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Purpose: This study explores the feasibility of using megavoltage fluoroscopy (MVF) for on‐line real‐time verification and guidance for gated/4D treatment delivery. Initial experiences in implementing MVF and methods developed to minimize required dose and optimize the signal‐to‐noise ratio will be presented. Methods and Materials: A Siemens LINAC retrofit with a flat panel imager in the beam direction was used to acquire MVF images with 10242 pixel matrix at 7 frames‐per‐second (fps) using 6 MV photons. Image processing software tools were developed to remove artifacts caused by beam pulsing, dead pixels and stuck bits and to average pixels and video frames. The beam delivery rate was adjusted during MVF acquisition in an attempt to minimize the dose per frame. A phantom designed to test the contrast resolution of kilovoltage fluoroscopy systems was used to assess the MVF image quality at different beam delivery rates. Results: Good quality MVF images with sequences longer than a typical respiratory cycle (5 sec) were obtained with an estimated dose as low as 0.6 cGy. Matching the beam pulse frequency to a multiple of the video frame rate minimized striping artifacts. Image sequences acquired at a frame rate of 3.5 fps and beam delivery rate of 12 MU/min had an SNR of 125; the minimum object resolvable during cine display was 0.063 inch in diameter, and the 0.31 inch diameter object was detectable at 2% contrast. Contrast detectability was improved by averaging video frames, reducing spatial resolution and displaying video in high‐speed cine mode. Conclusions: It is feasible to acquire good quality megavoltage fluoroscopy images using a low dose‐rate treatment beam, indicating that MVF can be used to verify or guide gated/4D treatment delivery in real time (e.g. prior or during treatment delivery). Conflict of Interest: This work was supported in part by Siemens OCS.

Purpose: Different CT modalities with varying image quality are being used to correct for interfractional variations in patient setup and anatomy changes, thereby reducing CTV‐to‐PTV margins, for prostate radiotherapy (RT). We explore how CT image quality affects patient repositioning and CTV‐to‐PTV margin. Method and Materials: Three CT‐based IGRT modalities routinely used in our institute for prostate RT are considered in this study: MV fan beam CT (Tomotherapy), MV cone beam CT (MVision, Siemens) and kV fan beam CT (CTVision, Siemens). Daily shifts are determined by manual registration to achieve the best soft tissue agreement. Effect of image quality on patient repositioning was determined by statistical analysis of daily shifts for 65 prostate cancer patients (34 Tomotherapy, 21 CTVision,10 MVision) treated in our clinics. The impact of soft tissue contrast on organ interface identification was evaluated by analyzing contours drawn by 7 users on the scans from each imaging modality. In addition, variability of soft tissue registration between 10 users was evaluated based on the registration of representative scan for each CT modality with its corresponding planning scan. CTV‐to‐PTV margin was defined as 1.96σ. Results: Inferior image quality with MV CT based IGRT leads to increased variations in daily shifts (3, 4, 5 mm for CTVision, Tomotherapy and MVision) and in prostate delineation (6, 3, 10 mm for CTVision, Tomotherapy and MVision). Superior image quality with the kV CT results in reduced variation between 10 users in soft tissue registration. Uniform margin introduced to account for the uncertainty in the identification of prostate edge are determined to be 2, 6 and 5 mm for CTVision, Tomotherapy and MVision. Conclusion: Image quality adversely affects the reproducibility of the manual registration for IGRT and necessitates a margin of 2 mm for kV CT and 6 mm for MV CT to ensure adequate coverage.

Purpose: Radiotherapy is sometimes carried out with the patient in prone position. The purpose of this work is to study the changes of respiratory organ motion for patients in prone versus in supine position. Method and Materials: We obtained 4D CT datasets for 15 patients, 5 in prone and 10 in supine position, using a GE LightSpeed 4D CT scanner. Patients in both positions were immobilized by Alpha cradle. Lung movement (the change in lung spatial dimensions between maximum and minimum inspiration) in the superior‐inferior (S/I) direction was measured on the 4D CT, as well as in the anterior‐posterior (A/P) and left‐right (L/R) directions in two transverse CT images near the diaphragm and near T4. We also measured the movement of the anterior chest wall with respect to the table top on the T4 transverse CT image as well as on the transverse image defined by the nipple. Results: Average lung movement changes from supine to prone position were: in 17.8 to 11.5 mm in S/I direction changes, and 1.6 to 0.5 mm in the A/P direction on the T4 image. In the transverse image defined by the nipple, we observed chest wall average movement of 0.1±0.4 mm in prone position versus 1.9±0.4 mm in supine position. Similarly, at the level of T4, the chest wall moved 0.3±0.3 mm in prone setup and 2.0±0.7 mm in supine position. Conclusion: Respiratory organ motions in thorax are generally reduced when patient position is changed from supine to prone. We have found a significant reduction in anterior chest wall movement for the prone position, an advantage of treating breast cancer in the prone position.

Purpose: We analyzed the daily setup uncertainty and dose verification for partial‐breast irradiation (PBI) in prone position using helical tomotherapy. Method and Materials: According to an in‐house protocol, early‐stage breast cancer patients received PBI treatments in the prone position on the TomoTherapy Hi‐Art system using megavoltage‐CT guidance (TomoTherapy, Inc., Madison, WI). For planning, kilovoltage‐CT scans were obtained with the involved breast suspended downward. Treatment plans were generated based on criteria from the NSABP B‐39/RTOG 0413 protocol; the PTV_eval was to receive 3.85 Gy per fraction over 10 fractions administered twice daily. Before each fraction, an MVCT scan was acquired and compared with the planning kVCT images to refine the patient position. Along each shift direction, a margin to estimate the setup uncertainty for treatments without MVCT guidance was calculated from the average and standard deviation among the daily shifts. The dose actually delivered in each fraction was reconstructed based on the daily MVCT, accounting for the daily shifts. Among all fractions, the average and standard deviation for specific DVH points were compiled for comparison with the plan DVH. Results: Among the MVCT‐guided shift data, the random setup uncertainty in general exceeds the systematic difference from the plan. The overall margin was as large as 24.5 mm among the cases analyzed. From the MVCT‐based recalculations, the reconstructed doses differed little from the planned doses for each breast, each lung, thyroid, and heart. Average reconstructed doses for PTV_eval were slightly lower than the planned dose, attributable to increased breast thickness for some fractions. Yet, at least 90% of PTV_eval received over 90% of the prescribed dose. Conclusion: The estimated margin to account for setup uncertainty motivates improvements for PBI positioning. With MVCT guidance for prone‐positioned PBI, the deviation of the delivered target and organ‐at‐risk doses from the planned dose is minimal.