P Chan

Repeat stereotactic body radiation therapy for patients with pulmonary malignancies who had previously received SBRT to the same or an adjacent tumor site.

OBJECTIVES Retrospective analysis of patients with recurrences at or closely adjacent to the site of prior lung stereotactic body radiation therapy (SBRT) who received repeat SBRT. MATERIALS AND METHODS Nine patients with non-small cell lung cancer (n = 8) or oligometastatic colonic adenocarcinoma (n = 1) were treated with image-guided lung SBRT to a median of 60 Gy (range, 30-60) in a median of 3 fractions (3-5). Patients developed in-field relapse (n = 3) or recurrence adjacent (≤ 3.5 cm away) to the previous tumor location (n = 6) and received 2 nd lung SBRT to a median of 60 Gy. RESULTS Median follow-up after 2 nd SBRT was 22 months (4-40). All completed prescribed course of repeat SBRT and acute toxicity was limited. There was no grade >3 late toxicity. 3 (33.3%) patients developed Grade 3 late reactions: 2 pulmonary and 1 chest wall pain. Late pulmonary toxicity included 2 (22.2%) patients with Grade 3 and 3 (33.3%) with Grade 2. One patient (11.1%) had late Grade 3 and 1 (11.1%) Grade 2 chest wall pain. 1 (11.1%) developed Grade 2 late brachial plexopathy. No myelopathy was observed. Two patients developed progression of tumors treated by 2 nd SBRT. Local recurrence free survival and overall survival was 75% and 68.6%, respectively at 2 years. Relative volume of ipsilateral lung receiving 5 Gy (V5) and V10 were lower for 2 nd SBRT. CONCLUSION Repeat image-guided SBRT for patients with small peripheral recurrences was feasible and severe toxicity was not observed. Additional studies are needed to evaluate the safety and efficacy of lung reirradiation using 2 nd SBRT.

SU-E-T-542: Comparison of Stereotactic Radiosurgery (SRS) of Brain Lesions Using Gamma Knife, VMAT, IMRT, and Conformal Arcs

PURPOSE To compare dose distributions in stereotactic radiation surgery of brain lesions using gamma Knife, VMAT, conformal arcs, and IMRT in order to provide an optimal treatment. METHODS Dose distributions from single shot of 4C model of Gamma Knife at the helmet collimation sizes of 4, 8, 14, and 18 mm in diameter were compared with full arcs with the square shapes of 4×4 (or 5×5), 8×8 (or 10×10), and spherical shapes of 16 or 20 mm in diameter using EDR3 films in the same gamma knife QA phantom. Plans for ten SRS cases with single and multiple lesions were created in gamma knife plans and Pinnacle plans. The external beam plans had enlarged field size by 2-mm and used single conformal full circle arc for solitary lesion and none coplanar arcs/beams for multiple lesions. Coverage, conformity index, dose to critical organs, and integral dose to the brain and nearby critical structures were compared on all plans. Structures and dose matrices were registered in a Velocity deformable image registration system. RESULTS Single full circle arc from Elekta beam-modulate MLC (4-mm leaf thickness) and agility MLC (5-mm leaf thickness) have larger penumbra and less flatness than that of Gamma Knife single shot. None-coplanar arcs or beams were required to achieve similar dose distribution. In general, Gamma Knife plans provided significant less integral dose than that of linac-based plans. Benefits of IMRT and VMAT versus gamma Knife and conformal arcs were not significant. CONCLUSION Our dose measurement and treatment planning evaluation clearly demonstrated dose distribution differences amount current popular SRS modalities for small solitary and multiple brain lesions. The trend of using MLC shape beams or arcs to replace conventional cones should be revisited in order to keep lower integral dose if the late correlates with some radiation-induced side effects. Pilot grant from Elekta LLC.

4D-Video Usefulness For Tracking Respiration Motion in 4DCT Scans and Targeting Small Mobile Tumors

4DCT-based treatment plan, 4DCBCT-guided patient setup, and real-time tracking of target motion are essential for accurate stereotactic body radiotherapy (SBRT) of small mobile tumors. However, there is lack of correlation between 4DCT-4DCBCT data in phases of regular respiration cycles and the real-time monitored irregular respiration motion data. We are now proposing a real-time 4D video technique to synchronize respiration patterns in 4DCT, 4DCBCT, and real-time target motion during the dose delivery. Results of feasibility studies with sequential surface images on several patients and volunteers demonstrated the capability of detecting the breathing patterns by the movements of the frontal surfaces of the thorax and abdomen. The deep breathing concurs with larger thorax surface shifts of > 5-mm or tilt angles of > 2-degrees and longer respiratory cycles with periods of >6- seconds. Normal breathing usually has small thorax surface shifts of < 2-mm and angles of < 2-degrees but wide ranges of abdomen surface shifts from 2 to 30-mm and angles from 1 to 5 degrees. At free breathing conditions, we also find that patient respiration cycles and abdomen surface movements are frequently changed with time. More importantly, the personal respiration patterns are predictable with the dynamic volumetric curves determined by the abdomen and thorax surface movements. The surface movements and changes of surface covered volume can be correlated with dynamic volumetric changes in 4DCT and 4DCBCT scans. Thus, the 4D video technique is potentially useful for real-time tracking of small mobile targets in SBRT through synchronization with 4DCT or 4DCBCT.

SU‐E‐J‐160: A Simple Technique for Quick In‐Vivo Dose Measurements for KV CBCT and Diagnostic X‐Ray Planar Imaging Using MicroMOSFET Dosimeters

Purpose: To present a practical method for dose measurement of kV CBCT scans and diagnostic X‐ray planar imaging according to typical IGRT in radiotherapy and chest and pelvis diagnostic imaging protocols by using microMOSFET dosimeters. Once calibrated, in‐vivo measurements can be easily performed on patients on surface and/or in internal cavities. Methods: The calibration procedure uses two ADCL calibrated ion chambers for diagnostic kV X‐ray beams, one 6 cc farm chamber and one large parallel plane chamber for absolute dose references, and the microMOSFET with high sensitivity settings for calibration in air and in phantom. The calibration covered the possible dose range from 1 to 25 mGy. The calibrated dosimeters were then placed at anterior, posterior, right lateral and center of an anthromorphic phantom to measure the dose for CBCT scans using the appropriate calibration file according to kVp and dose range. Also, the dosimeters were placed inside two cadavers for internal dose measurement for typical x‐ray imaging. Results: Our measured results showed that the dose from CBCT scans were typically 16% ‐ 39% higher than the nominal dose presented by the vendor per protocol. The dose from 4D CBCT scan, 2.16 cGy, was approximately 39.4% higher than the nominal dose of 1.55 cGy. The Prostate M10 scan of 3.458 cGy was approximately 16.8% higher than the nominal dose of 2.96 cGy. The Pelvis/Chest M20 scan 2.972 cGy, was approximately 35.1% higher than the nominal dose of 2.2 cGy. Large variation was observed for planar X‐ray imaging due to use of shielding and tissue attenuations. Conclusion: The ease of implementation and using microMOSFETs for CBCT scans and X‐ray imaging allows quick and accurate in‐vivo dose measurements. Vendor specified doses are verified by this technique and should be added to increase patient quality assurance and safety.

SU‐E‐T‐342: Differences of Dose to Pelvic Organs at Risk (OAR) with Use of Vaginal Multi‐Channel Cylinder Applicator for Adjuvant High‐Dose Rate (HDR) Brachytherapy to the Vaginal Cuff

PURPOSE We reviewed the use of Vaginal multi-channel cylinder applicator (VCMC) in adjuvant HDR brachytherapy with the goal of reducing dose to OAR while completely covering the entire proximal vaginal cuff. METHODS Two patients underwent total hysterectomy for endometrial carcinoma and received combined 5 fractions of Ir-192 HDR brachytherapy to the vaginal cuff as a portion of their postoperative regimens. Treatment was delivered using a commercially available Nucletron CT/MR compatible VCMC. The dwell time distribution for multiple lumens was optimized to maintain at least 95% of CTV within 95% of isodose line (V95 > 95%) while keeping dose to all OAR as low as possible. For comparison, plans were created using the central channel only simulating single-channel (SC) cylinder treatments. Related samples Wilcoxon signed rank test was used to test for statistical significance. RESULTS Average CTV V200 was 39.7 ± 1.5 % of prescribed dose for VCMC plans vs. 32.5 ± 0.5 % for SC (p=0.04). Average CTV V150, V95, and V90 were 58.4 ± 2.0 %, 98.1 ± 1.0 %, and 99.4 ± 0.5 % for VCMC and 51.1 ± 0.3 % (p=0.04), 96.5 ± 0.6 % (p=0.04) and 98.8 ± 0.4 % (p=0.08) for SC. Average maximum point vaginal surface dose as a percentage of prescribed dose was similar: 294.5 ± 62.4 % for VCMC and 295.9 ± 79.7 % for SC. We also calculated the average D0.1cc, D1cc, D2cc and D10cc for bladder, rectum, sigmoid, and small bowel. CONCLUSION Use of VCMC reduced D0.1cc, D1cc and D2cc for the rectum; however bladder D0.1cc and D10cc were lower for SC. While CTV V200 and V150 were higher for VCMC, maximum dose to vaginal surface was similar. Additional dosimetric and clinical studies of VCMC use are necessary.

WE‐C‐WAB‐08: Measurements and Quantifications for the Lung Nodule Motion Using 4DCBCT Scans Prior to Each SBRT Treatment

PURPOSE To measure the lung nodule motion using 4D CBCT, to quantify any potential influences on the motion trajectory, and to derive an analytical approximation for motion trajectory in order to synchronize with external real-time respiratory motion tracking. METHODS GTV from fast CT and ITV from CT-PET obtained from rigid or deformable co-registration with simulation CT are used for SBRT planning. Pretreatment 4D CBCT with ten phases are automatically registered according to the GTV mask. The nodule motion trajectory is quantified by the center, amplitudes, swiping area, major and minor axes in the principal plane (PP), and the PP normal vector. The setup errors and motion margins are assessed by the trajectory center and amplitude, respectively. Detected motion in the PP is analytically approximated by an ellipse with time parameter and the trajactory shape or axes and orentation may be correlated with the lung volume, nodule location, abdominal compression, and restriction force from the neighboring chest wall or other rigid tissue. Those variables in seventy-two 4D CBCT scans from 34 patients are retrospectively analyzed. RESULTS After 4D CBCT-guided setup adjustments of 10±8 mm, the remaining setup errors in any directions increase from 1±1 mm for upper and middle lobes to 1± 2 mm for the lower lobes while the average motion margins in the lateral, longitudinal, and posterior-anterior directions increase from (1, 2, 1) to (1, 6, 1) mm. No significant difference is observed between the left and right lungs and between the groups with and without abdominal compression. Importantly, most motion trajectories can be analytically approximated and its PP is predicable with the local force. CONCLUSION The clinical measurements and detail analysis validate the geometric accuracy and precision of current SBRT and our analytical approximation is potentially useful for synchronization with real-time respiratory motion tracking for future gated SBRT.

SU-E-T-648: Comparison of VMAT Vs Arc Treatment Plans for Patients Undergoing SBRT of Early-Stage Non-Small Cell Lung Cancer (NSCLC)

PURPOSE To evaluate the dosimetric implications of using VMAT (Volume Modulated Arc Therapy) treatment planning techniques compared to traditional Arc therapy methods for patients undergoing SBRT (Stereotactic Body Radiation Therapy) for early-stage Non-Small Cell Lung Cancer (NSCLC). METHODS Ten NSCLC cancer patients are planned with both VMAT and Arc techniques. The SBRT treatment plans comparison was quantified by several Dose-Volume Histogram (DVH) indicators including mean, maximum and minimum doses for GTV, ITV, PTV, OAR (Organs At Risk), and V95 (volume receiving at least 95% of the prescribed dose) for PTV. RESULTS On average VMAT plans require for treatment delivery 16.6 ± 20.2 % more monitor units (MU) than the traditional Arc plans. The average PTV minimum, maximum and mean doses as a percentage of prescribed dose are 94.5 ± 3.9 %, 114.1 ± 3.3 % and 106.6 ± 1.6 % for VMAT vs 91.6 ± 4.4 %, 119.5 ± 5.3 % and 109.5 ± 2.5 % for the Arc technique. The V95 PTV coverage for VMAT plans range from 99.4 % to 100 % with a mean of 99.7 %, compared with a range of 96.8 % to 100 % with a mean of 99 % for the Arc plans. The maximum dose received by the lungs, spinal cord and chest wall show on average significant increases for Arc plans as opposed to VMAT plans (5.7 ± 6 % increase for lungs, 4.4 ± 9.2 % for cord and 2.4 ± 6.3 % for chest wall). The average mean doses and minimum doses for the OAR are similar for both techniques. CONCLUSIONS The comparison of VMAT vs Arc plans for SBRT of NSCLC patients is subject to many variables, including GTV and PTV volume sizes, shape and their proximity relative to the OAR.

SU‐E‐T‐554: Systematic Evaluation of SBRT of NSCLC Using VMAT, IMRT, and Multiple Short Arcs Considering Uncertainties in CT and PET‐Based Planning, Body‐Frame and Abdomen Compressed Setup, and XVI‐ Guided and Optical‐Tracked On‐Line Adjustment

Purpose: to assess uncertainties of internal target volume (ITV) using CT and PET scans, target repositioning using immobilization devices and image guidance, and to evaluate difference among IMRT, VMAT, and multiple short arcs (MSA) techniques. Methods: Over sixty patients were accrued for ITV delineation using CT simulation scans and diagnosis PET‐CT scans. IMRT and MSA plans were both generated for early patients and VMAT plans were added for the last ten patients. Daily XVI was acquired prior to treatment and manually aligned with the planned ITV. On‐line 3D shifts were applied to early cases and additional 3D rotations were applied to the last 15 cases after installation of Elekta HexaPOD tabletop and iGUIDE optical tracking. Daily XVI image sets for 30 cases were registered in patient plans in exact treated positions and ITVs were delineated on daily XVI images. Composite DVHs were calculated and compared with the original planning DVHs. Results: ITV defined in PET could differ by up to 10 times for some cases. MSA plans provided higher BED to the ITV while maintaining the same V90 and different V50 to the lung and nearby structures. VMAT was the most efficient in dose delivery. Interfractional motions from 1 to 3 cm were corrected with XVI‐guidance. There were 2‐ mm intrafractional displacements for nodule in various locations shown in repeated daily XVI. Combined ITV was >99% if 3‐mm margin was used for planning target volume. Optical tracking, body‐frame, and abdomen compression had marginal effects Conclusions: ITV should be determined from the union of ITVs from PET and CT scans. Interfractional motion was eliminated by daily XVI guidance. A 3‐mm expansion of ITV was still required for planning target volume. IMRT and VMAT optimization offers rooms for improvement against MSA.

MO-F-214-01: A Novel Dosimetry System Useful for Accurate and Optimal Total-Skin Electron-Beam Therapy

Purpose: We are introducing a dosimetric system for calculation of dose distribution within patients from total skinelectron beam (TSEB) in order to avoid large dose variations and frequent setup changes for accurate and safe TSEB therapy of mycosis fundoides. Methods: Custom cylindrical phantoms were made of a set of buckets filled with water and wrapping with bolus outside. Films placed at depths of interest in the cylindrical phantoms and flat solid water phantom were irradiated using 6‐MeV high‐dose‐rate TSEB from an Elekta Synergy linear accelerator. Horizontal and vertical fluence profiles as well as depth‐dose curves were measured by positioning the flat phantom at various locations. Curvature factors were determined by the dose curves across the films in the cylindrical phantom and quantified as functions of the curvature radius and the angle between the TSEB and local surface norm. The off‐axis and curvature factors were used in calculation of the skin‐depth dose distribution in any body parts for individual beams. Sum of the doses from all beams provides the total skin dose distribution. The calculation requires only the toward beam body surfaces that can be captured with 3D cameras. Results: Repeated phantom experiments confirmed the dose calculation accuracy within 5%. Computer simulation for patients with different body shapes and treatment setup using the Stanford technique agreed with the results of in‐vivo dose measurements. Dose variation from different techniques including changes from 60 to 45 degree turntable angles for oblique beams in Stanford technique and rotation TSEB were simulated. Conclusions: This simple and accurate dosimetric system allows users to predict total skin dose distribution and distribution changes with different treatment techniques. Importantly, it allows users to plan and optimize TSEB treatment.

SU‐E‐T‐236: Evaluation of Electronic Portal Imaging Device in the Quality Assurance of Volumetric Modulated Arc Therapy

Purpose: To evaluate the feasibility of engaging electronic portal imaging device(EPID) in the routine VMAT QA programs. Methods: A small radiation field of 1.6×1.6cm was used to irradiate an 8mm diameter ball bearing positioned precisely at machine isocenter and projected images at different gantry angles were recorded using EPID. A dynamic‐MLC based picket fence pattern was created both in (a) static mode and (b) VMAT mode. In either mode, a total of 40 control points were evenly divided into ten strips of 2cm in width. The ratio of radiation fluence between two neighboring strips was set at 2. Elekta linear accelerators operate with binned dose rates. The dose rate transition is instant and in strict synchronization with the gantry rotation speed and MLC movement. The entire delivery of the picket fence pattern was recorded in accumulation mode with Elekta iView. Image analysis was performed using an in‐house program written in MATLAB. Results: During a full gantry rotation, EPID panel was found to move up to 1.3 mm in the gantry‐target direction but less than 0.5mm in the lateral direction. The MLC speed and positioning were well controlled in both static mode and VMAT mode. At any time point during radiation delivery, the positions of individual leaves in each bank were within 0.5mm of the planned position in static mode. Comparing the picket fence pattern in VMAT mode with that in static mode, we noted minimal discrepancy (<0.5mm) between the corresponding leaf edges. In addition, we observed a negligible impact of gantry rotation speed and MLC travel speed on the accuracy of leaf positioning. Conclusions: Through this study, we demonstrate that EPID is indeed a feasible tool in routine VMAT QA program given that the motion of EPID panel per se is carefully considered.