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MO‐FF‐A1‐03: Comparison of Inter‐ and Intra‐Observer Variability in Prostate Definition with Tissue Harmonic (H) and Brightness (B) Mode Imaging

Purpose: To compare the image quality of tissue harmonic (H) with brightness (B) mode transrectal ultrasound (TRUS) images of the prostate gland, by studying the interobserver and intraobserver variation in prostate delineation. Method and Materials: Transrectal ultrasound images of the prostates of 10 patients were acquired using both B and H ultrasound imaging modes. The prostates on all image sets were contoured by an experienced radiation oncologist (RO) and five equally trained observers. The volumes of the prostate glands were calculated and compared among the observers, and against the RO. Prostate contours for one of the patients were drawn five times by four observers in order to evaluate the intraobserver variability. The prostate volumes were also compared to observe the inter mode (B vs H) variation. Results: A one sample student t‐test showed that the volumes outlined by the five observers are in close (p>0.05) agreement with that of the RO in both imaging modes. Based on a paired student t‐test it was found the prostate volumes with H mode were significantly smaller (p=0.008) than that of B mode. Inter‐ and intraobserver consistency was estimated based on standard deviations as percentages of mean volumes and concordance indices (CIs). It was found that for smaller prostate glands (≤ 35 cc) H mode provides more consistency amongst observers (higher CI) compared to B mode; however for larger glands (≥ 35 cc) B mode provides more consistent results. Intraobserver consistency was also higher for H mode compared to B mode. Conclusion: Prostate volumes defined with H mode are significantly smaller compared to B mode. H mode improves inter‐ and intraobserver consistency in volume differentiation for smaller to medium prostate glands. In larger glands, H mode does not show any advantage over the B mode.

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SU‐GG‐I‐34: Comparison of Radiation Doses for Optimal and Dose Modulation Techniques in Abdominal CT Examinations

Purpose: To compare reduction in radiation dose using dose modulation techniques and optimal exposure parameters and its impact on image quality for biphasic abdominal CT examination performed using multidetector CT scanner. Method and Materials: A prospective study of 426 patients undergoing biphasic contrast‐enhanced abdominal examination involving arterial and venous regions were performed using optimal and dose modulation techniques in a six‐section multidetector CT scanner. For the optimal setting, images were acquired using constant tube potential (arterial phase with 90 kV and venous phase with 120 kV) and varying tube current‐time product according to patient's body weight. Using angular dose modulation (D‐DOM) and z‐axis dose modulation (Z‐DOM) techniques, images were acquired with constant tube potential similar to optimal settings. The effective doses were calculated using the DLP values. The signal to noise ratio (SNR) was also studied from the image obtained at the region of the liver. In a blinded review, one radiologist rated the CT scans for overall image quality. Radiation dose and image noise for optimal and dose modulation techniques with similar settings used for patients were also studied using an acrylic body phantom. Results: The effective dose using optimal, D‐DOM and Z‐DOM techniques were 11.3 mSv, 9.5 mSv and 8.2 mSv respectively for patient's of body weight 40 to 60 Kgs. The mean SNR value for arterial phase and (venous phase) was 7.7 (6.2), 8.3 (6.5) and 8.5 (7.9) for optimal, D‐DOM and Z‐DOM respectively for patient's of body weight 40 to 60 Kgs Conclusion: This study indicates the potential of significant dose reduction in abdominal CT. Use of dose modulation techniques resulted in a dose reduction of 16 – 28% with acceptable diagnostic quality in comparison to the optimal settings.

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SU‐GG‐I‐69: An Attempt to Establish Regional Diagnostic Reference Levels for CT Scanners in India

Purpose: To establish regional diagnostic reference levels for CT scanners from a survey in Tamil Nadu, South India. Method and Materials: A regional survey of 71 CT scanners in a few districts in Tamil Nadu was done over a period of one year as a part of an ongoing Atomic Energy Regulatory Board (AERB) funded project. The survey included dissemination of necessary data involving exposure parameters and radiation doses used routinely in each centre. Dose estimation in CT scanners was performed using a 32 cm CT dose index (CTDI) acrylic body phantom and a 10cc pencil ion chamber. Weighted CTDI (CTDIw) and effective doses were calculated using observed CTDI values. An attempt to establish regional diagnostic reference levels was made by adopting results from a local dose reference level obtained from the host institution where the study was conducted. Results: Out of the 71 CT scanners surveyed, 24 were conventional, 29 spiral and 18 multidetectors. The mean CTDIw values for biphasic abdominal CT using corresponding routine institutional protocols would be 14.55 mGy, 12.6 mGy and 17.18 mGy for conventional, spiral and multidetector CT scanners respectively. The third quartile values of effective dose for biphasic abdominal CT from the regional dose survey would be 17.15 mSv (range: 5.36 mSv – 46.81 mSv), 14.83 mSv (range: 3.68 mSv – 25.63 mSv) and 20.37 mSv (range: 4.31 mSv – 47.08 mSv) for conventional, spiral and multidetector CT scanners respectively. Conclusion: Regional diagnostic reference levels for body CT examinations could be established from prior knowledge of the local reference levels obtained. The wide variation of dose levels observed from various centres could contribute extensively to the population dose in the region in concern.

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SU‐FF‐I‐38: Calibration of An ASi Electronic Portal Imaging Device for Dosimetric Evaluation of Intensity Modulated Radiation Therapy

Purpose: To calibrate an amorphous Silicon (aSi) Electronic Portal Imaging Device (EPID) for dosimetric evaluation of Intensity Modulated Radiation Therapy (IMRT). Methods and Materials: This study was performed using a 6 MV photon beam of a Primus linear accelerator with an aSi EPID. A water‐filled penta step‐phantom of perspex was fabricated for calibrating the EPID. Absolute dosimetry was performed using a calibrated 0.14 cc ion chamber. A polystyrene slab with 21 bores (each separated by 1 cm) to accommodate the ion chamber was used for measuring dose below each step of the phantom. The EPID was calibrated at a source to detector distance of 150 cm. The calibration procedure involved: i) Acquisition of Electronic Portal Images (EPIs) with the centre of the step‐phantom positioned at isocentre for 1, 2 and 3 monitoring unit settings. ii) Measurement of the dose profile below the step‐phantom with the ion chamber placed at pre‐determined positions on the polystyrene slab iii) Acquisition of an EPI with the centre of the step‐phantom at isocentre and the polystyrene slab fixed onto the surface of the flat panel detector. This was used to locate the positions of the ion chamber with respect to the step‐phantom. iv) Plotting the calibration graph for pixel values (averaged over ROI of 10 × 12 pixels) and dose. EPIs of IMRT segments were acquired with the EPID in the ‘port‐during’ mode for each field. All the individually acquired segments were added using codes developed in MATLAB (version 6.5) to get the fluence map. Results: The pixel values of the EPIs were found to increase linearly with dose. The fluence maps acquired using the calibrated EPID were verified with the planned fluence maps. Conclusion: Therefore it is concluded that EPID could be used as a dosimetric verification tool for IMRT.

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