Couch Correction Research Articles

Feasibility of a Single-Fraction Stereotactic Dose of 30 Gy to Solitary Lung Lesions on Halcyon.

Purpose We sought to explore the feasibility of using the current co-planar Halcyon ring delivery system (RDS) with a novel multileaf collimator (MLC) aperture shape controller in delivering a single high dose of 30 Gy to solitary lung lesions via stereotactic body radiotherapy (SBRT). Materials and methods Thirteen non-small-cell lung cancer (NSCLC) patients previously treated with a single dose of 30 Gy to lung lesions via SBRT on the TrueBeam (6MV-FFF) using non-coplanar volumetric modulated arc therapy (VMAT) arcs were anonymized and replanned onto the Halcyon RDS (6MV-FFF) following RTOG-0915 single-fraction criteria. The Halcyon plans utilized a novel dynamic conformal arc (DCA)-based MLC-fitting approach before VMAT optimization with a user-defined aperture shape controller option. The clinical TrueBeam and Halcyon plans were compared via their protocol compliance, target conformity, gradient index, and dose to organs-at-risk (OAR). Treatment delivery efficacy and accuracy were assessed through end-to-end quality assurance (QA) tests on Halcyon and independent dose verification via in-house Monte Carlo (MC) second-check validation. Results All Halcyon lung SBRT plans met RTOG-0915 protocol's requirements for target coverage, conformity, and gradient indices, and maximum dose 2 cm away from the target (D2cm) while being statistically insignificant (p > 0.05) when compared to clinical TrueBeam plans. Additionally, Halcyon provided a similar dose to OAR except for the ribs, where Halcyon demonstrated a lower maximum dose (15.22 Gy vs 17.01 Gy, p < 0.001). However, Halcyon plans required a higher total monitor unit (8892 MU vs 7413 MU, p < 0.001), resulting in a higher beam modulation factor (2.96 MU/cGy vs 2.47 MU/cGy, p < 0.001) and an increase in beam-on time by a factor of 2.1 (11.11 min vs 5.3 min, p < 0.005). End-to-end QA measurements demonstrate that Halcyon plans were clinically acceptable with an average gamma passing rate of 99.8% for 2%/2mm criteria and independent MC 2nd checks within ±2.86%. Conclusion Our end-to-end testing and validation study demonstrates that by utilizing a DCA-based MLC aperture shape controller before VMAT optimization, Halcyon can be used for delivering a single dose of lung SBRT treatment. However, future improvements of Halcyon RDS are recommended to allow higher output rates, rotational couch corrections, and an integrated intrafraction motion management system that will further enhance Halcyon's capability for site-specific single dosage of SBRT.

Stereoscopic X-ray image and thermo-optical surface guidance for breast cancer radiotherapy in deep inspiration breath-hold

PurposeTo investigate the feasibility of a thermo-optical surface imaging (SGRT) system combined with room-based stereoscopic X‑ray image guidance (IGRT) in a dedicated breast deep inspiration breath-hold (DIBH) irradiation workflow. In this context, benchmarking of portal imaging (EPID) and cone-beam CT (CBCT) against stereoscopic X‑rays was performed.MethodsSGRT + IGRT data of 30 left-sided DIBH breast patients (1 patient with bilateral cancer) treated in 351 fractions using thermo-optical surface imaging and X-ray IGRT were retrospectively analysed. Patients were prepositioned based on a free-breathing surface reference derived from a CT scan. Once the DIBH was reached using visual feedback, two stereoscopic X‑ray images were acquired and registered to the digitally reconstructed radiographs derived from the DIBH CT. Based on this registration, a couch correction was performed. Positioning and monitoring by surface and X-ray imaging were verified by protocol-based EPID or CBCT imaging at selected fractions and the calculation of residual geometric deviations.ResultsThe median X‑ray-derived couch correction vector was 4.9 (interquartile range [IQR] 3.3–7.1) mm long. Verification imaging was performed for 134 fractions (216 RT field verifications) with EPID and for 37 fractions with CBCT, respectively. The median 2D/3D deviation vector length over all verification images was 2.5 (IQR 1.6–3.9) mm/3.4 (IQR 2.2–4.8) mm for EPID/CBCT, both being well within the planning target volume (PTV) margins (7 mm). A moderate correlation (0.49–0.65) was observed between the surface signal and X-ray position in DIBH.ConclusionDIBH treatments using thermo-optical SGRT and X-ray IGRT were feasible for breast cancer patients. Stereoscopic X‑ray positioning was successfully verified by standard IGRT techniques.

Feasibility and Safety of Single-Isocenter/Multi-Lesion (SIML) HyperArc Brain SRT: Clinical Implementation and Early Outcomes

Patients with multiple brain metastases may not tolerate relatively longer treatment times for traditional stereotactic radiation therapy (SRT) with individual isocenter plans for each lesion due to discomfort or co-morbidities. SRT using a single-isocenter/multi-lesion (SIML) HyperArc volumetric modulated arc therapy (VMAT) plan with flattening filter free (FFF) beam could significantly shorten overall treatment time, and improve patient comfort, compliance, and clinic efficiency. We report early clinical results of treating multiple brain metastases with SIML HyperArc SRT. Twenty-three patients with multiple brain metastatic tumors (range, 2-9 lesions; total treated lesions, n = 96) were simulated using Encompass support and Q-fix mask, and treated with a highly-conformal SIML VMAT SRT plans via non-coplanar HyperArc geometry. Mean tumor distance to isocenter was 5.3 cm, maximum up to 7 cm. Common prescriptions were 25-30 Gy/5 fractions, 24-27 Gy/3 fractions, and 20 Gy/1 fraction prescribed to each planning target volume (PTV) using 2 mm margin around standard gross tumor volume (GTV) delineated on contrasted enhanced MP-RAGE MRI fusion. Acuros dose calculation for 6MV-FFF beam was used for tissue heterogeneity corrections. Alliance A071801 criteria was used for dose constraints to organs at risk (OAR) and target conformality. Treatment was delivered every other day with CBCT-guidance, adjustments made with 6DOF couch corrections on a medical linear accelerator, and treatment delivery time within 15 minutes. Local control rates were reported, and toxicity profile rated based on CTCAE v5.0 for brain radionecrosis, optic neuropathy, and brainstem dysfunction. All plans met Alliance A071801 requirements for each tumor coverage, dose to OAR including optic apparatus, brainstem, and spinal cord. Mean GTV and PTV volume were 9.4 cc (range, 0.3-54.8 cc) and 16.13 cc (range, 1.0-80.2 cc). Patient-specific quality assurance results were 98.3% for gamma passing criteria of 2%/2mm. Independent in-house Monte Carlo physics second check agreed with HyperArc plans by ±3.0%. Mean follow up was 6 months (range, 0.0-18.6 months). Of the 23 patients treated, 17 (74%) had post-treatment MRI imaging to assess local control and toxicity. Local control was achieved in 69/73 (95%) of treated and followed lesions. CTCAE grade 2 radionecrosis occurred in 2 patients and were managed with dexamethasone. No CTCAE grade 3+ events of radionecrosis, optic pathway dysfunction, or brainstem toxicity were observed. SIML HyperArc Brain SRT for multiple brain metastases has excellent local control and low toxicity profile in our patients. It can significantly reduce treatment delivery time as compared to traditional multiple-isocenter brain SRT or chronologically separate treatment courses and thus, help to improve patient comfort, compliance, ease of care, and clinic workflow. Longer median follow up of SIML brain SRT on larger patient cohort is warranted.

First-Year Experience of IMRT/SBRT Treatments Using a Novel Biology-Guided Radiation Therapy System

This study presents the first-year experience of treating patients using intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) with the X1 system, the first biology-guided radiation therapy (BgRT) machine installed in a clinical setting. A total of 78 patients underwent IMRT and SBRT treatments on the X1 system from May 2021 to May 2022. Clinical and technical data, such as treatment sites, number of pre-treatments kVCT scans, beam on time, patient setup time, imaging time per kVCT, and couch shifts after kVCT match, were collected and analyzed. Additionally, daily machine output stability, patient-specific quality assurance (QA) results, machine uptime, and user survey were also documented and reported. The most commonly treated site was the head and neck (63%), followed by the pelvis (23%), thorax (6%), and abdomen (8%). All treatments, except for 5 pelvis patients (6%) who received SBRT treatments for bony metastases, were conventionally fractionated IMRT (CF IMRT). The average number of kVCT scans per fraction is 1.2 ± 0.5 for all treatments. The average beam on time in minutes was 9.2 ± 3.5 for all treatments, 8.4 ± 2.4 for head and neck, 6.7 ± 1.3 for thorax, 10.3 ± 1.6 for abdomen, 11.6 ± 5.1 for CF IMRT pelvis, and 10.8 ± 5.3 for SBRT pelvis. The average patient setup time and imaging time per kVCT was 4.8 ± 2.6 minutes and 4.6 ± 1.5 minutes, respectively. The average couch corrections based on kVCT images were 0.4 ± 4.4 mm, 1.0 ± 4.5 mm, and 1.3 ± 4.3 mm along the x, y, and z direction, respectively; the average couch rotation corrections were 0.1 ± 0.9° for pitch, 0.0 ± 0.9° for roll, and 0.2 ± 1.2° for yaw. The daily machine output was 0.4 ± 1.2% from the baseline. The patient QA had a gamma passing rate of 97.4 ± 2.8%. The machine uptime was 92% of the total treatment time. The kVCT image quality and daily QA process received the highest level of satisfaction, while the treatment workflow for therapists received the lowest level of satisfaction (table 1). At one year after the installation of the X1 system, this study reports successful treatment of 78 patients using IMRT/ SBRT. With the recent FDA clearance of BgRT, our institution is preparing to treat patients using PET-guidance via a new product release, which should address deficiencies in the current IGRT workflow.

Clinical Outcomes of Synchronous Lung Stereotactic Body Radiation Therapy Lesions via Single-Isocenter/Multi-Lesion Treatments

Lung cancer patients with synchronous primary or oligometastatic (< 5 lesions) with associated co-morbidities may not retain their treatment position for the extended stereotactic body radiotherapy (SBRT) treatment times with individual isocenter plans for each lesion due to discomfort or shortness of breath. SBRT using a single-isocenter/multi-lesion (SIML) volumetric arc therapy (VMAT) plan with flattening filter free (FFF) beam could significantly shorten overall treatment time, improve patient comfort and compliance and clinic efficiency. We report clinical results of treating multiple lung metastases or synchronous primary lung cancers with SIML lung SBRT. Eighty patients with synchronous primary lung cancers or oligometastatic lung tumors (two, n = 61; three, n = 10; four, n = 4; five n = 5; total treated lesions, n = 193) were simulated with abdominal compression and/or 4D-CT MIP images and treated with a highly-conformal SIML VMAT lung SBRT plans via 3-4 non-coplanar arcs. Common prescriptions were 50-55 Gy/5 fractions and 54 Gy/3 fractions prescribed 70-80% isodose line to the each PTV. Acuros dose calculation for 6MV-FFF beam was used for tissue heterogeneity corrections. RTOG-0618/0813 criteria were used to dose constraints to organs at risk (OAR) and target conformality. Treatment was delivered every other day or twice weekly with CBCT-guidance, adjustments made with 6DOF couch corrections on a medical linear accelerator, and treatment delivery time within 15 minutes. Local control rates and toxicity profile was evaluated using CTCAE v. 5.0 grading for pneumonitis, rib fracture and chest wall pain. All plans met RTOG-0618/0813 requirements for each tumor coverage, dose to OAR including normal lung and ribs. Mean follow up after last fraction of treatment was 16.9 months (range, 1.0-54.2 months). PTV volume ranged from 2.17 to 167.8 cc with mean volume of 16.1 cc. Of the 80 patients treated, 71 had adequate post-treatment thoracic CT imaging to assess local control. Local control was achieved in 167/175 (95.4%) of treated and followed lesions. CTCAE grade 1 asymptomatic pneumonitis was noted on thoracic CT scans in 42/71 (59.2%) of patients and occurred, on average 4.8 months after SBRT. Symptomatic pneumonitis and rib fracture did not occur in any patient. CTCAE grade 2 chest wall pain occurred in 4/80 (5.0%) treated patients and was managed conservatively with over-the-counter NSAIDS or acetaminophen. SIML lung SBRT for synchronous primary lung cancers or multiple lung metastases can be used as a variant to traditional multiple isocenter SBRT or chronologically separate treatment courses, and has excellent local control rates and low toxicity profile in our patient population. It can help improve comfort and compliance of the patients who have difficulty lying still for an extended treatment course, and significantly reduces treatment time via isocenter shifts/repeated CBCTs for image guidance, thus improving clinic efficiency.

Evaluation of a novel phantom for the quality assurance of a six-degree-of-freedom couch 3D-printed at multiple centres

This study aimed to validate a bespoke 3D-printed phantom for use in quality assurance (QA) of a 6 degrees-of-freedom (6DoF) treatment couch. A novel phantom design comprising a main body with internal cube structures, was fabricated at five centres using Polylactic Acid (PLA) material, with an additional phantom produced incorporating a PLA-stone hybrid material. Correctional setup shifts were determined using image registration by 3D-3D matching of high HU cube structures between obtained cone-beam computer tomography (CBCT) images to reference CTs, containing cubes with fabricated rotational offsets of 3.5°, 1.5° and −2.5° in rotation, pitch, and roll, respectively. Average rotational setup shifts were obtained for each phantom. The reproducibility of 3D-printing was probed by comparing the internal cube size as well as Hounsfield Units between each of the uniquely produced phantoms. For the five PLA phantoms, the average rot, pitch and roll correctional differences from the fabricated offsets were −0.3 ± 0.2°, −0.2 ± 0.5° and 0.2 ± 0.3° respectively, and for the PLA hybrid these differences were −0.09 ± 0.14°, 0.30 ± 0.00° and 0.03 ± 0.10°. There was found to be no statistically significant difference in average cube size between the five PLA printed phantoms, with the significant difference (P < 0.05) in HU of one phantom compared to the others attributed to setup choice and material density. This work demonstrated the capability producing a novel 3D-printed 6DoF couch QA phantom design, at multiple centres, with each unique model capable of sub-degree couch correction.

Spine Stereotactic Body Radiation Therapy Without Immobilization: Detailed Analysis of Intrafraction Motion Using High-Frequency kV Imaging During Irradiation

Spine SBRT requires high positioning accuracy and a stable patient to maximize target coverage and reduce excessive irradiation to organs at risk (OARs). Positional verification during spine SBRT delivery helps to ensure accurate positioning for all patients. We report our experience with non-invasive 3-dimensional (3D) target position monitoring during volumetric modulated arc therapy (VMAT) of spine metastases, in non-immobilized patients positioned using only a thin mattress and simple arm and knee supports. Fluoroscopic planar kV images were acquired at 7 frames/second using the on-board imaging system during VMAT spine SBRT. Template matching and triangulation were used to track the target in vertical, longitudinal and lateral direction. If the tracking trace deviated >1mm from the planned position in ≥1 direction, treatment was manually interrupted and 6D CBCT-based couch correction was performed. Tracking data was used to retrospectively analyze the target position. Positional data, agreement with CBCT, correlation between position of the couch and direction of any positional correction, and treatment times were analyzed. 175 fractions were analyzed. Delivery was interrupted 83 times in 66 fractions for a deviation >1mm. In 97% of cases the difference between tracking data and subsequent clinical shift performed after the CBCT match was ≤0.5mm. Lateral/longitudinal shift performed after intervention correlated with the couch roll/pitch at the start of treatment (correlation coefficient: -0.63/0.53). Mean (SD; range) time between start of first imaging and end of the last arc was 15.2 minutes (5.1;7.6-36.3). Spine tracking during irradiation can be used to prompt an intervention CBCT scan and repositioning, so that a spine SBRT target deviates by ≤1mm from the planned position, even in non-immobilized patients. kV tracking and CBCT are in good agreement. The data support verification CBCT after all 6 degrees-of-freedom positional corrections in non-immobilized spine SBRT patients.

Gantry triggered x-ray verification during single-isocenter stereotactic radiosurgery: Increased certainty for a no-margin strategy

BackgroundSingle-isocenter linac-based stereotactic radiosurgery (SRS) has emerged as a dedicated treatment option for multiple brain metastases. Consequently, image-guidance for patient positioning and motion management has become very important. The purpose of this study was to analyze intra-fraction errors measured with stereoscopic x-rays and their impact on the dose distribution. Materials and methodsTreatments were planned with non– coplanar dynamic conformal arcs for 33 patients corresponding to 127 brain lesions and 356 arcs. Intra-arc positioning errors were measuredusing stereoscopic x-rays (ExacTrac Dynamic, Brainlab), triggered during arc delivery. Couch corrections above 0.7 mm and 0.5° were always applied. Intra-arc positioning data was analyzed. The dose impact was evaluated by applying the measured errors to the dose given in each arc. ResultsMedian residual errors were 0.10 mm, 0.13 mm and 0.08 mm for the lateral, longitudinal and vertical directions and 0.10°, 0.08° and 0.13° for the pitch, roll and yaw angles respectively. 90% of the treatment arcs showed shifts of less than 0.4 mm and 0.4°in all directions. Dosimetric impact of motion showed the largest losses in coverage on small targets. All targets achieved at least 95% of the prescription dose to 95% of their volume, even when planned without margins. ConclusionsIntra-fractional errors measured during beam delivery were found to be notably low with a dose impact that showed acceptable target coverage when applying these intra-arc errors to the dose distributions of the individual treatment arcs. Using an adequate immobilization and intra-fraction imaging prior to and during irradiation, no margins need to be added to compensate for intra-fraction motion.

Effectiveness of CT-image guidance in proton therapy for liver cancer and the importance of daily dose monitoring for tumors and organs at risk.

It is important to have precise image guidance throughout proton therapy in order to take advantage of the therapy's physical selectivity. We evaluated the effectiveness of computed tomography (CT)-image guidance in proton therapy for patients with hepatocellular carcinoma (HCC) by assessing daily proton dose distributions. The importance of daily CT image-guided registration and daily proton dose monitoring for tumors and organs at risk (OARs) was investigated. A retrospective analysis was conducted using 570 sets of daily CT (dCT) images throughout whole treatment fractions for 38 HCC patients who underwent passive scattering proton therapy with either a 66 cobalt gray equivalent (GyE)/10 fractions (n=19) or 76 GyE/20 fractions (n=19) protocol. The actual daily delivered dose distributions were estimated by forward calculation using the dCT sets, their corresponding treatment plans, and the recorded daily couch correction information. We then evaluated the daily changes of the dose indices D99% , V30GyE , and Dmax for the tumor volumes, non-tumorous liver, and other OARs, that is, stomach, esophagus, duodenum, colon, respectively. Contours were created for all dCT sets. We validated the efficacy of the dCT-based tumor registrations (hereafter, "tumor registration") by comparing them with the bone registration and diaphragm registration as a simulation of the treatment based on the positioning using the conventional kV X-ray imaging. The dose distributions and the indices of three registrations were obtained by simulation using the same dCT sets. In the 66 GyE/10 fractions, the daily D99% value in both the tumor and diaphragm registrations agreed with the planned value with 3%-6% (SD), and the V30GyE value for the liver agreed within ±3%; the indices in the bone registration showed greater deterioration. Nevertheless, tumor-dose deterioration occurred in all registration methods for two cases due to daily changes of body shape and respiratory condition. In the 76 GyE/20 fractions, in particular for such a treatment that the dose constraints for the OARs have to be cared in the original planning, the daily D99% in the tumor registration was superior to that in the other registration (p<0.001), indicating the effectiveness of the tumor registration. The dose constraints, set in the plan as the maximum dose for OARs (i.e., duodenum, stomach, colon, and esophagus) were maintained for 16 patients including seven treated with re-planning. For three patients, the daily Dmax increased gradually or changed randomly, resulting in an inter-fractional averaged Dmax higher than the constraints. The dose distribution would have been improved if re-planning had been conducted. The results of these retrospective analyses indicate the importance of daily dose monitoring followed by adaptive re-planning when needed. The tumor registration in proton treatment for HCC was effective to maintain the daily dose to the tumor and the dose constraints of OARs, particularly in the treatment where the maintenance for the dose constraints needs to be considered throughout the treatment. Nevertheless daily proton dose monitoring with daily CT imaging is important for more reliable and safer treatment.

Simultaneous integrated boost (SIB) stereotactic body radiotherapy (SBRT) for prostate and pelvic nodes in high risk, node positive and oligometastatic prostate cancers: Safety, efficacy and quality of life outcome measures.

121 Background: SBRT in high risk (HR), node positive (N+) and oligometastatic (OM) prostate cancers is limited by pelvic nodal coverage, larger treatment volumes and use of hormone therapy. We report the safety, efficacy and patient reported quality of life measures (QOL) for SIB-SBRT for prostate and pelvic nodal treatment in HR, N+ and OM (non-regional node or skeletal deposits) cohorts. Methods: 22 patients of NCCN category HR, N+, OM adenocarcinoma prostate and all staged by MRI prostate and PSMA PETCT scan received SIB-SBRT from April 2019 to October 2021. All received long term androgen (ADT) deprivation therapy (80%) or surgical castration (20%). ADT was neo-adjuvant, concomitant and adjuvant for at least 2 years and for OM 2nd generation hormone treatment. SIB-SBRT to prostate and pelvic nodal regions (until common iliac level) was 35-36.25 Gy and 25 Gy in 5 fractions respectively. Post ADT gross PSMA avid nodes and skeletal deposits received 30 Gy in 5 fractions. SIB-SBRT was by volumetric modulated arc therapy (VMAT) with intrafraction imaging and hexapod robotic couch corrections on alternate days. Baseline IPSS and Expanded Prostate Cancer Index Composite for Clinical Practice (EPIC-CP) QOL recorded and then post SIB-SBRT at 6 weeks and then 3 monthly till 2 years. Dosimetry followed standard published literature. Results: 22 patients were treated over 3 years with median age of 70 years. The median PSA at baseline was 45ng/ml with 90% Gleason score 8+. The OM disease included nonregional nodes or 1-2 skeletal deposits. After median follow up of 20 months acute grade 2 genitourinary (GU) and gastrointestinal (GI) toxicities were 10% and 12%, respectively. Late grade 2 GU and GI toxicities were 4% and 9%, respectively. Late grade 3 GU and GI toxicity was seen in 2 patients each requiring medical management. 2- year overall survival and biochemical failure-free survival was 95% and 90% respectively. Baseline median IPSS score was 9 and median EPIC CP score was 12/60. Mean urinary incontinence domain scores worsened significantly from baseline to 6 months and obstructive scores improved from baseline to 6 months with more than 95% patients not using any pads at 6 months. Bowel frequency scores were maximum at 6 weeks post SBRT and it persisted till 6 months. Sexual function scores declined gradually and recovered only in a few at last follow up. Vitality/hormonal scores were unchanged from baseline till 6 months. Conclusions: SIB-SBRT for HR, N+ and OM prostate cancers is safe and effective, with good QOL. The encouraging PSA control and minimal grade 3 toxicity and favorable therapeutic ratio achieved reflect the feasibility of LINAC based SIB-SBRT. This warrants long term follow-up with effective bladder/rectal protocol measures and optimal use of ADT in a prospective setting.

The spatial accuracy of ring-mounted halcyon linac versus C-arm TrueBeam linac for single-isocenter/multi-target SBRT treatment.

Stereotactic body radiotherapy (SBRT) treatment of oligometastatic lesions via single-isocenter/multi-target (SIMT) plan is more efficient than using multi-isocenter/multitarget SBRT. This study quantifies the spatial positioning accuracy of 2 commercially available LINAC systems for SIMT treatment pertaining to the potential amplification of error as a function of the target's distance-to-isocenter. We compare the Ring-Gantry Halcyon LINAC equipped with the fast iterative conebeam-CT (iCBCT) for image-guided SIMT treatment, and the SBRT-dedicated C-Arm TrueBeam with standard pretreatment CBCT imaging. For both systems, Sun Nuclear's MultiMet Winston-Lutz Cube phantom with 6 metallic BBs distributed at different planes up to 7 cm away from the isocenter was used. The phantom was aligned and imaged via CBCT, and then couch corrections were applied. To treat all 6 BBs, an Eclipse 10-field 3D-conformal Field-in-Field (2×2 cm2 MLC field to each BB) plan for varying gantry, collimator, and couch (TrueBeam only) positions was developed for both machines with 6MV-FFF beam. The plan was delivered through ARIA once a week. The EPID images were analyzed via Sun Nuclear's software for spatial positioning accuracy. On TrueBeam, the treatment plan was delivered twice: once with 3DoF translational corrections and once with PerfectPitch 6DoF couch corrections. The average 3D spatial positioning accuracy was 0.55 ± 0.30 mm, 0.54 ± 0.24 mm, and 0.56 ± 0.28 mm at isocenter, and 0.59 ± 0.30 mm, 0.69 ± 0.30 mm, and 0.70 ± 0.35 mm at 7 cm distance-to-isocenter for Halcyon, TrueBeam 3DoF, and TrueBeam 6DoF, respectively. This suggests there are no clinically significant deviations of spatial uncertainty between the platforms with the distance-to-isocenter. On both platforms, our weekly independent measurements demonstrated the reproducibility for less than 1.0 mm positional accuracy of off-axis targets up to 7 cm from the isocenter. Due to this, no additional PTV-margin is suggested for lesions within 7 cm of isocenter. This study confirms that Halcyon can deliver similar positional accuracy to SBRT-dedicated TrueBeam to off-axis targets up to 7 cm from isocenter. These results further benchmark the spatial uncertainty of our extensively used SBRT-dedicated TrueBeam LINAC for SIMT SBRT treatments.

Treatment time of image-guided radiotherapy with a Halcyon 2.0 system

IntroductionThe treatment fraction time is a key indicator of the external beam radiotherapy process. The Halcyon system was designed to improve the clinical workflow, according to the manufacturer (Varian Medical Systems). Few works studied the actual delivery efficiency of the Halcyon system. This work analyzed the treatment time on a Halcyon 2.0 unit for a variety of sites along a period of 9 months. Materials and MethodsTreatment time included patient setup, image acquisition, image-guided online couch correction, and radiation delivery time. Data were extracted from the ARIA oncology information system and were studied as a function of the treatment site, the delivery modality, and the time from the first day of treatments with the Halcyon 2.0 system in our institution. ResultsA total of 8599 fractions were delivered during the analyzed period (69.5% from VMAT plans, and 30.5% from IMRT plans). The number of fractions by site ranged from 30 for anal canal to 1933 for prostate. Five sites (prostate, lung, pelvis with prostate, breast, and gynecological sites) accounted for the 84% of the fractions. After a 2-week adaptation period of the staff, the daily mean treatment time was reduced to less than 12 min. The mean treatment time of all the fractions amounted to 10.5 ± 3.8 min. ConclusionsThe Halcyon 2.0 allowed delivering online image-guided radiation therapy in all fractions with total treatment time consistently below the 12-min standard time slot, for most of the analyzed treatment sites.

A Fast 3D/3D Registration Method Based on Water Equivalent Path Length for Heavy-Ion Radiotherapy

<h3>Purpose/Objective(s)</h3> The accuracy of patient setup needs to be high in heavy-ion radiotherapy. Accordingly, there is an increasing demand for three-dimensional (3D) image-guided systems because of the limitations of the present two-dimensional imaging registration technology. One of the limitations is the inability to directly compare with planning computed tomography (CT) images. Due to this uncertainty in conformation, high dose per fraction radiotherapy is difficult. This study proposes a fast 3D/3D registration method comparing with water equivalent path length (WEPL) that uses planning and treatment CT images to minimize range variations by using the Lucas-Kanade (LK) algorithm. <h3>Materials/Methods</h3> A six-degrees-of-freedom (6DOF) couch correction was simulated for patient positioning at our center by performing automatic rigid image registration (RIR) and using the resultant translation/rotation. Our automatic RIR algorithm was used to align the planning CT and treatment CT images, simulating 6DOF. The alignment values (Δ<i>V</i>), which represents a six-dimensional vector of the amounts of 6DOF couch movement, were calculated by minimizing the following error function: E(<i>ΔV</i>)=Σ <i>_i</i>_∈Ω [<i>T_i</i> (<i>V+ΔV</i>)-<i>P_i</i> (<i>V</i>)]², where <i>V</i> is a six-dimensional vector that denotes a CT position and rotation in a treatment room; <i>T_i</i> (<i>V</i>) and <i>P_i</i> (<i>V</i>) are treatment and planning CT images respectively, whose position in the room is represented by <i>V; i</i> is a 3D vector which denotes a point in the room; and Ω is a set of calculation points, which was assigned a planning target volume (PTV) of Ω in this study. We minimized the error function by using the LK algorithm. To minimize the range variations, each pixel value of the CT images was changed into WEPL values and the alignment values were calculated in the same manner as the above algorithm. The method was evaluated using a dataset containing eight prostate cancer patient images, including a planning CT image with PTV and treatment CT images of four fraction with a fan-beam CT. The range variations were measured by calculating WEPL variations between the planning CT and treatment CT at (1) before registration and after registration based on (2) CT pixel values and (3) WEPL values, respectively. In addition, the calculation time was measured on a workstation with a single GPU. <h3>Results</h3> The mean WEL variation across the 32 cases (eight patients of four fractions) was (1) 1.8 ± 1.16 mm, (2) 1.08 ± 0.52 mm, and (3) 0.22 ± 0.17 mm. The worst WEPL variation were (1) 6.23 mm, (2) 3.06 mm, and (3) 1.17 mm. The mean calculation time were (2) 89 ms and (3) 1066 ms. <h3>Conclusion</h3> We developed a fast 3D/3D RIR method for reducing range variations to realize more accurate heavy-ion radiotherapy. We performed experiments involving eight prostate cancer patients, and our proposed method obtained high accuracy and short computation time.

Effectiveness of 6D couch with daily cone beam computed tomography in reducing PTV margins for glioblastoma multiforme.

Image-guided radiotherapy maximizes therapeutic index of brain irradiation by reducing setup errors during treatment. The aim of study was to analyze setup errors in the radiation treatment of glioblastoma multiforme and if decrease in planning target volume (PTV), margin is feasible using daily cone beam CT (CBCT) and 6D couch correction. Twenty-one patients (630 fractions of radiotherapy) were studied in which corrections were made in 6° of freedom. We determined setup errors, impact of setup errors of initial three fractions CBCT versus rest of the treatment with daily CBCT, and mean difference in setup errors with or without application of 6D couch and volumetric benefit of reduction of PTV margin from 0.5 cm to 0.3 cm. The mean shift in the conventional directions, namely, vertical, longitudinal, and lateral was 0.17 cm, 0.19 cm, and 0.11 cm. There was significant change in vertical shift when first three fractions were compared with rest of the treatment with daily CBCT. When the effect of 6D couch was nullified, all directions showed increased error with longitudinal shift being significant. The number of setup errors of magnitude >0.3 cm was more significant when only conventional shifts were applied as compared with 6D couch. There was significant decrease in volume of brain parenchyma irradiated when margin of PTV was reduced from 0.5 cm to 0.3 cm. Daily CBCT along with 6D couch correction can reduce setup error which allows reduction in PTV margin during radiotherapy planning in turn improving the therapeutic index.

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.

142: Assessment of Intrafraction Motion for Spine and Non-Spine Bone Metastases Treated with Image-Guided Stereotactic Ablative Radiotherapy Without 6 Degrees-of-Freedom Couch Correction

142: Assessment of Intrafraction Motion for Spine and Non-Spine Bone Metastases Treated with Image-Guided Stereotactic Ablative Radiotherapy Without 6 Degrees-of-Freedom Couch Correction

Residual image registration error by fiducial markers in accelerated partial breast irradiation using C-arm linac: a phantom study.

External beam accelerated partial breast irradiation (APBI) is an alternative treatment for patients with early-stage breast cancer. The efficacy of image-guided radiotherapy (IGRT) using fiducial markers, such as gold markers or surgical clips, has been demonstrated. However, the effects of respiratory motion during a single fraction have not been reported. This study aimed to evaluate the residual image registration error of fiducial marker-based IGRT by respiratory motion and propose a suitable treatment strategy. We developed an acrylic phantom embedded with surgical clips to verify the registration error under moving conditions. The frequency of the phase difference in the respiratory cycle due to sequential acquisition was verified in a preliminary study. Fiducial marker-based IGRT was then performed in ten scenarios. The residual registration error (RRE) was calculated on the basis of the differences in the coordinates of clips between the true position if not moved and the last position. The frequencies of the phase differences in 0.0-0.99, 1.0-1.99, 2.0-2.99, 3.0-3.99, and 4.0-5.0mm were 23%, 24%, 22%, 20%, and 11%, respectively. When assuming a clinical case, the mean RREs for all directions were within 1.0mm, even if respiratory motion of 5mm existed in two axes. For APBI with fiducial marker-based IGRT, the introduction of an image registration strategy that employs stepwise couch correction using at least three orthogonal images should be considered.

Conebeam CT‐guided 3D MLC‐based spatially fractionated radiation therapy for bulky masses

For fast, safe, and effective management of large and bulky (≥8 cm) non‐resectable tumors, we have developed a conebeam CT‐guided three‐dimensional (3D)‐conformal MLC‐based spatially fractionated radiation therapy (SFRT) treatment. Using an in‐house MLC‐fitting algorithm, Millennium 120 leaves were fitted to the gross tumor volume (GTV) generating 1‐cm diameter holes at 2‐cm center‐to‐center distance at isocenter. SFRT plans of 15 Gy were generated using four to six coplanar crossfire gantry angles 60° apart with a 90° collimator, differentially weighted with 6‐ or 10‐MV beams. A dose was calculated using AcurosXB algorithm, generating sieve‐like dose channels without post‐processing the physician‐drawn GTV contour within an hour of CT simulation allowing for the same day treatment. In total, 50 extracranial patients have been planned and treated using this method, comprising multiple treatment sites. This novel MLC‐fitting algorithm provided excellent dose parameters with mean GTV (V7.5 Gy) and mean GTV doses of 53.2% and 7.9 Gy, respectively, for 15 Gy plans. Average peak‐to‐valley dose ratio was 3.2. Mean beam‐on time was 3.32 min, and treatment time, including patient setup and CBCT to beam‐off, was within 15 min. Average 3D couch correction from original skin‐markers was <1.0 cm. 3D MLC‐based SFRT plans enhanced target dose for bulky masses, including deep‐seated large tumors while protecting skin and adjacent critical organs. Additionally, it provides the same day, safe, effective, and convenient treatment by eliminating the risk to therapists and patients from heavy gantry‐mounted physical GRID‐block—we recommend other centers to use this simple and clinically useful method. This rapid SFRT planning technique is easily adoptable in any radiation oncology clinic by eliminating the need for plan optimization and patient‐specific quality assurance times while providing dosimetry information in the treatment planning system. This potentially allows for dose‐escalation to deep‐seated masses to debulk unresectable large tumors providing an option for neoadjuvant treatment. An outcome study of clinical trial is underway.

94P Preliminary outcome data for deep inspiratory breath hold guided stereotactic radiotherapy (SBRT) with robotic couch correction for early stage non-small cell lung cancers (NSCLC)

Stereotactic radiotherapy (SBRT) remains a viable option for early stage inoperable non small cell lung cancers (NSCLC). Multitude of delivery platforms and motion management devices been in practice with their own pros and cons. We report the departmental prospectively maintained database for early stage (T1-2a,N0M0 8th edition) NSCLC treated by SBRT with deep inspiratory breathold (DIBH) and robotic couch correction.

HyperArc VMAT stereotactic radiotherapy for locally recurrent previously-irradiated head and neck cancers: Plan quality, treatment delivery accuracy, and efficiency.

PurposeThis paper demonstrates the clinical feasibility and efficacy of HyperArc VMAT treatments for locally recurrent, locally advanced, or previously irradiated head and neck cancers treated with stereotactic radiotherapy (SRT).Materials/MethodsFirst, an anthropomorphic SRS head phantom from the MD Anderson's IROC credentialing laboratory containing a 1.9 cm diameter spherical target, including in vivo dosimetry system, was imaged, planned, and irradiated (25 Gy in 1 fraction) using HyperArc VMAT with a 6 MV flattening filter free (FFF) beam. Second, RANDO phantom was imaged, planned, and irradiated (35 Gy in 5 fractions) by generating eight HyperArc VMAT plans (4 right, 4 left neck tumors) at different anatomical locations (C1–C4). Average tumor volume was 21.7 cm3 up to 32.3 cm3. Distance to isocenter from the central marker of the Encompass device down to neck was 25.8 cm up to 28.0 cm and 24.3 cm up to 27.1 cm for left‐ and right‐sided neck tumors, respectively, and 9 cm from both lateral markers defined by the patient protection zone. Third, seven recurrent head and neck cancer patients with 80.3 cm3 tumors on average, and up to 159 cm3, were imaged, planned, and treated with 30–40 Gy in 5 fractions with HyperArc SRT. Plan quality, treatment delivery accuracy, and efficiency are reported herein.ResultsPhantom irradiation results met all the compliance requirements set forth by the IROC for HyperArc SRS treatment. For end‐to‐end RANDO phantom tests, a highly conformal target dose distribution with 50% isodose fall‐off within 5 mm from the surface of the target was obtained. Average beam modulation factor, beam‐on‐time, and overall treatment time were 2.9, 2.56 min, and 13.96 min with 99.1% pre‐treatment quality assurance pass rate for the 2%/2 mm gamma criteria, respectively. Immediately adjacent critical structures, such as the spinal cord (maximum, 3.9 Gy and 0.35 cm3 of cord, 3.7 Gy) and skin (maximum, 10.3 Gy and 10 cm3 of skin, 5.7 Gy), were spared. Similar results were found on the patient's HyperArc VMAT plans including highly conformal target coverage, sharp dose fall‐off, and low doses to the adjacent critical organs such as the spinal cord (< 5 Gy). Average perfect pitch couch correction was <1.5 mm and 2° in each direction. Average beam‐on‐time was approximately 3.21 min and treatments were completed within 15 min.ConclusionFor recurrent head and neck SRT treatments, HyperArc VMAT provided highly conformal dose distributions, rapid dose fall‐off, excellent sparing of adjacent critical organs, and highly accurate treatments that could be delivered down to the C4 vertebral level. This could potentially allow for delivery of HyperArc SRT to patients with glomus tumors as well to those who may not tolerate frame‐based SRS treatment. Clinical follow up of these patients is ongoing to confirm the therapeutic benefits of this novel treatment option.