Dosimetric Changes Research Articles

Intentional deep overfit learning for patient-specific dose predictions in adaptive radiotherapy.

The framework of adaptive radiation therapy (ART) was crafted to address the underlying sources of intra-patient variation that were observed throughout numerous patients' radiation sessions. ART seeks to minimize the consequential dosimetric uncertainty resulting from this daily variation, commonly through treatment planning re-optimization. Re-optimization typically consists of manual evaluation and modification of previously utilized optimization criteria. Ideally, frequent treatment plan adaptation through re-optimization on each day's computed tomography (CT) scan may improve dosimetric accuracy and minimize dose delivered to organs at risk (OARs) as the planning target volume (PTV) changes throughout the course of treatment. Re-optimization in its current form is time-consuming and inefficient. In response to this ART bottleneck, we propose a deep learning based adaptive dose prediction model that utilizes a head and neck (H&N) patient's initial planning data to fine-tune a previously trained population model towards a patient-specific model. Our fine-tuned, patient-specific (FT-PS) model, which is trained using the intentional deep overfit learning (IDOL) method, may enable clinicians and treatment planners to rapidly evaluate relevant dosimetric changes daily and re-optimize accordingly. An adaptive population (AP) model was trained using adaptive data from 33 patients. Separately, 10 patients were selected for training FT-PS models. The previously trained AP model was utilized as the base model weights prior to re-initializing model training for each FT-PS model. Ten FT-PS models were separately trained by fine-tuning the previous model weights based on each respective patient's initial treatment plan. From these 10 patients, 26 ART treatment plans were withheld from training as the test dataset for retrospective evaluation of dose prediction performance between the AP and FT-PS models. Each AP and FT-PS dose prediction was compared against the ground truth dose distribution as originally generated during the patient's course of treatment. Mean absolute percent error (MAPE) evaluated the dose differences between a model's prediction and the ground truth. MAPE was calculated within the 10% isodose volume region of interest for each of the AP and FT-PS models dose predictions and averaged across all test adaptive sessions, yielding 5.759% and 3.747% respectively. MAPE differences were compared between AP and FT-PS models across each test session in a test of statistical significance. The differences were statistically significant in a paired t-test with two-tailed p-value equal to and 95% confidence interval (CI) equal to [-2.483, -1.542]. Furthermore, MAPE was calculated using each individually segmented structure as an ROI. Nineteen of 24 structures demonstrated statistically significant differences between the AP and FT-PS models. We utilized the IDOL method to fine-tune a population-based dose prediction model into an adaptive, patient-specific model. The averaged MAPE across the test dataset was 5.759% for the population-based model versus 3.747% for the fine-tuned, patient-specific model, and the difference in MAPE between models was found to be statistically significant. Our work demonstrates the feasibility of patient-specific models in adaptive radiotherapy, and offers unique clinical benefit by utilizing initial planning data that contains the physician's treatment intent.

Dosimetric Impact of Source Displacement in GammaTile Surgically Targeted Radiation Therapy for Gliomas.

Background This study aims to evaluate dosimetric changes that happened during the first month after GammaTile surgically targeted radiation therapy (STaRT) for gliomas due to Cesium-131 (Cs-131) seed displacement caused by cavity shrinkage in brain brachytherapy. Methodology In this study, 10 glioma patients had 4-11 GammaTiles placed along the resection bed after maximal safe resection during craniotomy. Each GammaTile is composed of four Cs-131 seeds embedded in a biodegradable collagen sponge to minimize seed movement and maintain seed-to-cavity surface distance. The Cs-131 seed positions were identified using VariSeed on day one. On day 30, post-implant computed tomography (CT) images and dosimetry parameters were calculated. An iterative closest point (ICP) algorithm was used to compute rigid transformation between the day one and day 30 seed clouds. The seed displacement was calculated after registration. The volume receiving 100% of the prescription dose (V100), the dose received by 90% of the planning target volume (D90_PTV), the planning target volume receiving 100% of the prescription dose (V100_PTV), and the dose to organs at risk (OARs) were calculated for both CT images to determine the dosimetric changes from any seed displacement. Results The mean seed displacement of 1.8 ± 1.0 mm for all patients was observed between day one and day 30. The maximum seed displacement for each patient ranged from 2.3 mm to 7.3 mm. The mean V100 difference between day one and day 30 was 2.5 cc (range = 0.5-6.5 cc). The mean D90_PTVs were 95.5% (range = 69.0%-131.0%) and 98.1% (range = 19.9%-149.0%) on day one and day 30, respectively. The mean V100_PTVs were 88.4% (range = 81.3%-99.1%) and 87.9% (range = 47.0%-99.7%) on day one and day 30, respectively. On day one, the brainstem dose was 63.5 Gy for one case and 28.1 Gy for another case; while on day 30, the brainstem dose was 55.8 Gy and 20.6 Gy for the same patients, contributing to 7.7 Gy (12.8%) and 7.5 Gy (12.5%) dose reductions to brainstem for these patients, respectively. Only two patients received a dose to the optic nerves (34.1 Gy and 5.2 Gy). There were small changes (1.8 Gy and 0.5 Gy, respectively) in the dose to optic nerves when comparing the dose calculated on day one and the dose calculated on day 30 CT images. The same two patients received 30.4 Gy and 6.8 Gy to the chiasm, respectively. Small changes in the dose to the chiasm (≤1.1 Gy) were noted between day one and day 30. Conclusions A maximum seed displacement of up to 7.3 mm and a mean seed displacement of 1.8 mm caused by cavity shrinkage were observed during the first month after GammaTile STaRT for gliomas. There were noticeable changes in dosimetry parameters. Changes in the doses to OARs, particularly the brainstem, were large (up to 12.8% of the prescription dose). These changes in dosimetry should be considered when evaluating treatment outcomes and planning future GammaTile treatments.

PO-1860 Individual monitoring of dosimetric changes in head and neck adaptive proton therapy

PO-1860 Individual monitoring of dosimetric changes in head and neck adaptive proton therapy

The use of in-vivo dosimetry to identify head and neck cancer patients needing adaptive radiotherapy

Background and purposeHead and neck cancer (HNC) patients experiencing anatomical changes during their radiotherapy (RT) course may benefit from adaptive RT (ART). We investigated the sensitivity of an electronic portal imaging device (EPID)-based in-vivo dosimetry (EIVD) system to detect patients that require ART and identified its limitations. Materials and methodsA retrospective study was conducted for 182 HNC patients: laryngeal cancer without elective lymph nodes (group A), postoperative RT (group B) and primary RT including elective lymph nodes (group C). The effect of anatomical changes on the dose distribution and volumetric changes was quantified. The receiver operating characteristic curve was used to obtain the optimal cut-off value for the gamma passing rate (%GP) with a dose difference of 3% and a distance to agreement of 3 mm. ResultsFifty HNC patients receiving ART were analyzed: 1 in group A, 10 in group B and 39 in group C. Failed fractions (FFs) occurred in 1/1, 6/10 and 23/39 cases before ART in group A, B and C respectively. In the four cases in group B without FFs, only minor dosimetric changes were observed. One of the cases in group C without FFs had significant dosimetric changes (false negative). Three cases received ART because of clinical reasons that cannot be detected by EIVD. The optimal cut-off value for the %GP was 95%/95.2% for old/new generation machines respectively. ConclusionEIVD combined with 3D imaging techniques can be synergistic in the detection of anatomical changes in HNC patients who benefit from ART.

PD15-08 PROSTATE BED DEFORMATIONS AND DOSIMETRIC IMPLICATIONS: AN EXPLORATORY ANALYSIS OF A PHASE II CLINICAL TRIAL (SCIMITAR)

You have accessJournal of UrologyCME1 Apr 2023PD15-08 PROSTATE BED DEFORMATIONS AND DOSIMETRIC IMPLICATIONS: AN EXPLORATORY ANALYSIS OF A PHASE II CLINICAL TRIAL (SCIMITAR) Tommy Jiang, Lauren Smith, Luca Valle, Leslie Ballas, John Nikitas, Michael Steinberg, Robert Reiter, Minsong Cao, and Amar Kishan Tommy JiangTommy Jiang More articles by this author , Lauren SmithLauren Smith More articles by this author , Luca ValleLuca Valle More articles by this author , Leslie BallasLeslie Ballas More articles by this author , John NikitasJohn Nikitas More articles by this author , Michael SteinbergMichael Steinberg More articles by this author , Robert ReiterRobert Reiter More articles by this author , Minsong CaoMinsong Cao More articles by this author , and Amar KishanAmar Kishan More articles by this author View All Author Informationhttps://doi.org/10.1097/JU.0000000000003262.08AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookLinked InTwitterEmail Abstract INTRODUCTION AND OBJECTIVE: One major challenge of postoperative radiotherapy is the deformable nature of the bladder and rectum. These changes can potentially lead to underdosing of the prostate bed and overdosing of the organs at risk (OARs: bladder and rectum). Our objective was to quantify the dosimetric impact of interfractional prostate bed and OAR deformation. METHODS: SCIMITAR (NCT03541850) was a prospective phase II clinical trial evaluating stereotactic body radiotherapy (SBRT) for post-prostatectomy patients. This analysis included men receiving CT-guided SBRT. All patients received five fractions of 6−6.8 Gy to the prostate bed. The clinical target volume (CTV) and OARs were retrospectively contoured on fractional CBCT images using MIMVista. Volumetric, shape (via the dice similarity coefficient [DSC]) and dosimetric changes were quantified. Student’s t-test was used to analyze the differences between planning and daily treatment outcomes RESULTS: 31 patients were eligible for this analysis. We found the CTV volume remained stable (median change 1.1%; IQR: -15.1%–16.1%), while the shape was highly deformable (DSC of 0.76 [IQR: 0.71–0.79]). The bladder and rectum exhibited similar changes with median volume change of 5.7% (IQR: -24.3%–51.0%) and 5.5% (IQR: -8.7%-21.9%) respectively and median DSC of 0.77 (IQR: 0.68–0.84) and 0.74 (IQR: 0.69–0.80) respectively. Notably, the CTV received less radiation than planned (volume receiving 95% of dose 93.2% vs 99.6%, p<0.01). Additionally, 39% (56/145) of total fractions and 52% (15/29) of patients met criteria for under coverage (volume receiving 95% of the dose <93%). The rectum received more radiation than planned on several dosimetric parameters (volume receiving 27.5 Gy increased from 15.4% to 21.0% [p = 0.009] and volume receiving 32.5 Gy increased from 6.0% to 10.9% [p = 0.006]) (Table 1). CONCLUSIONS: We found underdosing of the prostate CTV and overdosing of the rectum in a fair percentage of patients receiving postoperative radiation, despite modern radiotherapy planning and delivery techniques. While future work will correlate these dosimetric consequences with toxicity and efficacy, these data suggest that approaches to improving dose delivery, such as adaptive radiotherapy, may be beneficial. Source of Funding: AUA Medical Student Fellowship © 2023 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 209Issue Supplement 4April 2023Page: e421 Advertisement Copyright & Permissions© 2023 by American Urological Association Education and Research, Inc.MetricsAuthor Information Tommy Jiang More articles by this author Lauren Smith More articles by this author Luca Valle More articles by this author Leslie Ballas More articles by this author John Nikitas More articles by this author Michael Steinberg More articles by this author Robert Reiter More articles by this author Minsong Cao More articles by this author Amar Kishan More articles by this author Expand All Advertisement PDF downloadLoading ...

Geometric and dosimetric evaluation of deep learning based auto-segmentation for clinical target volume on breast cancer.

Recently, target auto-segmentation techniques based on deep learning (DL) have shown promising results. However, inaccurate target delineation will directly affect the treatment planning dose distribution and the effect of subsequent radiotherapy work. Evaluation based on geometric metrics alone may not be sufficient for target delineation accuracy assessment. The purpose of this paper is to validate the performance of automatic segmentation with dosimetric metrics and try to construct new evaluation geometric metrics to comprehensively understand the dose-response relationship from the perspective of clinical application. A DL-based target segmentation model was developed by using 186 manual delineation modified radical mastectomy breast cancer cases. The resulting DL model were used to generate alternative target contours in a new set of 48 patients. The Auto-plan was reoptimized to ensure the same optimized parameters as the reference Manual-plan. To assess the dosimetric impact of target auto-segmentation, not only common geometric metrics but also new spatial parameters with distance and relative volume ( ) to target were used. Correlations were performed using Spearman's correlation between segmentation evaluation metrics and dosimetric changes. Only strong (|R2 |>0.6, p<0.01) or moderate (|R2 |>0.4, p<0.01) Pearson correlation was established between the traditional geometric metric and three dosimetric evaluation indices to target (conformity index, homogeneity index, and mean dose). For organs at risk (OARs), inferior or no significant relationship was found between geometric parameters and dosimetric differences. Furthermore, we found that OARs dose distribution was affected by boundary error of target segmentation instead of distance and to target. Current geometric metrics could reflect a certain degree of dose effect of target variation. To find target contour variations that do lead to OARs dosimetry changes, clinically oriented metrics that more accurately reflect how segmentation quality affects dosimetry should be constructed.

Evaluation of dosimetric and volumetric changes in target volumes and organs at risk during adaptive radiotherapy in head and neck cancer: A prospective study

BackgroundDuring radiation therapy for head and neck malignancies, most patients experience significant anatomical alterations due to loss of weight, changes in tumor volumes, and immobilization issues. Adaptive radiotherapy adapts to the patient's actual anatomy through repetitive imaging and replanning. In the present study, dosimetric and volumetric changes in target volumes and organs at risk during adaptive radiotherapy in head and neck cancer was evaluated. Material and methodsThirty-four locally advanced Head and neck carcinoma patients with histologically proven Squamous Cell Carcinoma for curative treatment were included. Rescan was done at the end of 20 fractions of treatment. All quantitative data were analyzed with paired t-Test and Wilcoxon Signed Rank (Z) test. ResultsMost patients had oropharyngeal carcinoma (52.9%). There were significant volumetric changes in all the parameters - GTV-primary (10.95, p < 0.001), GTV- nodal (5.81, p = 0.001), PTV High Risk (26.1, p < 0.001), PTV – Intermediate Risk (46.9, p = 0.006), PTV – Low Risk (43.9, p = 0.003), lateral neck diameter (0.9, p < 0.001), right parotid volumes (6.36, p < 0.001) and left parotid volumes (4.93, p < 0.001). Dosimetric changes in the organs at risk were non-significant. ConclusionAdaptive replanning has been seen to be labour intensive. However, the changes in the volumes of both target and the OARs credit a mid-treatment replanning to be done. Long term follow-up is required to assess locoregional control after adaptive radiotherapy in head and neck cancer.

Liver SBRT dose accumulation to assess the impact of anatomic variations on normal tissue doses and toxicity in patients treated with concurrent sorafenib

Background and PurposeUnexpected liver volume reductions occurred during trials of liver SBRT and concurrent sorafenib. The aims were to accumulate liver SBRT doses to assess the impact of these anatomic variations on normal tissue dose parameters and toxicity. Materials and MethodsThirty-two patients with hepatocellular carcinoma (HCC) or metastases treated on trials of liver SBRT (30–57 Gy, 6 fractions) and concurrent sorafenib were analyzed. SBRT doses were accumulated using biomechanical deformable registration of daily cone-beam CT. Dose deviations (accumulated-planned) for normal tissues were compared for patients with liver volume reductions > 100 cc versus stable volumes, and accumulated doses were reported for three patients with grade 3–5 luminal gastrointestinal toxicities. ResultsPatients with reduced (N = 12) liver volumes had larger mean deviations of 0.4–1.3 Gy in normal tissues, versus −0.2–0.4 Gy for stable cases (N = 20), P > 0.05. Deviations > 5% of the prescribed dose occurred in both groups. Two HCC patients with toxicities to small and large bowel had liver volume reductions and deviations to the maximum dose of 4% (accumulated 36.9 Gy) and 3% (accumulated 33.4 Gy) to these organs respectively. Another HCC patient with a toxicity of unknown location plus tumor rupture, had stable liver volumes and deviations to luminal organs of −6% to 4.5% (accumulated < 30.5 Gy). ConclusionLiver volume reductions during SBRT and concurrent sorafenib were associated with larger increases in accumulated dose to normal tissues versus stable liver volumes. These dosimetric changes may have further contributed to toxicities in HCC patients who have higher baseline risks.

Spot delivery error predictions for intensity modulated proton therapy using robustness analysis with machine learning.

The purpose of this work is to assess the robustness of treatment plans when spot delivery errors were predicted with a machine learning (ML) model for intensity modulated proton therapy (IMPT). Over 6000 machine log files from delivered IMPT treatment plans were included in this study. From these log files, over 4.1 106 delivered proton spots were used to train the ML model. The presented model was tested and used to predict the spot position as well as the monitor units (MU) per spot, based on the original planning parameters. Two patient plans (one accelerated partial breast irradiation [APBI] and one ependymoma) were recalculated with the predicted spot position/MUs by the ML model and then were re-analyzed for robustness. Plans with ML predicted spots were less robust than the original clinical plans. In the APBI plan, dosimetric changes to the left lung and heart were not clinically relevant. In the ependymoma plan, the hot spot in the brainstem decreased and the hot spot in the cervical cord increased. Despite these differences, after robustness analysis, both ML spot delivery error plans resulted in >95% of the CTV receiving >95% of the prescription dose. The presented workflow has the potential benefit of including realistic spots information for plan quality checks in IMPT. This work demonstrates that in the two example plans, the plans were still robust when accounting for spot delivery errors as predicted by the ML model.

Dosimetric Comparison of Different Dose Calculation Algorithms in Postmastectomy Breast Cancer Patients Using Conformal Planning Techniques.

The aim of the current study was to compare three different dose-calculating algorithms, i.e., superposition (SP), fast SP (FSP), and convolution (CV), for breast cancer patients treated with intensity-modulated radiotherapy (IMRT) and field-in-Field forward plan IMRT (FiF-FP-IMRT). The current retrospective study involved 100 postmastectomy breast cancer patients who were given radiotherapy using IMRT and FiF-FP-IMRT planning techniques. All the initially SP-calculated plans were recalculated with the same monitor units for FSP and CV algorithm without change in any of the other planning parameters. The isodose distribution and various plan evaluating parameters, for example, conformity index (CI), homogeneity index, and uniformity index target volume and normal structure doses were compared and analyzed for all the different algorithm calculated plans. In the IMRT plans, all the target and normal structure dose-volume parameters showed a significant difference between all the three different algorithms with P < 0.05. In the FiF-FP-IMRT plans, CV algorithm showed a significant difference in most of the target and normal structure dose-volume parameters. Among quality indexes, only CI showed a significant difference between all the algorithms in both the planning techniques. R50 showed a significant difference with the CV algorithm in both the planning techniques. The change in the dose calculation algorithm resulted in dosimetric changes which must be evaluated by the medical physicists and oncologists while evaluating treatment plans. In the current study with breast patients, the results obtained for target and normal structure doses using the CV algorithm are overestimated as compared to SP and FSP algorithms, producing variable results in air and bony normal structures. However, the ipsilateral lung V5 parameter and the ipsilateral humeral head mean dose were found to be underestimated by the CV algorithm as compared to the SP and FSP algorithm in both the planning techniques.

Total marrow irradiation versus total body irradiation using intensity-modulated helical tomotherapy

BackgroundTotal body irradiation (TBI) is often a component of the conditioning regimen prior to hematopoietic stem cell transplantation in patients with hematological malignancies. However, total marrow irradiation (TMI) could be an alternative method for reducing radiation therapy-associated toxicity, as it specifically targets the skeleton and thus could better protect organs at risk. Here, we compared dosimetric changes in irradiation received by the target volume and organs at risk between TBI and TMI plans.Materials and methodsTheoretical TMI plans were calculated for 35 patients with various hematological malignancies who had already received TBI in our clinic. We then statistically compared irradiation doses between the new TMI plans and existing TBI plans. We examined whether TMI provides greater protection of organs at risk while maintaining the prescribed dose in the targeted skeletal area. We also compared beam-on times between TBI and TMI.ResultsTMI planning achieved significant reductions in the mean, minimum, and maximum irradiation doses in the lungs, kidneys, liver, spleen, and body (i.e., remaining tissue except organs and skeleton). In particular, the mean dose was reduced by 49% in the liver and spleen and by 55–59% in the kidneys. Moreover, TMI planning reduced the corpus beam-on time by an average of 217 s.ConclusionTMI planning achieved significant dose reduction in organs at risk while still achieving the prescribed dose in the target volume. Additionally, TMI planning reduced the beam-on time for corpus plans despite a high modulation factor.Graphical abstract

Evaluation of interfractional organ motion during neoadjuvant radiotherapy for rectal cancer patients.

To investigate interfractional motion of the mesorectum and bladder and to assess dosimetric changes using cone-beam computed tomography (CBCT) during neoadjuvant radiotherapy for locally advanced rectal cancer patients. Twenty-one patients who underwent volumetric arc therapy with CBCT imaging protocol were retrieved. The mesorectum and bladder were delineated on every CBCT image, and treatment plans were recalculated for all CBCTs. The organ motion was analyzed as a mean shift on the X-Y-Z axes. The volume changes were evaluated using the DICE index. Mann-Whitney U test was used in pairwise comparison analysis and ANOVA was used to compare shifts in each direction. A total of 105 CBCTs were evaluated retrospectively. The movement of the total mesorectum was found to be 1.5 mm, 4 mm, and 5 mm on the X-Y-Z-axes, respectively. In the subgroup analysis, the movement of the 1/3 upper mesorectum on the Y-axis was significantly higher (mean movement 8 mm, p = 0.005). Mean bladder displacements were 2 mm, 4 mm, and 8 mm on the X-Y-Z-axes, respectively. In the D2, D95, and D98 doses, there was no statistically significant change depending on the motion. During radiotherapy planning, the mesorectal movement should not be forgotten and PTV margins should be determined accordingly (Tab. 6, Ref. 22). Text in PDF www.elis.sk Keywords: cone-beam computed tomography, rectal cancer, mesorectum, interfractional organ motion, neoadjuvant radiotherapy.

Investigation of Computed Tomography Numbers on Multiple Imaging Systems using Single and Multislice Methods.

The aim of this study is to determine the variation in Hounsfield values with single and multi-slice methods using in-house software on fan-beam computed tomography (FCT), linear accelerator (linac) cone-beam computed tomography (CBCT), and Icon-CBCT datasets acquired using Gammex and advanced electron density (AED) phantoms. The AED phantom was scanned on a Toshiba computed tomography (CT) scanner, five linac-based CBCT X-ray volumetric imaging systems, and Leksell Gamma Knife Icon. The variation between single and multi-slice methods was assessed by comparing scans acquired using Gammex and AED phantoms. The variation in Hounsfield units (HUs) between seven different clinical protocols was assessed using the AED phantom. A CIRS Model 605 Radiosurgery Head Phantom (TED) phantom was scanned on all three imaging systems to assess the target dosimetric changes due to HU variation. An in-house software was developed in MATLAB to assess the HU statistics and the trend along the longitudinal axis. The FCT dataset showed a minimal variation (central slice ± 3 HU) in HU values along the long axis. A similar trend was also observed between the studied clinical protocols acquired on FCT. Variation among multiple linac CBCTs was insignificant. In the case of the water insert, a maximum HU variation of -7.23 ± 68.67 was observed for Linac 1 towards the inferior end of the phantom. All five linacs appeared to have a similar trend in terms of HU variation from the proximal to the distal end of the phantom, with a few outliers for Linac 5. Among three imaging modalities, the maximum variation was observed in gamma knife CBCTs, whereas FCT showed no appreciable deviation from the central value. In terms of dosimetric comparison, the mean dose in CT and Linac CBCT scans differed by <0.5 Gy, whereas at least a 1 Gy difference was observed between CT and gamma knife CBCT. This study shows a minimal variation with FCT between single, volume-based, and multislice methods, and hence the current approach of determining the CT-electron density curve based on a single-slice method would be sufficient for producing a HU calibrations curve for treatment planning. However, CBCTs acquired on linac, and in particular, gamma knife systems, show noticeable variations along the long axis, which is likely to affect the dose calculations performed on CBCTs. It is highly recommended to assess the Hounsfield values on multiple slices before using the HU curve for dose calculations.

Gated radiation therapy for patients with breast cancer to reduce the dose to the lung and heart: A comparative cross-sectional study

ABSTRACT JOURNAL/crsat/04.03/02201859-202306020-00005/figure1/v/2023-08-03T140821Z/r/image-tiff Background: Deep inspiratory breath hold (DIBH) is an established technique to treat left breast cancer with fewer side-effects to the surrounding organs at risk (OARs). However, studies assessing DIBH in resource-poor settings, especially in a government setup, are scarce. Objectives: Our primary objective was to assess and compare the dosimetric changes in the DIBH technique compared to the free breathing technique among patients with left-sided breast cancer undergoing radiation therapy. Materials and Methods: This was a comparative cross-sectional study conducted in the Department of Radiation Oncology, Kidwai Memorial Institute of Oncology (Bengaluru, India), over a period of 24 months. A total of 65 patients were treated with the three-dimensional conformal radiation therapy (3D-CRT) technique either using DIBH or free breathing as per the standard protocol. The dosimetric comparisons for the surrounding OARs, in terms of mean percentage volume receiving 15 Gy (V15Gy) and 20 Gy (V20Gy) in the left lung, V25Gy, V30Gy, V5Gy in the heart the mean dose in Gy received by 25% (V25%) and 40% (V40%) of the heart, V10% and V30% of the left anterior descending artery (LAD) and mean values of Dmean dose (Gy) received by the left lung, heart, and LAD artery were compared between both the techniques. Results: Among 65 patients, 25 (38.4%) were treated by the free breathing technique and 40 (61.6%) by DIBH. Various dosimetric parameters that were significantly lower in the DIBH compared to free breathing techniques included the mean percentage volume of left lung receiving 20 Gy (V20 Gy) (29.5% vs. 23.5%), heart receiving 25 Gy (V25Gy) (6.7% vs. 2.9%) and 30 Gy (12.8% vs. 2.0%), mean values of Dmean received by the heart (7.8 Gy vs. 4.5 Gy) and LAD artery (31.3 Gy vs. 16.9 Gy), mean dose received by 10% volume of LAD (V10%) (36.3 Gy vs. 29.0 Gy) and 30% of LAD artery (V30%) (34.3 Gy vs. 22.2 Gy) (P &lt; 0.001). Conclusion: DIBH significantly decreases the radiation dose delivered to the heart, left lung, and LAD artery. Thus, in patients with left-sided breast cancer, DIBH leads to a lower radiation dose to the OARs, leading to potentially less side-effects.

Introduce a rotational robust optimization framework for spot-scanning proton arc (SPArc) therapy

Objective. Proton dosimetric uncertainties resulting from the patient’s daily setup errors in rotational directions exist even with advanced image-guided radiotherapy techniques. Thus, we developed a new rotational robust optimization SPArc algorithm (SPArcrot) to mitigate the dosimetric impact of the rotational setup error in Raystation ver. 6.02 (RaySearch Laboratory AB, Stockholm, Sweden). Approach. The initial planning CT was rotated ±5° simulating the worst-case setup error in the roll direction. The SPArcrot uses a multi-CT robust optimization framework by taking into account of such rotational setup errors. Five cases representing different disease sites were evaluated. Both SPArcoriginal and SPArcrot plans were generated using the same translational robust optimized parameters. To quantitatively investigate the mitigation effect from the rotational setup errors, all plans were recalculated using a series of pseudo-CT with rotational setup error (±1°/±2°/±3°/±5°). Dosimetric metrics such as D98% of CTV, and 3D gamma analysis were used to assess the dose distribution changes in the target and OARs. Main results. The magnitudes of dosimetric changes in the targets due to rotational setup error were significantly reduced by the SPArcrot compared to SPArc in all cases. The uncertainties of the max dose to the OARs, such as brainstem, spinal cord and esophagus were significantly reduced using SPArcrot. The uncertainties of the mean dose to the OARs such as liver and oral cavity, parotid were comparable between the two planning techniques. The gamma passing rate (3%/3 mm) was significantly improved for CTV of all tumor sites through SPArcrot. Significance. Rotational setup error is one of the major issues which could lead to significant dose perturbations. SPArcrot planning approach can consider such rotational error from patient setup or gantry rotation error by effectively mitigating the dose uncertainties to the target and in the adjunct series OARs.

Preliminary Evaluation of PTV Margins for Online Adaptive Radiation Therapy of the Prostatic Fossa

In modern trials, traditional planning target volume (PTV) margins for postoperative prostate radiation therapy have been large (7-10 mm) to account for both daily changes in patient positioning and target deformation. With daily adaptive radiation therapy, these interfractional changes could be minimized, potentially reducing the margins required for treatment and improving adjacent normal-tissue dosimetry. A single-center retrospective study was conducted from March 2021 to November 2021. Patients receiving conventionally fractionated postoperative radiation therapy (PORT) for prostate cancer with pretreatment and posttreatment cone beam computed tomography (CBCT) imaging (pre-CBCT and post-CBCT, respectively) were included (248 paired images). Pretreatment and posttreatment clinical target volumes (pre-CTVs and post-CTVs) were contoured by a single observer on all CBCTs and verified by a second observer. Motion was calculated from pre-CTV to that of the post-CTV, and predicted margins were calculated with van Herk's formula. Adequate coverage of the proposed planning target volume (PTV) margin expansions (pre-PTV) were verified by determining overlap with post-CTV. In a smaller cohort (25 paired images), dosimetric changes with the proposed online adaptive margins were compared with conventional plans in the Ethos emulator environment. The estimated margins predicted to achieve ≥95% CTV coverage for 90% of the population were 1.6 mm, 2.0 mm, and 2.2 mm (x-, y-, and z -xes, respectively), with 95% of the absolute region of interest displacement being within 1.9 mm, 2.8 mm, and 2.1 mm. After symmetrically expanding all pre-CTVs by 3 mm, the percentage of paired images achieving ≥95% CTV coverage was 97.1%. When comparing adaptive plans (3-mm margins) with scheduled plans (7-mm margins), rectum dosimetry significantly improved, with an average relative reduction in V40Gy[cc] of 59.2% and V65Gy[cc] of 79.5% (where V40Gy and V65Gy are defined as the volumes receiving 40 Gy and 65 Gy or higher dose, respectively). Online daily adaptive radiation therapy could significantly decrease PTV margins for prostatic PORT and improve rectal dosimetry, with a symmetrical expansion of 3 mm achieving excellent coverage in this cohort. These results need to be validated in a larger prospective cohort.

On the feasibility of improved target coverage without compromising organs at risk using online adaptive stereotactic partial breast irradiation (A-SPBI).

Describe an early-adopting institution's experience with online adaptive radiation for stereotactic partial breast irradiation. Retrospective review of 22 women treated between May 2021 and March 2022 with adaptive stereotactic partial breast irradiation. A total of 106 of 110 fractions were evaluated for dosimetric changes in target coverage and organ-at-risk (OAR) dose. Patient set up with stereotactic wooden frame and adapted per fraction. Treatment and planning times were collected prospectively by radiation therapists. Scheduled PTV30Gy was <95% in 72.1% and <90% in 38.5% of fractions, and both PTV and CTV coverage were improved significantly after adaption, and 83.7% of fractions were delivered as adapted per physician choice. There was no difference in OAR coverage. Average adaptive treatment planning took 15min and average time-on-couch was 34.4min. Adaptive stereotactic breast irradiation resulted in improved target coverage with equivalent dosing to OARs in an efficient and tolerated treatment time. Improved target coverage allowed for decreased PTV margins compared to prior trial protocols that may improve acute and late toxicities.

PO23 Presentation Time: 7:40 AM: Dosimetric Effects of Model-Based TG-186 Vaginal Cylinder Treatment Plans

<h3>Purpose</h3> The vaginal cuff is the main location of relapse after curative surgery for early-stage endometrial cancer. Adjuvant vaginal cylinder brachytherapy is increasingly used to decrease the risk of postoperative recurrence due to low vaginal cuff recurrence rates and low gastrointestinal toxicity. Current clinical treatment planning follows TG-43 guidance, which assumes that all material is water. Cylinder composition can vary substantially from water equivalent materials, highlighting the importance of investigations into the dosimetric impact of cylinder composition. In the present study, we evaluated dosimetric characteristics of plans utilizing TG-186, which provides a model-based calculation approach to account for heterogeneities in cylinder configuration. <h3>Materials and Methods</h3> Ten graphically planned vaginal cylinder plans were retrospectively evaluated in this study. Each patient was fitted with an Elekta cylinder (part# 084350) comprised primarily of polysulfone (PSU, 1.29g/cc) and secondarily of stainless steel (8.00g/cc) for the stem and rings. Cylinder diameters ranged from 2.5cm to 3.5cm, and treatment lengths from 4.0cm to 7.5cm. Dose prescription was 7Gy/fx x 3fx at 5mm depth for the most proximal 2cm of the cylinder and at cylinder surface for the remainder of the treatment length. Treatment planning was conducted in Oncentra Brachy 4.5.3. The original TG-43 plans (which assume a water-equivalent material, 1.00g/cc) were recalculated using Oncentra's Advanced Collapsed-Cone Engine (ACE) according to TG-186's guidance to account for differences in density. Dwell times were unchanged, so heterogeneity correction accounts for any dosimetric changes. Points placed at the middle of the 5mm depth portion and the middle of the surface depth portion - prescription dose for TG-43 - were compared, as were rectum points. In addition, all isodose lines were converted to contours and compared for the following metrics: percent volume change, Hausdorff distance (HD), mean distance to agreement (MDA), and Dice coefficient. A paired Wilcoxon signed-rank test assessed for significance. <h3>Results</h3> Recalculated TG-186 plans were found to have consistently and statistically significantly (p<0.05) less dose: 5mm depth: -10.5% +/- 2.4%, surface: -15.9% +/- 3.5%, and rectum: -12.1% +/- 3.4% average dose reduction. Compared to TG-43, the TG-186 100% isodose lines as contours show an average volume decrease of 22.2% +/- 2.5%, HD 4.6mm +/- 1.2mm, MDA 1.5mm +/- 0.3mm, and Dice 0.87 +/- 0.02. The high dose region as defined by 150% of prescription (25.7% volume decrease, HD 4.4mm, MDA 1.3mm) and the low dose region of 50% prescription (19.5% volume decrease, HD 8.2mm, MDA 1.9mm) show similar trends. <h3>Conclusions</h3> Ten TG-43 graphical vaginal cylinder plans were recalculated using TG-186 to account for the material composition of the cylinders. ACE calculations show that the dose delivered to target prescription points and to OARs may be less than originally planned, although the impact on treatment efficacy is presently unclear. Further studies on other cylinder models will be useful to evaluate the generalizability of these findings, as well as any potential effect on treatment.

Do HDR Tandem and Ring Interfractional Displacements Predict the Need to Re-Plan?

<h3>Purpose/Objective(s)</h3> To evaluate the effect of changes in HDR tandem and ring applicator position on doses to nearby organs at risk. <h3>Materials/Methods</h3> CT imaging studies (29) and dosimetry of 6 women ages 44 to 86 undergoing tandem/ring based HDR for cervical cancer were used retrospectively. Measures of applicator positional changes (translation and rotation) relative to the pelvis were obtained using image fusion in a commercial image analysis software. Dosimetric data were calculated in the commercial image analysis software using the original (first fraction) dose distribution applied to organs at risk (rectum and bladder) via deformable registration of the original contours. The new contours were examined visually for appropriateness. Differences in translation and rotation between the planning CT and subsequent individual fractions were recorded and plotted against dose changes between each fraction of treatment and the original (first) fraction. Percent differences in positional shifts vs dosimetric changes were calculated and a correlation coefficient between dose parameters and measured positional shifts of the applicator was determined. <h3>Results</h3> Dosimetric parameters for rectum and bladder (D_2cc and V₅₀) calculated using a commercial treatment planning system for HDR brachytherapy and commercial image analysis software gave similar results within 1% of the original plan (Fraction 1 of 5). A summary of positional changes and average and maximum dose changes for each dosimetric endpoint is found in Table 1. Average percent difference between planned dose and fractional doses obtained through deformable registration were 12%, 16%, 4% and 49% for bladder D_2cc, Bladder V₅₀, Rectum D_2cc and Rectum D₅₀, respectively. Correlation between dose parameters and measured positional changes for bladder D₅₀ and displacement and bladder D₅₀ and sagittal plane rotation gave an r² of 0.3. Applicator displacement of approximately 1.6 times greater than average applicator displacement across all plans studied have shown a significant increase in bladder dose. Bladder dose increased by a maximum of 30 Gy and 54.5 Gy for bladder D_2cc and bladder V₅₀, respectively for the patient with the largest catheter shifts. <h3>Conclusion</h3> We have quantified the effects of applicator positional changes on dose differences for bladder and rectum. Even large changes did not result in significant changes in dose to organs at risk, meaning adaptive re-planning is not required in most cases below a certain displacement threshold.

Osteoradionecrosis rate in oropharynx cancer treated with dose volume histogram based constraints

ObjectivesMandibular dose constraints are designed to limit high dose to small volumes to avoid osteoradionecrosis (ORN). Based upon a published experience, intermediate-dose constraints were introduced but have not been independently validated. We hypothesize that these constraints lower ORN rate without compromising other organs at risk (OAR). MethodsOropharyngeal cancer patients treated with standard fractionation adjuvant/definitive VMAT from 01/2014–08/2020 were included. In 09/2017, mandibular dose constraint was changed from historical constraint (HC) of D 0.1 cc < 70 Gy to modified constraints (MC) of V 44 Gy < 42%, V 58 Gy < 25%, D 0.5 cc < 70 Gy. OAR dosimetric changes and ORN development were evaluated. Regression modelling predicted long-term ORN cases in MC group. ResultsThere were 174 patients, 71 in MC group. Seven cases of ORN in HC group at a median follow up (FU) of 39 months and 1 case of ORN in MC group at a median FU of 11 months were observed. More patients in the MC group met V 44 Gy (87% vs 62%, p < 0.01) and V 58 Gy constraints (92% vs 73%, p < 0.01). Mean doses to OARs did not rise. Mandible V 44 Gy and V 58 Gy were significantly associated with ORN (p < 0.01 and p = 0.03, respectively) across all patients. In the HC group, V 44 Gy was independently associated with ORN (p = 0.01). To account for shorter FU in MC group, logistic regression of ORN based on V 44 Gy in HC patients was performed. This predicts 3.2 ORN cases in the MC group (95% CI: 0.00–6.4). ConclusionAchieving V 44 Gy and V 58 Gy was successful in 87% of cases without sacrificing target coverage or OARs and resulted in non-significant ORN decrease.