Technical note: Sources of systemic error in total body irradiation and total skin electron therapy in vivo measurements using nanoDot optically stimulated luminescence dosimeters within high-efficiency clinics.

PURPOSE: To identify sources of systemic errors and estimate their effects, especially the vendor-provided sensitivity Ss , i ,vendor , on total body irradiation (TBI) and total skin electron therapy (TSET) in vivo OSLD measurements. MATERIALS: Calibration nanoDot OSLDs were irradiated 50-300cGy under reference conditions. Raw OSLD readings Mraw were corrected by Ss , i ,vendor to obtain corrected readings Mcorr . A quadratic calibration curve relating Mcorr to delivered dose Dw was established and commissioned for clinical use. For clinical measurements, directly adjacent pairs of nanoDot OSLDs were placed on TBI and TSET patients with a medical tape with or without 1.5 cm of bolus respectively before treatment. Used OSLDs were bleached between each use until cumulative dose of 15 Gy. Relative difference in corrected counts (∆Mcorr,rel  = pair-difference/mean) was fitted multi-linearly versus possible sources of systemic errors (Ss , i ,vendor , bleaching history, cumulative dose, and age differences). Total of 101 TBI and 110 TSET measurement pairs from calibrated batches were analyzed. RESULTS: Ss , i ,vendor introduced a residual systemic error to corrected readings Mcorr (-0.98% per +0.01, p = 4e-12). Given Ss , i ,vendor distribution is σ = ±0.025, measured dose 1-σ error is ±2.5%, compared to ±2.8% uncertainty reported in the literature which may include this systemic error. Bleaching or cumulative dose did not affect Mcorr significantly after adjusting for Ss , i ,vendor . Adjusting for the systemic error in Ss , i ,vendor decreased two-sample mean Dw median absolute error from ±2.6% to ±1.9% and 95-percentile absolute error from ±7.1% to ±5.5%. Variability between paired clinical OSLDs was larger for TBI versus TSET at σpd  = ±4.7% and ±6.3%, respectively, despite similar predictor distributions. CONCLUSION: Our findings suggest that Mraw correction via vendor-provided sensitivity results in a small but significant systemic error. Dosimeters with outlier sensitivities should be excluded during batch calibration to minimize error. Bleaching and cumulative dose likely minimally affect measurements if cumulative dose is controlled below 15 Gy. Random errors were higher for TSET than TBI.

Initial Evaluation of a Novel Cone-Beam CT-Based Semi-Automated Online Adaptive Radiotherapy System for Head and Neck Cancer Treatment - A Timing and Automation Quality Study.

Introduction A novel on-line adaptive radiotherapy (ART) system based on O-ring linear accelerator (LINAC) and cone-beam CT (CBCT) was evaluated for treatment and management of head & neck (H&N) cancer in an emulated environment accessed via remote desktop connection. In this on-line ART system, organs-at-risk (OARs) and target contours and radiotherapy (RT) plans are semi-automatically generated based on the patient CBCT, expediting a typically hours-long RT planning session to under half an hour. In this paper, we describe our initial experiences with the system and explore optimization strategies to expedite the process further. Methods We retroactively studied five patients with head and neck cancers, treated 16-35 fractions to 50-70 Gys. For each patient, on-line ART was simulated with one planning CT and three daily CBCT images taken beginning, middle, and end of treatment (tx). Key OAR (mandible, parotids, and spinal cord) and target (planning target volume (PTV) = clinical target volume (CTV) + 3 mm margin) contours were auto-generated and adjusted as needed by therapist/dosimetrist and attending physician, respectively. Duration of OAR contouring, target contouring, and plan review was recorded. Key OAR auto-contours were qualitatively rated from 1 (unacceptable) - 5 (perfect OAR delineation), and then quantitatively compared to human-adjusted "ground truth" contours via dice similarity coefficient (DSC) and 95-percentile Hausdorff distance (HD95%). Once contours were approved, adapted RT plans were auto-generated for physician review. Simulated doses to OARs and targets from the adapted plan were compared to that from the original (un-adapted) plan. Results Median on-line ART planning duration in the remote emulated environment was 19 min 34 sec (range: 13 min 10 sec - 31 min 20 sec). Automated key OAR quality was satisfactory overall (98% scored ≥3; 82% ≥4), though mandible was rated lower than others (p < 0.05). Most key OARs and all targets were within 2 mm margin of human-adjusted contours, but a few parotid and spinal cord contours deviated up to 5 mm. Anatomical changes over tx course further increased auto-contour error (p < 0.05, ΔHD95% = 0.77 mm comparing start and end of tx). Further optimizing auto-contoured OAR and target quality could reduce the on-line treatment planning duration by ~5 min and ~4.5 min, respectively. Dosimetrically, adapted plan spared OARs at a rate much greater than random chance compared to the original plan (χ2 = 22.3, p << 0.001), while maintaining similar therapeutic dose to treatment target CTV (χ2 = 1.14, p > 0.05). In addition, a general decrease in accumulated OAR dose was observed with adaptation. Unsupervised adapted plans where contours were auto-generated without human review still spared OAR at a greater rate than the original plans, suggesting benefits of adaptation can be maintained even with some leniency in contour accuracy. Conclusion Feasibility of a novel, semi-automated on-line ART system for various head and neck (H&N) cancer sites was demonstrated in terms of treatment duration, dosimetric benefits, and automated contour accuracy in a remote emulator environment. Adaptive planning duration was clinically viable at 19 min and 34 sec, but further improvements in automated contour accuracy and performance improvements of plan auto-generation may reduce adaptive planning duration by up to 10 minutes.

An investigation of a novel reusable radiochromic sheet for 2D dose measurement.

PURPOSE: Radiochromic film remains a useful and versatile clinical dosimetry tool. Current film options are single use. Here, we introduce a novel prototype two-dimensional (2D) radiochromic sheet, which optically clears naturally at room temperature after irradiation and can be reused. We evaluate the sheets for potential as a 2D dosimeter and as a radiochromic bolus with capability for dose measurement. METHODS: A novel derivative of reusable Presage® was manufactured into thin sheets of 5 mm thickness. The sheets contained 2% cumin-leucomalachitegreen-diethylamine (LMG-DEA) and plasticizer (up to 25% by weight). Irradiation experiments were performed to characterize the response to megavoltage radiation, including dose sensitivity, temporal decay rate, consistency of repeat irradiations, intra and inter-sheet reproducibility, multi-modality response (electrons and photons), and temperature sensitivity (22°C to 36°C). The local change in optical-density (ΔOD), before and after radiation, was obtained with a flat-bed film scanner and extracting the red channel. Repeat scanning enabled investigation of the temporal decay of ΔOD. Additional studies investigated clinical utility of the sheets through application to IMRT treatment plans (prostate and a TG119 commissioning plan), and a chest wall electron boost treatment. In the latter test, the sheet performed as a radiochromic bolus. RESULTS: The radiation induced OD change in the sheets was found to be proportional to dose and to exponentially decay to baseline in ~24 h (R2 = 0.9986). The sheet could be reused and had similar sensitivity (within 1% after the first irradiation) for at least eight irradiations. Importantly, no memory of previous irradiations was observed within measurement uncertainty. The consistency of dose response from photons (6 and 15 MV) and electrons (6-20 MeV) was found to be within calibration uncertainty (~1%). The dose sensitivity of the sheets had a temperature dependence of 0.0012 ΔOD/°C. For the short (1 min) single field IMRT QA verification, good agreement was observed between the Presage sheet and EBT film (gamma pass rate 97% at 3% 3 mm dose-difference and distance-to-agreement tolerance, with a 10% threshold). For the longer (~13 min) TG-119 9-field IMRT verification the gamma agreement was lower at 93% pass rate at 5% 3 mm, 10% threshold, when compared with Eclipse. The lower rate is attributed to uncertainty arising from signal decay during irradiation and indicates a current limitation. For the electron cutout treatment, both Presage and EBT agreed well (within 2% RMS difference) but differed from the Eclipse treatment plan (~7% RMS difference) indicating some limitations to the Eclipse modeling in this case. The worst case estimates of uncertainty introduced by the signal decay for deliveries of 2, 5, and 10 min are 0.6%, 1.4%, and 2.8% respectively. CONCLUSIONS: Reusable Presage sheets show promise for 2D dose measurement and as a radiochromic bolus for in vivo dose measurement. The current prototype is suitable for deliveries of length up to 5 min, where the uncertainty introduced by signal decay is anticipated to be ~1% (worst case 1.4%), or for longer deliveries where there is no temporal modulation (e.g. physical compensators, or open beams). Additionally, spatial resolution is limited by sheet thickness and scanner resolution, resulting in a practical resolution of 0.8 mm.

Delivered Dose Distribution Visualized Directly With Onboard kV-CBCT: Proof of Principle.

PURPOSE: To demonstrate proof of principle of visualizing delivered 3-dimensional (3D) dose distribution using kilovoltage (kv) cone beam computed tomography (CBCT) mounted onboard a linear accelerator. We apply this technique as a unique end-to-end verification of multifocal radiosurgery where the coincidence of radiation and imaging systems is quantified comprehensively at all targets. METHODS AND MATERIALS: Dosimeters (9.5-cm diameter N-isopropylacrylamide) were prepared according to standard procedures at one facility and shipped to a second (remote) facility for irradiation. A 4-arc volumetric modulated arc therapy (VMAT) multifocal radiosurgery plan was prepared to deliver 20 Gy with 6-MV photons to 6 targets (1-cm diameter). A dosimeter was aligned via CBCT and irradiated, followed by 3 CBCT scans acquired immediately, with total time between pre-CBCT and final CBCT <30 minutes. Image processing included background subtraction and low-pass filters. A dose-volume structure was created per target with the same volume as the planned prescription dose volume, and their spatial agreement was quantified using volume centroid and the Jaccard index. For comparison, 5 diagnostic computed tomography (CT) scans were also acquired after >24 hours with the same spatial analysis applied; comparison with planned doses after absolute dose calibration also was conducted. RESULTS: Regions of high dose were clearly visualized in the average CBCT with a contrast-to-noise ratio of 1.7 ± 0.7, which increased to 5.8 ± 0.5 after image processing, and 11.9 ± 3.7 for average diagnostic CT. Centroids of prescription isodose volumes agreed with the root mean square difference of 1.1 mm (range, 0.8-1.7 mm) for CBCT and 0.7 mm (0.4-0.8 mm) for diagnostic CT. The dose was proportional to density above 10 to 12 Gy with a 3D gamma pass rate of 94.0% and 99.5% using 5% for 1-mm and 3% for 2-mm criteria, respectively (threshold = 15 Gy, using global dose criteria). CONCLUSIONS: This work demonstrates for the first time the potential to visualize in 3D delivered dose using onboard kV-CBCT (0.5 × 0.5 × 1 mm3 voxel size) immediately after irradiation with a sufficient contrast-to-noise ratio to measure radiation and imaging system coincidence to within 2 mm.