3D Dosimetry for Electron Flash Radiotherapy: Assessment of Radiochromic Dosimeter Phantoms with Optical CT Scanning as a 3D Dosimetry System

PURPOSE/OBJECTIVE(S)

Many of the dosimeters used in conventional radiation therapy exhibit dose rate dependence which prohibits their use in ultra-high-dose-rate (FLASH) radiation therapy. Radiochromic plastic dosimeter PRESAGE® has been used for 3D dosimetry for many years. We hypothesized that these phantoms would show dose-rate independence throughout both the conventional and FLASH RT regimes, indicating these phantoms exhibit qualities useful for relative 3D dosimetry in FLASH electron beams.

MATERIALS/METHODS

FLASH experiments were performed using a commercially available linear accelerator, converted to deliver an ultra-high-dose-rate 10 MeV electron beam. The LINAC delivered approximately 0.7 Gy/pulse for FLASH irradiations. Dose rate was varied from about 40 Gy/s to 240 Gy/s by changing the repetition rate. PRESAGE phantoms were irradiated en face at six FLASH dose rates: 40 Gy/s, 80 Gy/s, 120 Gy/s, 160 Gy/s, 200 Gy/s, and 240 Gy/s. EBT film and scintillator measurements were used to verify dose delivered. The optical response of the PESAGE phantom versus delivered dose was evaluated with various known doses. A novel parallel-beam optical CT scanner, utilizing fiber optic taper for collimated images, was developed for fast, high resolution, and accurate readout of 3D dosimeters. Percent depth dose curves for various FLASH dose rates and conventional dose rate beams were generated and compared based on the optical response versus dose measurements. Percent depth dose curves from Monte Carlo calculation of the presage phantom were also compared.

RESULTS

As shown in Table 1, the percent depth dose as a function of depth for six FLASH dose rates (240-40 Gy/s) are nearly identical, indicating that optical response of PRESAGE is dose-rate independent. The optical density of PRESAGE phantom was confirmed to be linear with absorbed dose for all FLASH dose rates, consistent with the observation at regular treatment dose rates.

CONCLUSION

PRESAGE phantoms show dose-rate independence in electron beams for a wide range of dose rates from conventional to ultra-high-dose-rates, indicating these phantoms can be useful for relative 3D dose measurements in FLASH electron beams. Future experiments will be undertaken as part of the commissioning of a commercially available FLASH radiotherapy unit.

Creation and Implementation of an Interdisciplinary Workflow for CBCT-Based Online Adaptive Radiotherapy

PURPOSE/OBJECTIVE(S)

CBCT-based online adaptive radiotherapy (OART) is an emerging treatment strategy to replan based on the anatomy of the day while the patient remains on the couch. OART is not just an add-on to the current workflow; it necessitates a new approach across the patient's path of care, from CT simulation to treatment delivery. OART requires the addition of duties to clinical personnel, strategies to create auto-plan templates, and monitoring the "black box" adaptation process. Studies have shown that OART implementation is limited by its resource-intensive nature and the risks associated with the treatment approach. We hypothesized that the implementation of an interdisciplinary, streamlined workflow and checklists would enhance the OART treatment efficiency, prevent medical errors from the adaptation, and minimize the burden on clinicians.

MATERIALS/METHODS

An interdisciplinary OART working group comprising radiation oncologists, medical physicists, dosimetrists, and therapists was created to enable weekly knowledge sharing, workflow design, implementation, and continuous process improvement. 213 adaptive sessions from 5 treatment sites (pancreas, bladder, prostate, rectum, anus) were treated on a CBCT-based OART platform in a single institutional study. An evaluation of the treatment safety and workflow time was performed for each adaptive session.

RESULTS

The OART workflow was divided into four sub-workflows: 1) pre-treatment site-specific template preparation, 2) pre-treatment initial planning and verification, 3) on-treatment procedure, and 4) post-treatment evaluation. The sub-processes involved 4, 8, 13, and 4 separate, sequentially tasks, respectively, and a total of 11 task checklists. The template preparation is a new process developed for site-specific, standardized physician template directives, automated planning template development, and testing for its accuracy and robustness. The planning templates generated high-quality initial plans automatically within minutes once structures were segmented on the planning CT. This process was replicated during treatment using the CBCT. The median (interquartile range) online procedure time, defined as the time from initial CBCT to plan approval, of the five treatment sites (pancreas, bladder, prostate, rectum, anus) was 22.1 (19.2-24.8) min, 16.5 (15.3-17.5) min, 14.7 (13.9-17.4) min, 17 (15.3-19.7) min, and 24 (21.4-25.8) min, respectively. Safety assessment determined that no treatment deviations were observed.

CONCLUSION

Creating an interdisciplinary, standardized workflow and checklists allowed the safe delivery of OART with clinically feasible online procedure time and significantly reduced initial planning time compared with traditional EBRT. The unique workflow is essential to minimize the burden on the care team, increase patient safety, and access to OART.

CT-Guided Online Adaptive Stereotactic Body Radiotherapy for Pancreas Ductal Adenocarcinoma: Dosimetric and Initial Clinical Experience

PURPOSE/OBJECTIVE(S)

Retrospective analysis suggests that dose escalation to a biologically effective dose of more than 70 Gy may improve overall survival in patients with pancreatic ductal adenocarcinoma (PDAC), but such treatments in practice are limited by proximity of organs at risk (OARs). We hypothesized that CT-guided online adaptive radiotherapy (OART) can account for interfraction movement of OARs, reduce dose to OARs, and improve coverage of targets.

MATERIALS/METHODS

This is a single institution retrospective analysis of patients with PDAC treated with OART on a CBCT-based OART platform. All patients were treated to 40 Gy in 5 fractions. PTV overlapping with a 5 mm planning risk volume expansion on the stomach, duodenum and bowel received 25 Gy. Initial treatment plans were created conventionally. For each fraction, PTV and OAR volumes were recontoured with AI assistance after initial cone beam CT (CBCT). The adapted plan was calculated, underwent QA, and then compared to the scheduled plan. A second CBCT was obtained prior to delivery of the selected plan. Total treatment time (first CBCT to end of radiation delivery) and active physician time (first to second CBCT) were recorded. PTV_4000 V95%, PTV_2500 V95%, and D0.03 cc to stomach, duodenum and bowel were reported for scheduled (S) and adapted (A) plans. CTCAEv5.0 toxicities were recorded. Statistical analysis was performed using a two-sided T test and α of 0.05.

RESULTS

Seven patients with unresectable or locally-recurrent PDAC were analyzed, with a total of 35 fractions. Average total time was 33:00 minutes (22:25-49:40) and average active time was 22:48 minutes (14:15-39:34). All fractions were treated with adapted plans. All adapted plans met PTV_4000 V95.0% > 95.0% coverage goal and OAR dose constraints. Dosimetric data for scheduled and adapted plans per fraction are in Table 1. Median follow up was 1.7 months. 5 (71%) patients experienced either Grade 1 or 2 toxicities. No patients experienced Grade 3+ toxicities.

CONCLUSION

Daily OART significantly reduced dose OARs while achieving superior PTV coverage. Treatment was generally well tolerated with no grade 3+ acute toxicity, and required only 22:48 minutes on average of active physician time. Our initial clinical experience demonstrates OART allows for safe dose escalation in the treatment of PDAC.

Adaptive Radiation Therapy: A Review of CT-based Techniques

Adaptive radiation therapy is a feedback process by which imaging information acquired over the course of treatment, such as changes in patient anatomy, can be used to reoptimize the treatment plan, with the end goal of improving target coverage and reducing treatment toxicity. This review describes different types of adaptive radiation therapy and their clinical implementation with a focus on CT-guided online adaptive radiation therapy. Depending on local anatomic changes and clinical context, different anatomic sites and/or disease stages and presentations benefit from different adaptation strategies. Online adaptive radiation therapy, where images acquired in-room before each fraction are used to adjust the treatment plan while the patient remains on the treatment table, has emerged to address unpredictable anatomic changes between treatment fractions. Online treatment adaptation places unique pressures on the radiation therapy workflow, requiring high-quality daily imaging and rapid recontouring, replanning, plan review, and quality assurance. Generating a new plan with every fraction is resource intensive and time sensitive, emphasizing the need for workflow efficiency and clinical resource allocation. Cone-beam CT is widely used for image-guided radiation therapy, so implementing cone-beam CT-guided online adaptive radiation therapy can be easily integrated into the radiation therapy workflow and potentially allow for rapid imaging and replanning. The major challenge of this approach is the reduced image quality due to poor resolution, scatter, and artifacts. Keywords: Adaptive Radiation Therapy, Cone-Beam CT, Organs at Risk, Oncology © RSNA, 2023.

Physician-driven artificial intelligence enabled planning for intraprostatic dose escalation in under ten minutes

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Background: Intraprostatic radiation dose escalation is an area of clinical interest. Dose escalation within the prostate must be balanced with maintaining acceptable dose to the organs at risk, OAR (bladder, rectum, and urethra). Treatment planning therefore requires simultaneous consideration of multiple competing plan optimization goals, for which iterative, interdisciplinary treatment planning tasks may take significant physician, physicist, and dosimetrist time. Semi-automated treatment planning using artificial intelligence has the potential to significantly reduce treatment planning time for technically complex treatments. Methods: A prostate SBRT planning template was created using the Varian ETHOS treatment planning system (TPS) combined with an in-house RapidPlan SBRT prostate model. Prostate dose was prescribed to 36.25 Gy over 5 fractions with 95% coverage to the PTV. To respect standard SBRT normal tissue toxicity constraints while simultaneously escalating intraprostatic dose, the TPS automatically created an intraprostatic boost structure (PTV_SIB), derived from the PTV by excluding OARs with a pre-determined margin. Physicians were trained to perform treatment planning using the prostate SBRT planning template. Treatment planning was performed on 5 unique patients. The time spent from initiation to end of treatment planning and dosimetric parameters were recorded. Results: For each patient, the ETHOS TPS generated two SBRT plans (9 field static IMRT and 3 VMAT arc) with intraprostatic dose escalation in an average of 9.3 minutes [range 8.4-11.8]. Static field and VMAT plans were comparable. PTV_SIB was escalated to above 50 Gy in all cases. Relevant dosimetry for each patient’s static IMRT plan is shown. Conclusions: Physician-driven ETHOS treatment planning was able to produce boosted internal PTV doses using autosegmented volumes. The ETHOS TPS was able to generate dose-escalated plans that reconciled complex OAR and PTV goals within 8-12 minutes. Hence, the ETHOS TPS opens the possibility of rapid physician-driven treatment planning throughput. [Table: see text]

NIMG-67. DISAPPEARING DOTS – TRANSIENT LATE ENHANCING LESIONS YEARS AFTER BRAIN RADIOTHERAPY

BACKGROUND

Late-delayed radiation effects appear 6 months to years following radiotherapy. We characterize a species of small enhancing lesions in the late-delayed phase of post-radiotherapy that are distinct from the classic descriptions of radiation necrosis or pseudoprogression associated with mass effect and edema. These “disappearing dots” are small, do not exert mass effect nor edema, and spontaneously resolve.

METHOD

We retrospectively describe a series of cases with “disappearing dots” following brain radiotherapy.

RESULTS

There were 10 cases (4 men), median age 42 years (range 29-63). Diagnoses were glioblastoma (3); low grade astrocytoma, anaplastic astrocytoma, and anaplastic oligodendroglioma (2 each); and solitary fibrous tumor (1). All patients received 54-60 Gy (Gray) of external beam radiotherapy, except one (proton beam therapy to 60 cobalt Gray equivalent). Disappearing dots appeared at a median of 27 months (range 5-197) post-radiotherapy. Lesions were relatively small (~< 1 cm3), peri-ventricular, and within the radiotherapy field. Most enlarged before resolving. Advanced MR imaging and fluorodeoxyglucose (FGD)-PET results were inconsistent. Lesions persisted a median of 8.5 months (range 1-49) before spontaneous resolution. All were asymptomatic. Biopsy in one case revealed treatment effects rather than recurrent tumor.

CONCLUSIONS

Asymptomatic small periventricular enhancing lesions can develop and remit spontaneously, years following brain radiotherapy. Such disappearing dots should be part of the differential diagnosis along with tumor recurrence. of new enhancing lesions in the late-delayed phase post-radiotherapy.

RADI-14. FRAMELESS STEREOTACTIC RADIOSURGERY ON THE GAMMA KNIFE ICON: EARLY EXPERIENCE FROM 42 PATIENTS WITH BRAIN METASTASES

BACKGROUND: The Gamma Knife (GK) Icon uses a Cone-Beam CT (CBCT) scanner and an infrared camera system to support the delivery of frameless radiosurgery. There are limited data on patients treated with frameless GK radiosurgery (GKRS) for brain metastases. OBJECTIVE: To describe the early experience, process, technical details, and short-term outcomes with frameless GKRS for brain metastases at our institution. METHODS: We describe our patient selection and workflow for frameless GKRS in detail. Because of the short interval of follow-up, we provide crude rates of local control. RESULTS: 42 patients had a total of 96 brain metastases. Median age was 69. 77 intact lesions were treated definitively, 18 cavities postoperatively, and 1 had GKRS for recurrence after resection. 11 patients underwent repeat GKRS to the same area. Median dose was 20Gy in 1 fraction (range: 14–21), 24Gy in 3 fractions (range: 19.5–27), and 25Gy in 5 fractions (Range: 25–30). Median treatment time was 23.7 minutes (Range: 7.3 – 85.5). 29 patients had a follow-up MRI in our records after completing GKRS. Median follow-up time was 105 days (Range: 16 – 314). 16 local recurrences (LR) were identified in 9 patients. An additional 6 patients had distant brain recurrence without LR. Crude mean time between GKRS and LR was 101 days (range 44–161 days). There were 6 patients with grade 1, 3 with grade 2, 2 with grade 3, and 1 with grade 4 toxicity. We found an improvement in workflow and a greater number of patients eligible for GKRS due to the ability to fractionate treatments. CONCLUSION: We report a large cohort of consecutive patients with brain metastases treated with frameless GKRS. We look forward to studies with longer follow-up to provide valuable data on clinical outcomes and to further our understanding of the radiobiology of hypofractionation in the brain.

Mice and the A-Bomb: Irradiation Systems for Realistic Exposure Scenarios

Validation of biodosimetry assays is normally performed with acute exposures to uniform external photon fields. Realistically, exposure to a radiological dispersal device or reactor leak will include exposure to low dose rates and likely exposure to ingested radionuclides. An improvised nuclear device will likely include a significant neutron component in addition to a mixture of high- and low-dose-rate photons and ingested radionuclides. We present here several novel irradiation systems developed at the Center for High Throughput Minimally Invasive Radiation Biodosimetry to provide more realistic exposures for testing of novel biodosimetric assays. These irradiators provide a wide range of dose rates (from Gy/s to Gy/week) as well as mixed neutron/photon fields mimicking an improvised nuclear device.

Stereotactic body radiotherapy for pancreatic cancer: Analysis of toxicity

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Background: Pancreatic ductal adenocarcinoma (PDAC) carries a poor prognosis, with significant morbidity and mortality from local progression. Radiotherapy can be administered for local control, with stereotactic body radiotherapy (SBRT) increasingly being utilized. We aim to compare the toxicity of SBRT with intensity-modulated radiotherapy (IMRT) in this setting. Methods: A retrospective analysis of patients with PDAC receiving IMRT or SBRT at our institution between April 2011 and November 2015 was performed. 81 patients were identified. Clinical notes were reviewed for treatment details and acute and late toxicities. Late toxicity data was available for 62 patients. Toxicity was assessed using Common Terminology Criteria for Adverse Events version 4.0. Radiology reports were reviewed to assess for the presence of recurrence and/or progression. Results: Median follow-up from RT was 29 months (95% CI 22-36 months), and median survival from RT was 25 months (95% CI 22-34 months). IMRT patients received 37.5-54 Gy in 15-30 fractions. SRBT patients received 19.8-39.6 Gy in 3-6 fractions. 45% of IMRT and 29% of SBRT patients had local failure (p = 0.1329). IMRT patients were significantly more likely to have Grade ≥2 acute gastrointestinal (GI) toxicity than SBRT patients (RR 1.70; 95% CI 0.98-2.96, p = 0.0489), although one SBRT patient had to have treatment stopped due to extrahepatic stricture. There were no significant differences in late GI toxicity (67% Grade 2 or higher vs. 57%, respectively, p = 0.4244), although one IMRT patient died due to late GI bleeding.IMRT patients were more likely to lose weight during RT(Median -0.03% vs. -0.004%, p = 0.0001) and have moreEmergency Department (ED) visits after RT (Range 0-8 vs. 0-3, p = 0.0019). There were no significant differences in dermatitis, fatigue, or hematologic toxicity. Conclusions: Patients with locally advanced PDAC treated with IMRT versus SBRT had significantly more acute gastrointestinal toxicity, median weight loss, and ED visits after RT, but no significant differences in late gastrointestinal toxicity, dermatitis, fatigue, or hematologic toxicity. SBRT is a relatively tolerable and more convenient alternative for patients with pancreatic cancer.

Estimated risks of radiation-induced fatal cancer from pediatric CT

OBJECTIVE

In light of the rapidly increasing frequency of pediatric CT examinations, the purpose of our study was to assess the lifetime cancer mortality risks attributable to radiation from pediatric CT.

MATERIALS AND METHODS

Organ doses as a function of age-at-diagnosis were estimated for common CT examinations, and estimated attributable lifetime cancer mortality risks (per unit dose) for different organ sites were applied. Standard models that assume a linear extrapolation of risks from intermediate to low doses were applied. On the basis of current standard practice, the same exposures (milliampere-seconds) were assumed, independent of age.

RESULTS

The larger doses and increased lifetime radiation risks in children produce a sharp increase, relative to adults, in estimated risk from CT. Estimated lifetime cancer mortality risks attributable to the radiation exposure from a CT in a 1-year-old are 0.18% (abdominal) and 0.07% (head)-an order of magnitude higher than for adults-although those figures still represent a small increase in cancer mortality over the natrual background rate. In the United States, of approximately 600,000 abdominal and head CT examinations annually performed in children under the age of 15 years, a rough estimate is that 500 of these individuals might ultimately die from cancer attributable to the CT radiation.

CONCLUSION

The best available risk estimates suggest that pediatric CT will result in significantly increased lifetime radiation risk over adult CT, both because of the increased dose per milliampere-second, and the increased lifetime risk per unit dose. Lower milliampere-second settings can be used for children without significant loss of information. Although the risk-benefit balance is still strongly tilted toward benefit, because the frequency of pediatric CT examinations is rapidly increasing, estimates that quantitative lifetime radiation risks for children undergoing CT are not negligible may stimulate more active reduction of CT exposure settings in pediatric patients.