Corrigendum: A single-institution experience with 177Lu RPT workflow improvements and qualifying the SPECT/CT imaging for dosimetry

[This corrects the article DOI: 10.3389/fonc.2024.1331266.].

Clinical application of an institutional fractionated stereotactic radiosurgery (FSRS) program for brain metastases delivered with MRIdianⓇ BrainTx™

Single-fraction stereotactic radiosurgery (SRS) or fractionated SRS (FSRS) are well established strategies for patients with limited brain metastases. A broad spectrum of modern dedicated platforms are currently available for delivering intracranial SRS/FSRS; however, SRS/FSRS delivered using traditional CT-based platforms relies on the need for diagnostic MR images to be coregistered to planning CT scans for target volume delineation. Additionally, the on-board image guidance on traditional platforms yields limited inter-fraction and intra-fraction real-time visualization of the tumor at the time of treatment delivery. MR Linacs are capable of obtaining treatment planning MR and on-table MR sequences to enable visualization of the targets and organs-at-risk and may subsequently help identify anatomical changes prior to treatment that may invoke the need for on table treatment adaptation. Recently, an MR-guided intracranial package (MRIdian A3i BrainTxTM) was released for intracranial treatment with the ability to perform high-resolution MR sequences using a dedicated brain coil and cranial immobilization system. The objective of this report is to provide, through the experience of our first patient treated, a comprehensive overview of the clinical application of our institutional program for FSRS adaptive delivery using MRIdian's A3i BrainTx system-highlights include reviewing the imaging sequence selection, workflow demonstration, and details in its delivery feasibility in clinical practice, and dosimetric outcomes.

A single-institution experience with 177Lu RPT workflow improvements and qualifying the SPECT/CT imaging for dosimetry

Background and purpose

Implementing any radiopharmaceutical therapy (RPT) program requires a comprehensive review of system readiness, appropriate workflows, and training to ensure safe and efficient treatment delivery. A quantitative assessment of the dose delivered to targets and organs at risk (OAR) using RPT is possible by correlating the absorbed doses with the delivered radioactivity. Integrating dosimetry into an established RPT program demands a thorough analysis of the necessary components and system fine-tuning. This study aims to report an optimized workflow for molecular radiation therapy using 177Lu with a primary focus on integrating patient-specific dosimetry into an established radiopharmaceutical program in a radiation oncology setting.

Materials and methods

We comprehensively reviewed using the Plan-Do-Check-Act (PDCA) cycle, including efficacy and accuracy of delivery and all aspects of radiation safety of the RPT program. The GE Discovery SPECT/CT 670DR™ system was calibrated per MIM protocol for dose calculation on MIM SurePlan™ MRT software. Jaszcak Phantom with 15-20 mCi of 177Lu DOTATATE with 2.5 µM EDTA solution was used, with the main energy window defined as 208 keV ±10% (187.6 to 229.2 keV); the upper scatter energy window was set to 240 keV ±5% (228 to 252 keV), while the lower scatter energy window was 177.8 keV ±5% (168.9 to 186.7 keV). Volumetric quality control tests and adjustments were performed to ensure the correct alignment of the table, NM, and CT gantry on SPECT/CT. A comprehensive end-to-end (E2E) test was performed to ensure workflow, functionality, and quantitative dose accuracy.

Results

Workflow improvements and checklists are presented after systematically analyzing over 400 administrations of 177Lu-based RPT. Injected activity to each sphere in the NEMA Phantom scan was quantified, and the MIM Sureplan MRT reconstruction images calculated activities within ±12% of the injected activity. Image alignment tests on the SPECT/CT showed a discrepancy of more than the maximum tolerance of 2.2 mm on any individual axis. As a result of servicing the machine and updating the VQC and COR corrections, the hybrid imaging system was adjusted to achieve an accuracy of <1 mm in all directions.

Conclusion

Workflows and checklists, after analysis of system readiness and adequate training for staff and patients, are presented. Hardware and software components for patient-specific dosimetry are presented with a focus on hybrid image registration and correcting any errors that affect dosimetric quantification calculation. Moreover, this manuscript briefly overviews the necessary quality assurance requirements for converting diagnostic images into dosimetry measurement tools and integrating dosimetry for RPT based on 177Lu.

Commissioning Intracranial Stereotactic Radiosurgery for a Magnetic Resonance-Guided Radiation Therapy (MRgRT) System: MR-RT Localization and Dosimetric End-to-End Validation

PURPOSE

This is the first reporting of the MRIdian A3iTM intracranial package (BrainTxTM) and benchmarks the end-to-end localization and dosimetric accuracy for commissioning an magnetic resonace (MR)-guided stereotactic radiosurgery program. We characterized the localization accuracy between MR and radiation (RT) isocenter through an end-to-end hidden target test, relative dose profile intercomparison, and absolute dose validation.

METHODS AND MATERIALS

BrainTx consists of a dedicated head coil, integrated mask immobilization system, and high-resolution MR sequences. Coil and baseplate attenuation was quantified. An in-house phantom (Cranial phantOm foR magNetic rEsonance Localization of a stereotactIc radiosUrgery doSimeter, CORNELIUS) was developed from a mannequin head filled with silicone gel, film, and MR BB with pinprick. A hidden target test evaluated MR-RT localization of the 1×1×1 mm3 TrueFISP MR and relative dose accuracy in film for a 1 cm diameter (International Electrotechnical Commission (IEC)-X/IEC-Y) and 1.5 cm diameter (IEC-Y/IEC-Z) spherical target. Two clinical cases (irregular-shaped target and target abutting brainstem) were mapped to the CORNELIUS phantom for feasibility assessment. A 2-dimensional (2D)-gamma compared calculated and measured dose for spherical and clinical targets with 1 mm/1% and 2 mm/2% criteria, respectively. A small-field chamber (A26MR) measured end-to-end absolute dose for a 1 cm diameter target.

RESULTS

Coil and baseplate attenuation were 0.7% and 2.7%, respectively. The displacement of MR to RT localization as defined through the pinprick was 0.49 mm (IEC-X), 0.27 mm (IEC-Y), and 0.51 mm (IEC-Z) (root mean square 0.76 mm). The reproducibility across IEC-Y demonstrated high fidelity (<0.02 mm). Gamma pass rates were 97.1% and 95.4% for 1 cm and 1.5 cm targets, respectively. Dose profiles for an irregular-shaped target and abutting organ-at-risk-target demonstrated pass rates of 99.0% and 92.9%, respectively. The absolute end-to-end dose difference was <1%.

CONCLUSIONS

All localization and dosimetric evaluation demonstrated submillimeter accuracy, per the TG-142, TG-101, MPPG 9.a. criteria for SRS/SRT systems, indicating acceptable delivery capabilities with a 1 mm setup margin.

Accelerated Hypofractionated Magnetic Resonance Guided Adaptive Radiation Therapy for Ultracentral Lung Tumors

Radiotherapy for ultracentral lung tumors represents a treatment challenge, considering the high rates of high-grade treatment-related toxicities with stereotactic body radiation therapy (SBRT) or hypofractionated schedules. Accelerated hypofractionated magnetic resonance-guided adaptive radiation therapy (MRgART) emerged as a potential game-changer for tumors in these challenging locations, in close proximity to central organs at risk, such as the trachea, proximal bronchial tree, and esophagus. In this series, 13 consecutive patients, predominantly male (n = 9), with a median age of 71 (range (R): 46-85), underwent 195 MRgART fractions (all 60 Gy in 15 fractions) to metastatic (n = 12) or primary ultra-central lung tumors (n = 1). The median gross tumor volumes (GTVs) and planning target volumes (PTVs) were 20.72 cc (R: 0.54-121.65 cc) and 61.53 cc (R: 3.87-211.81 cc), respectively. The median beam-on time per fraction was 14 min. Adapted treatment plans were generated for all fractions, and indications included GTV/PTV undercoverage, OARs exceeding tolerance doses, or both indications in 46%, 18%, and 36% of fractions, respectively. Eight patients received concurrent systemic therapies, including immunotherapy (four), chemotherapy (two), and targeted therapy (two). The crude in-field loco-regional control rate was 92.3%. No CTCAE grade 3+ toxicities were observed. Our results offer promising insights, suggesting that MRgART has the potential to mitigate toxicities, enhance treatment precision, and improve overall patient care in the context of ultracentral lung tumors.

Surgically targeted radiation therapy (STaRT) for recurrent brain metastases: Initial clinical experience

PURPOSE

This study evaluates the outcomes of recurrent brain metastasis treated with resection and brachytherapy using a novel Cesium-131 carrier, termed surgically targeted radiation therapy (STaRT), and compares them to the first course of external beam radiotherapy (EBRT).

METHODS

Consecutive patients who underwent STaRT between August 2020 and June 2022 were included. All patients underwent maximal safe resection with pathologic confirmation of viable disease prior to STaRT to 60 Gy to a 5-mm depth from the surface of the resection cavity. Complications were assessed using CTCAE version 5.0.

RESULTS

Ten patients with 12 recurrent brain metastases after EBRT (median 15.5 months, range: 4.9-44.7) met the inclusion criteria. The median BED10Gy90% and 95% were 132.2 Gy (113.9-265.1 Gy) and 116.0 Gy (96.8-250.6 Gy), respectively. The median maximum point dose BED10Gy for the target was 1076.0 Gy (range: 120.7-1478.3 Gy). The 6-month and 1-year local control rates were 66.7% and 33.3% for the prior EBRT course; these rates were 100% and 100% for STaRT, respectively (p < 0.001). At a median follow-up of 14.5 months, there was one instance of grade two radiation necrosis. Surgery-attributed complications were observed in two patients including pseudomeningocele and minor headache.

CONCLUSIONS

STaRT with Cs-131 presents an alternative approach for operable recurrent brain metastases and was associated with superior local control than the first course of EBRT in this series. Our initial clinical experience shows that STaRT is associated with a high local control rate, modest surgical complication rate, and low radiation necrosis risk in the reirradiation setting.

A study on inter-planner plan quality variability using a manual planning- or Lightning dose optimizer-approach for single brain lesions treated with the Gamma Knife® Icon™

PURPOSE

The purpose of this study is to investigate inter-planner plan quality variability using a manual forward planning (MFP)- or fast inverse planning (FIP, Lightning)-approach for single brain lesions treated with the Gamma Knife® (GK) Icon™.

METHODS

Thirty patients who were previously treated with GK stereotactic radiosurgery or radiotherapy were selected and divided into three groups (post-operative resection cavity, intact brain metastasis, and vestibular schwannoma [10 patients per group]). Clinical plans for the 30 patients were generated by multiple planners using FIP only (1), a combination of FIP and MFP (12), and MFP only (17). Three planners (Senior, Junior, and Novice) with varying experience levels re-planned the 30 patients using MFP and FIP (two plans per patient) with planning time limit of 60 min. Statistical analysis was performed to compare plan quality metrics (Paddick conformity index, gradient index, number of shots, prescription isodose line, target coverage, beam-on-time (BOT), and organs-at-risk doses) of MFP or FIP plans among three planners and to compare plan quality metrics between each planner's MFP/FIP plans and clinical plans. Variability in FIP parameter settings (BOT, low dose, and target max dose) and in planning time among the planners was also evaluated.

RESULTS

Variations in plan quality metrics of FIP plans among three planners were smaller than those of MFP plans for all three groups. Junior's MFP plans were the most comparable to the clinical plans, whereas Senior's and Novice's MFP plans were superior and inferior, respectively. All three planners' FIP plans were comparable or superior to the clinical plans. Differences in FIP parameter settings among the planners were observed. Planning time was shorter and variations in planning time among the planners were smaller for FIP plans in all three groups.

CONCLUSIONS

The FIP approach is less planner dependent and more time-honored than the MFP approach.

Accelerated hypofractionated magnetic resonance-guided adaptive radiotherapy for oligoprogressive non-small cell lung cancer

Given the positive results from recent randomized controlled trials in patients with oligometastatic, oligoprogressive, or oligoresidual disease, the role of radiotherapy has expanded in patients with metastatic non-small cell lung cancer (NSCLC). While small metastatic lesions are commonly treated with stereotactic body radiotherapy (SBRT), treatment of the primary tumor and involved regional lymph nodes may require prolonged fractionation schedules to ensure safety especially when treating larger volumes in proximity to critical organs-at-risk (OARs). We have developed an institutional MR-guided adaptive radiotherapy (MRgRT) workflow for these patients. We present a 71-year-old patient with stage IV NSCLC with oligoprogression of the primary tumor and associated regional lymph nodes in which MR-guided, online adaptive radiotherapy was performed, prescribing 60 Gy in 15 fractions. We describe our workflow, dosimetric constraints, and daily dosimetric comparisons for the critical OARs (esophagus, trachea, and proximal bronchial tree [PBT] maximum doses [D0.03cc]), in comparison to the original treatment plan recalculated on the anatomy of the day (i.e., predicted doses). During MRgRT, few fractions met the original dosimetric objectives: 6.6% for esophagus, 6.6% for PBT, and 6.6% for trachea. Online adaptive radiotherapy reduced the cumulative doses to the structures by 11.34%, 4.2%, and 5.62% when comparing predicted plan summations to the final delivered summation. Therefore, this case study presets a workflow and treatment paradigm for accelerated hypofractionated MRgRT due to the significant variations in daily dose to the central thoracic OARs to reduce treatment-related toxicity associated with radiotherapy.

Treatment of glioblastoma using MRIdian® A3i BrainTx™: Imaging and treatment workflow demonstration

For patients with newly diagnosed glioblastoma, the current standard-of-care includes maximal safe resection, followed by concurrent chemoradiotherapy and adjuvant temozolomide, with tumor treating fields. Traditionally, diagnostic imaging is performed pre- and post-resection, without additional dedicated longitudinal imaging to evaluate tumor volumes or other treatment-related changes. However, the recent introduction of MR-guided radiotherapy using the ViewRay MRIdian A3i system includes a dedicated BrainTx package to facilitate the treatment of intracranial tumors and provides daily MR images. We present the first reported case of a glioblastoma imaged and treated using this workflow. In this case, a 67-year-old woman underwent adjuvant chemoradiotherapy after gross total resection of a left frontal glioblastoma. The radiotherapy treatment plan consisted of a traditional two-phase design (46 Gy followed by a sequential boost to a total dose of 60 Gy at 2 Gy/fraction). The treatment planning process, institutional workflow, treatment imaging, treatment timelines, and target volume changes visualized during treatment are presented. This case example using our institutional A3i system workflow successfully allows for imaging and treatment of primary brain tumors and has the potential for margin reduction, detection of early disease progression, or to detect the need for dose adaptation due to interfraction tumor volume changes.

OptImal Gamma kNife lIghTnIng sOlutioN (IGNITION) score to characterize the solution space of the Gamma Knife FIP optimizer for stereotactic radiosurgery

OBJECTIVES

The objective of this study is to evaluate the user-defined optimization settings in the Fast Inverse Planning (FIP) optimizer in Leksell GammaPlan® and determine the parameters that result in the best stereotactic radiosurgery (SRS) plan quality for brain metastases, benign tumors, and arteriovenous malformations (AVMs).

METHODS

Thirty patients with metastases and 30 with benign lesions-vestibular schwannoma, AVMs, pituitary adenoma, and meningioma-treated with SRS were evaluated. Each target was planned by varying the low dose (LD) and beam-on-time (BOT) penalties in increments of 0.1, from 0 to 1. The following plan quality metrics were recorded for each plan: Paddick conformity index (PCI), gradient index (GI), BOT, and maximum organ-at-risk (OAR) doses. A novel objective score matrix was calculated for each target using a linearly weighted combination of the aforementioned metrics. A histogram of optimal solutions containing the five best scores was extracted.

RESULTS

A total of 7260 plans were analyzed with 121 plans per patient for the range of LD/BOT penalties. The ranges of PCI, GI, and BOT across all metastatic lesions were 0.58-0.97, 2.1-3.8, and 8.8-238 min, respectively, and were 0.13-0.97, 2.1-3.8, and 8.8-238 min, respectively, for benign lesions. The objective score matrix showed unique optimal solutions for metastatic lesions and benign lesions. Additionally, the plan metrics of the optimal solutions were significantly improved compared to the clinical plans for metastatic lesions with equivalent metrics for all other cases.

CONCLUSION

In this study, FIP optimizer was evaluated to determine the optimal solution space to maximize PCI and minimize GI, BOT and OAR doses simultaneously for single metastatic/benign/non-neoplastic targets. The optimal solution chart was determined using a novel objective score which provides novice and expert planners a roadmap to generate the most optimal plans efficiently using FIP.

Implementation of superficial radiation therapy (SRT) using SRT-100 Vision™ for non-melanoma skin cancer in a Radiation Oncology clinic

PURPOSE

This article describes our experience in implementation of superficial radiation therapy (SRT) using SRT-100 Vision™ for non-melanoma skin cancer.

METHODS

Following the American Association of Physicists in Medicine Task Group-61 protocol, absolute output (absorbed dose to water at surface (cGy/min)) was measured for three energies (50, 70, and 100 kV) and for six applicators (1.5-5.0 cm in diameter). Percent depth dose (PDD) and profiles were also measured. Timer testing and ultrasound testing were performed. A treatment time calculation worksheet was created. Quality assurance (QA) of SRT-100 Vision was implemented. After treatment workflow for our clinic was developed, end-to-end (E2E) testing was performed using a Rando phantom. Considerations for treatment using SRT-100 Vision were made.

RESULTS

Absolute output (cGy/min) decreases as energy increases and applicator size decreases. Due to scatter from the applicator, PDD at depths ≤5 mm does not follow conventional trends but PDD at depths ≥15 mm increases with increasing applicator size. Profiles for the 5 cm applicator do not have strong dependence on depth except profiles at 5 mm for 50 kV. Timer/end errors are negligible for all three energies. Ultrasound images confirm allowed field of view and depth as well as no image artifacts and spatial integrity. Daily, monthly and annual QA of SRT-100 Vision implemented in our clinic is listed in a table format. E2E testing results (<1%) demonstrate the functionality and performance of our treatment workflow. Our considerations for SRT treatment include patient, applicator size and energy selections, patient setup, and shields.

CONCLUSIONS

This article is expected to serve as guidance for Radiation Oncology and/or Dermatology clinics aspiring to initiate an SRT program in their clinics.

RADT-13. CLINICAL OUTCOMES COMPARING RADIOSURGERY TECHNIQUES FOR MANAGING LARGE BRAIN METASTASES

INTRODUCTION

Large brain metastases (BM) are associated with poor local control with single-fraction radiosurgery (SRS) alone. We sought to compare outcomes with various alternative management strategies to assess disease control rates.

METHODS

Consecutive patients diagnosed with BM ≥ 2 cm in maximum diameter between 3/2019-2/2021 and treated at a single tertiary institution were included. Primary endpoints included freedom-from-local-failure (FFLF) calculated on a per-lesion basis from the date of initial intervention to local failure, and overall survival (OS). Kaplan-Meier analysis and the log-rank test were used.

RESULTS

86 patients with 102 large BM met inclusion criteria. The median age was 66 years (range: 31-98), median KPS was 90 (50-100), and 57.8% were female. The most common primary tumor was lung in 40 (39.2%) lesions. The median tumor volume was 9.7 cm3 (2.1-35.5 cm3) and the median maximum tumor diameter was 3.3 cm (2.9-5.6 cm). 37 (36.3%) lesions were treated with staged-SRS, 31 (30.4%) with resection and postoperative-SRS, 21 (20.6%) with preoperative-SRS and resection, and 13 (12.7%) with fractionated-SRS (FSRS). Lesions treated with preoperative-SRS or postoperative-SRS were associated with higher rates of no- or stable-extracranial disease, and larger tumor volumes (preoperative-SRS: median 14.5 cm3 [4.2-33.1]; postoperative-SRS: 18.0 cm3 [2.5-35.5]; FSRS: 5.9 cm3 [2.1-24.6]; staged-SRS: 6.5 cm3 [2.2-27.8]).With a median follow-up of 15.6 months, 12 (11.8%) local failures and 42 (41.2%) deaths occurred. 1-year FFLF and OS for entire cohort were 85.6% (95% CI: 77.9-93.3%) and 64.4% (95% CI: 54.7-74.1%), respectively. The 1-year FFLF (p=0.542)/OS (p=0.043) rates were 94.1%/100% for preoperative-SRS, 84.0%/43.4% for staged-SRS, 81.8%/74.1% for FSRS, and 81.5%/63.4% for postoperative-SRS, respectively.

CONCLUSION

Although limited by patient numbers, selection bias, and high attrition rate, preoperative-SRS had the numerically most favorable outcomes, supporting this approach for large BM. Prospective evaluation is warranted to determine the optimal management strategy.

CTIM-21. NRG-BN010: A SAFETY RUN-IN AND PHASE II STUDY EVALUATING THE COMBINATION OF TOCILIZUMAB, ATEZOLIZUMAB, AND FRACTIONATED STEREOTACTIC RADIOTHERAPY IN RECURRENT GLIOBLASTOMA – TRIAL IN PROGRESS

BACKGROUND

The glioblastoma (GBM) tumor microenvironment (TME) is characterized by a paucity of effector T cells and massive infiltration of immunosuppressive tumor-associated macrophages (TAMs). Inhibition of the interleukin-6 receptor (IL6R) has been demonstrated in preclinical models to repolarize TAMs toward an immunostimulatory phenotype, rendering GBM more susceptible to PD-1/PD-L1 inhibition. Additional preclinical data suggest that fractionated stereotactic radiotherapy (FSRT) can stimulate the release and presentation of tumor-specific antigens, acting as an in-situ vaccine and potentially converting a “cold” TME to a T cell-inflamed state. We designed a safety run-in and phase II study to evaluate the efficacy, safety, and impact on the TME of the combination of IL6R inhibition (tocilizumab), PD-L1 inhibition (atezolizumab), and FSRT in patients with recurrent GBM (rGBM).

METHODS

NRG-BN010 (NCT04729959) is open to enrollment for adults with rGBM following prior radiotherapy and has 3 components: a safety run-in to determine the recommended phase II dose (RP2D) of the treatment regimen, a phase II single-arm nonsurgical cohort to assess efficacy of the treatment regimen at the RP2D, and a window-of-opportunity surgical cohort. Once the safety run-in (n=12, 3 + 3 design) has determined the RP2D, the phase II and surgical cohorts will open. Phase II patients (n=25, Simon 2-stage design, primary endpoint objective radiographic response) receive an initial pre-FSRT cycle of tocilizumab (plus atezolizumab if included in the RP2D). Within 3-7 days later, the patient receives FSRT (8 Gy x 3 fx), followed by resumption of systemic therapy. Surgical cohort patients (n=16, 1:1 randomization) receive a neoadjuvant cycle of atezolizumab with (n=8) vs. without (n=8) tocilizumab, then FSRT 3-7 days later, then surgical resection 7-14 days after FSRT. Post-operatively, all patients resume systemic therapy at the RP2D. Fresh tumor tissues will be subjected to deep immune profiling to understand the impact of tocilizumab on TAMs in the GBM TME.

RADT-03. GOLDILOCKS AND THE THREE DOSES OF RADIOSURGERY FOR SMALL BRAIN METASTASES

INTRODUCTION

The optimal dose for single-fraction radiosurgery (SRS) for small brain metastases (BM) (&lt; 2 cm) has not been established considerable variability in dose selection.

METHODS

Consecutive patients diagnosed with intact BM &lt; 2 cm in maximum dimension treated with SRS to prescription doses of 24, 22, or 20 Gy on the Gamma Knife® between 3/2019-1/2021 were reviewed. Institutional dosing prescriptions were: 24 Gy for &lt; 10 lesions, 22 Gy for 10-20, and 20 Gy for &gt;20). Endpoints included freedom-from-local-failure (FFLF) and radionecrosis on a per-lesion basis calculated from date of SRS to event. Univariable and multivariable analyses were performed using the Kaplan-Meier method and Cox proportional hazards regression.

RESULTS

133 patients with 863 BM met inclusion criteria. Prescription dose was 24 Gy for 416 (48.2%) lesions, 22 Gy for 266 (30.8%), and 20 Gy for 181 (21.0%). 20 Gy was associated (all p&lt; 0.001) with younger age, presence of extracranial-disease, progressive extracranial-disease, lung primary, higher number of BM per SRS course (median 17 for 20 Gy, 14 for 22 Gy, 6 for 24 Gy), and smaller per-lesion tumor volume. With a median follow-up of 15 months, 76 (8.8%) LFs occurred. Actuarial 1-year FFLF was 91.2% (95%CI: 88.6-94.8%) for 24 Gy, 92.5% (89.1-95.9%) for 22 Gy, and 77.9% (71.2-84.6%) for 20 Gy (p&lt; 0.001). On multivariable analysis, factors independently associated with adverse FFLF (all p&lt; 0.05) were male sex, presence of extracranial-disease, breast or lung primary, and 20 Gy prescription dose (HR: 2.52, 95%CI: 1.34-4.76). The cumulative 1-year radionecrosis rate was 7.0% for 24 Gy, 4.0% for 22 Gy, and 3.8% for 20 Gy (p=0.035).

CONCLUSION

For BM &lt; 2 cm, we propose a testable hypothesis that 22 Gy prescription dose may represent the “Goldilocks zone”- superior to 20 Gy regarding LF and superior to 24 Gy concerning radionecrosis.

Fractionated Total Body Irradiation on an Infant Using Tomotherapy

Total body irradiation (TBI) is used with chemotherapy to induce immunosuppression for hematopoietic cell transplantation and is often administered using lead blocks to minimize lung dose in adults and children. This technique is challenging in infants and young children. A 13-month-old female with acute lymphoblastic leukemia (ALL) was treated with fractionated TBI to a dose of 12 Gy in eight fractions delivered twice daily. Multiple TBI techniques for delivering treatment were considered. Ultimately, treatment using helical tomotherapy was selected in order to spare and accurately quantify the dose to the lung, meet lung dose constraints, and ensure adequate TBI dose coverage. With anesthesia, this technique provided a comfortable and reproducible set-up for the young child. The treatment plan was delivered with intensity-modulated radiotherapy, where 96.4% of the target volume received a prescription dose with a total beam-on time of 16.8 minutes. The mean lung dose was 7.7 Gy for a total lung volume of 245cc. This report describes the challenges faced during the treatment planning and delivery, and how they were resolved.

Dedicated isotropic 3-D T1 SPACE sequence imaging for radiosurgery planning improves brain metastases detection and reduces the risk of intracranial relapse

BACKGROUND

Stereotactic radiosurgery (SRS) is increasingly used for brain metastases (BM) patients, but distant intracranial failure (DIF) remains the principal disadvantage of this focal therapeutic approach. The objective of this study was to determine if dedicated SRS imaging would improve lesion detection and reduce DIF.

METHODS

Between 02/2020 and 01/2021, SRS patients at a tertiary care institution underwent dedicated treatment planning MRIs of the brain including MPRAGE and SPACE post-contrast sequences. DIF was calculated using the Kaplan-Meier method; comparisons were made to a historical consecutive cohort treated using MPRAGE alone (02/2019-01/2020).

RESULTS

134 patients underwent 171 SRS courses for 821 BM imaged with both MPRAGE and SPACE (primary cohort). MPRAGE sequence evaluation alone detected 679 lesions. With neuroradiologists evaluating SPACE and MPRAGE, an additional 108 lesions were identified (p<0.001). Upon multidisciplinary review, 34 additional lesions were identified. Compared to the historical cohort (103 patients, 135 SRS courses, 479 BM), the primary cohort had improved median time to DIF (13.5 vs. 5.1 months, p=0.004). The benefit was even more pronounced for patients treated for their first SRS course (18.4 vs. 6.3 months, p=0.001). SRS using MPRAGE and SPACE was associated with a 60% reduction in risk of DIF compared to the historical cohort (HR: 0.40; 95%CI: 0.28-0.57, p<0.001).

CONCLUSIONS

Among BM patients treated with SRS, a treatment planning SPACE sequence in addition to MPRAGE substantially improved lesion detection and was associated with a statistically significant and clinically meaningful prolongation in time to DIF, especially for patients undergoing their first SRS course.

Impact of MRI timing on tumor volume and anatomic displacement for brain metastases undergoing stereotactic radiosurgery

Background

The objective of this study was to evaluate the impact of the time interval between planning imaging and stereotactic radiosurgery (SRS) delivery on tumor volumes and spatial anatomic displacements of brain metastases (BM).

Methods

Consecutive patients diagnosed with BM treated with SRS over a 3-year period were evaluated. Only patients who underwent an institutionally standardized diagnostic MRI (MRI-1) and a treatment planning MRI (MRI-2) were included. The impact of histology, inter-scan time interval, lesion location, tumor volume, and diameter were evaluated on final lesion diameter, volume, anatomic displacement, and ultimate need for change in management (ie, expanding margins, rescanning).

Results

101 patients (531 lesions) with a median inter-scan time interval of 8 days (range: 1-42 days) met the inclusion criteria. The median percentage increase in BM diameter and volume were 9.5% (IQR: 2.25%-24.0%) and 20% (IQR: 0.7%-66.7%). Overall, 147 lesions (27.7%) in 57 patients (56.4%) required a change in management. There was a statistically significant relationship between initial tumor diameter (cm) and change in management (OR: 2.69, 95% CI: 1.93-3.75; P < .001). Each day between MRI-1 and MRI-2 was associated with a change in management with an OR of 1.05 (95% CI: 1.03-1.07; P < .001).

Conclusions

Changes in tumor diameter, volume, and spatial position occur as a function of time. Planning imaging for SRS is recommended to occur in close temporal proximity to treatment; for those with delays, a larger setup margin may need to be used to ensure tumor coverage and account for positional changes.

Systematic evaluation and plan quality assessment of the Leksell® gamma knife® lightning dose optimizer

To compare stereotactic radiosurgery (SRS) plan quality metrics of manual forward planning (MFP) and Elekta Fast Inverse Planning™ (FIP)-based inversely optimized plans for patients treated with Gamma Knife®. Clinically treated, MFP SRS plans for 100 consecutive patients (115 lesions; 67 metastatic and 48 benign) were replanned with the FIP dose optimizer based on a convex linear programming formulation. Comparative plans were generated to match or exceed the following metrics in order of importance: Target Coverage (TC), Paddick Conformity Index (PCI), beam-on time (BOT), and Gradient Index (GI). Plan quality metrics and delivery parameters between MFP and FIP were compared for all lesions and stratified into subgroups for further analysis. Additionally, performance of FIP for multiple punctate (<4 mm) metastatic lesions on a subset of cases was investigated. A Wilcoxon signed-rank test for non-normal distributions was used to assess the statistical differences between the MFP and FIP treatment plans. Overall, 76% (87/115) of FIP plans showed a statistically significant improvement in plan quality compared to MFP plans. As compared to MFP, FIP plans demonstrated an increase in the median PCI by 1.1% (p<0.01), a decrease in GI by 3.7% (p< 0.01), and an increase in median number of shots by 74% (p< 0.01). TC and BOT were not statistically significantly different between MFP and FIP plans (p>0.05). FIP plans showed a statistically significant increase in use of 16 mm (p< 0.01) and blocked shots (p< 0.01), with a corresponding decrease in 4 mm shots (p< 0.01). Use of multiple shots per coordinate was significantly higher in FIP plans (p<0.01). The FIP optimizer failed to generate a clinically acceptable plan in 4/115 (3.5%) lesions despite optimization parameter changes. The mean optimization time for FIP plans was 5.0 min (Range: 1.0 - 10.0 min). In the setting of multiple punctate lesions, PCI for FIP was significantly improved (p<0.01) by changing the default low-dose/BOT penalty optimization setting from a default of 50/50 to 75-85/40. FIP offers a significant reduction in manual effort for SRS treatment planning while achieving comparable plan quality to an expert planner-substantially improving overall planning efficiency. FIP plans employ a non-intuitive increased use of blocked sectors and shot-in-shot technique to achieve high quality plans. Several FIP plans failed to achieve clinically acceptable treatments and warrant further investigation.

Dose-limiting toxicity after hypofractionated dose-escalated radiotherapy in non-small-cell lung cancer

PURPOSE

Local failure rates after radiation therapy (RT) for locally advanced non-small-cell lung cancer (NSCLC) remain high. Consequently, RT dose intensification strategies continue to be explored, including hypofractionation, which allows for RT acceleration that could potentially improve outcomes. The maximum-tolerated dose (MTD) with dose-escalated hypofractionation has not been adequately defined.

PATIENTS AND METHODS

Seventy-nine patients with NSCLC were enrolled on a prospective single-institution phase I trial of dose-escalated hypofractionated RT without concurrent chemotherapy. Escalation of dose per fraction was performed according to patients' stratified risk for radiation pneumonitis with total RT doses ranging from 57 to 85.5 Gy in 25 daily fractions over 5 weeks using intensity-modulated radiotherapy. The MTD was defined as the maximum dose with ≤ 20% risk of severe toxicity.

RESULTS

No grade 3 pneumonitis was observed and an MTD for acute toxicity was not identified during patient accrual. However, with a longer follow-up period, grade 4 to 5 toxicity occurred in six patients and was correlated with total dose (P = .004). An MTD was identified at 63.25 Gy in 25 fractions. Late grade 4 to 5 toxicities were attributable to damage to central and perihilar structures and correlated with dose to the proximal bronchial tree.

CONCLUSION

Although this dose-escalation model limited the rates of clinically significant pneumonitis, dose-limiting toxicity occurred and was dominated by late radiation toxicity involving central and perihilar structures. The identified dose-response for damage to the proximal bronchial tree warrants caution in future dose-intensification protocols, especially when using hypofractionation.

SU-E-J-144: Recurrence Quantification Analysis of Lung Cancer Patients' Breathing Pattern

PURPOSE

To demonstrate that Recurrence quantification analysis (RQA) can be used as a quantitative decision making tool to classify patients breathing pattern and select treatment strategy for maneuvering the tumor motion : (a) MIP based treatment (b) 4D treatment using non-linear prediction only (c) 4D treatment non-linear control prediction and breathing control.

METHOD AND MATERIALS

In our previous work we established that breathing patterns can be described as a 5-6 dimensional nonlinear, stationary and deterministic system that exhibits sensitive dependence on initial conditions. Recurrence plots enable one to investigate an m-dimensional state space trajectory through a two-dimensional representation of its recurrences where the value of a specific pixel is 1 if the distance between the two corresponding trajectory points is less than a threshold value ε. Important measures calculated are: Recurrence Rate (RR), %Determinism, Divergence, Shannon Entropy, LMean, and Renyi entropy (K2). Time Resolved RQA: By implementing a sliding window design, each of the above calculated parameters is computed multiple times. Alignment of those parameters with the time series reveals details not obvious in the 1 -dimensional time series data. The breathing pattern for seven randomly chosen volunteers were recorded using the RPM system for 15 minutes. Non-linear prediction was performed and the normalized root mean square error (NRMSE) was calculated for each of the volunteer data.

RESULTS

The threshold value ε was chosen such that the Recurrence Rate was equal to 1%. There is a strong correlation of NRMSE with Entropy, Determinism and LMean. Time resolved RR locates strong Unstable Periodic Orbits(UPOs), i.e. patterns of uninterrupted equally spaced diagonal lines.

CONCLUSIONS

RQAs could prove to be a very powerful tool for design of personalized treatment regimen. Entropy, Determinism in conjunction with strong UPOs can be used to determine if patients are suitable candidates for prediction and chaos control.

Adaptive planning using megavoltage fan-beam CT for radiation therapy with testicular shielding

This study highlights the use of adaptive planning to accommodate testicular shielding in helical tomotherapy for malignancies of the proximal thigh. Two cases of young men with large soft tissue sarcomas of the proximal thigh are presented. After multidisciplinary evaluation, preoperative radiation therapy was recommended. Both patients were referred for sperm banking and lead shields were used to minimize testicular dose during radiation therapy. To minimize imaging artifacts, kilovoltage CT (kVCT) treatment planning was conducted without shielding. Generous hypothetical contours were generated on each "planning scan" to estimate the location of the lead shield and generate a directionally blocked helical tomotherapy plan. To ensure the accuracy of each plan, megavoltage fan-beam CT (MVCT) scans were obtained at the first treatment and adaptive planning was performed to account for lead shield placement. Two important regions of interest in these cases were femurs and femoral heads. During adaptive planning for the first patient, it was observed that the virtual lead shield contour on kVCT planning images was significantly larger than the actual lead shield used for treatment. However, for the second patient, it was noted that the size of the virtual lead shield contoured on the kVCT image was significantly smaller than the actual shield size. Thus, new adaptive plans based on MVCT images were generated and used for treatment. The planning target volume was underdosed up to 2% and had higher maximum doses without adaptive planning. In conclusion, the treatment of the upper thigh, particularly in young men, presents several clinical challenges, including preservation of gonadal function. In such circumstances, adaptive planning using MVCT can ensure accurate dose delivery even in the presence of high-density testicular shields.

Independent quality assurance of a helical tomotherapy machine using the dose magnifying glass

PURPOSE

Helical tomotherapy is a complex delivery technique, integrating CT image guidance and intensity modulated radiotherapy in a single system. The integration of the CT detector ring on the gantry not only allows patient position verification but is also often used to perform various QA procedures. This convenience lacks the rigor of a machine-independent QA process.

METHODS

In this article, a Si strip detector, known as the Dose Magnifying Glass (DMG), was used to perform machine-independent QA measurements of the multileaf collimator alignment, leaf open time threshold, and leaf fluence output factor (LFOF).

RESULTS

The DMG measurements showed good agreements with EDR2 film for the MLC alignment test while the CT detector agrees well with DMG measurements for leaf open time threshold and LFOF measurements. The leaf open time threshold was found to be approximately 20 ms. The LFOF measured with the DMG agreed within error with the CT detector measured LFOF.

CONCLUSIONS

The DMG with its 0.2 mm spatial resolution coupled to TERA ASIC allowed real-time high temporal resolution measurements of the tomotherapy leaf movement. In conclusion, DMG was shown to be a suitable tool for machine-independent QA of a tomotherapy unit.

Prior image constrained scatter correction in cone-beam computed tomography image-guided radiation therapy

X-ray scatter is a significant problem in cone-beam computed tomography when thicker objects and larger cone angles are used, as scattered radiation can lead to reduced contrast and CT number inaccuracy. Advances have been made in x-ray computed tomography (CT) by incorporating a high quality prior image into the image reconstruction process. In this paper, we extend this idea to correct scatter-induced shading artifacts in cone-beam CT image-guided radiation therapy. Specifically, this paper presents a new scatter correction algorithm which uses a prior image with low scatter artifacts to reduce shading artifacts in cone-beam CT images acquired under conditions of high scatter. The proposed correction algorithm begins with an empirical hypothesis that the target image can be written as a weighted summation of a series of basis images that are generated by raising the raw cone-beam projection data to different powers, and then, reconstructing using the standard filtered backprojection algorithm. The weight for each basis image is calculated by minimizing the difference between the target image and the prior image. The performance of the scatter correction algorithm is qualitatively and quantitatively evaluated through phantom studies using a Varian 2100 EX System with an on-board imager. Results show that the proposed scatter correction algorithm using a prior image with low scatter artifacts can substantially mitigate scatter-induced shading artifacts in both full-fan and half-fan modes.

The effect and stability of MVCT images on adaptive TomoTherapy

Use of helical TomoTherapy‐based MVCT imaging for adaptive planning is becoming increasingly popular. Treatment planning and dose calculations based on MVCT require an image value to electron density calibration to remain stable over the course of treatment time. In this work, we have studied the dosimetric impact on TomoTherapy treatment plans due to variation in image value to density table (IVDT) curve as a function of target degradation. We also have investigated the reproducibility and stability of the TomoTherapy MVCT image quality over time. Multiple scans of the TomoTherapy “Cheese” phantom were performed over a period of five months. Over this period, a difference of 4.7% in the HU values was observed in high‐density regions while there was no significant variation in the image values for the low densities of the IVDT curve. Changes in the IVDT curves before and after target replacement were measured. Two clinical treatment sites, pelvis and prostate, were selected to study the dosimetric impact of this variation. Dose was recalculated on the MVCTs with the planned fluence using IVDT curves acquired before and after target change. For the cases studied, target replacement resulted in an overall difference of less than 5%, which can be significant for hypo‐fractionated cases. Hence, it is recommended to measure the IVDT curves on a monthly basis and after any major repairs/replacements.

PACS numbers: 87.55.Qr, 87.56.bd, 87.57.C, 87.57.Q

Streaking artifacts reduction in four-dimensional cone-beam computed tomography

Cone-beam computed tomography (CBCT) using an "on-board" x-ray imaging device integrated into a radiation therapy system has recently been made available for patient positioning, target localization, and adaptive treatment planning. One of the challenges for gantry mounted image-guided radiation therapy (IGRT) systems is the slow acquisition of projections for cone-beam CT (CBCT), which makes them sensitive to any patient motion during the scans. Aiming at motion artifact reduction, four-dimensional CBCT (4D CBCT) techniques have been introduced, where a surrogate for the target's motion profile is utilized to sort the cone-beam data by respiratory phase. However, due to the limited gantry rotation speed and limited readout speed of the on-board imager, fewer than 100 projections are available for the image reconstruction at each respiratory phase. Thus, severe undersampling streaking artifacts plague 4D CBCT images. In this paper, the authors propose a simple scheme to significantly reduce the streaking artifacts. In this method, a prior image is first reconstructed using all available projections without gating, in which static structures are well reconstructed while moving objects are blurred. The undersampling streaking artifacts from static structures are estimated from this prior image volume and then can be removed from the phase images using gated reconstruction. The proposed method was validated using numerical simulations, experimental phantom data, and patient data. The fidelity of stationary and moving objects is maintained, while large gains in streak artifact reduction are observed. Using this technique one can reconstruct 4D CBCT datasets using no more projections than are acquired in a 60 s scan. At the same time, a temporal gating window as narrow as 100 ms was utilized. Compared to the conventional 4D CBCT reconstruction, streaking artifacts were reduced by 60% to 70%.

High temporal resolution and streak-free four-dimensional cone-beam computed tomography

Cone-beam computed tomography (CBCT) has been clinically used to verify patient position and to localize the target of treatment in image-guided radiation therapy (IGRT). However, when the chest and the upper abdomen are scanned, respiratory-induced motion blurring limits the utility of CBCT. In order to mitigate this blurring, respiratory-gated CBCT, i.e. 4D CBCT, was introduced. In 4D CBCT, the cone-beam projection data sets acquired during a gantry rotation are sorted into several respiratory phases. In these gated reconstructions, the number of projections for each respiratory phase is significantly reduced. Consequently, undersampling streaking artifacts are present in the reconstructed images, and the image contrast resolution is also significantly compromised. In this paper, we present a new method to simultaneously achieve both high temporal resolution ( approximately 100 ms) and streaking artifact-free image volumes in 4D CBCT. The enabling technique is a newly proposed image reconstruction method, i.e. prior image constrained compressed sensing (PICCS), which enables accurate image reconstruction using vastly undersampled cone-beam projections and a fully sampled prior image. Using PICCS, a streak-free image can be reconstructed from 10-20 cone-beam projections while the signal-to-noise ratio is determined by a denoising feature of the selected objective function and by the prior image, which is reconstructed using all of the acquired cone-beam projections. This feature of PICCS breaks the connection between the temporal resolution and streaking artifacts' level in 4D CBCT. Numerical simulations and experimental phantom studies have been conducted to validate the method.