Virtual unenhanced dual-energy computed tomography for photon radiotherapy: The effect on dose distribution and cone-beam computed tomography based position verification.

Background and Purpose: Virtual Unenhanced images (VUE) from contrast-enhanced dual-energy computed tomography (DECT) eliminate manual suppression of contrast-enhanced structures (CES) or pre-contrast scans. CT intensity decreases in high-density structures outside the CES following VUE algorithm application. This study assesses VUE's impact on the radiotherapy workflow of gynecological tumors, comparing dose distribution and cone-beam CT-based (CBCT) position verification to contrast-enhanced CT (CECT) images. Materials and Methods: A total of 14 gynecological patients with contrast-enhanced CT simulation were included. Two CT images were reconstructed: CECT and VUE. Volumetric Modulated Arc Therapy (VMAT) plans generated on CECT were recalculated on VUE using both the CECT lookup table (LUT) and a dedicated VUE LUT. Gamma analysis assessed 3D dose distributions. CECT and VUE images were retrospectively registered to daily CBCT using Chamfer matching algorithm.. Results: Planning target volume (PTV) dose agreement with CECT was within 0.35% for D2%, Dmean, and D98%. Organs at risk (OARs) D2% agreed within 0.36%. A dedicated VUE LUT lead to smaller dose differences, achieving a 100% gamma pass rate for all subjects. VUE imaging showed similar translations and rotations to CECT, with significant but minor translation differences (<0.02 cm). VUE-based registration outperformed CECT. In 24% of CBCT-CECT registrations, inadequate registration was observed due to contrast-related issues, while corresponding VUE images achieved clinically acceptable registrations. Conclusions: VUE imaging in the radiotherapy workflow is feasible, showing comparable dose distributions and improved CBCT registration results compared to CECT. VUE enables automated bone registration, limiting inter-observer variation in the Image-Guided Radiation Therapy (IGRT) process.

Magnetic resonance imaging-guided radiotherapy for intermediate- and high-risk prostate cancer: Trade-off between planning target volume margin and online plan adaption.

Magnetic resonance-guided radiotherapy with daily plan adaptation for intermediate- and high-risk prostate cancer is time and labor intensive. Fifty adapted plans with 3 mm planning target volume (PTV)-margin were compared with non-adapted plans using 3 or 5 mm margins. Adequate (V95% ≥ 95%) prostate coverage was achieved in 49 fractions with 5 mm PTV without plan adaptation, however, coverage of the seminal vesicles (SV) was insufficient in 15 of 50 fractions. There was no insufficient coverage for prostate and SV using plan adaptation with 3 mm. Hence, daily adaptation is recommended to obtain adequate SV-coverage when using 3 mm PTV.

Combined Inter- and Intrafractional Plan Adaptation Using Fraction Partitioning in Magnetic Resonance-guided Radiotherapy Delivery.

Magnetic resonance-guided radiation therapy (MRgRT) not only allows for superior soft-tissue setup and online MR-guidance during delivery but also for inter-fractional plan re-optimization or adaptation. This plan adaptation involves repeat MR imaging, organs at risk (OARs) re-contouring, plan prediction (i.e., recalculating the baseline plan on the anatomy of that moment), plan re-optimization, and plan quality assurance. In contrast, intrafractional plan adaptation cannot be simply performed by pausing delivery at any given moment, adjusting contours, and re-optimization because of the complex and composite nature of deformable dose accumulation. To overcome this limitation, we applied a practical workaround by partitioning treatment fractions, each with half the original fraction dose. In between successive deliveries, the patient remained in the treatment position and all steps of the initial plan adaptation were repeated. Thus, this second re-optimization served as an intrafractional plan adaptation at 50% of the total delivery. The practical feasibility of this partitioning approach was evaluated in a patient treated with MRgRT for locally advanced pancreatic cancer (LAPC). MRgRT was delivered in 40Gy in 10 fractions, with two fractions scheduled successively on each treatment day. The contoured gross tumor volume (GTV) was expanded by 3 mm, excluding parts of the OARs within this expansion to derive the planning target volume for daily re-optimization (PTVOPT). The baseline GTVV95% achieved in this patient was 80.0% to adhere to the high-dose constraints for the duodenum, stomach, and bowel (V33 Gy <1 cc and V36 Gy <0.1 cc). Treatment was performed on the MRIdian (ViewRay Inc, Mountain View, USA) using video-assisted breath-hold in shallow inspiration. The dual plan adaptation resulted, for each partitioned fraction, in the generation of PlanPREDICTED1, PlanRE-OPTIMIZED1 (inter-fractional adaptation), PlanPREDICTED2, and PlanRE-OPTIMIZED2 (intrafractional adaptation). An offline analysis was performed to evaluate the benefit of inter-fractional versus intrafractional plan adaptation with respect to GTV coverage and high-dose OARs sparing for all five partitioned fractions. Interfractional changes in adjacent OARs were substantially larger than intrafractional changes. Mean GTV V95% was 76.8 ± 1.8% (PlanPREDICTED1), 83.4 ± 5.7% (PlanRE-OPTIMIZED1), 82.5 ± 4.3% (PlanPREDICTED2),and 84.4 ± 4.4% (PlanRE-OPTIMIZED2). Both plan re-optimizations appeared important for correcting the inappropriately high duodenal V33 Gy values of 3.6 cc (PlanPREDICTED1) and 3.9 cc (PlanPREDICTED2) to 0.2 cc for both re-optimizations. To a smaller extent, this improvement was also observed for V25 Gy values. For the stomach, bowel, and all other OARs, high and intermediate doses were well below preset constraints, even without re-optimization. The mean delivery time of each daily treatment was 90 minutes. This study presents the clinical application of combined inter-fractional and intrafractional plan adaptation during MRgRT for LAPC using fraction partitioning with successive re-optimization. Whereas, in this study, interfractional plan adaptation appeared to benefit both GTV coverage and OARs sparing, intrafractional adaptation was particularly useful for high-dose OARs sparing. Although all necessary steps lead to a prolonged treatment duration, this may be applied in selected cases where high doses to adjacent OARs are regarded as critical.

Toward planning target volume margin reduction for the prostate using intrafraction motion correction with online kV imaging and automatic detection of implanted gold seeds.

PURPOSE: The imaging application Auto Beam Hold (ABH) allows for the online analysis of 2-dimensional kV images acquired during treatment. ABH can automatically detect fiducial markers and initiate a beam interrupt. In this study, we investigate the practical use and results of this intrafraction monitoring tool for patients with prostate cancer who have implanted gold seeds treated with a RapidArc technique. METHODS AND MATERIALS: A total of 105 patients were included. For setup, the seeds were lined up using 2 orthogonal 2-dimensional kV images. After the setup procedure, ABH was applied at an interval of 3 seconds. The software requires a maximum-allowed deviation to be defined for each seed, which is referred to as a deviation limit (DL). Online, the ABH application evaluates the position of the seeds and indicates for each seed whether or not it exceeds the DL. Patients were divided in 3 groups. For the first group ABH was used with the DL at 6 mm, which corresponds to the planning target volume (PTV) margin. For the second group, the DL was set at 5 mm with an unchanged PTV margin of 6 mm. For the third group, the PTV margin was reduced to 5 mm with a DL of 5 mm. Offline, we performed an analysis of the number of beam stops and resulting re-setups. RESULTS: ABH initiated a beam interrupt 223 times (13%) during a total of 1736 sessions. By decreasing the DL from 6 mm to 5 mm, the amount of workload for re-setups increased from 6% (group 1) to 14% (groups 2 and 3). Re-setup, 3-dimensional shifts larger than the PTV margin were found in 44%, 35%, and 45% for groups 1,2, and 3, respectively. CONCLUSIONS: Intrafraction imaging of prostate position during treatment using automatic detection of implanted gold seeds was successfully implemented. PTV margins were safely reduced from 6mm to 5mm without a substantial increase in workload.

Phased versus midventilation attenuation-corrected respiration-correlated PET for patients with non-small cell lung cancer.

UNLABELLED: Respiration-correlated PET (RCPET) can reduce motion artifacts, but image quality generally decreases. The use of phase-by-phase attenuation correction (PAC) for RCPET using respiration-correlated CT (RCCT) requires large computational resources, and tumor positions will not always match correctly because of different binning methods for CT and PET. In this study, we investigated whether PAC for RCPET can be replaced by midventilation attenuation correction (MidV-AC) for a group of lung cancer patients. METHODS: RCPET/CT scans of 19 non-small cell lung cancer patients were performed. List-mode PET and CT data were binned and reconstructed into 8 phases. Two AC methods for RCPET were applied. First, the corresponding 8 RCCT phases were used for PAC. Then MidV-AC was used. Analyses were performed in terms of standardized uptake values (SUVs), volume recovery, contrast, and signal-to-noise ratio (SNR). RESULTS: Average differences between PAC and MidV-AC for mean and maximum SUV were 1.0% and 0.9% (P = 0.007 and P = 0.002), respectively, whereas SNR, contrast, and volume did not differ significantly (P >or= 0.2). Large motion amplitudes and irregular breathing revealed larger differences between phase 1 and MidV-AC values. CONCLUSION: Differences in SUV, volume, SNR, and contrast between PAC as available in currently used clinical software and MidV-AC for RCPET are small. MidV-AC provides an excellent surrogate for PAC for most lung cancer patients encountered in clinical practice.