studied. As a result, the study showed that the values of the parameters associated with the&#x0D; constancy of the proton beam energy selection fluctuated within much smaller limits compared to&#x0D; those proposed in TG-224.&#x0D; Conclusion: The obtained result may allow us to reconsider the approach to proton therapy planning&#x0D; system in the FSBI FSCCRO of the FMBA of Russia, in particular, to the choice of PTV margins. The new&#x0D; refined tolerances for the maximum deviations of the analyzed parameters are proposed in this research work

Online adaptive radiotherapy of anal cancer: Normal tissue sparing, target propagation methods, and first clinical experience

Background and purposeOnline adaptive radiotherapy (oART) potentially spares OARs as PTV margins are reduced. This study evaluates dosimetric benefits, compared to standard non-adaptive radiotherapy (non-ART), target propagation methods, and first clinical treatments of CBCT-guided oART of anal cancer. Materials and methodsTreatment plans with standard non-ART and reduced oART PTV margins were retrospectively generated for 23 consecutive patients with anal cancer. For five patients randomly selected among the 23 patients, weekly CBCT-guided oART sessions were simulated, where the targets were either deformed or rigidly propagated. Preferred target propagation method and dose to OARs were evaluated. Ten consecutive patients with anal cancer were treated with CBCT-guided oART. Target propagation methods and oART procedure time were evaluated. ResultsFor the retrospective treatment plans, oART resulted in median reductions in bowel bag V45Gy of 11.4 % and bladder V35Gy of 16.1%. Corresponding values for the simulated sessions were 7.5% and 27.1%. In the simulated sessions, 35% of all targets were deformed while 65% were rigidly propagated. Manual editing and rigid propagation were necessary to obtain acceptable target coverage. In the clinical treatments, the primary and some elective targets were rigidly propagated, while other targets were deformed. The median oART procedure time, measured from CBCT acquisition to completion of plan review and QA, was 23 min. ConclusionsSimulated oART reduced the dose to OARs, indicating potential reduction in toxicity. Rigid propagation of targets was necessary to reduce the need for manual edit. Clinical treatments demonstrated that oART of anal cancer is feasible but time-consuming.

Accuracy and potential improvements of surface-guided breast cancer radiotherapy in deep inspiration breath-hold with daily image-guidance

Objective. Radiotherapy of left-sided breast cancer in deep inspiration breath-hold (DIBH) reduces the heart dose. Surface guided radiotherapy (SGRT) can guide the DIBH, but the accuracy is subject to variations in the chest wall position relative to the patient surface. Approach. In this study, ten left-sided breast cancer patients received DIBH radiotherapy with tangential fields in 15–18 fractions. After initial SGRT setup in free breathing an orthogonal MV/kV image pair was acquired during SGRT-guided breath-hold. The couch was corrected to align the chest wall during another breath-hold, and a new SGRT reference surface was acquired for the gating. The chest wall position error during treatment was determined from continuous cine MV images in the imager direction perpendicular to the cranio-caudal direction. A treatment error budget was made with individual contributions from the online registration of the setup MV image, the difference in breath-hold level between setup imaging and SGRT reference surface acquisition, the SGRT level during treatment, and intra-fraction shifts of the chest wall relative to the SGRT reference surface. In addition to the original setup protocol (Scenario A), SGRT was also simulated with better integration of image-guidance by capturing either the new reference surface (Scenario B) or the SGRT positional signal (Scenario C) simultaneously with the setup MV image, and accounting for the image-guided couch correction by shifting the SGRT reference surface digitally. Main results. In general, the external SGRT signal correlated well with the internal chest wall position error (correlation coefficient >0.7 for 75% of field deliveries), but external-to-internal target position offsets above 2 mm occasionally occurred (13% of fractions). The PTV margin required to account for the treatment error was 3.5 mm (Scenario A), 3.4 mm (B), and 3.1 mm (C). Significance. Further integration of SGRT with image-guidance may improve treatment accuracy and workflow although the current study did not show large accuracy improvements of scenario B and C compared to scenario A.

Clinical application of a sub-fractionation workflow for intrafraction re-planning during prostate radiotherapy treatment on a 1.5 Tesla MR-Linac: A practical method to mitigate intrafraction motion

BackgroundIntrafraction motion during radiotherapy limits margin reduction and dose escalation. Magnetic resonance (MR)-guided linear accelerators (MR-Linac) have emphasised this issue by enabling intrafraction imaging. We present and clinically apply a new workflow to counteract systematic intrafraction motion during MR-guided stereotactic body radiotherapy (SBRT). Materials and methodsWith the sub-fractionation workflow, the daily dose is delivered in multiple sequential parts (sub-fractions), each adapted to the latest anatomy. As each sub-fractionation treatment plan complies with the dose constraints, no online dose accumulation is required. Imaging and treatment planning are executed in parallel with dose delivery to minimise dead time, enabling an efficient workflow. The workflow was implemented on a 1.5 T MR-Linac and applied in 15 prostate cancer (PCa) patients treated with 5 × 7.25 Gy in two sub-fractions of 3.625 Gy (10 × 3.625 Gy in total). Intrafraction clinical target volume (CTV) motion was determined and compared to a workflow with single-plan delivery. Furthermore, required planning target volume (PTV) margins were determined. ResultsAverage on-table time was 42.7 min. Except for two fractions, all fractions were delivered within 60 min. Average intrafraction 3D CTV displacement (±standard deviation) was 1.1 mm (± 0.7) with the sub-fractionation workflow, whereas this was up to 3.5 mm (± 2.4) without sub-fractionation. Calculated PTV margins required with sub-fractionation were 1.0 mm (left–right), 2.4 mm (cranial-caudal), and 2.6 mm (anterior-posterior). ConclusionFeasibility of the sub-fractionation workflow was demonstrated in 15 PCa patients treated with two sub-fractions on a 1.5 T MR-Linac. The workflow allows for significant PTV margin reduction in these patients by reducing systematic intrafraction motion during SBRT.

A concept for anisotropic PTV margins including rotational setup uncertainties and its impact on the tumor control probability in canine brain tumors

Objective. In this modelling study, we pursued two main goals. The first was to establish a new CTV-to-PTV expansion which considers the closest and most critical organ at risk (OAR). The second goal was to investigate the impact of the planning target volume (PTV) margin size on the tumor control probability (TCP) and its dependence on the geometrical setup uncertainties. The aim was to achieve a smaller margin expansion close to the OAR while allowing a moderately larger expansion in less critical areas further away from the OAR and whilst maintaining the TCP. Approach. Imaging data of radiation therapy plans from pet dogs which had undergone radiation therapy for brain tumor were used to estimate the clinic specific rotational setup uncertainties. A Monte-Carlo methodology using a voxel-based TCP model was used to quantify the implications of rotational setup uncertainties on the TCP. A combination of algorithms was utilized to establish a computational CTV-to-PTV expansion method based on probability density. This was achieved by choosing a center of rotation close to an OAR. All required software modules were developed and integrated into a software package that directly interacts with the Varian Eclipse treatment planning system. Main results. Several uniform and non-isotropic PTVs were created. To ensure comparability and consistency, standardized RT plans with equal optimization constraints were defined, automatically applied and calculated on these targets. The resulting TCPs were then computed, evaluated and compared. Significance. The non-isotropic margins were found to result in larger TCPs with smaller margin excess volume. Further, we presented an additional application of the newly established CTV-to-PTV expansion method for radiation therapy of the spinal axis of human patients.

Factors Affecting Isocenter Displacement and Planning Target Volume Margin for Patients With Rectal Cancer Receiving Radiation Therapy

PurposeSetup errors are inherent in the process of daily radiation therapy (RT) delivery. Pelvic RT for rectal cancer is one of the body sites associated with the largest shift among other body sites. This study aimed to evaluate interfraction random and systematic errors and hence propose the optimum planning target volume (PTV) in patients with rectal cancer. Methods and MaterialsTranslational and angular isocenter displacements were retrospectively collected for 189 patients. Random and systematic errors were determined, and then the PTV margin was computed. Effect of positioning, body mass index (BMI), and type of immobilization were studied. Portal images before and after online correction were used to define PTV for no-daily image-guided radiotherapy (IGRT) and daily IGRT respectively. ResultsBefore the online correction, the systematic errors were 2.5, 2.8, and 3.0 mm for superior-inferior (SI), right-left (RL), and anterior-posterior (AP) directions, respectively, compared with 2.1, 1.7, and 1.8 mm after online correction. The random errors were 6.2, 7.4, and 8.2 mm in SI, RL, and AP, respectively, before online correction, compared with 4, 4.2, and 4.5 mm after online correction. The recommended PTV margin was 0.7 and 1.0 cm for daily IGRT and no-daily IGRT, respectively. The prone position and BMI >30 kg/m2 warrant higher margins in no-daily IGRT cases, 1.2 and 1.4 cm, respectively. ConclusionsThe prone position, BMI >30 kg/m2, and belly board device are associated with larger daily setup errors warranting higher PTV margins for no-daily IGRT; however, that can be avoided by using daily IGRT.

Clinical utility of a new immobilization method in image-guided intensity-modulated radiotherapy for breast cancer patients after radical mastectomy.

To investigate clinical utility of a new immobilization method in image-guided intensity-modulated radiotherapy (IMRT) for breast cancer patients after radical mastectomy. Forty patients with breast cancer who underwent radical mastectomy and postoperative IMRT were prospectively enrolled. The patients were randomly and equally divided into two groups using both a carbon-fiber support board and a hollowed-out cervicothoracic thermoplastic mask (Group A) and using only the board (Group B). An iSCOUT image-guided system was used for acquiring and correcting pretreatment setup errors for each treatment fraction. Initial setup errors and residual errors were obtained by aligning iSCOUT images with digitally reconstructed radiograph (DRR) images generated from planning CT. Totally 600 initial and residual errors were compared and analyzed between two groups, and the planning target volume (PTV) margins before and after the image-guided correction were calculated. The initial setup errors of Group A and Group B were (3.14±3.07), (2.21±1.92), (2.45±1.92) mm and (3.14±2.97), (2.94±3.35), (2.80±2.47) mm in the left-right (LAT), superior-inferior (LONG), anterior-posterior (VERT) directions, respectively. The initial errors in Group A were smaller than those in Group B in the LONG direction (P < 0.05). No significant difference was found in the distribution of three initial error ranges (≤3 mm, 3-5 mm and > 5 mm) in each of the three translational directions for the two groups (P > 0.05). The residual errors of Group A and Group B were (1.74±1.03), (1.62±0.92), (1.66±0.91) mm and (1.70±0.97), (1.68±1.18), (1.58±0.98) mm in the three translational directions, respectively. No significant difference was found in the residual errors between two groups (P > 0.05). With the image-guided correction, PTV margins were reduced from 8.01, 5.44, 5.45 mm to 3.54, 2.99, 2.89 mm in three translational directions of Group A, respectively, and from 8.14, 10.89, 6.29 mm to 2.67, 3.64, 2.74 mm in those of Group B, respectively. The use of hollowed-out cervicothoracic thermoplastic masks combined with a carbon-fiber support board showed better inter-fraction immobilization than the single use of the board in reducing longitudinal setup errors for breast cancer patients after radical mastectomy during IMRT treatment course, which has potential to reduce setup errors and improve the pretreatment immobilization accuracy for breast cancer IMRT after radical mastectomy.

Setup uncertainties and appropriate setup margins in the head-tilted supine position of whole-brain radiotherapy (WBRT).

Various applications of head-tilting techniques in whole-brain radiotherapy (WBRT) have been introduced. However, a study on the setup uncertainties and margins in head-tilting techniques has not been reported. This study evaluated the setup uncertainties and determined the appropriate planning target volume (PTV) margins for patients treated in the head-tilted supine (ht-SP) and conventional supine position (c-SP) in WBRT. Thirty patients who received WBRT at our institution between October 2020 and May 2021 in the c-SP and ht-SP were investigated. The DUON head mask (60124, Orfit Industries, Wijnegem, Belgium) was used in the c-SP, and a thermoplastic U-Frame Mask (R420U, Klarity Medical & Equipment Co. Ltd., Lan Yu, China) was used in the ht-SP. Daily setup verification using planning computed tomography (CT) and cone-beam CT was corrected for translational (lateral, longitudinal, and vertical) and rotational (yaw) errors. In the c-SP, the means of systematic errors were -0.80, 0.79, and 0.37 mm and random errors were 0.27, 0.54, and 0.39 mm in the lateral, longitudinal, and vertical translational dimensions, respectively. Whereas, for the ht-SP, the means of systematic errors were -0.07, 0.73, and -0.63 mm, and random errors were 0.75, 1.39, 1.02 mm in the lateral, longitudinal, and vertical translational dimensions, respectively. The PTV margins were calculated using Stroom et al.'s [2Σ+0.7σ] and van Herk et al.'s recipe [2.5Σ+0.7σ]. Appropriate PTV margins with van Herk et al.'s recipe in WBRT were <2 mm and 1.5° in the c-SP and <3 mm and 2° in the ht-SP in the translational and rotational directions, respectively. Although the head tilt in the supine position requires more margin, it can be applied as a sufficiently stable and effective position in radiotherapy.

Robust optimization of VMAT for prostate cancer accounting for geometric uncertainty.

The aim of this study was to propose optimal robust planning by comparing the robustness with setup error with the robustness of a conventional planning target volume (PTV)‐based plan and to compare the robust plan to the PTV‐based plan for the target and organ at risk (OAR). Data from 13 patients with intermediate‐to‐high‐risk localized prostate cancer who did not have T3b disease were analyzed. The dose distribution under multiple setup error scenarios was assessed using a conventional PTV‐based plan. The clinical target volume (CTV) and OAR dose in moving coordinates were used for the dose constraint with the robust plan. The hybrid robust plan added the dose constraint of the PTV‐rectum to the static coordinate system. When the isocenter was shifted by 10 mm in the superior–inferior direction and 8 mm in the right‐left and anterior directions, the doses to the CTV, bladder, and rectum of the PTV‐based plan, robust plan, and hybrid robust plan were compared. For the CTV D99% in the PTV‐based plan and hybrid robust plan, over 95% of the prescribed dose was secured in all directions, except in the inferior direction. There was no significant difference between the PTV‐based plan and the hybrid robust plan for rectum V70Gy, V60Gy, and V40Gy. This study proposed an optimization method for patients with prostate cancer. When the setup error occurred within the PTV margin, the dose robustness of the CTV for the hybrid robust plan was higher than that of the PTV‐based plan, while maintaining the equivalent OAR dose.

QUANTIFICATION OF 6D INTER-FRACTION TUMOUR LOCALISATION ERRORS IN TONGUE AND PROSTATE CANCER USING DAILY KV-CBCT FOR 1000 IMRT AND VMAT TREATMENT FRACTIONS.

This study aimed to evaluate the 6D inter-fraction tumour localisation errors in 20 tongue and 20 prostate cancer patients treated with intensity-modulated radiation therapy and volumetric-modulated arc therapy. The patient tumour localisation errors in lateral, longitudinal and vertical translation axes and pitch, roll and yaw rotational axes were analysed by automatic image registration of daily pretreatment kilovoltage cone-beam computed tomography (kV-CBCT) with planning CT in 1000 fractions. The overall mean error (M), systematic error (Σ), random error (σ) and planning target volume (PTV) margins were evaluated. The frequency distributions of setup errors were normally distributed about the mean except for pitch in the tongue and prostate. The overall 3D vector length≥5mm was 14.2 and 49.8% in the ca-tongue and ca-prostate, respectively. The frequency of rotational errors ≥1 degree was a maximum of 37 and 59.5%, respectively, in ca-tongue and ca-prostate. The M, Σ and σ for all translational and rotational axes decreased with increasing frequency of verification correction in ca-tongue and ca-prostate patients. Similarly, the PTV margin was reduced with no correction to alternate day correction from a maximum of 4.7 to 2.5mm in ca-tongue and from a maximum of 8.6 to 4.7mm in ca-prostate. The results emphasised the vital role of the higher frequency of kV-CBCT based setup correction in reducing M, Σ, σ and PTV margins in ca-tongue and ca-prostate patients.

Evaluation of Hybrid VMAT Advantages and Robustness Considering Setup Errors Using Surface Guided Dose Accumulation for Internal Lymph Mammary Nodes Irradiation of Postmastectomy Radiotherapy

ObjectivesSetup error is a key factor affecting postmastectomy radiotherapy (PMRT) and irradiation of the internal mammary lymph nodes is the most investigated aspect for PMRT patients. In this study, we evaluated the robustness, radiobiological, and dosimetric benefits of the hybrid volumetric modulated arc therapy (H-VMAT) planning technique based on the setup error in dose accumulation using a surface-guided system for radiation therapy.MethodsWe retrospectively selected 32 patients treated by a radiation oncologist and evaluated the clinical target volume (CTV), including internal lymph node irradiation (IMNIs), and considered the planning target volume (PTV) margin to be 5 mm. Three different planning techniques were evaluated: tangential-VMAT (T-VMAT), intensity-modulated radiation therapy (IMRT), and H-VMAT. The interfraction and intrafraction setup errors were analyzed in each field and the accumulated dose was evaluated as the patients underwent daily surface-guided monitoring. These parameters were included while evaluating CTV coverage, the dose required for the left anterior descending artery (LAD) and the left ventricle (LV), the normal tissue complication probability (NTCP) for the heart and lungs, and the second cancer complication probability (SCCP) for contralateral breast (CB).ResultsWhen the setup error was accounted for dose accumulation, T-VMAT (95.51%) and H-VMAT (95.48%) had a higher CTV coverage than IMRT (91.25%). In the NTCP for the heart, H-VMAT (0.04%) was higher than T-VMAT (0.01%) and lower than IMRT (0.2%). However, the SCCP (1.05%) of CB using H-VMAT was lower than that using T-VMAT (2%) as well as delivery efficiency. And T-VMAT (3.72) and IMRT (10.5).had higher plan complexity than H-VMAT (3.71).ConclusionsIn this study, based on the dose accumulation of setup error for patients with left-sided PMRT with IMNI, we found that the H-VMAT technique was superior for achieving an optimum balance between target coverage, OAR dose, complication probability, plan robustness, and complexity.