Y Kang

Measurements of in‐air output ratios for a linear accelerator with and without the flattening filter

The in-air output ratio (Sc) for photon beams from linear accelerators describes the change of in-air output as a function of the collimator settings. The physical origin of the Sc is mainly due to the change in scattered radiation that can reach the point of measurement as the geometry of the head changes. The flattening filter (FF) and primary collimator are the major sources of scattered radiation. The change in amount of backscattered radiation from the collimator into the beam-monitoring chamber also contributes to the variation of output. In this work, we measured the Sc and backscatter factors (Sb) into the beam-monitoring chamber for a linear accelerator with and without the FF. We measured the Sc with a Farmer-type chamber in a miniphantom at the depth of 10 g/cm2 for 6- and 18-MV x-ray beams from a Varian Clinac 2100EX linear accelerator. The Sb were measured with a universal pulse counter and a diode array with build-in counting hardware and software. The head scatter component (Sh) was then derived from the relationship Sc= Sh x Sb, where Sb was the linear fit of measured results. Significant differences were observed for Sc with and without the FF. Within the range of experimental uncertainty, the Sb was similar with and without the FF. The variations in Sh differed significantly over the range of field sizes of 3 X 3 to 40 X 40 cm2 with and without the FF; for the 6-MV beam, it was 8% vs 3%, and for the 18-MV beam, 7% vs 1%. By analyzing the contributions of backscatter factor and total in-air output ratios with and without the FF, we directly gained insight into the contributions of different components to the total variations in Sc of a linear accelerator. Sc, Sb, and Sh are basic and useful dosimetric quantities for delivery of intensity-modulated radiation therapy using a linear accelerator operating in a mode without the FF.

WE-E-J-6C-06: Proton Treatment Planning for Mobile Lung Tumors

Purpose: Traditional treatment planning methods may lead to lung proton treatment plans in which the apparent and actual dose distributions may be significantly different due to respiratory motion. We are developing strategies for designing compensator-based 3D proton treatment plans using 4D CTs (composed of 3D CTs at a sequence of respiratory phases) for mobile lung tumors and assessing the validity of these strategies using 4D dose computation methods. Method and Materials: 4D CTs for a population of lung cancer patients were used to obtain tumor targets and critical structures. The internal target volume (ITV) was the composite of target volumes on the 4D CT. For each patient, we evaluated four compensator design and planning strategies based on (1) the average CT obtained by averaging all phases of the 4D CT; (2) free breathing CT; (3) maximum intensity projection (MIP) CT; and (4) the average CT with the CT numbers inside the tumor volume replaced by the corresponding MIP CT numbers. For each strategy, the resulting apparent dose distribution was compared with the corresponding 4D dose distribution computed by deforming dose distributions of each phase to the reference phase and summing. Results: The composite 4D dose coverage of the target was significantly superior for method (4) while normal tissue doses were slightly higher though well below the limits. A seemingly conservative compensator design using MIP for the entire image, not just the target volume (Method 3), resulted in poor proximal target coverage due to over-estimation of the densities of intervening tissues. Conclusion: 3D proton plans based on the CT obtained by averaging the 3D CTs comprising the 4D CT, and with the CT numbers in the tumor volume replaced by the corresponding MIP CT numbers, is an effective approach to achieve good tumor coverage and acceptable normal tissue sparing.

SU‐EE‐A3‐04: On‐Line CT‐Guided Adaptive Re‐Planning Based On Deformed Original Dose Distributions

Purpose: Daily image‐guided setup based on soft tissue target may improve target coverage. However, when significant anatomical changes occur, a simple isocenter shift may not be adequate. In this study, we propose an online IMRT replanning strategy for prostate cancer which uses the deformed dose distributions from the original treatment plan as its objective. Method and Materials: The replanning procedure used was as follows: (1) the dose distribution on the planning CT was deformed to the daily CT and used as the reference objective dose distribution for IMRT replanning; (2) the prescription isodose line on the reference dose distribution was auto‐segmented and used as the fictitious “target volume” to set the initial MLC leaf positions; (3) we developed and implemented a voxel‐by‐voxel dose‐based cost function. The IMRT treatment plan was optimized using the direct machine parameter optimization algorithm to achieve the following goals: (a) inside the region enclosed by the original prescription dose line, the replanned dose distribution was optimized to match with the reference objective; (b) outside this region, the objective function was chosen to lower the dose value and to penalize the dose value exceeding the reference dose for each voxel. The replanning process does not need re‐auto‐segmentation of patients anatomy, although re‐segmented anatomic contours were used to evaluate the effectiveness of this approach. Results: We compared the dose distributions and the DVHs of the original plan and the daily plans using (1) image‐guided setup based on prostate alignment, (2) deformed dose distribution from the original plan, and (3) our re‐planning method. We found that our re‐planning strategy matched well with the original plan. Conclusion: The replanning strategy using the original dose distribution as the goal for optimization produces dose distributions similar to the original approved plan and is an effective approach for on‐line CT‐guided adaptive radiotherapy.

TH-C-ValB-06: Thoracic Proton Treatment Planning Strategies Based On the 4D CT Information

Purpose: Particularly in the case of thoracic radiation therapy, there are substantial inter‐ and intra‐fractional variations in shape, volume and position of treatment targets and the intervening and surrounding normal tissues. The purpose of this work is to develop protontreatment planning strategies for mobile tumors with and without mobile intervening structures based on 4D CT, and to assess the planning strategies using 4D CT data and daily in‐room CT information. Method and Materials: Five treatment planning strategies were evaluated based on (1) free breathing CT with small smearing, (2) free breathing CT with large smearing, (3) average CT with small smearing, (4) average CT with CT numbers inside the tumor volume replaced by higher CT numbers, and (5) maximum intensity projection CT. For a lung patient with large, 1.6 cm, tumor motion and immobile surrounding tissues,treatment plans were designed using strategies 1, 2 and 4. Each treatment plan was recalculated on five daily CTimages using bony structure alignment. For an esophagus patient with 3.5 cm tumor motion and large cardiac, liver and spleen motion, treatment plans were designed using strategies 1, 2, 3 and 5. Each treatment plan was recalculated in all 10 phases of the 4D CT.Results: For the lung patient, the tumor coverage evaluated using the five daily CTs is superior when using strategies 2 and 4 as compared to strategy 1. However, the lung sparing is superior using strategy 4 compared to strategy 2. For the esophagus patient, the treatment plans using strategy 2 and 5 covered the targets for each 4D CT phase while the plan using strategy 1 and 3 caused significant under‐dosing. Conclusion: With the strategies developed using 4D CT data, good tumor coverage was achieved for the thoracic patients with large tumor and surrounding tissue motion using proton therapy.

SU‐FF‐T‐87: Software Tools for Transferring Treatment Plans Between Two Planning Systems

Purpose: There is often a need to transfer treatment plan data from one system to another. However, even when both systems are claimed to be compliant with a standard (e.g., DICOM‐RT), the system‐specific implementations may be incompatible . The purpose of this work is to develop and evaluate tools to seamlessly transfer plans designed on one commercial treatment planning system (TPS) to another TPS and vice versa. Method and materials: Pinnacle and Eclipse are the two TPS used in this study. From Pinnacle to Eclipse, a filter to make the Pinnacle DICOM‐RT plans conform to Eclipse implementation was developed. From Eclipse to Pinnacle, a tool to covert a DICOM‐RT plan file to Pinnacle Script file was developed. To evaluate these tools, ten prostate patients planned and treated using IMRT at our institution were used. The 10 IMRT plans were first transferred from Pinnacle to Eclipse, and the doses of the 10 plans were recalculated on Eclipse. New IMRT plans were designed on Eclipse also. The latter were transferred back to Pinnacle and doses were re‐calculated there. Also, optimal fluence distributions generated on Eclipse were transferred to Pinnacle, and the Pinnacles leaf sequencer was used to generate new leaf sequences. To evaluate the differences between Pinnacle and Eclipse plans, dose and dose‐volume indices were used. Results: The dosimetric data for the plans transferred to Eclipse from Pinnacle do not differ significantly from the original plans, and vice versa. For plans of similar quality, the ones designed on Eclipse had 56% fewer segments than the plans designed on Pinnacle. The Pinnacle generated 23% more segments than the Eclipse using the same optimized fluence distributions imported from Eclipse. Conclusions: DICOM‐RT implementations are often not complete and, therefore, compatible among different commercial planning systems. Special tools are needed to make the plans interchangeable.

SU‐FF‐T‐344: Impact of Inter‐Fractional Motion of the Anatomy On Prostate Proton Dose

Purpose: The purpose of this study is to determine whether the inter‐fractional motion of the anatomy has a more significant impact on proton dose distributions compared to IMRT dose distributions. A secondary objective is to evaluate the impact of CTV‐to‐PTV margins on plans for protons produced with the double scattering technique. Method and Materials: Repeat CT scans of prostate patients acquired with a CT‐on‐Rails were used for this study. First, proton and IMRT plans were designed using (1) the CTV‐to‐PTV margin standard at our institution (normally 8 mm except at the rectum‐prostate interface, where it is 5.8 mm) and (2) a small uniform margin (3mm). The proton and IMRT plans were then applied to 8 daily CTimages aligned either to skin marks or the center of prostate. The doses for the 8 daily CTimages were recalculated using the same beam configurations (aperture, compensators, gantry angles, Monitor Units etc). Results: For proton plans, a 3mm margin appears to be adequate for tumor coverage even when a conventional skin mark alignment technique is used. The proton plans with 3mm margins lead to nearly the same coverage as in the IMRT plans with standard margins. For prostate center of volume‐based alignment, this coverage was 98.6% for protons vs. % 98.1 for IMRT and 96.6% vs. 96.3% for skin marks‐based alignment. Conclusion: With the double scattering technique, the dose distribution from the proton plan is not very sensitive to the daily variation of the patient anatomy as compared to IMRT plans. The 3 mm CTV‐to‐PTV margin is acceptable for proton plans but not for IMRT plans for both alignment methods. The preliminary data show that the CTV‐to‐PTV margin can be reduced to 3mm if we use daily image guided set‐up.

MO‐D‐T‐617‐01: Measurement of In‐Air Output Ratios for a Linear Accelerator with and Without the Flattening Filter

Purpose: In‐air output ratios (Sc) for photon beams from linear accelerators describe the change of in‐air output as a function of the collimator settings. The physical origin of in‐air output ratios is mainly due to the change in scattered radiation that can reach the point of measurement as the geometry of the head changes. The flattening filter and primary collimator are the major sources of scattered radiation. The change of amount of backscattered radiation from the collimator into the monitor ion chamber also contributes to the variation of output. To have a better understanding of the contribution of various components to Sc, we measured Sc and backscatter factor Sb for a linear accelerator with and without flattening filter. Method and Materials: In‐air output ratio (Sc) measurements were carried out with a Farmer type ion chamber in a mini‐phantom at 10 g/cm2 depth for 6 MV and 18 MV x‐ray beams from a Varian 2000EX linear accelerator.Backscatter factor (Sb) were measured with a universal pulse counter and a diode array with build‐in counting hardware and software. The scatter component Sh was then derived from the relation, S c = S h × S b , where Sb was the linear fit of measured results. Results: Significant differences are observed for Sc with and without the flattening filter. Within experimental uncertainty, the backscatter factors, Sb, are similar with and without the flattening filter. There are significant differences in variations of Sh over the range of field size 3×3 to 40×40 cm2 with and without the flattening filter; for 6 MV it is 8% versus 3%, and for 18 MV 7% versus 1%. Conclusion: By analyzing the backscatter contribution and total in‐air output ratios with and without flattening filter, we gained insight on contributions of different components to the total variation of Sc.