Program Information
Adapting High-Definition MLC VMAT Plans to the Millennium MLC for Single-Fraction Treatment Without Re-Optimization
S French1,2*, M Podgorsak1,2 , S Bhagroo2 , J Zielan1 , D Nazareth1,2 , (1) Roswell Park Cancer Institute, Buffalo, NY (2) State University of New York at Buffalo, Buffalo, NY
Presentations
PO-BPC-Exhibit Hall-20 (Saturday, March 18, 2017) Room: Exhibit Hall
Purpose: Linacs occasionally require repair during treatment hours. Our department’s operational policy does not allow the cancellation of a patient’s daily treatment under these circumstances. This necessitates the transfer of patient plans to another linac for a single-fraction treatment. Transferring a VMAT plan from a high definition (HD) MLC linac to a standard Millenium MLC linac requires re-optimization. When several VMAT plans require transferring, this process may be unacceptably time-consuming. We have developed a time-efficient method for adapting HD plans to standard MLCs which does not involve re-optimization.
Methods: The VMAT plan’s DICOM RT file is exported from the TPS. For each control point, the positions of every pair of adjacent HD leaves are averaged to provide the new standard leaf position. A modified DICOM file is created with Matlab and imported into the TPS under a new standard linac plan. The dose is then calculated and normalized similarly to the original plan. Evaluations were performed by applying our method to 6 clinical prostate VMAT plans and examining the resulting PTV and OAR DVHs.
Results: The PTV DVHs of the modified plans exhibited a slightly increased shouldering effect, due to reduced MLC resolution. This resulted in an average increase of the maximum PTV dose of 9.8%. The OAR doses similarly showed small increases. For example, the mean rectum dose increased by an average of 2.0%. These single fraction dose deviations were considered acceptable by the attending radiation oncologist since the composite dose distribution for a patient’s entire treatment course remained well within all dose constraints.
Conclusion: We have developed a time- efficient method for single-fraction transfer of VMAT plans from HD to standard Millenium MLC linacs. This process would prove useful in the event of linac downtime, for clinics equipped with one HD and one or more standard linacs.
Contact Email: