Blocked field effects on collimator scatter factors.

In routine dosimetry we assume separability of the collimator (Sc) and phantom (Sp) scatter components that together comprise the total scatter factor (Sc,p). In practice, the addition of blocking also affects the photon fluence attributable to the treatment head and flattening filter in a complicated way. The reduced aperture blocks out some of the head scatter contribution, while the block and tray add back secondary scatter. In the following we present techniques for directly measuring the aperture effect on Sc in air or in a full-scatter phantom. The change in Sc is found to be a scaleable quantity that can be modelled as a simple linear fit to the ratio of projected open-to-blocked equivalent square fields. Measurements have been made for 6, 18 and 24 MV photon beams on one Varian 2500 and two Varian 2100c accelerators. Results indicate a progressive loss of collimator scatter contribution with increased field blocking that is amplified with increasing energy. Block and tray scatter only contribute significantly to Sc for large fields and treatment distances of 80 cm or less. Application of these corrections in monitor unit calculations is presented.

Calculation of depth dose and dose per monitor unit for irregularly shaped electron fields.

A new dosimetric quantity, the lateral build-up ratio (LBR), has been introduced to calculate depth dose distribution for any shaped field. Factors to account for change in incident fluence with collimation are applied separately. The LBR data for a small circular field are used to extract radial spread of the pencil beam, sigma(r), as a function of depth and energy. By using the relationship between LBR, sigma(r), energy and depth, a formalism is developed to calculate dose per monitor unit for any shaped field. Criteria for lateral scatter equilibrium are also developed which are useful in clinical dosimetry.

WE-G-218-01: Management of RT Patients with Implanted Cardiac Devices: Updated Recommendations?

It has been long time since AAPM published TG-34 on cardiac pacemakers of older technology, which has been the standard document for clinical use, even today, for pacemakers (ICPs) and for defibrillators (ICDs), alike. Management of RT patients with recent technology cardiac devices has been widely published in literature without the provision of a new comprehensive and concise set of recommendations. The various effects of interaction of non-ionizing and ionizing radiations with those devices are crucial to be studied and accounted for during RT treatment deliveries. Thus, AAPM has formed the new TG-203 to work on this issue and provide recommendations to the clinical user for management of patients with cardiac devices when receiving RT. It has been numerous postings that we see in medical physics list server groups inquiring advices on dealing with these devices during patient imaging and radiation treatments. As treatment delivery technologies (IMRT, SBRT, dose escalations, proton beams, etc) and ICP/ICD technology advance, the need to address the management of patients with such devices receiving radiation treatment becomes increasingly important. ICDs offer the same functionality as ICPs, but they are also able to deliver a high-voltage shock to the heart, if needed. Finally, major discrepancies exist among manufacturer recommendations and wide variations exist among radiation therapy facilities regarding patient management precautions. LEARNING OBJECTIVES: 1. Provide a review on sources of potential malfunctions of modern ICPs and ICDs, including malfunction mechanisms from high-LET radiations and transient effects attributed to medical imaging procedures for radiotherapy. 2. Provide a review on management of radiotherapy patients with cardiac implanted devices. 3. Utilize recently available data and computation methods of out-of-field/peripheral dose by scattered photons and secondary neutrons in order to assess cumulative doses on the ICPs and ICDs, during current treatment deliveries (IMRT, SBRT, proton beam therapy, etc). Risk of failure associated with these doses will be discussed. 4. Provide recommendations for management of radiotherapy patients with implanted cardiac devices including the initial patient evaluation stage, dosimetric evaluation to the ICP/ICD during treatment simulation, treatment planning and treatment delivery. Recommendations for the final evaluation of the integrity and functionality of the device after treatment completion will be assessed.

SU-E-T-179: MVCB Pelvic Surface Dosimetry.

PURPOSE: To compare commonly clinically available methods of estimating skin dose for a Megavoltage Conebeam CT of the pelvis. METHODS: The clinical 6 MV conebeam uses 15 MU on a Siemens Oncor linac. Film tests using XV-2 film were done on a solid water phantom, as were tests using a parallel plate ion chamber at 2 mm depth. A Rando pelvic phantom was set up for a MVCB CT, and a cylindrically symmetric Isorad 6-12 MV PDM diode was placed at various angles around the phantom and irradiated using a 150 MU 6 MV arc beam with the same geometry as the clinical conebeam, giving higher and more accurate readings than would be obtained with the 15 MU conebeam. A comparison was made with the Pinnacle 8.0m treatment planning system. A strip of Optically Stimulated Luminescence dosimeters provides an accurate check of the surface dose distribution. RESULTS: All methods revealed an expected lateral asymmetry in the dose due to the starting and stopping angles of 270 (-90) and 110 for the conebeam. On the sides of the phantom, the diode dose was comparable to the Pinnacle-calculated dose at a depth of 3-5 mm. Near the anterior portion the diode dose was about 5% higher than the maximum Pinnacle-calculated dose at that angle. This difference is partly due to the increased diode response at shorter SSDs and higher dose rates, and to the geometry with the arc beam radiation being mainly anterior to posterior. The skin dose, corresponding to a depth of about 2 mm, is expected to be somewhat lower. CONCLUSIONS: To estimate pelvic skin dose in the MV CB geometry, corrections for measurement depth and geometry can be used to improve the dosimetry for these common clinically available dosimeters.

SU-E-T-463: Biological-Based Optimization and VMAT is Unnecessary for Stereotactic Body Radiation Therapy.

PURPOSE: This study shows that there is no clear dosimetric benefit of biological-based optimization for either fixed-beam IMRT or VMAT. Other than shorter delivery times, even VMAT does not offer additional advantage to fixed-beam IMRT. METHODS: A small number of patients for lung, pancreas, spine and brain CA were planned with fixed-beam IMRT, optimized with (gEUD) and without (DV) biological objectives and, also planned for VMAT with and without gEUD, for comparison. For the lung and brain cases, a non-coplanar 7-11 beam arrangement was used for fixed- beam IMRT and a coplanar 'hybrid' arc simulated VMAT with beams set every 5° spacing. For the other treatment sites, all beams were coplanar. For each case, the fixed-beam IMRT and VMAT plans were optimized with the same objectives. It is important to note that, only 2 segments/beam were allowed for each plan, in order to create small fluence modulation, appropriate for small target volumes during SBRT. RESULTS: For all plans we noticed that there were minor or no dosimetric differences between fixed- beam IMRT and VMAT, whether DV or gEUD objectives were used or whether fixed-beam IMRT or VMAT is used. Keeping the level of beam modulation as-low-as possible, for small SBRT targets, one can show that VMAT with or without gEUD optimization does not offer any dosimetric advantage against fixed-beam IMRT with multiple non-coplanar beams. This is against the expectation that gEUD-optimization can Result superior plans than DV-optimization. The difference is that, for small target volumes like those encountered in SBRT, the complexity of the fluence is not as high as in large field intensity modulated cases. CONCLUSIONS: The fact that VMAT with or without gEUD can produce as good plans as fixed-IMRT does not make VMAT a preferred treatment modality, other than the fact that requires reduced treatment time.

SU-E-I-90: Fast and Robust Algorithm Towards Vessel Lumen and Stent Strut Detection in Optical Coherence Tomography.

PURPOSE: Optical Coherence Tomography (OCT) is a catheter-based imaging method that employs near-infrared light to produce high-resolution cross-sectional intravascular images. We propose a new segmentation technique for automatic lumen area extraction and stent strut detection in intravascular OCT images for the purpose of quantitative analysis of neointimal hyperplasia (NIH). METHODS: Two clinical dataset of frequency-domainOCT scans of the human femoral artery were analyzed. First, a segmentation method based on Fuzzy C-Means (FCM) clustering and Wavelet Transform (WT) was applied towards inner luminal contour extraction. Subsequently, stent strut positions were detected by utilizing metrics derived from the local maxima of the wavelet transform into the FCM membership function. RESULTS: The inner lumen contour and the position of stent strut were extracted with very high accuracy. Compared with manual segmentation by an expert physician, the automatic segmentation had an average overlap value of 0.917 ± 0.065 for all OCT images included in the study. The strut detection procedure successfully identified 6.7 ± 0.5 struts for each OCT image. CONCLUSIONS: A new fast and robust automatic segmentation technique combining FCM and WT for lumen border extraction and strut detection in intravascular OCT images was designed and implemented. The proposed algorithm may be employed for automated quantitative morphological analysis of in-stent neointimal hyperplasia.