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SU‐E‐T‐616: Shifting MLC to Follow the Prostate Movements while Concurrently Treating Pelvic Lymph Nodes
Author(s) -
Huang G.S.,
Qi P,
Shang Q,
Olsen L Sheplan,
Stephans K,
Tendulkar R,
Xia P
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612579
Subject(s) - isocenter , medicine , prostate , nuclear medicine , rectum , lymph , radiation treatment planning , collimated light , pelvis , cone beam computed tomography , radiation therapy , radiology , surgery , computed tomography , cancer , physics , laser , psychiatry , optics
Purpose: Shifting MLC leaves is a viable option for adaptive radiation therapy, particularly for slow motion organs such as prostate. The purpose of this paper is to evaluate effectiveness of the MLC shifting method for concurrent treatment of prostate and pelvic lymph nodes using daily kilo‐voltage cone beam CT (KV‐CBCT). Methods: We retrospectively evaluated 6 patients with 44 fractions of kV‐CBCT images. For each KV‐CBCT, contours of the prostate, bladder, and rectum were manually delineated, and the contour of lymph nodes was transferred from the planning CT after bone‐based rigid image registration. The daily prostate displacement was obtained using dual image registrations: alignment to the bone and alignment to the prostate. For each segment of an IMRT plan, an in‐house program automatically identified MLC pairs that were collimated to the prostate and adjusted the positions of these leaf pairs according to the measured shifts. The shifted MLC positions were input back to the planning system and applied to the KV‐CBCT for dose calculation. Results: The MLC‐shift method and isocenter shift method achieved a comparable dose coverage for prostate. The daily D95 was within 3% differences from the planned doses. For pelvic lymph nodes, daily D95 was within 1.5% from the planned dose for the MLC‐shift method, compared to 8% for the isocenter shift method. Compared to the planning dose, the differences for the MLC‐ shift method in D50 of the bladder and rectum ranged from −10.7% to 29.1% (mean −1.3%) and from 16.6% to 30.8% (mean 21.8%), respectively; and for the isocenter shift method, they ranged from −14.1% to 11.3% (mean −7.2%) and from 16.3% to 31.1% (mean 21.2%), respectively. Conclusions: The MLC‐shift method is effective for concurrent treatment of two independently moving targets, especially for patients with large organ motion. The process does not require re‐planning and can be automated.