Engineering topics

Xia, P

Publications and source records attributed to Xia, P.

22 records · Page 2

DOE OSTI2014

SU-E-J-245: Is Off-Line Adaptive Radiotherapy Sufficient for Head and Neck Cancer with IGRT?

Purpose: Radiation doses delivered to patients with head and neck cancer (HN) may deviate from the planned doses because of variations in patient setup and anatomy. This study was to evaluate whether off-line Adaptive Radiotherapy (ART) is sufficient. Methods: Ten HN patients, who received IMRT under daily imaging guidance using CT-on-rail/KV-CBCT, were randomly selected for this study. For each patient, the daily treatment setup was corrected with translational only directions. Sixty weekly verification CTs were retrospectively analyzed. On these weekly verification CTs, the tumor volumes and OAR contours were manually delineated by a physician. With the treatment iso-center placed on the verification CTs, according to the recorded clinical shifts, the treatment beams from the original IMRT plans were then applied to these CTs to calculate the delivered doses. The electron density of the planning CTs and weekly CTs were overridden to 1 g/cm3. Results: Among 60 fractions, D99 of the CTVs in 4 fractions decreased more than 5% of the planned doses. The maximum dose of the spinal cord exceeded 10% of the planned values in 2 fractions. A close examination indicated that the dose discrepancy in these 6 fractions was due to patient rotations, especially shoulder rotations. After registering these 6 CTs with the planning CT allowing six degree of freedoms, the maximum rotations around 3 axes were > 1.5° for these fractions. With rotation setup errors removed, 4 out of 10 patients still required off-line ART to accommodate anatomical changes. Conclusion: A significant shoulder rotations were observed in 10% fractions, requiring patient re-setup. Off-line ART alone is not sufficient to correct for random variations of patient position, although ART is effective to adapt to patients' gradual anatomic changes. Re-setup or on-line ART may be considered for patients with large deviations detected early by daily IGRT images. The study is supported in part by Siemens Medical Solutions.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2014

SU-E-J-249: Correlation of Mean Lung Ventilation Value with Ratio of Total Lung Volumes

Purpose: Lung ventilation function measured from 4D-CT and from breathing correlated CT images is a novel concept to incorporate the lung physiologic function into treatment planning of radiotherapy. The calculated ventilation functions may vary from different breathing patterns, affecting evaluation of the treatment plans. The purpose of this study is to correlate the mean lung ventilation value with the ratio of the total lung volumes obtained from the relevant CTs. Methods: A ventilation map was calculated from the variations of voxel-to-voxel CT densities from two breathing phases from either 4D-CT or breathing correlated CTs. An open source image registration tool of Plastimatch was used to deform the inhale phase images to the exhale phase images. To calculate the ventilation map inside lung, the whole lung was delineated and the tissue outside the lung was masked out. With a software tool developed in house, the 3D ventilation map was then converted in the DICOM format associated with the planning CT images. The ventilation map was analyzed on a clinical workstation. To correlate ventilation map thus calculated with lung volume change, the total lung volume change was compared the mean ventilation from our method. Results: Twenty two patients who underwent stereotactic body irradiation for lung cancer was selected for this retrospective study. For this group of patients, the ratio of lung volumes for the inhale (Vin ) and exhale phase (Vex ) was shown to be linearly related to the mean of the local ventilation (Vent), Vin/Vex=1.+0.49*Vent (R2=0.93, p<0.01). Conclusion: The total lung volume change is highly correlated with the mean of local ventilation. The mean of local ventilation may be useful to assess the patient's lung capacity.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2014

SU-E-T-405: Robustness of Volumetric-Modulated Arc Therapy (VMAT) Plans to Systematic MLC Positional Errors

Purpose: To evaluate the dosimetric impact of systematic MLC positional errors (PEs) on the quality of volumetric-modulated arc therapy (VMAT) plans. Methods: Five patients with head-and-neck cancer (HN) and five patients with prostate cancer were randomly chosen for this study. The clinically approved VMAT plans were designed with 2–4 coplanar arc beams with none-zero collimator angles in the Pinnacle planning system. The systematic MLC PEs of 0.5, 1.0, and 2.0 mm on both MLC banks were introduced into the original VMAT plans using an in-house program, and recalculated with the same planned Monitor Units in the Pinnacle system. For each patient, the original VMAT plans and plans with MLC PEs were evaluated according to the dose-volume histogram information and Gamma index analysis. Results: For one primary target, the ratio of V100 in the plans with 0.5, 1.0, and 2.0 mm MLC PEs to those in the clinical plans was 98.8 ± 2.2%, 97.9 ± 2.1%, 90.1 ± 9.0% for HN cases and 99.5 ± 3.2%, 98.9 ± 1.0%, 97.0 ± 2.5% for prostate cases. For all OARs, the relative difference of Dmean in all plans was less than 1.5%. With 2mm/2% criteria for Gamma analysis, the passing rates were 99.0 ± 1.5% for HN cases and 99.7 ± 0.3% for prostate cases between the planar doses from the original plans and the plans with 1.0 mm MLC errors. The corresponding Gamma passing rates dropped to 88.9 ± 5.3% for HN cases and 83.4 ± 3.2% for prostate cases when comparing planar doses from the original plans and the plans with 2.0 mm MLC errors. Conclusion: For VMAT plans, systematic MLC PEs up to 1.0 mm did not affect the plan quality in term of target coverage, OAR sparing, and Gamma analysis with 2mm/2% criteria.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2014

SU-E-T-534: Dosimetric Effect of Multileaf Collimator Leaf Width On Volumetric Modulated Arc Stereotactic Radiotherapy for Spine Tumors

Purpose: To study the dosimetric impact of MLC leaf width in patients treated with Volumetric Modulated Arc Therapy (VMAT) for spine Stereotactic Body radiation Therapy (SBRT). Methods: Twelve spine SBRT patients were retrospectively selected for this study. The patients were treated with IMRT following the RTOG-0631 of spine metastasis. The prescription dose was 16 Gy in one fraction to 90% of the target volume (V16 > 90%). The maximum spinal cord dose of 14 Gy and 10% of the cord receiving < 10 Gy (V10) were set as dose constraints. For purpose of this study, three dual arc VMAT plans were created for each patient using three different MLC leaf widths: 2.5 mm, 4mm, and 5mm. The compliance to RTOG 0631, conformal index (CI), dose gradient index (DGI), and number of monitor units (MUs) were compared. Results: The average V16 of the target was 91.91±1.36%, 93.73±2.38%, and 92.25±2.49% for 2.5 mm, 4 mm, and 5 mm leaf widths, respectively (p=0.39). Accordingly, the average CI was 1.36±0.39, 1.36±0.34, and 1.41±0.3 (0.96), respectively. The average DGI was 0.24 ± 0.05, 0.22 ± 0.05, and 0.23 ± 0.04, respectively (p=0.86). The average spinal cord maximum dose was 12.10 ± 0.88 Gy, 12.52 ± 1.15 Gy, and 12.05 ± 1.12 (p=0.75) and V10 was 2.69 ± 1.71 cc, 5.43 ± 2.16 cc, and 3.71 ± 2.34 cc (p=0.15) for 2.5 mm, 4 mm, and 5 mm leaf widths, respectively. According, the average number of MUs was 4255 ± 431 MU, 5049 ± 1036 MU, and 4231 ± 580 MU respectively (p=0.17). Conclusion: The use of 2.5 mm, 4 mm, and 5 mm MLCs achieved similar VMAT plan quality as recommended by RTOG-0631. The dosimetric parameters were also comparable for the three MLCs.

60 APPLIED LIFE SCIENCES↗