Purpose
Half‐beam block is a field matching technique frequently used in radiotherapy. With no setup error, a well calibrated linac, and no internal organ motion, two photon fields can be matched seamlessly dosimetry‐wise with their central axes passing the match line. However, in actual clinical situations, internal organ motion is often inevitable. This study was conducted to investigate its influence on radiation dose to patient internal points directly under the matching line.
Methods
A clinical setting is modeled as two half‐space (x<0 and x<0) radiation fields that are turned on sequentially with a time gap of integer times of the patient internal organ motion period (T0). Our point of interest moves with patient internal organs periodically and evenly in and out of the radiation fields, resulting in an average location at x=0. When the fields are delivered without any motion management, the initial phase of the point's movement is unknown. Statistical methods are used to compute the expected value (
Purpose: To evaluate the target positional differences between 3D/4D planning CTs (pCTs) and daily CBCTs, and assess ITV coverage of daily GTVs, for lung SBRT patients with misaligned tumor positions between the 3D and 4D pCTs. Methods: Simulation 3D and 4D pCTs, both under free breathing, were acquired on a Sensation Open (Siemens) with RPM (Varian). ITV_All is the union of the GTVs delineated on the 3D pCT (GTV_3D) and on all phases of the 4D pCT (ITV_4D). While for most patients GTV_3D is included in ITV_4D, for some patients GTV_3D shows a substantial misalignment in the axial plane from ITV_4D with spine‐based rigid registration. Under IRB approval, pCTs of 51 patients were retrospectively reviewed to assess the frequency of such misalignments. For one patient with daily kV CBCTs available, all planning and treatment CTs were rigidly registered. GTV_3D, ITV_4D, GTV_F1 through GTV_F5 (on daily CBCTs of Fractions 1–5) were delineated. ITV_All and ITV_5mm (5mm uniform expansion of GTV_3D) were generated. Volumes and centers‐of‐mass (COMs) of all structures were analyzed. The inclusion relations (percentage of Structure‐A included in Structure‐B) were studied between GTV_F1 to GTV_F5 (Structure‐As) and GTV_3D, ITV_4D, ITV_All, ITV_5mm (Structure‐Bs). Results: Three patients showed substantial target misalignments between the 3D and 4D pCTs. For the analyzed patient, daily GTV positions agreed better with GTV_3D than with ITV_4D, with a median (range) COM vector difference of 2.45mm (1.00–4.58) and 16.06mm (15.39–17.44), respectively. The average inclusion of daily GTVs was 99.47% and 69.53% in ITV_5mm and ITV_All, respectively; although ITV_5mm was ∼35% larger than ITV_All for the ∼2cc tumor (GTV_3D). Conclusion: Substantial lung tumor misalignments were observed between 3D and 4D pCTs for some patients. For one such patient, daily tumor positions agreed better with the 3D pCT. Using spinebased localization for SBRT, ITV_All may provide insufficient internal margin.
Purpose: Randomness in patient internal organ motion phase at the beginning of non‐gated radiotherapy delivery may introduce uncertainty to dose received by the patient. Concerns of this dose deviation from the planned one has motivated many researchers to study this phenomenon although unified theoretical framework for computing it is still missing. This study was conducted to develop such framework for analyzing the effect. Methods: Two reasonable assumptions were made: a) patient internal organ motion is stationary and periodic; b) no special arrangement is made to start a non ‐gated radiotherapy delivery at any specific phase of patient internal organ motion. A statistical ensemble was formed consisting of patient's non‐gated radiotherapy deliveries at all equally possible initial organ motion phases. To characterize the patient received dose, statistical ensemble average method is employed to derive formulae for two variables: expected value and variance of dose received by a patient internal point from a non‐gated radiotherapy delivery. Fourier Series was utilized to facilitate our analysis. Results: According to our formulae, the two variables can be computed from non‐gated radiotherapy generated dose rate time sequences at the point's corresponding locations on fixed phase 3D CT images sampled evenly in time over one patient internal organ motion period. The expected value of point dose is simply the average of the doses to the point's corresponding locations on the fixed phase CT images. The variance can be determined by time integration in terms of Fourier Series coefficients of the dose rate time sequences on the same fixed phase 3D CT images. Conclusion: Given a non‐gated radiotherapy delivery plan and patient's 4D CT study, our novel approach can predict the expected value and variance of patient radiation dose. We expect it to play a significant role in determining both quality and robustness of patient non‐gated radiotherapy plan.
Purpose: To study gain calibration variation over time for an MV flat‐panel‐detector (FPD). Methods: Gain calibration images (1024×1024 pixels) of a FPD (PerkinElmer AN‐9 on a Siemens ARTISTE) acquired in 40 consecutive months were studied. Using Python programming language, the images were processed to analyze the central 900×900 pixels with a threshold applied to exclude dead pixels. Month1 was set as the base image, then the difference images between it and the following months were calculated. The pixel intensity mean and standard deviation of the difference images were used to study the gain variation over time. Two other months’ images were also randomly selected as the base image and the above analyses were repeated. Finally, to investigate the equivalence of monthly calibration and quarterly calibration, the results from comparing neighboring months’ images, such as (month2‐month1), (month3 ‐month2) et al, were compared with those from comparing images taken in 3‐month intervals, such as (month4‐month1), (month7‐month4) et al, using Welch's t‐test. Results: For most months’ gain calibration images, the differences (mean and standard deviation) from base images were constant. In three months, the differences were relatively larger compared to other months but seemed instantaneous without any worsening trend. In those 3 months no clinical portal image quality degradation was observed. The difference between monthly and quarterly calibration tested by Welch's t‐test were insignificant for both means (p=0.9) and standard deviations (p=0.8). Conclusion: Long term stability of the FPD gain calibration images was observed. Therefore, the vendor‐recommended calibration frequency of 2–4 weeks is unnecessary. Less frequent, such as quarterly, calibration is sufficient.
Purpose: Stereotactic body radiation therapy (SBRT) has been suggested to provide high rates of local control for locally advanced pancreatic cancer. However, the close proximity of highly radiosensitive normal tissues usually causes the labor‐intensive planning process, and may impede further escalation of the prescription dose. The present study evaluates the potential of an automatic planning system as a dose escalation strategy. Methods: Ten pancreatic cancer patients treated with SBRT were studied retrospectively. SBRT was delivered over 5 consecutive fractions with 6 ∼ 8Gy/fraction. Two plans were generated by Pinnacle Auto‐Planning with the original prescription and escalated prescription, respectively. Escalated prescription adds 1 Gy/fraction to the original prescription. Manually‐created planning volumes were excluded in the optimization goals in order to assess the planning efficiency and quality simultaneously. Critical organs with closest proximity were used to determine the plan normalization to ensure the OAR sparing. Dosimetric parameters including D100, and conformity index (CI) were assessed. Results: Auto‐plans directly generate acceptable plans for 70% of the cases without necessity of further improvement, and two more iterations at most are necessary for the rest of the cases. For the pancreas SBRT plans with the original prescription, autoplans resulted in favorable target coverage and PTV conformity (D100 = 96.3% ± 1.48%; CI = 0.88 ± 0.06). For the plans with the escalated prescriptions, no significant target under‐dosage was observed, and PTV conformity remains reasonable (D100 = 93.3% ± 3.8%, and CI = 0.84 ± 0.05). Conclusion: Automatic planning, without substantial human‐intervention process, results in reasonable PTV coverage and PTV conformity on the premise of adequate OAR sparing for the pancreas SBRT plans with escalated prescription. The results highlight the potential of autoplanning as a dose escalation strategy for pancreas SBRT treatment planning. Further investigations with a larger number of patients are necessary. The project is partially supported by Philips Medical Systems
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.