Differential dosing of prostate and seminal vesicles using dynamic multileaf collimation

Purpose: We have investigated the potential of applying different doses to the prostate (PTV2) and prostate/ seminal vesicles (PTV1) using multileaf c ollimation (MLC) for intensity modulated radiation therapy (IMRT). Current dose-escalation studies call for treatment of the PTV1 to 54 Gy in 27 fract ions followed by 20 Gy minimum to the PTV2. A daily minimum PTV dose of 2 G y using a 7-field technique (4 obliques, opposed laterals, and an ant-post field) is delivered. This requires monitor unit calculations, paper and ele ctronic chart entry, and quality assurance for a total of 14 fields. The go al of MLC IMRT is to improve efficiency and deliver superior dose distribut ions. Acceptance testing and commissioning of the dynamic MLC (DMLC) option on a dual energy accelerator was accomplished. Most of the testing was per formed using segmental MLC (SMLC) IMRT with stop-and-shoot sequences built within the dynamic mode of the DMLC. Methods and Materials: The MLC IMRT fields were forward planned using a thr ee-dimensional treatment planning system. The 14 fields were condensed to 7 SMLC IMRT fields,vith two segments each. In this process, steps were creat ed by moving the leaves to the reduced held positions. No dose (<0.01%) was delivered during this motion. The monitor units were proportioned accordin g to the planned treatment weights. Film and ionization chamber dosimetry w ere used to analyze leaf positional accuracy and speed, output, and depth-d ose characteristics. A geometric phantom,vas used for absolute and relative measurements. We obtained a volumetric computerized tomography (CT) scan o f the phantom, performed 3D planning, and then delivered a single treatment fraction. Results: The acceptance testing and commissioning demonstrated that the lea ves move to programmed positions accurately and in a timely manner. We did find an <similar to>1 mm offset of the set leaf position and radiation edge (50%) due to the curved-end nature and calibration limitations. The 7-fiel d SMLC IMRT treatment duplicated the 14-field static plan dose distribution with variations no greater than 1.5%. Conclusions: The MLC IMRT approach will improve efficiency because the numb er of electronic and chart entries has decreased by a factor of 2. Portal i mages are able to capture the initial and final MLC segments. The question of differential daily dose to the prostate and seminal vesicles remains. (C ) 2000 Elsevier Science Inc.