Water Equivalent Path Length Variations Research Articles

Evaluating the Impact of Bowel Gas Variations for Wilms’ Tumor in Pediatric Proton Therapy

(1) Background: Proton therapy, a precise form of radiation treatment, can be significantly affected by variations in bowel content. The purpose was to identify the most beneficial gantry angles that minimize deviations from the treatment plan quality, thus enhancing the safety and efficacy of proton therapy for Wilms’ tumor patients. (2) Methods: Thirteen patients with Wilms’ tumor, enrolled in the SJWT21 clinical trial, underwent proton therapy. The variations in bowel gas were systematically monitored using daily Cone Beam Computed Tomography (CBCT) imaging. Air cavities identified in daily CBCT images were analyzed to construct daily verification plans and measure water equivalent path length (WEPL) changes. A worst-case scenario simulation was conducted to identify the safest beam angles. (3) Results: The study revealed a maximum decrease in target dose (ΔD100%) of 8.0%, which corresponded to a WEPL variation (ΔWEPL) of 11.3 mm. The average reduction in target dose, denoted as mean ΔD100%, was found to be 2.8%, with a standard deviation (SD) of 3.2%. The mean ΔWEPL was observed as 3.3 mm, with an SD of 2.7 mm. The worst-case scenario analysis suggested that gantry beam angles oriented toward the patient’s right and posterior aspects from 110° to 310° were associated with minimized WEPL discrepancies. (4) Conclusions: This study comprehensively evaluated the influence of bowel gas variability on treatment plan accuracy and proton range uncertainties in pediatric proton therapy for Wilms’ tumor.

A Fast 3D/3D Registration Method Based on Water Equivalent Path Length for Heavy-Ion Radiotherapy

<h3>Purpose/Objective(s)</h3> The accuracy of patient setup needs to be high in heavy-ion radiotherapy. Accordingly, there is an increasing demand for three-dimensional (3D) image-guided systems because of the limitations of the present two-dimensional imaging registration technology. One of the limitations is the inability to directly compare with planning computed tomography (CT) images. Due to this uncertainty in conformation, high dose per fraction radiotherapy is difficult. This study proposes a fast 3D/3D registration method comparing with water equivalent path length (WEPL) that uses planning and treatment CT images to minimize range variations by using the Lucas-Kanade (LK) algorithm. <h3>Materials/Methods</h3> A six-degrees-of-freedom (6DOF) couch correction was simulated for patient positioning at our center by performing automatic rigid image registration (RIR) and using the resultant translation/rotation. Our automatic RIR algorithm was used to align the planning CT and treatment CT images, simulating 6DOF. The alignment values (Δ<i>V</i>), which represents a six-dimensional vector of the amounts of 6DOF couch movement, were calculated by minimizing the following error function: E(<i>ΔV</i>)=Σ <i>_i</i>_∈Ω [<i>T_i</i> (<i>V+ΔV</i>)-<i>P_i</i> (<i>V</i>)]², where <i>V</i> is a six-dimensional vector that denotes a CT position and rotation in a treatment room; <i>T_i</i> (<i>V</i>) and <i>P_i</i> (<i>V</i>) are treatment and planning CT images respectively, whose position in the room is represented by <i>V; i</i> is a 3D vector which denotes a point in the room; and Ω is a set of calculation points, which was assigned a planning target volume (PTV) of Ω in this study. We minimized the error function by using the LK algorithm. To minimize the range variations, each pixel value of the CT images was changed into WEPL values and the alignment values were calculated in the same manner as the above algorithm. The method was evaluated using a dataset containing eight prostate cancer patient images, including a planning CT image with PTV and treatment CT images of four fraction with a fan-beam CT. The range variations were measured by calculating WEPL variations between the planning CT and treatment CT at (1) before registration and after registration based on (2) CT pixel values and (3) WEPL values, respectively. In addition, the calculation time was measured on a workstation with a single GPU. <h3>Results</h3> The mean WEL variation across the 32 cases (eight patients of four fractions) was (1) 1.8 ± 1.16 mm, (2) 1.08 ± 0.52 mm, and (3) 0.22 ± 0.17 mm. The worst WEPL variation were (1) 6.23 mm, (2) 3.06 mm, and (3) 1.17 mm. The mean calculation time were (2) 89 ms and (3) 1066 ms. <h3>Conclusion</h3> We developed a fast 3D/3D RIR method for reducing range variations to realize more accurate heavy-ion radiotherapy. We performed experiments involving eight prostate cancer patients, and our proposed method obtained high accuracy and short computation time.

Beam angle evaluation to improve inter-fraction motion robustness in pelvic lymph node irradiation with proton therapy

Background: Proton therapy dose distributions are sensitive to range variations, e.g. arising from inter-fraction organ motion. The aim of this study was to evaluate the inter-fraction motion robustness of proton beams from different beam angles in irradiation of pelvic lymph nodes (LNs).Material and methods: Planning CT (pCT) and multiple repeat CT (rCT) scans of 18 prostate cancer patients were used. Considering left and right LNs separately, the average water equivalent path length (WEPL) over all ray paths in the beams eye view of the LNs were calculated for all gantry/couch angle combinations across all rCTs versus the corresponding pCT. Single beam proton plans were optimized on the pCT for all gantry angles (0° couch) and were re-calculated on all rCTs for each respective patient. WEPL and dose parameters were extracted and a statistical clustering analysis performed to identify patient sub-populations in terms of patterns in which angles were robust.Results: The WEPL analysis showed a general pattern of least variation for 0° couch beam angles where three minima were found across gantry angles for the left LNs and two for the right LNs. The clustering analysis identified three patient sub-groups for the left LNs and two groups for the right LNs. The dose calculations showed similar results as the WEPL variation, e.g. for the left LNs angles around 25°–35°, 100°–110°, and 160°–170° were consistently preferable for both target and organs at risk.Conclusions: Sub-populations of patients with similar patterns of WEPL variations across beam angles were identified. The most robust angles found for WEPL variations were also confirmed by the dose/volume analysis.

Internal target volume margins for liver tumours treated with gated scanned carbon-ion radiotherapy

A promising approach to treating liver cancer with radiotherapy, is using a combination of a conformal technique, such as carbon ion-beam therapy, and beam gating. This study aims to assess the dosimetric implications, in a dose planning and verification study, of the use of gated internal target volume (ITVgate) when beam gating is used. Patients were immobilised with abdominal compression and a subset of these patients were treated using beam gating. The delivered dose was reconstructed. Significant dose degradation in the clinical target volume (CTV) due to the patient’s motion was identified. In addition, new plan optimisations were performed for the PTVgate, obtained from the geometric union of the phase-CTVs in the gating window. Finally, the concept of CTV expansion, including water equivalent path length variations (PTVgWEPL), was evaluated. When breathing was regular in amplitude, dose planning to the PTVgate resulted in a reduction in the dose delivered to healthy liver tissue without compromising the target coverage. In contrast, the use of PTVgWEPL concept resulted in a delivered plan with minor deviations from the planned treatment. The planned dose for the two concepts was verified in a 3D-motion water phantom. The mean relative deviations between measurements and the reconstructed dose were below 5% and were therefore within the clinically acceptable range for treatment.

Water equivalent path length calculations using scatter-corrected head and neck CBCT images to evaluate patients for adaptive proton therapy

Proton therapy has dosimetric advantages due to the well-defined range of the proton beam over photon radiotherapy. When the proton beams, however, are delivered to the patient in fractionated radiation treatment, the treatment outcome is affected by delivery uncertainties such as anatomic change in the patient and daily patient setup error. This study aims at establishing a method to evaluate the dosimetric impact of the anatomic change and patient setup error during head and neck proton therapy. Range variations due to the delivery uncertainties were assessed by calculating water equivalent path length (WEPL) to the distal edge of tumor volume using planning CT and weekly treatment cone-beam CT (CBCT) images. Specifically, mean difference and root mean squared deviation (RMSD) of the distal WEPLs were calculated as the weekly range variations. To accurately calculate the distal WEPLs, an existing CBCT scatter correction algorithm was used. An automatic rigid registration was used to align the planning CT and treatment CBCT images, simulating a six degree-of-freedom couch correction at treatments. The authors conclude that the dosimetric impact of the anatomic change and patient setup error was reasonably captured in the differences of the distal WEPL variation with a range calculation uncertainty of 2%. The proposed method to calculate the distal WEPL using the scatter-corrected CBCT images can be an essential tool to decide the necessity of re-planning in adaptive proton therapy.

Definition of a threshold for plan adaptation in particle therapy

Introduction Charged particle beam therapy has been used for its twofold advantageous behavior: offering high precision to the target volume and minimum doses to organs at risk. Treatment outcome of pancreatic patients is compromised by dose limits of the GI tract and dose escalation is critical due to inter-fractional uncertainties. Purpose The aim was to assess tissue variations by the accumulated water equivalent path-length variation (Δ-accWEPL) and find a dosimetric threshold point where corrective action should be taken i.e. re-optimization of the plan, with less computational work and time consumption. Materials and methods Two types of analysis were performed by comparing planning-CT and rigidly-registered weekly-CTs for 11 patients: (A) Dose Forward calculation performed using research TPS TRiP98, from the treatment plan clinically applied to the patients. The dose evaluation parameters were V95%PTV, V95%CTV and Gamma-index criteria. (B) WEPL maps were generated using the MeVisLab framework to quantify densities variation as a percentage of voxels, within our defined threshold (±3 mm), of the Δ-accWEPL. Results Both Gamma-index and V95%PTV showed tendency to increase or decrease with Δ-accWEPL by strong correlation of 0.73 and 0.6, respectively. A threshold of Δ-accWEPL as 56% interprets more than 3% variations in V95%PTV and gamma passing rate lower than 96.5%. OAR analysis showed high dose impact on large intestine V20, up to 20% increased dose. Conclusion The studied method proved to be a prediction tool for physicians in decision making for plan re-optimization need due to inter-fractional density variation, with less computational time and without contouring structures. Acknowledgment This work was funded by DFG within KFO 214 and partially supported by BMBF SPARTA (grant agreement number 01lB13001G) Julia Bauer, Wenjing Chen. We would also like to thank the clinical and medical physics team at HIT. Disclosure No disclosure.

Using CBCT for pretreatment range check in proton therapy: a phantom study for prostate treatment by anterior-posterior beam.

This study explores the potential of cone‐beam computed tomography (CBCT) for monitoring relative beam range variations due to daily changes in patient anatomy for prostate treatment by anterior proton beams. CBCT was used to image an anthropomorphic pelvic phantom, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then after it was shifted by 10 mm translation randomly along one of the X, Y, or Z directions. The filling of the phantom bladder with water was not refreshed from day to day, inducing gradual change of the water‐equivalent path length (WEPL) across the bladder. MIMvista (MIM) software was used to perform image registration and re‐alignment of all the scans with the scan from the first session. The XiO treatment planning system was used to perform data analysis. It was found that, although the Hounsfield unit numbers in CBCT have substantially larger fluctuations than those in diagnostic CT, CBCT datasets taken for daily patient positioning could potentially be used to monitor changes in patient anatomy. The reproducibility of the WEPL, computed using CBCT along anterior–posterior (AP) paths across and around the phantom prostate, over a volume of 360 cc, is sufficient for detecting daily WEPL variations that are equal to or larger than 3 mm. This result also applies to CBCT scans of the phantom after it is randomly shifted from the treatment position by 10 mm. limiting the interest to WEPL variation over a specific path within the same CBCT slice, one can detect WEPL variation smaller than 1 mm. That is the case when using CBCT for tracking daily change of the WEPL across the phantom bladder that was induced by spontaneous change in the bladder filling due to evaporation. In summary, the phantom study suggests that CBCT can be used for monitoring day to day WEPL variations in a patient. The method can detect WEPL variation equal to or greater than 3 mm. The study calls for further investigation using the CBCT data from real patients. If confirmed with real patients' data, CBCT could become, in addition to patient setup, a standard tool for proton therapy pretreatment beam range check.PACS number: 87.55.Tm

A method for evaluation of proton plan robustness towards inter-fractional motion applied to pelvic lymph node irradiation

ABSTRACTBackground. The benefit of proton therapy may be jeopardized by dose deterioration caused by water equivalent path length (WEPL) variations. In this study we introduced a method to evaluate robustness of proton therapy with respect to inter-fractional motion and applied it to irradiation of the pelvic lymph nodes (LNs) from different beam angles. Patient- versus population-specific patterns in dose deterioration were explored.Material and methods. Patient data sets consisting of a planning computed tomography (pCT) as well as multiple repeat CT (rCT) scans for three patients were used, with target volumes and organs at risk (ORs) outlined in all scans. Single beam spot scanning proton plans were optimized for the left and right LN targets separately, across all possible beam angle configurations (5° angle intervals). Isotropic margins of 0, 3, 5 and 7 mm from the clinical target volume (CTV) to the planning target volume (PTV) were investigated. The optimized fluence maps for the pCT for each beam were applied onto all rCTs and the dose distributions were re-calculated. WEPL variation for each beam angle was computed by averaging over beams eye view WEPL distributions.Results. Similarity in deterioration patterns were found for the investigated patients, with beam angles delivering less dose to rectum, bladder and overall normal tissue identified around 40° and around 150°–160° for the left LNs, and corresponding angles for the right LNs. These angles were also associated with low values of WEPL variation.Conclusion. We have established and explored a method to quantify the robustness towards inter-fractional motion of single beam proton plans treating the pelvic LNs from different beam configurations and with different CTV to PTV margins. For the patients investigated we were able to identify beam orientations that were robust to dose deterioration in the target and ORs.

Surrogate-driven deformable motion model for organ motion tracking in particle radiation therapy

The aim of this study is the development and experimental testing of a tumor tracking method for particle radiation therapy, providing the daily respiratory dynamics of the patient’s thoraco-abdominal anatomy as a function of an external surface surrogate combined with an a priori motion model. The proposed tracking approach is based on a patient-specific breathing motion model, estimated from the four-dimensional (4D) planning computed tomography (CT) through deformable image registration. The model is adapted to the interfraction baseline variations in the patient’s anatomical configuration. The driving amplitude and phase parameters are obtained intrafractionally from a respiratory surrogate signal derived from the external surface displacement. The developed technique was assessed on a dataset of seven lung cancer patients, who underwent two repeated 4D CT scans. The first 4D CT was used to build the respiratory motion model, which was tested on the second scan. The geometric accuracy in localizing lung lesions, mediated over all breathing phases, ranged between 0.6 and 1.7 mm across all patients. Errors in tracking the surrounding organs at risk, such as lungs, trachea and esophagus, were lower than 1.3 mm on average. The median absolute variation in water equivalent path length (WEL) within the target volume did not exceed 1.9 mm-WEL for simulated particle beams. A significant improvement was achieved compared with error compensation based on standard rigid alignment. The present work can be regarded as a feasibility study for the potential extension of tumor tracking techniques in particle treatments. Differently from current tracking methods applied in conventional radiotherapy, the proposed approach allows for the dynamic localization of all anatomical structures scanned in the planning CT, thus providing complete information on density and WEL variations required for particle beam range adaptation.

A method for selection of beam angles robust to intra-fractional motion in proton therapy of lung cancer

Background. Proton therapy offers the potential for sparing the normal tissue surrounding the target. However, due to well-defined proton ranges around the Bragg peak, dose deposition is more sensitive to changes in the water equivalent path length (WEPL) than with photons. In this study, we assess WEPL variations caused by breathing-induced motion for all possible beam angles in a series of lung cancer patients. By studying the association between measures for WEPL variation and breathing-induced target dose degradation we aimed to develop and explore a tool to identify beam angles that are robust to patient-specific patterns of intra-fractional motion.Material and methods. Using four-dimensional computed tomography (4DCT) images of three lung cancer patients we evaluated the impact of the WEPL changes on target dose coverage for a series of coplanar single-beam plans. The plans were optimised for the internal target volume (ITV) at the maximum intensity projection (MIP) CT for every 3° gantry interval. The plans were transferred to the ten 4DCT phases and the average reduction in ITV V95 over the ten phases, relative to the original MIP CT calculation, was quantified. The target dose reduction was associated with the mean difference between the WEPL and the phase-averaged WEPL computed for all beam rays across all possible gantry-couch angle combinations.Results. The gantry-couch angle maps showed areas of both high and low WEPL variation, with overall quite similar patterns yet with individual differences reflecting differences in tumour position and breathing-induced motion. The coplanar plans showed a strong association between WEPL changes and ITV V95 reduction, with a correlation coefficient ranging between 0.92 and 0.98 for the three patients (p < 0.01).Conclusion. We have presented a 4DCT-based method to quantify WEPL changes during the breathing cycle. The method identified proton field gantry-couch angle combinations that were either sensitive or robust to WEPL changes. WEPL variations along the beam path were associated with target under-dosage.

A serial 4DCT study to quantify range variations in charged particle radiotherapy of thoracic cancers

Weekly serial 4DCT scans were acquired under free breathing conditions to assess water-equivalent path length (WEL) variations due to both intrafractional and interfractional changes in tissue thickness and density and to calculate proton dose distributions resulting from anatomical variations observed in serial 4DCT. A template of region of interests (ROIs) was defined on the anterior–posterior (AP) beam's eye view, and WEL measurements were made over these ROIs to quantify chest wall thickness variations. Interfractional proton dose distributions were calculated to assess changes in the expected dose distributions caused by range variations. Mean intrafractional chest wall WEL changes during respiration varied by: −4.1 mm (<−10.2 mm), −3.6 mm (<−7.1 mm), −3.2 mm (<−5.6 mm) and −2.5 mm (<−5.1 mm) during respiration in the ITV, upper, middle and lower lung regions, respectively. The mean interfractional chest wall WEL variation at Week 6 decreased by −4.0 mm (<−8.6 mm), −9.1 mm (<−17.9 mm), −9.4 mm (<−25.3 mm) and −4.5 mm (<−15.6 mm) in the ITV, upper, middle and lower lung regions, respectively. The variations were decomposed into anterior and posterior chest wall thickness changes. Dose overshoot beyond the target was observed when the initial boli was applied throughout the treatment course. This overshoot is due to chest wall thickness variations and target positional variations. The radiological path length can vary significantly during respiration as well as over the course of several weeks of charged particle therapy. Intrafractional/interfractional chest wall thickness changes can be a significant source of range variation in treatment of lung tumors with charged particle beams, resulting in dose distribution perturbations from the initial plan. Consideration of these range variations should be made in choosing the therapeutic charged particle beam range.

SU‐C‐110‐05: Analysis of Intra‐ and Inter‐Fractional Range Variations to Fiducials in Proton Radiotherapy of Liver Tumors

Purpose: To quantify radiological pathlength variations to fiducials in proton beam treatment of liver tumors. Methods: Serial 4DCT scans of 10 patients with primary liver tumors were analyzed. The water equivalent pathlength (WEL) was quantified from entrance surface to radio‐opaque clips 1) for different respiratory phases 2) for serial 4DCT scans 3) for each treatment angle (typically AP or PA and a single lateral field) 4) for different immobilization methods. WEL measurements to fiducials is well defined, Vs range to the distal surface of a deformably registered target, but results in fewer measurements (typically 3 clips). A statistical analysis of the mean range variations and spread is performed. Results: AP range at T50 to clips is reproducible from day to day with a mean variation of 1.8mm, SD 1.3mm. Lateral field range variations from day to day at T50 were 1.3mm SD 2.6mm. Range variations at inhale were more variable, with AP T00 mean WEL variation of 3.5mm SD 4.4mm and lateral mean variations of 3.2mm SD 1.7mm. An 8mm lateral range variation was observed in one patient. Range variation during respiration was also studied. The mean AP and lateral range variation was 3mm SD 2.4 and 4.1 mm SD 2.1mm. A 13mm variation was observed in one patient, and attributed to clip motion during respiration without significant change in the entrance surface. Conclusions: Several sources of range uncertainty are routinely included in treatment range determination for proton therapy. These include uncertainties in CT HU numbers, relative stopping powers of tissues, compensator misalignment, etc. Organ motion is explicitly taken into account in the aperture plane, but not in the beam direction. This study elucidates the magnitude and causes of range variations to well defined points. Inclusion of this uncertainty should be considered in determining treatment range.The project described was supported by Award Number P01CA021239 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

Quantification and Visualization of Charged Particle Range Variations

Range variations during respiration affect the penetration of charged particle beams and can result in beam overshoot or undershoot to the target. We have developed analysis tools to quantify the water equivalent pathlength (WEL) variations resulting from respiration (WEL analyzer, Aqualyzer), as well as a data explorer to view WEL variations interactively. The metrics to characterize and quantify penetration of a charged particle beam during respiration were calculated semiautomatically. The analysis involved the generation of images that (1) encode the radiologic pathlength across a beam's eye view image during the respiratory phase, (2) display the variation of the radiologic pathlength relative to a reference respiratory phase, (3) display isopenetration as a function of breathing, and (4) show range fluctuation images for a compensating bolus when applied to four-dimensional computed tomography. Additional quantities relevant to the analysis of charged particle beams in a breathing patient that are calculated include the beam overshoot volume and beam overshoot distance. These quantities are calculated as a function of time, gantry angle, and position. The software was applied to test cases to illustrate its utility in the analysis of range variations of charged particle beams in the treatment of lung tumors. WEL analysis is useful in the rapid assessment of range variations in the treatment of lung tumors and in determining the optimal gantry angle and respiratory gating window. The extension of encoding range fluctuations to a beam's eye view display is helpful in designing plans that are more robust in the presence of motion.

Quantitation of water equivalent pathlength variations from serial CT for prostate treatment with proton beams

Quantitation of water equivalent pathlength variations from serial CT for prostate treatment with proton beams