2835 Optimizing Beam Angles for IMRT Treatment Planning

K. Pesola

Varian Medical Systems, Helsinki, Finland

Purpose/Objective(s): In conventional IMRT treatment planning, beam angles are selected manually. This selection isfollowed by an optimization of beam profiles (fluences) using inverse optimization methods in order to minimize the value ofan objective function (OF). Typically, the OF value is proportional to the fulfillment of the user-given dose-volume histogram(DVH) based constraints. Several trial-and-error attempts in selecting beam angles may be needed in order to produce anacceptable treatment plan. This study presents a novel approach for optimizing beam angles for IMRT treatments. Specialattention has been paid to the quality of dose calculation within beam angle optimization while maintaining the runtime of thealgorithm acceptable for routine clinical work.

Materials/Methods: There are two modes in the current algorithm: the global optimization mode and the local optimizationmode. In the global mode, either a 2D (plane) or a 3D search space may be used. The initial search space is covered with a presetnumber of uniformly distributed fields. Thereafter, a few fluence optimization iterations are calculated in order to produce anoptimal beam profile for each field. Throughout the optimization process, the 3D dose distribution is completely calculated inorder to fully account for long range scatter effects. The relative importance of each field is determined by removing the fieldfrom the plan and calculating the corresponding OF value. The fields with a low importance value are thereafter removed. Thebeam angle optimization iterations are continued until the desired number of fields has been reached. The local beam angleoptimization mode is used for locally expanding the initially discrete search space into a continuous one in order to minimizethe value of the objective function. Two possibilities for the optimization algorithm to be used in the local optimization modehave been implemented: the downhill Simplex method and the Powell method.

Results: In order to clarify the goodness of the plans obtained with the novel beam angle optimization algorithm, an evaluationstudy has been carried out for 10 prostate cases. In addition to the PTV, the rectum, the bladder and the femoral heads weresegmented and set with DVH-based constraints. Beam angles were optimized for treatment plans including 5, 7 and 9 fields.The optimized beam angles were compared both with equispaced field setups and with class solution beam directions. Theinitial field geometry in beam angle optimization consisted of 72 fields in a 2D (coplanar) field geometry. The results show thatwith beam angle optimization it is possible to obtain 11 % decrease in the OF value for five field plans and a 14 % decreasein the OF value for seven field plans when compared to class solution plans. The decrease in the OF value can be observed alsoin the corresponding DVHs for PTV, bladder, rectum and femoral heads.

Conclusions: This study shows that the novel beam angle optimization algorithm is able to improve the OF values obtainedwith the class solution plans for five field and for seven field prostate treatments. The runtime of the novel beam angleoptimization algorithm (around 10–30 min) makes it usable in routine clinical work. By taking the beam angle optimizationalgorithm into clinical use, the workload of the personnel constructing the IMRT plans would be significantly reduced.

Author Disclosure: K. Pesola, Varian Medical Systems, A. Employment.

2836 Restricted Field IMRT Dramatically Enhances IMRT Planning for Mesothelioma Patients

M. A. Czerminska, D. Schofield, F. L. Hacker, E. H. Baldini, A. Allen

Dana Farber/Brigham and Women’s Cancer Center, Boston, MA

Purpose/Objective(s): IMRT has been shown to improve local control after extrapleural pneumonectomy(EPP) (Stevens et alASTRO ’05). However, planning is extremely complex because of the normal tissue constraints of the heart, liver and lung.Previous experience of IMRT to 54Gy following EPP produced a high rate of fatal pneumonitis (46%) (IJROBP 5/06) with lunglimits of V20�20% and mean lung dose (MLD) �15 Gy. We sought to devise a novel method of delivering IMRT (restrictedfield IMRT) that would not compromise target coverage but would dramatically limit the contralateral lung dose.

Materials/Methods: All patients were initially irradiated with 54 Gy to the PTV with selected areas boosted to 60Gy. The liverwas kept to a mean dose of 31Gy. The lung constraints are reported above. In order to decrease the contralateral lung dose threesteps were taken. First, the optimization criteria were modified to limit the lung to MLD�9.5 , and V5 � 60%. Secondly, the7–9beam angles that were used were limited to a maximum of 10–20 degrees into the contralateral side. Most importantly,independent jaws were used to restrict beams that were passing through a large volume of lung superiorly down to the levelof the heart on left sided cases and down to the level of the liver on right-sided cases. The superior portion of the target wastreated only by the unrestricted beams (primarily AP-PA), that did not pass through lung tissue. Optimization was done withECLIPSE (Varian, CA) software.

Results: Six patient cases with mesothelioma s/p EPP underwent replanning with the above described “Restricted Field IMRT”technique. The mean value for MLD, V20 and V5 were 8.8 Gy, 8.0%, and 59.7% respectively, compared with 14.6 Gy MLD,V20 of 16.5% and V5 of 94.5% in our previous series. Constraints for all other organs at risk were maintained.

Conclusions: The Restricted Field IMRT technique dramatically reduces contralateral lung dose without sacrificing targetcoverage in pleural mesothelioma cases. This technique results in much lower lung doses than can be achieved withoptimization alone.

Author Disclosure: M.A. Czerminska, None; D. Schofield, None; F.L. Hacker, None; E.H. Baldini, None; A. Allen, None.

S680 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006