Breath Gating Research Articles

Selection of motion management in liver stereotactic body radiotherapy and its impact on treatment time.

Reduction of respiratory tumour motion is important in liver stereotactic body radiation therapy (SBRT) to reduce side effects and improve tumour control probability. We have assessed the distribution of use of voluntary exhale breath hold (EBH), abdominal compression (AC), free breathing gating (gating) and free breathing (FB), and the impact of these on treatment time. We assessed all patients treated in a single institution with liver SBRT between September 2017 and September 2021. Data from pre-simulation motion management assessment using fluoroscopic assessment of liver dome position in repeat breath holds, and motion with and without AC, was reviewed to determine liver dome position consistency in EBH and the impact of AC on motion. Treatment time was assessed for all fractions as time from first image acquisition to last treatment beam off. Of 136 patients treated with 145 courses of liver SBRT, 68% were treated in EBH, 20% with AC, 7% in gating and 5% in FB. AC resulted in motion reduction<1mm in 9/26 patients assessed. Median treatment time was higher using EBH (39min) or gating (42min) compared with AC (30min) or FB (24min) treatments. Motion management in liver SBRT needs to be assessed per-patient to ensure appropriate techniques are applied. Motion management significantly impacts treatment time therefore patient comfort must also be taken into account when selecting the technique for each patient.

Deep inspiration breath-hold radiation therapy in left-sided breast cancer patients: a single-institution retrospective dosimetric analysis of organs at risk doses

BackgroundRadiotherapy can induce cardiac injury in left-sided breast cancer cases. Cardiac-sparing irradiation using the deep inspiration breath-hold (DIBH) technique can achieve substantial dose reduction to vulnerable cardiac substructures compared with free breathing (FB). This study evaluated the dosimetric differences between both techniques at a single institution.MethodsFrom 2017 to 2019, 130 patients with left-sided breast cancer underwent breast-conserving surgery (BCS; n = 121, 93.1%) or mastectomy (ME; n = 9, 6.9%) along with axillary lymph node staging (n = 105, 80.8%), followed by adjuvant irradiation in DIBH technique; adjuvant systemic therapy was included if applicable. 106 (81.5%) patients received conventional and 24 (18.5%) hypofractionated irradiation. Additionally, 12 patients received regional nodal irradiation. Computed tomography (CT) scans in FB and DIBH position were performed for all patients. Intrafractional 3D position monitoring of the patient surface in deep inspiration and breath gating was performed using Sentinel and Catalyst HD 3D surface scanning systems (C-RAD, Catalyst, C‑RAD AB, Uppsala, Sweden). Individual coaching and determination of breathing amplitude during the radiation planning CT was performed. Three-dimensional treatment planning was performed using standard tangential treatment portals (6 or 18 MV). The delineation of cardiac structures and both lungs was done in both the FB and the DIBH scan.ResultsAll dosimetric parameters for cardiac structures were significantly reduced (p < 0.01 for all). The mean heart dose (Dmean) in the DIBH group was 1.3 Gy (range 0.5–3.6) vs. 2.2 Gy (range 0.9–8.8) in the FB group (p < 0.001). The Dmean for the left ventricle (LV) in DIBH was 1.5 Gy (range 0.6–4.5), as compared to 2.8 Gy (1.1–9.5) with FB (p < 0.001). The parameters for LV (V10 Gy, V15 Gy, V20 Gy, V23 Gy, V25 Gy, V30 Gy) were reduced by about 100% (p < 0.001). The LAD Dmean in the DIBH group was 4.1 Gy (range 1.2–33.3) and 14.3 Gy (range 2.4–37.5) in the FB group (p < 0.001). The median values for LAD such as V15 Gy, V20 Gy, V25 Gy, V30 Gy, and V40 Gy decreased by roughly 100% (p < 0.001). An increasing volume of left lung in the DIBH position resulted in dose sparing of cardiac structures.ConclusionFor all ascertained dosimetric parameters, a significant dose reduction could be achieved in DIBH technique.

Uloga stereotaksične radioterapije u lečenju metastaza u jetri kolorektalnog karcinoma

Colorectal cancer is third most common malignant disease and second leading cause of cancer-related deaths worldwide. In 2020, there were 5900 new cases in Serbia and around 3300 number of deaths related to this disease. Metastatic disease is most frequently located in liver. Surgery is first option if complete resection of liver metastases is achievable. Since liver metastases are resectable in 10 - 20% of cases, there is a possibility of implementation of other treatment modality. Alternative for surgery in local treatment of unresectable metastases are stereotactic body radiation therapy (SBRT), interstitial and intraluminal brachytherapy, transarterial chemoembolization, hepatic arterial infusion chemotherapy, selective internal radiation therapy with yttrium-90 resin microspheres, cryoablation, radiofrequency, chemical, and microwave ablation. Candidates for SBRT are patients with unresectable liver metastatic disease and patients with comorbidities which disable surgical treatment, with adequate function of uninvolved liver tissue. Respiration induced motion of target volume can be reduced by introduction of motion management strategies such as infrared markers, deep inspiration breath hold, abdominal compression, respiratory tracking and gating. CyberKnife, TomoTherapy machine and modified linear accelerators are used for delivering SBRT. These units allow us to deliver dose more precisely and to make dose escalation. Different regimes of fractionation are optional, from single fraction to hypo fractionation regimes, and doses are typically around 30 - 60 Gy in 3 fractions. Low toxicity rates in patients with liver metastases treated with SBRT are in relation with precise treatment planning, dose prescription and fractionation. Results of research suggest that delivery of large doses can provide high rate of local response, but on the other hand there is possibility of disease progression out of target volumes. With adequate selection of patients with unresectable liver metastases, the implementation of SBRT, especially in combination with effective systematic treatment modalities, can provide better local control with extension of survival.

Abstract 14294: Quantification Correction for Rapid Myocardial T1ρ Mapping in Mice Using the T1ρ* Relaxation Pathway

Introduction: Cardiac MRI offers detailed tissue characterization and high sensitivity to macromolecules using T 1ρ based imaging. However, fast and accurate T 1ρ mapping in the myocardium is still a major challenge in small animal models. The sequence design based on prospective triggering and breath gating causes a corruption of T 1ρ relaxation leading to quantification errors in the in vivo experiment. In this study, we present a T 1ρ quantification correction using the T 1ρ * relaxation pathway. Methods: A new signal equation was derived by analytical means describing spin-lock prepared fast gradient echo readouts: S(t SL ) = S 0 / [ exp(t SL /T 1ρ ) - cos(α) NR exp(-(N R T R +T rec )/T 1 ) ] The equation was validated in phantom experiments and used in vivo for corrected T 1ρ mapping in mice. Here, the correlation of T 1ρ and the breath dependent spin recovery time T rec was examined. The results were compared to the conventional monoexponential T 1ρ fitting. Results: In the phantom study, the novel signal equation provided improved quantification accuracy compared to monoexponential fitting. The mean quantification error could be reduced from -7.4% to -0.97%. In vivo, a correlation of T 1ρ with the respiratory cycle was observed for monoexponential fitting (PCC=0.804). Using the new correction method, this correlation was reduced (PCC=0.495) and the standard deviation was decreased from ±4.2% to ±1.3%. Conclusions: Our corrected formalism enables fast and accurate T 1ρ quantification for small animals and can improve the comparability of study results. The new technique thus offers a reasonable tool for assessing myocardial diseases. Besides, the derived signal equation could be used for sequence optimization or for subsequent correction of study results.

Application of surface image guided radiotherapy (SIGRT) for deep inspiration breath-hold and free breathing gating technique in Pantai Hospital Kuala Lumpur

Pantai Hospital Kuala Lumpur (PHKL) has integrated Surface Image Guided Radiotherapy (SIGRT), C-RAD Catalyst HD camera in radiotherapy treatment over the 12 months. It eases us in setting up patients before treatment, monitoring intrafraction treatment and allows Deep Inspiration Breath-Hold (DIBH) treatment delivery. Positioning patients using Catalyst gives correctable error in 6-dimension and the shifts were small and acceptable. However, C-RAD is not the gold standard for positioning compared to CBCT but it has the advantage in terms of monitoring patient surface motion during treatment especially DIBH treatments. Target volume coverage for DIBH plan is improved by 0.6% to 1.8% compared to FB plan. This study also compare dose to the heart using DIBH technique over the free breathing (FB) technique for left breast cases. Maximum dose to the heart in DIBH is reduced by 18% compared to FB. Treatment of left breast with DIBH results in significant reductions in doses to the heart, since the distance between the target and the heart is increase compare to FB technique.

Radiation induced cardiotoxicity in left sided breast cancer - Where do we stand?

Breast cancer is the commonest cancer in women, with around a million new cases diagnosed each year worldwide. Adjuvant radiotherapy (RT) is an important component of therapy for many women with early-stage breast cancer. With improving survival rates following breast cancer, patients are increasingly likely to die of other causes. As a result, long-term adverse effects of treatment are of major concern. To determine which treatment is optimal, clinician need to be aware of long-term risks and benefits of adjuvant therapies. An awareness of the potential cardiotoxicity of RT led to the application of improved RT techniques that minimize the irradiation to the heart. Although new techniques, including intensity-modulated RT combined with free breathing gating and helical tomotherapy may further reduce radiation-induced cardiac toxicities, the most important factors in limiting cardiac radiation are associated with the techniques used and the skill of the radiation oncologist.

Passive breath gating equipment for cone beam CT-guided RapidArc gastric cancer treatments

Background and purposeTo report preliminary results of passive breath gating (PBG) equipment for cone-beam CT image-guided gated RapidArc gastric cancer treatments. Material and methodsHome-developed PBG equipment integrated with the real-time position management system (RPM) for passive patient breath hold was used in CT simulation, online partial breath hold (PBH) CBCT acquisition, and breath-hold gating (BHG) RapidArc delivery. The treatment was discontinuously delivered with beam on during BH and beam off for free breathing (FB). Pretreatment verification PBH CBCT was obtained with the PBG-RPM system. Additionally, the reproducibility of the gating accuracy was evaluated. ResultsA total of 375 fractions of breath-hold gating RapidArc treatments were successfully delivered and 233 PBH CBCTs were available for analysis. The PBH CBCT images were acquired with 2–3 breath holds and 1–2 FB breaks. The imaging time was the same for PBH CBCT and conventional FB CBCT (60s). Compared to FB CBCT, the motion artifacts seen in PBH CBCT images were remarkably reduced. The average BHG RapidArc delivery time was 103s for one 270-degree arc and 269s for two full arcs. ConclusionsThe PBG-RPM based PBH CBCT verification and BHG RapidArc delivery was successfully implemented clinically. The BHG RapidArc treatment was accomplished using a conventional RapidArc machine with high delivery efficiency.

Cardiac avoidance in breast radiotherapy: many choices for a worthwhile objective.

In this Research Topic, Goyal and Haffty have collected a series of papers on the emerging field of cardio-oncology. Indeed, Darby et al.’s paper demonstrating an incremental 7% increase in risk of ischemic events per gray increase in mean heart dose has been a watershed moment in our efforts to improve the therapeutic ratio of adjuvant breast radiotherapy (1). While a 0.07 increase over baseline risk per Sievert may seem high, it is important to understand this relative increase in risk in the context of the absolute baseline risk. Darby and colleagues do not provide a denominator for eligible patients in the two population registries from which they drew their cases and controls. Instead, they estimate the baseline risk using data from 15 Western European nations, and in Table S12 of the Supplementary Material, go on to estimate the absolute risk increase by age 80 years in women exposed to RT at various ages and with various co-morbid risk profiles. The excess absolute risks appear to be modest at first glance. For example, for a young 40-year-old woman receiving a high mean heart dose of 10 Gy, the estimated absolute excess risk of dying from cardiac disease is about 1.4%. Should the same woman have at least one co-morbid risk factor, her excess risk is 2.3%. These numbers may seem small, but are certainly relevant at the population level, especially given that the mortality benefit of adjuvant radiotherapy is also modest (2). Current efforts at reducing the risks of incidental cardiac irradiation have included advanced radiotherapy techniques for cardiac avoidance such as breath hold (3), gating treatments (4), proton therapy (5), prone positioning (6), and combinations thereof such as respiratory gating in the prone position (7). Cardiac avoidance techniques are illustrative of the general potential that technological innovations can have on human health. Going back to the very development of megavoltage machines, improvements in radiation delivery have consistently improved the therapeutic ratio in any number of settings. Recent reports have demonstrated fewer late second malignancies in children treated with proton therapy (8), lower rates of desquamation in breast cancer patients treated with IMRT (9), higher rates of local control in lung cancer patients treated with SBRT (10), and improved biochemical control in patients treated with highly conformal, high-dose radiotherapy for prostate cancer (11). Similar improvements in image-guided gynecological brachytherapy (12), IMRT in head/neck (13), GI (14), and gynecological malignancies (15), as well as intracranial SRS (16) have all demonstrated better outcomes compared with control data. Even as the calls for controlling costs become ever more constant, it is important to remember that the current excitement for a genomically driven model of cancer care has become possible only because of technological improvements in sequencing technologies. As such, continued funding, both federal and private, for technology innovations is critical and should not be relegated to lower tiers of priority. Coming back to breast cancer patients and the cardiac risks they face from radiotherapy, one additional (seemingly obvious) point needs to be made. While we can invoke continually advancing technologies for the purposes of cardiac avoidance (17), sometimes a return to simpler solutions may be all that is needed. Many women with early-stage breast cancer are eligible for off-protocol accelerated partial breast irradiation as a standard of care option (18). As one would expect, irradiating a smaller volume of breast tissue leads to lower incidental heart doses (19). Current studies and protocols examining, for example, breath hold parameters or prone positioning often include a large contingent of women who are candidates for partial breast irradiation. One rather elegant way to avoid treating the heart is to simply not treat it.

In the context of radiosurgery – Pros and cons of rescanning as a solution for treating moving targets with scanned particle beams

The treatment of mobile targets with scanned particle beams is challenging, and the effects of motion will be especially pronounced in hypo-fractionated treatment regimes due to the lack of statistical smoothing through fractionation and the prolonged delivery times per session. Therefore, motion mitigation techniques will play a major role for radiosurgery approaches. This article concentrates on the motion mitigation technique called rescanning. It alludes the existence of many scanning/rescanning flavors and raises awareness of the importance of an optimized flavor choice. Furthermore, it is discussed that rescanning can compensate for the lack of statistical wash-out, target dose conformity, however, will remain degraded. Therefore, especially in the context of radiosurgery, rescanning should be combined with other motion mitigation techniques like breath hold, gating and/or tracking.

Technical concepts for vascular electromagnetic navigated interventions: Aortic in situ fenestration and transjugular intrahepatic porto-systemic shunts

This work presents concepts for complex endovascular procedures using electromagnetic navigation technology (EMT). Navigation software interfacing a standard commercially available navigation system was developed, featuring registration, electromagnetic field distortion correction, breathing motion detection and gating, and state-of-the-art 3D imaging post processing. Protocols for endovascularly placed, in-situ fenestrated abdominal aortic stent grafts and an EMT guided transjugular intrahepatic portosystemic shunt (TIPSS) creation have been designed. A dedicated set of interventional devices was developed for each of the procedures: For aortic in-situ fenestration a combination of high-porosity stentgrafts, steerable catheters and electromagnetically navigated guidewires was used, for TIPSS a dual-navigated (sheath and stylet) TIPSS-device was designed and manufactured. The developed devices underwent phantom testing, in preparation for animal experiments to prove the feasibility of the approach. Once established, these systems could aid in performing these challenging interventional radiology procedures, exploiting the unique characteristics of electromagnetic navigation and solving multiple of the problems associated with these interventions being performed under X-ray fluoroscopy, such as lacking real-time 3D information or extensive exposure to ionizing radiation.

Kidney motion during free breathing and breath hold for MR-guided radiotherapy

Current treatments for renal cell carcinoma have a high complication rate due to the invasiveness of the treatment. With the MRI-linac it may be possible to treat renal tumours non-invasively with high-precision radiotherapy. This is expected to reduce complications. To deliver a static dose distribution, radiation gating will be used. In this study the reproducibility and efficiency of free breathing gating and a breath hold treatment of the kidney was investigated. For 15 patients with a renal lesion the kidney motion during 2 min of free breathing and 10 consecutive expiration breath holds was studied with 2D cine MRI. The variability in kidney expiration position and treatment efficiency for gating windows of 1 to 20 mm was measured for both breathing patterns. Additionally the time trend in free breathing and the variation in expiration breath hold kidney position with baseline shift correction was determined. In 80% of the patients the variation in expiration position during free breathing is smaller than 2 mm. No clinically relevant time trends were detected. The variation in expiration breath hold is for all patients larger than the free breathing expiration variation. Gating on free breathing is, for gating windows of 1 to 5 mm more efficient than breath hold without baseline correction. When applying a baseline correction to the breath hold it increases the treatment efficiency. The kidney position is more reproducible in expiration free breathing than non-guided expiration breath hold. For small gating windows it is also more time efficient. Since free breathing also seems more comfortable for the patients it is the preferred breathing pattern for MRI-Linac treatments of the kidney.

Four dimensional radiotherapy: a review of current technologies and modalities

Organ motion is a substantial concern in the treatment of thoracic tumours using radiotherapy. A number of technologies have evolved in order to address this concern in both the fields of CT imaging and radiation delivery. This review paper investigates the technologies which have been developed for the delivery of radiotherapy as well as the accuracy and workload implications of their use. Treatment techniques investigated include: breath hold, breath gating, robotic compensation and MLC manipulation. Each technique has its own advantages and drawbacks in regards to accuracy, treatment time, linac alterations and workload. Further, some treatment techniques have specific requirements for what kind of CT scans needs to be used in the planning process. This, along with the aforementioned considerations, could influence the decision as to implement some of these treatment techniques in the clinic.

Implement of Passive Breath Gating Equipment for Cone Beam CT Guided Volumetric Modulated Radiation Therapy in Gastric Cancer Treatment: Preliminary Results

Implement of Passive Breath Gating Equipment for Cone Beam CT Guided Volumetric Modulated Radiation Therapy in Gastric Cancer Treatment: Preliminary Results

SU-E-J-131: Implement a Passive Breath Gating Equipment for Gated CBCT Imaging: A Feasibility Study.

To investigate the feasibility of using a patient passive breath gating (PBG) equipment in kilo-voltage cone-beam CT (kVCBCT) image acquisition to reduce the respiratory induced motion artifacts for image-guided radiotherapy. A PBG equipment developed in our department was used to passively block patient's breathing. Assisted by this device, patients were able to hold their breaths for 15-25 seconds. An infrared block was placed on the patient's body and the breath-hold was triggered by the signal from the Varian RPM system. Three sets of kVCBCT images were acquired using the same imaging parameters with the OBI system on Varian Trilogy for a gastric cancer patient: free breathing (FB), with two-breath-hold (BH1) and three-breath-hold (BH2). Patient was allowed to breathe normally for 5-10s between breath-hold. A planning CT was obtained with one breath-hold on a conventional 4D CT scanner. The diaphragm position on FB, BH1 and BH2 CBCT images was compared with its position on the planning CT to assess the motion artifacts. The acquisition time for each of the three kVCBCTs was 60 seconds. The patient tolerated the breath-hold CBCT scanning well. Severe image blurriness and streak artifacts caused by respiratory motion were observed near the diaphragm on FB CBCT. However, for the breath-hold CBCT images, the motion artifacts were reduced and the diaphragm edge was restored. Two-breath-hold scan (BH1) produced better image quality than three-breath-hold (BH2). The details of the diaphragm edge on BH1 images were comparable to the planning CT. Diaphragm motion causes image distortion in FB CBCT. The PBG can be used in kVCBCT acquisition to reduce motion artifacts. The breath-hold kVCBCT can produce higher image quality for image-guided radiotherapy and adaptive radiotherapy in the thorax and abdominal regions. Conflict of Interest (only if applicable): Research supported by Varian Medical System. This work is partly supported by Varian Medical System.

A real-time respiration position based passive breath gating equipment for gated radiotherapy: A preclinical evaluation

To develop a passive gating system incorporating with the real-time position management (RPM) system for the gated radiotherapy. Passive breath gating (PBG) equipment, which consists of a breath-hold valve, a controller mechanism, a mouthpiece kit, and a supporting frame, was designed. A commercial real-time positioning management system was implemented to synchronize the target motion and radiation delivery on a linear accelerator with the patient's breathing cycle. The respiratory related target motion was investigated by using the RPM system for correlating the external markers with the internal target motion while using PBG for passively blocking patient's breathing. Six patients were enrolled in the preclinical feasibility and efficiency study of the PBG system. PBG equipment was designed and fabricated. The PBG can be manually triggered or released to block or unblock patient's breathing. A clinical workflow was outlined to integrate the PBG with the RPM system. After implementing the RPM based PBG system, the breath-hold period can be prolonged to 15-25 s and the treatment delivery efficiency for each field can be improved by 200%-400%. The results from the six patients showed that the diaphragm motion caused by respiration was reduced to less than 3 mm and the position of the diaphragm was reproducible for difference gating periods. A RPM based PBG system was developed and implemented. With the new gating system, the patient's breath-hold time can be extended and a significant improvement in the treatment delivery efficiency can also be achieved.

Manifold learning for image-based breathing gating in ultrasound and MRI

Respiratory motion is a challenging factor for image acquisition and image-guided procedures in the abdominal and thoracic region. In order to address the issues arising from respiratory motion, it is often necessary to detect the respiratory signal. In this article, we propose a novel, purely image-based retrospective respiratory gating method for ultrasound and MRI. Further, we apply this technique to acquire breathing-affected 4D ultrasound with a wobbler probe and, similarly, to create 4D MR with a slice stacking approach. We achieve the gating with Laplacian eigenmaps, a manifold learning technique, to determine the low-dimensional manifold embedded in the high-dimensional image space. Since Laplacian eigenmaps assign to each image frame a coordinate in low-dimensional space by respecting the neighborhood relationship, they are well suited for analyzing the breathing cycle. We perform the image-based gating on several 2D and 3D ultrasound datasets over time, and quantify its very good performance by comparing it to measurements from an external gating system. For MRI, we perform the manifold learning on several datasets for various orientations and positions. We achieve very high correlations by a comparison to an alternative gating with diaphragm tracking.

The Interplay Effect in Gated and Non-gated IMRT for Breast Cancer: Impact of Breathing Motion and Gating Parameters

The Interplay Effect in Gated and Non-gated IMRT for Breast Cancer: Impact of Breathing Motion and Gating Parameters

WE-E-202-01: Virtual Simulator CT QA

The virtual CT simulation process is a vital component of radiation oncology planning . It is during this step that a patient's CT scan is performed, which is then transferred to a virtual simulation system. Depending on the vendor and configuration the system may be used for applications such as image registration, structure delineation, field arrangement and anatomical measurements. A robust quality assurance program ensures that images and structures created during this step are rendered and transferred accurately. Practical methods of performing standard QA tests which evaluate parameters such as image uniformity, resolution, and distortion will be presented. New technology has brought multi slice scanners, PET‐CT scanners, breath gating systems and image fusion. Tests that address the quality and accuracy of these newer technologies will be also presented. SRS and SBRT require an enhanced arsenal of QA tools due to a superior requirement in spatial and anatomical accuracy. Tests will be described that address these requirements as well. Learning Objectives: 1. Explain how to perform tests that evaluate certain aspects of Virtual Simulator QA 2. Describe the components of the Virtual CT simulation process that require testing 3. Describe the effects that poor a QA program could have on treatment planning

SU-FF-J-104: Feasibility of Utilizing Pneumatic Abdominal Bellows for Quantitative 4DCT

Purpose: To determine if a breathing surrogate can be utilized in place of a spirometer for quantifying 4DCT. Method and Materials: Using an established four dimensional computed tomography (4DCT) protocol, forty one patient data sets were acquired while measuring the breathing cycle using both spirometry and a pneumatic abdominal bellows. The image datasets were segmented to find the total lung volume (air content) and the bellows internal pressure signal was normalized into tidal volume for the entire scan session by correlating against the spirometry data subdivided into sequential 18.75 s periods. A threshold tolerance of 2% in the correlation of each 18.75s period was applied to ignore times where the spirometry apparatus leaked or failed to record meaningful data. This threshold provided a conservative criterion in identifying regions of abnormal spirometry response. The tidal volume was compared against the air content to validate the normalized bellows signal. Results: The 2% threshold correlation excluded the time period where the spirometry signal was unreliable. Overall, 86% of the time periods are within the 2% threshold, providing sufficient data to determine the relationship between tidal volume and air content. The measured ratio of air content to tidal volume was 1.11±0.08 for all patients. The theoretical value is 1.11 based on the air density differences between room air and air inside the lungs.Conclusion: The bellows and spirometry data correlated well when the spirometry apparatus was functioning properly. The ratio of air content to spirometry‐measured tidal volume was exactly that expected using the ratio of air densities inside and outside the lungs. These data support the use of a non‐spirometry method of breathing gating such as the pneumatic bellows as long as the entire lungs are scanned to provide the air content data. This work supported in part by NIHR01CA116712 and NIHR01CA96679.

WE-C-AUD B-10: Which Patients Benefit From Prospective Gating in Treatment for Loco-Regional Breast Irradiation - a Radiobiological Assessment

Purpose: To determine a selection procedure based on radiobiological parameters to triage patients that benefit maximally from the use of free breathing gating in inspiration during treatment of the breast, internal mammary and medial supraclavicular lymph nodes. Methodology: Twenty patients undergoing external treatment to the breast were prospectively selected in this study, 10 patients with involvement in the right breast, and 10 in the left‐hand side. All patients underwent a classical non‐gated CT scan, followed by a CT scan using prospective gating with coached breathing. To perform prospective gating a large bore Siemens Sensation Open™ was utilized, adapted to work with the Varian RPM™ gating system, which is also installed on our Varian Linacs. For each patient, dose distributions for the same field setup were calculated on each CT‐scan. Differential DVH's for heart and lung, were analyzed providing an NTCP using a serial model with parameters: s=1, gamma=1.28, and D50=52.3 for the heart (predicting an increased risk of late cardiac mortality), and s=0.06, gamma=0.90, and D50=34.0 for the lung respectively (predicting clinical radiation pneumonitis). Results: The mean NTCP for the heart for all patients was 0.80% for a non‐‐gated treatment and was reduced to 0.26% in gated treatments. This improvement was significant (p=0.0008). Five patients had a reduction of NTCP of 1% or more for heart complications. All of these patients were treated to the left breast, but not all left breast patients seemed to benefit. For lung NTCP overall improvement (p=0.0006) could be shown, reducing mean NTCP from 2.1% to 1.4%. Conclusion: We have shown that it is possible to have an objective criterium to apply free breathing gating for breast cancer patients resulting in an effective and economically acceptable use of machine time for gating.