Verification CTs Research Articles

Pencil beam scanning proton therapy for mediastinal lymphomas in deep inspiration breath-hold: a retrospective assessment of plan robustness.

The aim of this study was to evaluate pencil beam scanning (PBS) proton therapy (PT) in deep inspiration breath-hold (DIBH) for mediastinal lymphoma patients, by retrospectively evaluating plan robustness to the clinical target volume (CTV) and organs at risk (OARs) on repeated CT images acquired throughout treatment. Methods: Sixteen mediastinal lymphoma patients treated with PBS-PT in DIBH were included. Treatment plans (TPs) were robustly optimized on the CTV (7 mm/4.5%). Repeated verification CTs (vCT) were acquired during the treatment course, resulting in 52 images for the entire patient cohort. The CTV and OARs were transferred from the planning CT to the vCTs with deformable image registration and the TPs were recalculated on the vCTs. Target coverage and OAR doses at the vCTs were compared to the nominal plan. Deviation in lung volume was also calculated. The TPs demonstrated high robust target coverage throughout treatment with D98%,CTV deviations within 2% for 14 patients and above the desired requirement of 95% for 49/52 vCTs. However, two patients did not achieve a robust dose to CTV due to poor DIBH reproducibility, with D98%,CTV at 78 and 93% respectively, and replanning was performed for one patient. Adequate OAR sparing was achieved for all patients. Total lung volume variation was below 10% for 39/52 vCTs. PBS PT in DIBH is generally a robust technique for treatment of mediastinal lymphomas. However, closely monitoring the DIBH-reproducibility during treatment is important to avoid underdosing CTV and achieve sufficient dose-sparing of the OARs.

Daily Head and Neck Treatment Assessment for Optimal Proton Planning Robustness

Daily setup variation can affect the quality of treatment and robust optimization is used to ensure adequate target coverage. We aimed to assess quality assurance by evaluating fractional plan quality and its impact on plan robustness. Eligible head and neck plans with full field of view cone beam CT (CBCT) for >75% of daily fractions were classified into 2 categories 1) Primary only (PO) coverage and 2) Primary and neck nodal (PNN) coverage. Registration used at the machine was utilized for each daily CBCT to generate a synthetic CT (sCT), and the clinical target volumes (CTVs) and organs at risk (OARs) were manually edited by the reviewing physician. The daily dose for the CTVs and OARs were compared to the expected range from the robustness bands using 3.5% range uncertainty and 3mm versus 5mm setup uncertainty, fractional error was defined as volume and dose outside of uncertainty constraints. Statistical significance was calculated using students t-test. There was 203 eligible daily fractions from 6 patients were included for analysis. The average percentage of fractions that deviated from planning CTV volumes as well as average error in delivered dose using 3mm and 5mm robustness can be seen in (Table 1). In PO plans, there was minimal change in percentage of fractions that deviated from planned CTV volumes and overall low average error in delivered dose at both 3mm and 5mm robustness, 0.35% and 0.26%, respectively. In comparison, PNN plans had greater percentage of fractions that deviated from planning CTV and consequently greater average error in delivered dose, 1.00% versus 0.37% at 3mm and 5mm robustness, respectively. There was a significant difference in the percentage of fractional error with 3mm robustness between PO patients, 0.22%, and PNN patients, 0.98%, p<0.01. No statistical difference in percentage of fractional error was found with 5mm robustness for PO and PNN patients, 0.24% and 0.52%, respectively. All OARS maintained mean daily dose within or below 3mm robustness calculations. Based on our analysis, there was a difference in error of delivered dose between 3mm and 5mm robustness for head and neck proton plans, particularly for PNN coverage. Clinically, up to 1% error to CTV volumes is often not significantly meaningful and treating centers should consider utilizing 3mm robustness to maximize OAR sparing while maintaining proper target coverage. sCTs analysis is important for determining planning guidelines and may be able to identify patients needing an adaptive plan instead of currently utilized verification CTs.

Dose evaluation of inter- and intra-fraction prostate motion in extremely hypofractionated intensity-modulated proton therapy for prostate cancer.

Inter- and intra-fractional prostate motion can deteriorate the dose distribution in extremely hypofractionated intensity-modulated proton therapy. We used verification CTs and prostate motion data calculated from 1024 intra-fractional prostate motion records to develop a voxel-wise based 4-dimensional method, which had a time resolution of 1s, to assess the dose impact of prostate motion. An example of 100 fractional simulations revealed that motion had minimal impact on planning dose, the accumulated dose in 95% of the scenarios fulfilled the clinical goals for target coverage (D95>37.5Gy). This method can serve as a complementary measure in clinical setting to guarantee plan quality.

Deep-learning based fast and accurate 3D CT deformable image registration in lung cancer.

Deformable Image Registration (DIR) is an essential technique required in many applications of radiation oncology. However, conventional DIR approaches typically take several minutes to register one pair of 3D CT images and the resulting deformable vector fields (DVFs) are only specific to the pair of images used, making it less appealing for clinical application. A deep-learning-based DIR method using CT images is proposed for lung cancer patients to address the common drawbacks of the conventional DIR approaches and in turn can accelerate the speed of related applications, such as contour propagation, dose deformation, adaptive radiotherapy (ART), etc. METHODS: A deep neural network based on VoxelMorph was developed to generate DVFs using CT images collected from 114 lung cancer patients. Two models were trained with the weighted mean absolute error (wMAE) loss and structural similarity index matrix (SSIM) loss (optional) (i.e., the MAE model and the M+S model). In total, 192 pairs of initial CT (iCT) and verification CT (vCT) were included as a training dataset and the other independent 10 pairs of CTs were included as a testing dataset. The vCTs usually were taken 2weeks after the iCTs. The synthetic CTs (sCTs) were generated by warping the vCTs according to the DVFs generated by the pre-trained model. The image quality of the synthetic CTs was evaluated by measuring the similarity between the iCTs and the sCTs generated by the proposed methods and the conventional DIR approaches, respectively. Per-voxel absolute CT-number-difference volume histogram (CDVH) and MAE were used as the evaluation metrics. The time to generate the sCTs was also recorded and compared quantitatively. Contours were propagated using the derived DVFs and evaluated with SSIM. Forward dose calculations were done on the sCTs and the corresponding iCTs. Dose volume histograms (DVHs) were generated based on dose distributions on both iCTs and sCTs generated by two models, respectively. The clinically relevant DVH indices were derived for comparison. The resulted dose distributions were also compared using 3D Gamma analysis with thresholds of 3mm/3%/10% and 2mm/2%/10%, respectively. The two models (wMAE and M+S) achieved a speed of 263.7±163 / 265.8±190ms and a MAE of 13.15±3.8 / 17.52±5.8 HU for the testing dataset, respectively. The average SSIM scores of 0.987±0.006 and 0.988±0.004 were achieved by the two proposed models, respectively. For both models, CDVH of a typical patient showed that less than 5% of the voxels had a per-voxel absolute CT-number-difference larger than 55 HU. The dose distribution calculated based on a typical sCT showed differences of ≤2cGy[RBE] for clinical target volume (CTV) D95 and D5 , within ±0.06% for total lung V5 , ≤1.5cGy[RBE] for heart and esophagus Dmean , and ≤6cGy[RBE] for cord Dmax compared to the dose distribution calculated based on the iCT. The good average 3D Gamma passing rates (>96% for 3mm/3%/10% and>94% for 2mm/2%/10%, respectively) were also observed. A deep neural network-based DIR approach was proposed and has been shown to be reasonably accurate and efficient to register the initial CTs and verification CTs in lung cancer.

Improving workflow for adaptive proton therapy with predictive anatomical modelling: A proof of concept

PurposeTo demonstrate predictive anatomical modelling for improving the clinical workflow of adaptive intensity-modulated proton therapy (IMPT) for head and neck cancer. Methods10 radiotherapy patients with nasopharyngeal cancer were included in this retrospective study. Each patient had a planning CT, weekly verification CTs during radiotherapy and predicted weekly CTs from our anatomical model. Predicted CTs were used to create predicted adaptive plans in advance with the aim of maintaining clinically acceptable dosimetry. Adaption was triggered when the increase in mean dose (Dmean) to the parotid glands exceeded 3 Gy(RBE). We compared the accumulated dose of two adaptive IMPT strategies: 1) Predicted plan adaption: One adaptive plan per patient was optimised on a predicted CT triggered by replan criteria. 2) Standard replan: One adaptive plan was created reactively in response to the triggering weekly CT. ResultsStatistical analysis demonstrates that the accumulated dose differences between two adaptive strategies are not significant (p > 0.05) for CTVs and OARs. We observed no meaningful differences in D95 between the accumulated dose and the planned dose for the CTVs, with mean differences to the high-risk CTV of −1.20 %, −1.23 % and −1.25 % for no adaption, standard and predicted plan adaption, respectively. The accumulated parotid Dmean using predicted plan adaption is within 3 Gy(RBE) of the planned dose and 0.31 Gy(RBE) lower than the standard replan approach on average. ConclusionPrediction-based replanning could potentially enable adaptive therapy to be delivered without treatment gaps or sub-optimal fractions, as can occur during a standard replanning strategy, though the benefit of using predicted plan adaption over the standard replan was not shown to be statistically significant with respect to accumulated dose in this study. Nonetheless, a predictive replan approach can offer advantages in improving clinical workflow efficiency.

Composite minimax robust optimization of VMAT improves target coverage and reduces non-target dose in head and neck cancer patients

Background and purposeTo assess the potential of composite minimax robust optimization (CMRO) compared to planning target volume (PTV)-based optimization for head and neck cancer (HNC) patients treated with volumetric modulated arc therapy (VMAT). Materials and methodsTen HNC patients previously treated with a PTV-based VMAT plan were studied. In addition to the PTV-plan a VMAT plan was created with CMRO. For both plans an adapted planning strategy was also investigated, including a plan adaptation during the third week of treatment. The PTV-plans and CMRO-plans (adapted and non-adapted) were evaluated by means of the estimated actually given dose (EAGD). Therefore, the dose was calculated on daily acquired CBCTs, mapped onto the planning CT and accumulated. The plans were compared by dosimetric parameters and normal tissue complication probabilities (NTCPs) for tube feeding dependence, grade 2–4 dysphagia and xerostomia. The accuracy of CBCT-based dose accumulation was further quantified by comparisons of dose accumulation on weekly verification CTs. ResultsOn average, CMRO significantly increased (1.5 Gy) the D98% of the EAGD to the clinical target volume and significantly decreased the mean dose of the ipsilateral parotid (2.8 Gy), inferior pharynx constrictor muscle (0.7 Gy) and the oral cavity (0.8 Gy). This translated into significantly reduced NTCP of tube feeding dependence (0.9%) and xerostomia (2.8%). The differences in EAGD derived from evaluation CTs or CBCTs were minimal. ConclusionMinimax robust optimization led to improved target coverage and dose reduction in organs at risk in HNC patients treated with VMAT.

Proton pencil beam scanning for mediastinal lymphoma: treatment planning and robustness assessment

ABSTRACTBackground: Modern radiotherapy (RT) for lymphoma is highly personalized. While advanced imaging is largely employed to define limited treatment volumes, the use of proton pencil beam scanning (PBS) for highly conformal lymphoma RT is still in its infancy. Here, we assess the dosimetric benefits and feasibility of PBS for mediastinal lymphoma (ML).Materials and methods: Ten patients were planned using PBS for involved-site RT. The initial plans were calculated on the average four-dimensional computed tomography (4D-CT). PBS plans were compared with 3D conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and proton double scattering (DS). In order to evaluate the feasibility of PBS and the plan robustness against inter- and intra-fractional uncertainties, the 4D dose was calculated on initial and verification CTs. The deviation of planned dose from delivered dose was measured. The same proton beamline was used for all patients, while another beamline with larger spots was employed for patients with large motion perpendicular to the beam.Results: PBS provided the lowest mean lung dose (MLD) and mean heart dose (MHD) for all patients in comparison with 3D-CRT, IMRT, and DS. For eight patients, internal target volume (ITV) D98% was degraded by <3%; and the MLD and MHD deviated by <10% of prescription over the course of treatment when the PBS field was painted twice in each session. For one patient with target motion perpendicular to the beam (>5 mm), the degradation of ITV D98% was 9%, which was effectively mitigated by employing large spots. One patient exhibited large dose degradation due to pericardial effusion, which required replanning across all modalities.Conclusions: This study demonstrates that PBS plans significantly reduce MLD and MHD relative to 3D-CRT, IMRT, and DS and identifies requirements for robust free-breathing ML PBS treatments, showing that PBS plan robustness can be maintained with repainting and/or large spots.

SU‐E‐T‐42: A Method to Determine the Optimal Proton Scanned Beam Angle to Ensure Robust Treatment Planning

Purpose:The ability of pencil beam scanning (PBS) to deliver highly conformal dose distributions may be affected by patient‐ and physics‐related uncertainties. In clinical practice, selection of proton beam angles is determined qualitatively. This study investigates whether an optimal proton PBS beam angle could be quantitatively determined to ensure robust planning for pelvic targets.Methods:PBS beam angles were optimized based on two independent criteria; shortest and most homogeneous path from the patient surface to the distal edge of the target. The beam angle optimization criteria for gantry angles between 90°‐270° were quantified in 10° increments for each ray, calculated as the straight line distance from the surface of the skin to the CTV's distal edge. The goal was to minimize the path length of a proton PBS beam from the patient surface to the distal edge of the CTV, relative to the entry point, while minimizing HU inhomogeneity along the ray. HU homogeneity (i.e. HU variation) was quantitatively defined as the standard deviation of the average intra‐ray HU intensity distribution of the rays comprising a single beam. This method was validated relative to inter‐fraction changes on ten consecutive, locally advanced, rectal cancer patients, who underwent an average 4 verification CTs. The displacement of the 95–98% isodose lines was determined from forward calculated dose distributions on verification CTs.Results:The posterior beam (180°) had the average shortest path length, 132.7±17.2mm, and the most homogenous path, 31.9±4.3HU. The 95–98% isodose lines from all plans verified our path length to within 2.3±1.2% and HU homogeneity to within 1.2±0.5%.Conclusion:The proposed optimization algorithm determined the posterior beam dose distribution as the most robust relative to inter‐fraction variation for large pelvic targets treated with PBS and was validated via verification CT for our patient cohort. Future work will focus on further algorithm development.

TU-F-CAMPUS-J-01: Dosimetric Effects of HU Changes During the Course of Proton Therapy for Lung Cancer

Purpose: To characterize the changes in Hounsfield unit (HU) in lung radiotherapy with proton beams during the course of treatment and to study the effect on the proton plan dose distribution. Methods: Twenty consecutive patients with non-small cell lung cancer treated with proton radiotherapy who underwent multiple CT scans including the planning CT and weekly verification CTs were studied. HU histograms were computed for irradiated lung volumes in beam paths for all scans using the same treatment plan. Histograms for un-irradiated lung volume were used as control to characterize inter-scan variations. HU statistics were calculated for both irradiated and un-irradiated lung volumes for each patient scan. Further, multiple CT scans based on the same planning CT were generated by replacing the HU of the lung based on the verification CT scans HU values. Using the same beam arrangement, we created plans for each of the altered CT scans to study the dosimetric effect using the dose volume histogram. Results: Lung HU decreased for irradiated lung volume during the course of radiotherapy. The magnitude of this change increased with total irradiation dose. On average, HU changed by −53.8 in the irradiated volume. This change resulted in less than 0.5mm of beam overshoot in tissue for every 1cm beam traversed in the irradiated lung. The dose modification is about +3% for the lung, and less than +1% for the primary tumor. Conclusion: HU of the lung decrease throughout the course of radiation therapy. This change results in a beam overshoot (e.g. 3mm for 6cm of lung traversed) and causes a small dose modification in the overall plan. However, this overshoot does not affect the quality of plans since the margins used in planning, based on proton range uncertainty, are greater. HU needs to change by 150 units before re-planning is warranted.

Prevention of needle displacement in multifraction high-dose-rate prostate brachytherapy: A prospective volumetric analysis and technical considerations

PurposeWe conducted a prospective study to evaluate needle displacements between prostate high-dose-rate (HDR) brachytherapy fractions and offer technical recommendations to help prevent displacements from the outset. Methods and materialsPlanning computed tomography and verification computed tomography scans were obtained at 1-mm slice thickness and prospectively assessed for interfraction needle movement for each fraction of a 2-fraction HDR prostate boost. For both the planning and verification CTs, distances from each needle tip to the centroid of 3 implanted prostate gold seeds were measured. We determined the mean and range of the displacement distances. ResultsThirty-three consecutive patients (66 fractions, 540 needle-pair positions for a total of 1080 needles) were evaluated for changes in the length between the needle tip and centroid displacement. Overall, only 0.2% of the needles had any change greater than 3.5 mm between the needle tip and centroid. The mean amount of displacement was 0.97 mm, with a standard deviation of 0.76 mm. Among the patients, no fraction had more than 1 needle with a variation greater than 3.0 mm. ConclusionsNeedle displacements in HDR prostate brachytherapy have been reported by numerous institutions using various techniques. We report the first study to demonstrate needle displacement of less than 1 mm on average, and we describe our process of care surrounding the implantation.

Impact of Intrafraction and Residual Interfraction Effect on Prostate Proton Pencil Beam Scanning

To quantitatively evaluate the impact of interplay effect and plan robustness associated with intrafraction and residual interfraction prostate motion for pencil beam scanning proton therapy. Ten prostate cancer patients with weekly verification CTs underwent pencil beam scanning with the bilateral single-field uniform dose (SFUD) modality. A typical field had 10-15 energy layers and 500-1000 spots. According to their treatment logs, each layer delivery time was <1 s, with average time to change layers of approximately 8 s. Real-time intrafraction prostate motion was determined from our previously reported prospective study using Calypso beacon transponders. Prostate motion and beam delivering sequence of the worst-case scenario patient were synchronized to calculate the "true" dose received by the prostate. The intrafraction effect was examined by applying the worst-case scenario prostate motion on the planning CT, and the residual interfraction effect was examined on the basis of weekly CT scans. The resultant dose variation of target and critical structures was examined to evaluate the interplay effect. The clinical target volume (CTV) coverage was degraded because of both effects. The CTV D99 (percentage dose to 99% of the CTV) varied up to 10% relative to the initial plan in individual fractions. However, over the entire course of treatment the total dose degradation of D99 was 2%-3%, with a standard deviation of <2%. Absolute differences between SFUD, intensity modulate proton therapy, and one-field-per-day SFUD plans were small. The intrafraction effect dominated over the residual interfraction effect for CTV coverage. Mean dose to the anterior rectal wall increased approximately 10% because of combined residual interfraction and intrafraction effects, the interfraction effect being dominant. Both intrafraction and residual interfraction prostate motion degrade CTV coverage within a clinically acceptable level. One-field-per-day SFUD delivered twice is as robust as the bilateral SFUD plan treated daily over the course of treatment.

SU-E-J-229: Quantitative Assessment for Timely Adaptive Re-Planning Using Weekly Dose Monitoring for Head and Neck Cancer

Purpose: For patients with head and neck (HN) cancer, mid-course adaptive radiation therapy (ART) is a common practice in our institution to accommodate anatomic changes. The aim of the study is to evaluate whether dose re-calculation on weekly verification images can provide quantitative assessment for timely adaptive re-planning with daily image-guided intensity modulated radiotherapy (IMRT). Methods: We retrospectively selected sixty daily verification images acquired on CT-on-rail/CBCT from ten HN patients. These image sets were typically a week apart. Among these patients, six patients received a mid-course ART. Contours of the tumors and organ-at-risks (OARs) were manually delineated by a physician on each verification CT. After placing the treatment iso-center on the verification CTs according to the recorded clinical shifts, daily dose was re-calculated with the same beam configuration as the original plan. For the purpose of this study, electron densities for both verification CTs and planning CTs were set to 1.0 g/cm3. Results: Two patients had D99 of the CTV 45 Gy, two out ten patients required ART at week 2 and two patients required ART at week 3, respectively. Conclusion: Weekly dose monitoring with re-calculation on verification images can provide quantitative dose guidance for timely adaptive re-planning. Future work will include accumulative dose analysis for the decision of adaptive re-planning. The study is supported in part by Siemens Medical Solutions.« less

SU-E-J-245: Is Off-Line Adaptive Radiotherapy Sufficient for Head and Neck Cancer with IGRT?

Purpose: Radiation doses delivered to patients with head and neck cancer (HN) may deviate from the planned doses because of variations in patient setup and anatomy. This study was to evaluate whether off-line Adaptive Radiotherapy (ART) is sufficient. Methods: Ten HN patients, who received IMRT under daily imaging guidance using CT-on-rail/KV-CBCT, were randomly selected for this study. For each patient, the daily treatment setup was corrected with translational only directions. Sixty weekly verification CTs were retrospectively analyzed. On these weekly verification CTs, the tumor volumes and OAR contours were manually delineated by a physician. With the treatment iso-center placed on the verification CTs, according to the recorded clinical shifts, the treatment beams from the original IMRT plans were then applied to these CTs to calculate the delivered doses. The electron density of the planning CTs and weekly CTs were overridden to 1 g/cm3. Results: Among 60 fractions, D99 of the CTVs in 4 fractions decreased more than 5% of the planned doses. The maximum dose of the spinal cord exceeded 10% of the planned values in 2 fractions. A close examination indicated that the dose discrepancy in these 6 fractions was due to patient rotations, especially shoulder rotations. After registeringmore » these 6 CTs with the planning CT allowing six degree of freedoms, the maximum rotations around 3 axes were > 1.5° for these fractions. With rotation setup errors removed, 4 out of 10 patients still required off-line ART to accommodate anatomical changes. Conclusion: A significant shoulder rotations were observed in 10% fractions, requiring patient re-setup. Off-line ART alone is not sufficient to correct for random variations of patient position, although ART is effective to adapt to patients' gradual anatomic changes. Re-setup or on-line ART may be considered for patients with large deviations detected early by daily IGRT images. The study is supported in part by Siemens Medical Solutions.« less

SU‐E‐T‐14: A Feasibility Study of Using Modified AP Proton Beam for Post‐Operative Pancreatic Cancer Therapy

Purpose:Due to the unpredictability of bowel gas movement, the PA beam direction is always favored for robust proton therapy in post‐operative pancreatic cancer treatment. We investigate the feasibility of replacing PA beam with a modified AP beam to take the bowel gas uncertainty into account.Methods:Nine post‐operative pancreatic cancer patients treated with proton therapy (5040cGy, 28 fractions) in our institution were randomly selected. The original plan uses PA and lateral direction passive‐scattering proton beams. Beam weighting is about 1:1. All patients received weekly verification CTs to assess the daily variations(total 17 verification CTs). The PA direction beam was replaced by two other groups of AP direction beam. Group AP: takes 3.5% range uncertainty into account. Group APmod: compensates the bowel gas uncertainty by expanding the proximal margin to 2cm more. The 2cm margin was acquired from the average bowel diameter in from 100 adult abdominal CT scans near pancreatic region (+/‐ 5cm superiorly and inferiorly). Dose Volume Histograms(DVHs) of the verification CTs were acquired for robustness study.Results:Without the lateral beam, Group APmod is as robust as Group PA. In Group AP, more than 10% of iCTV D98/D95 were reduced by 4–8%. LT kidney and Liver dose robustness are not affected by the AP/PA beam direction. There is 10% of chance that RT kidney and cord will be hit by AP proton beam due to the bowel gas. Compared to Group PA, APmod plan reduced the dose to kidneys and cord max significantly, while there is no statistical significant increase in bowel mean dose.Conclusion:APmod proton beam for the target coverage could be as robust as the PA direction without sacrificing too much of bowel dose. When the AP direction beam has to be selected, a 2cm proximal margin should be considered.

Daily dose monitoring with atlas‐based auto‐segmentation on diagnostic quality CT for prostate cancer

To evaluate the feasibility of daily dose monitoring using a patient specific atlas-based autosegmentation method on diagnostic quality verification images. Seven patients, who were treated for prostate cancer with intensity modulated radiotherapy under daily imaging guidance of a CT-on-rails system, were selected for this study. The prostate, rectum, and bladder were manually contoured on the first six and last seven sets of daily verification images. For each patient, three patient specific atlases were constructed using manual contours from planning CT alone (1-image atlas), planning CT plus first three verification CTs (4-image atlas), and planning CT plus first six verification CTs (7-image atlas). These atlases were subsequently applied to the last seven verification image sets of the same patient to generate the auto-contours. Daily dose was calculated by applying the original treatment plans to the daily beam isocenters. The autocontours and manual contours were compared geometrically using the dice similarity coefficient (DSC), and dosimetrically using the dose to 99% of the prostate CTV (D99) and the D5 of rectum and bladder. The DSC of the autocontours obtained with the 4-image atlases were 87.0% ± 3.3%, 84.7% ± 8.6%, and 93.6% ± 4.3% for the prostate, rectum, and bladder, respectively. These indices were higher than those from the 1-image atlases (p < 0.01) and comparable to those from the 7-image atlases (p > 0.05). Daily prostate D99 of the autocontours was comparable to those of the manual contours (p = 0.55). For the bladder and rectum, the daily D5 were 95.5% ± 5.9% and 99.1% ± 2.6% of the planned D5 for the autocontours compared to 95.3% ± 6.7% (p = 0.58) and 99.8% ± 2.3% (p < 0.01) for the manual contours. With patient specific 4-image atlases, atlas-based autosegmentation can adequately facilitate daily dose monitoring for prostate cancer.

SU-E-T-724: Evaluate the Impact of Bowel Gas Movement in Pancreatic Proton Therapy

Purpose: Current pancreatic proton planning protocol overrides bowel gas' CT Hounsfield Unit (HU) to water, based on the assumption of random bowel gas movement during whole treatment course. However, no follow‐up dosimetric study has been done to investigate the impact of the gas movement of each patient in his/her treatment period. The goal of this study is to assess passive‐scattering proton therapy (PT) dose variation due to the gas movement during the patient's treatment course. Methods: Five pancreatic cancer patients who received passive‐scattering PT using double scattering and uniform scanning system were identified. All of the patients received two verification CTs during the whole treatment course to evaluate dose distribution change. Two to three beam arrangements based on individual cases were used (two‐beam: posterior, right posterior oblique; Three‐beam: posterior, right posterior oblique and left posterior oblique). The proton plans were calculated using the Eclipse Proton Convolution Superposition algorithm and delivered 50.4Gy to the initial volume. Theinitial treatment plans with all beam parameters calculated on the CT with bowel gas overridden to water (compensator, MLC, beam line settings, etc) were recalculated using the same beam parameters to the verification CTs. Results: This study shows that the bowel mean dose varies from −135cGy to +342cGy due to the bowel gas movement compared to the initial proton plan. A large kidney dose variations were also observed due to inter‐fraction organ motions. Bowel gas movement doesn't affect target coverage with posterior proton beams proton plan. All plan's PTV D95 and iCTV D95 varies within 1% throughout all verification CTs. Conclusion: This evaluation method using verification CT data sets during the treatment course should also be useful in assessing dose distribution in other anatomic sites. Our quantitative results will help evaluate the effects of transient bowel gas movement in pancreatic proton therapy treatment course.

SU‐E‐T‐445: Prostate Motion Effect Evaluation in Proton Pencil Beam Scanning Delivery

Purpose: To quantitatively assess the impact of interplay effect associated with the intra‐and inter‐fraction prostate motion and the delivery of the scanning proton beam. Methods: A cohort of ten prostate patients with weekly verification CTs was treated by proton pencil beam scanning with bi‐lateral single field uniform dose modality. A typical field has 10–15 energy layers and 500–1000 spots. According to patient treatment logs, each layer delivery time was less than 1 second with average time to change layers of about 8 seconds. Real‐time intrafraction prostate motions were determined from a previously reported prospective study using the Calypso beacon transponder. The worst‐case‐scenario patient prostate motion was adopted in this study. The prostate motion and the beam delivering sequence were synchronized to demonstrate the interplay effect. Results: CTV coverage was altered due to intrafraction motion. In each individual fraction, the CTV D&lt;sub&gt;99%&lt;/sub&gt; can vary more than 10% relative to the initial plan. However, over the entire course of treatment the total dose degradation of D&lt;sub&gt;99%&lt;/sub&gt; had a mean of −1.5% and standard deviation of 1.2%. The large variation generally happens where one beam spot is significantly heavier weighted relative to the surrounding spots. The magnitude of deviation also depends on individual patient. The consideration of both intrafraction and interfraction motion further reduced the CTV coverage D&lt;sub&gt;99%&lt;/sub&gt;, with mean of ‐ 1.7% and standard deviation of 1.3%. The mean dose to the anterior rectal wall varied about 10% due to either intrafraction or interfraction motion. Conclusion: The intrafraction and interfraction prostate motion have comparable dosimetric impact. The dose over the course of treatment can reduce ∼2% for CTV coverage and increase about 10% for anterior rectal wall.

SU-E-T-315: Planning and Verification of CT-Based HDR Intraluminal Brachytherapy Treatment for Malignant Obstructive Jaundice.

To present our experience using CT to plan and verify intraluminal HDR treatment for a patient with obstructive jaundice. Due to the obstruction's proximity to the small bowel, along with small bowel adhesions from past surgical history, it was imperative to verify source position relative to the bowel before each treatment. Treatment was administered to a total dose of 2000cGy in 5 fractions via a 6F intraluminal catheter inserted into the patient's 14F percutaneous drainage catheter. Graduations on the intraluminal catheter were used to measure the exact length of catheter inserted in to the patient's drainage tube allowing reproducibility. Dummy seeds inserted during CT were identified by iteratively aligning the planning system's 3D reconstruction axis to the catheter at multiple points as it snaked through the liver. Taking in to account the known offset between actual dwell positions and dummy source positions, we determined what dwell positions to activate for planning. CT verification was performed prior to each treatment to insure that the drainage catheter had not moved and that the distance from treatment site to small bowel was adequate. Dummy seeds and anatomical landmarks were identified on the scout image and correlated to the CT. Verification CTs showed remarkable consistency in the day-to-day drainage catheter position. The physician was able to easily identify the small bowel of concern on the CT and determine if a safe distance existed for treatment. The method outlined in this work provides a safe means by which to treat bile duct obstructions using HDR when critical structures are nearby. We were prepared to make real-time adjustments to our treatment plan to account for significant variation, but found it unnecessary to do so in this particular case.