T He

TU‐E‐BRB‐05: Impact of Interfractional Tumor Motion on Respiratory Gated Stereotactic Body Radiation Therapy for Lung Tumors

PURPOSE To investigate the impact of interfractional tumor motion on dose delivery of gated lung SBRT. METHODS 4DCT scan for five lung patient was performed without breathing control at simulation and prior to each treatment. Gated treatment plans were performed on the end-exhale (50% phase) simulation CT with a 30% duty cycle. ITV was created by combining the GTVs at 40%, 50% and 60% phases. PTV was created by adding a 5 mm uniform margin to the ITV. All plans were normalized such that 60 Gy (3 fractions) was prescribed to the 85% isodose line. To calculate the accumulated dose over the treatment course, the original plan parameters were copied to the 40%, 50% and 60% CTs obtained prior to each treatment. In order to eliminate the effect of setup error to dose delivery, treatment isocenters at each fraction were determined by aligning the tumors on the slow CTs obtained prior to each treatment to that on the slow simulation CT. Doses recalculated on the 40% and 60% CTs at each fraction were warped through deformable CT image registration to their corresponding 50% CT to compose the 4D dose at that fraction. Those fractional 4D doses were warped to the 50% simulation CT to compose the accumulated 4D dose over the treatment course. RESULTS The minimum tumor doses over the treatment course were 59.9, 45.1, 68.9, 41.9 and 47.8 Gy respectively. Tumor V60s were 99.7, 92.2, 100, 97.2 and 93.0% respectively. The corresponding mean lung doses were 3.8, 6.4, 3.7, 4.4 and 3.7 Gy respectively. CONCLUSIONS Change in tumor motion pattern over the treatment course results in tumor underdosing. Tight margins are normally used in lung SBRT. Therefore monitoring of the reproducibility of interfractional tumor motion is critical to the success of dose delivery.

SU‐E‐J‐27: Effects of Metal Artifacts of KV and MV CT Images on Structure Delineation and Tissue Electron/Mass Density Calculation

PURPOSE To quantitatively evaluate effects of image artifacts of hip prostheses on the accuracy of structure delineation and tissue density calculation on kV and MV CT images. METHODS Five hip prostheses made of stainless steel, titanium and cobalt chrome alloys were positioned inside a water tank and scanned respectively on a Philips CT and a Tomotherapy Hi-Art unit. Prostheses were positioned to mimic single and bilateral implantations. Rods of tissue materials of lung, water and bone were placed at locations next and distal to metal implants near femoral head, neck and stem of prostheses. kV and MV CT scans were repeated for each placement. On CT images, cross-sectional outlines of metal implants and tissue rods were delineated. Densities of rod materials were determined and compared to the true values. RESULTS Metal artifacts were severe on kV CTs and minimal on MV CTs. Cross-sectional outlines of metal implants and tissue rods on kV CTs were severely distorted by artifacts while those on MV CTs remained clearly identifiable. For kV CTs, deviations of measured tissue density from true value were up to 51.3%, 30.6% and 40.9% respectively for lung, bone and solid water. The magnitude of deviation was generally larger at locations closer to metal implants and greater with bilateral implants than single implant. For MV CTs, deviations of measured density from true value were less than 6% for all three tissue materials either with single or bilateral implants. Magnitude of deviation appeared to be uniform and independent of locations relative to metal implants. CONCLUSIONS High Z metal artifacts on kV CTs can have severe impact on the accuracy of structure delineation and tissue density calculation, while on MV CTs, the impact is substantially less and insignificant. MV CTs should be considered for treatment planning on patients with high Z metal implants.

SU‐E‐T‐727: Evaluation of Eclipse AAA Dose Calculation Accuracy in the Presence of a Titanium Spinal Fixation Device

Purpose: To determine whether the Eclipses AAA algorithm can accurately predict doses in the presence of hardware. Methods: A titanium spinal fixation device was attached to a spine model to mimic its spatial orientation in patient. The model was immobilized to a plastic plate to form a rigid device. Posterior spine irradiation was simulated by positioning the device inside a PTW water tank with 270 degree gantry irradiation. Absolute doses and relative dose distributions were measured using ion chamber and OSLD. Lateral spine irradiation was simulated by placing the device in a water tank with 0 degree gantry irradiation. Planar doses were measured using a SunNuclear MapCheck device for 6 MV open and intensity‐modulated beams. Above experiement setups were reproduced in Eclipse from CTimages with correct electron densities assigned to materials used in the experiments, e.g. electron density 4.0 for the titanium device. Doses were calculated using AAA 8.9 algorithm and compared with measurements. Results: In posterior spine irradiation, doses were measured respectively for 7×10 and 10×15 cm open fields. Measured dose outputs agreed with calculations within 2.5% at all selected locations along the spinal canal and under the titanium cage. Excellent agreements ( 98% agreements between the measured and the calculated planar doses for all tested open and intensity‐modulated fields. Conclusions: Eclipses AAA 8.9 algorithm can reasonably accurately predict doses in the spine region in the presence of a titanium spinal fixation device.

SU‐E‐T‐222: Performance Comparison of In‐Vivo Dosimeters, including TLDs, MOSFETs, and OSLDs for Patients Receiving Total Body Irradiation

Purpose: To compare accuracy of in vivo dosimeters such as TLDs,MOSFETs & OSLDs for patients receiving Total Body Irradiation. Methods: Three types of dosimeters were considered for this study, LiF Thermoluminescent Dosimeters, Modified Field Effect Transistors(MOSFET ‐Thomson&Nielsen TN‐502RD) and Optically Stimulated Luminescence Dosimeters(OSLD ‐ Landuers 2% screened nanoDOTs). All three dosimeters were calibrated under full bulildup conditions at 100cm SSD, dmax = 3.4cm for 18MV photon energy and appropriate correction factors were applied for TBI treatment condition. This study is performed on a 30×30×22cm solid water phantom and a rando phantom at the TBI treatment distance. The separations at different locations for phantom were measured and MUs were calculated to deliver 100cGy at the midplane . Dosimeters were placed on the entrance side of the phantoms to measure dmax dose. Each type of dosimeters were irradiated at five locations in the rando phantom under the same treatment setup. The reproducibility of the dosimeters was verified by solidwater phantom measurements. Ten TBI cases, which were monitored using each dosimeter type were selected and tabulated the percentage difference of measured to expected dose for 5 different patient locations. Results: The reproducibility for TLDs,MOSFETs and OSLDs were 118.3cGy +/− 2.7%(1SD), 118.3cGy +/−2.1%(1SD) and 118.3cGy +/− 1.8%(1SD) resp after 10 consecutive measurements in solidwater phantom (Table 1). From rando phantom measurements, maximum percentage variation of the measured to expected dose with 1SD values are TLDs (1.4 +/− 1.4), MOSFETs (3.9+/− 2.2) and OSLDs (1.6 +/− 1.8) (Table 2). Percentage difference of the measured dose to expected dose for 10 clinical cases were TLDs (2.3+/− 2.9), MOSFETs (3.7+/− 4.9) and OSLDs (1.8+/− 2.9%) (Table 3). Conclusions: Measured values were within +/−5% tolerance level for all dosimeters. OSLDs can give more accurate and reproducible results with an added advantage of ease of use and shorter readout period.

SU‐GG‐J‐25: Motion Pattern of Electromagnetic Transponders Implanted in the Prostate Gland

Purpose: To investigate inter‐transponder motions over the period from implantation to the last treatment. Method and Materials: The inter‐transponder motions were assessed by the changes in the inter‐transponder distances (ITD).ITDs were obtained from simulation CTs, CBCTs and/or the Calypso System. Total 8 patients who were CT‐scanned on the same day of implantation were studied. ITDs obtained from initial CT scans were used as reference to evaluate the changes in ITDs measured at subsequent treatments. ITD variations relative to the day of implantation were plotted versus the number of days post implantation. Results: Over the 8 to 9 weeks long treatment course, most observed ITD variations were negative in values indicating reduction in the ITDs since implantation. Some patients had minimal ITD reductions (< 0.2 cm) over the entire course while some were found to be as large as 1.0 cm. The most rapid and substantial reduction in ITDs, if any, occurred during the first 2 weeks post implantation with a magnitude as large as 0.8 cm. From then on, the ITDs continued to decrease over time in most patients, but the reduction rate appeared to slow down with time. The ITDs eventually varied within a narrow range of ±0.1 cm for the rest of course. This stability, however, was not seen in one patient, where the ITD between RmB and LmB transponders continued to decrease to the last treatment day with no sign of stopping. Conclusion: The ITDs tend to reduce continuously throughout the entire course of treatment but appear to stabilize over time in most patients with few exceptions. The most rapid and substantial changes in ITD occur in the first two weeks post implantation. CT on patients for treatment planning use is preferred at least two weeks post implantation to reduce uncertainties in target delineation and localization.

SU‐GG‐T‐570: Selection of CT Image Sets for Treatment Planning in Stereotactic Body Radiation Therapy of Lung Cancer

Purpose: This study investigated the impact on tumordose coverage of treatment plans performed on various CTimage sets in stereotactic body radiation therapy(SBRT) of lungcancer.Methods and Materials: Five patients underwent SBRT for lungcancers were retrospectively investigated. For each patient, a free breathing (FB) CT and a four‐dimensional (4D) CT were acquired. Based on the 4DCT scans, two post‐processing CTimages were reconstructed: average intensity projection CT (AIP) and a low pitch, slow‐scan CT (SCT). The gross target volumes (GTVs) were delineated on the 4DCT images and combined to create the internal target volume (ITV). The planning target volume (PTV) was created by adding a 5 mm margin to the ITV. Treatment plans were performed on the FB CT, ATP CT, and SCT. Plan quality was evaluated by calculating and comparing the 4D dose for each plan using deformable‐image registration. Results: No matter which CTimage sets were used in treatment planning,lungtumors always receive at least the prescribed dose. The average difference in tumor D100 (minimum dose received by 100% of the tumor) is 0.28±0.61Gy (p=0.363) between the plans performed on AIP CT and those on FB CT, 0.62±1.35Gy (p=0.379) between the plans performed on AIP CT and those on SCT, and 0.34±0.77Gy (p=0.379) between the plans performed on FB CT and those on SCT. As for the mean lungdose, the average difference is −0.07±0.15Gy (p=0.390), 0.07±0.11Gy (p=0.221), and 0.14±0.15Gy (p=0.107) respectively. For the total lung V20, the average difference is −0.19±0.26% (p=0.186), 0.13±0.25% (p=0.321), and 0.31±0.27% (p=0.06) respectively. Conclusions: The differences in tumor and lungdose coverage for treatment plans performed on ATP CT, FB CT and SCT are indistinguishable. Those three CTdata sets are equally well in term of dose coverage for treatment planning in lungSBRT.

SU‐EE‐A3‐03: Comparison of CBCT and Electromagnetic Transponders for Prostate Localization

Purpose: In prostate cancerradiotherapy, it is unknown how large the PTV margins must be to account for the isocenter correction tolerance and intrafraction motion. The risk of geographic miss can be minimized by the placement of fiducial markers in the prostate gland for daily pretreatment localization and adjustment of patient position if necessary. In this study, we assess the magnitude of interfraction and intrafraction isocenter displacement using implanted electromangnetic transponders, and validate the accuracy of interfraction localization using cone beam CT.Method and Materials: Fifteen supine prostate IMRT patients with three implanted transponders each were studied. Initial daily localization was based on three laser and skin marks. Daily localization error distribution was determined from offsets between the initial setup position and that determined by Calypso. Post setup with the Calypso system, isocenter localization was immediately independently verified by imaging the radio‐opaque transponders using an integrated cone beam CTimaging system. Both localization techniques produced lateral, longitudinal, and vertical target offsets from machine isocenter. Organ motion or patient movement during treatment was continuously monitored by the Calypso system at a 4‐mm threshold. Results: The mean interfraction displacement (± SD) in cm in the lateral, vertical, and longitudinal directions were −0.2 ± 0.6, 1.8 ± 1.3, and 0.3 ± 0.9, respectively. After any necessary isocenter corrections, the mean isocenter placement error relative to the cone beam CT (± SD) in cm in the lateral, vertical, and longitudinal directions were 0.0 ± 0.1, 0.1 ± 0.2, and 0.0 ± 0.1, respectively. Conclusion: Compared with use of skin marks, electromagnetic isocenter repositioning provides an increased degree of isocenter localization. Good agreement was observed between cone beam CT isocenter localization and electromagnetic repositioning. However, the electromagnetic technique, with real time continuous tracking, has the added advantage of threshold‐based intervention with no additional radiation dose.