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Normal Tissue Dose-Volume Constrains for Inverse Planning of Acoustic Neuroma Stereotactic Radiosurgery (SRS)


H Liu

H Liu*, D Andrews, M Werner-Wasik, Y Xiao, Y Yu, W Shi, Thomas Jefferson Univ Hospital, Philadelphia, PA

SU-E-T-678 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Stereotacic radiosurgery (SRS) of acoustic neuroma was traditionally planned using forward planning techniques, either with Elekta GammaKnife ball-packing technique, or with multiple conformal dynamic arcs technique using micro-MLC and Linac (BrainLAB Novalis). As the recent development of Volumetric Modulated Arc Therapy (VMAT), inverse planning using multiple-arcs VMAT technique for SRS becomes commercially available, and it has the advantage of faster delivery with comparable dosimetry. However, planners need proper dose volume constrains to drive the optimizer to achieve the dosimetric goals, both clinically acceptable and technically achievable. This study is investigating how to set up the proper dose volume constraints for SRS plans of acoustic neuroma.

Methods:
Twenty cases were selected with their volumes cover the general range of acoustic neuroma SRS (0.1 cc to 5cc). Each case was planned using Gammaknife Perfexion with sector optimization. Prescription is 12Gy to 50% isodose, with 100% target coverage. The total tissue volumes that receive greater than 12, 8, and 6 Gy (V12, V8, and V6) were recorded. The relationship between the target volumes (Vtarget) and V12, V8, and V6 were plotted and linear regressions were fitted for each parameter.

Results:
A set of linear relationships between the target volume and the volumes that receive greater than 12, 8, and 6Gy was constructed as the following: V12 = 1.476Vtarget (R2 = 0.995), V8 = 3.04Vtarget +0.16(R2 = 0.990), and V6 = 4.64 Vtarget + 0.35 (R2 = 0.982).

Conclusions:
Traditional plan quality indices such as conformity index, selectivity, etc. are good for evaluating forward plans, but cannot be used to drive the inverse planning engine to optimize inverse plans. The normal tissue dose volume constraints obtained in this study, combined with clinical constraints for brainstem and cochlea, can provide inverse planner with objectives to obtain an inverse plan for acoustic neuroma SRS.


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