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Study of MLC Apertures and Their Significance for Volumetric Arc Therapy in the Management of Non-Small Cell Lung Cancer Using Stereotactic Body Radiotherapy

H Malhotra

J Spaans, L Mackowiak, H Malhotra*, Roswell Park Cancer Institute, Buffalo, NY

SU-E-T-477 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Electronic disequilibrium concerns limit the minimum field sizes to 3.5 x 3.5 cm² for NSCLC patients per various RTOG protocols which is easily achievable using 3DCRT. However, the inverse planning nature of VMAT does not provide any control on the field-size or MLC shapes and may often violate it. This study aims to analyze the difference between VMAT and 3DCRT planning techniques by evaluating aperture size & hence conformity with RTOG protocols.

Methods: 25 patients with NSCLC [5 tumors per lobe] who were treated using 3DCRT were retrospectively inverse planned for VMAT. Dosimetrically, VMAT plans were almost identical with that of clinical 3DCRT plans. DICOM plans were imported into MATLAB to determine the actual MLC area of the active leaves per control point and its relative weight in the overall dose delivery of that VMAT arc. The effective-weight of all the MLC areas which were less than 3.5 x 3.5 cm² was summated (VMAT3.5fs). We also studied the conditions where the total MLC area did satisfy the minimum requirement of 3.5 x 3.5 cm² but where any two opposite MLC leaf positions were less than 3.5 cm apart (VMAT3.5MLCwt). These evaluations are contained in VMAT3.5MLCwt which gives the weighted sum of all small MLC segments in the entire VMAT arc. A zero value of both ensures that the VMAT arc satisfies the RTOG requirement of minimal jaws size in its entirety.

Results: Respective values of VMAT3.5fs & VMAT3.5MLCwt were found to be 0.88 ± 0.23 & 0.9 ± 0.21 for 63 VMAT arcs. Only 2 patients had a value of 0 for both indices implying compliance to RTOG field size limits.

Conclusion: Proposed indices provide an objective method to evaluate the suitability of VMAT plans in satisfying RTOG constraints for NSCLC treatment using SBRT.

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