Impact of collimator leaf width on stereotactic radiosurgery and 3D conformal radiotherapy treatment plans

Purpose: The authors undertook a study to analyze the impact of collimator leaf width on stereotactic radiosurgery and 3D conformal radiotherapy treat ment plans. Methods and Materials: Twelve cases involving primary brain tumors, metasta ses, or arteriovenous malformations that had been planned with BrainLAB's c onventional circular collimator-based radiosurgery system were re-planned u sing a beta-version of BrainLAB's treatment planning software that is compa tible with MRC Systems' and BrainLAB's micro-multileaf collimators. These c ollimators have a minimum leaf width of 1.7 mm and 3.0 mm, respectively, at isocenter. The clinical target volumes ranged from 2.7-26.1 cc and the num ber of static fields ranged from 3-5. In addition, for 4 prostate cancer ca ses, 2 separate clinical target volumes were planned using MRC Systems' and BrainLAB's micro-multileaf collimators and Varian's multileaf collimator: the smaller clinical target volume consisted of the prostate gland and the larger clinical target volume consisted of the prostate and seminal vesicle s. For the prostate cancer cases, treatment plans were generated using eith er 6 or 7 static fields. A "PITV ratio," which the Radiation Therapy Oncolo gy Group defines as the volume encompassed by the prescription isodose surf ace divided by the clinical target volume, was used as a measure of the qua lity of treatment plans (a PITV ratio of 1.0-2.0 is desirable). Bladder and rectal volumes encompassed by; the prescription isodose surface, isodose d istributions and dose volume histograms were also analyzed for the prostate cancer patients. Results: In 75% of the cases treated with radiosurgery, a PITV ratio betwee n 1.0-2.0 could be achieved using a micro-multileaf collimator with a leaf width of 1.7;3.0 mm at isocenter and 3-5 static fields. When the clinical t arget volume consisted of the prostate gland, the micro-multileaf collimato r with a minimum leaf width of 3.0 mm allowed one to decrease the median vo lume of bladder and rectum within the prescription isodose surface by 26% a nd 17%, respectively, compared to the multileaf collimator with a leaf widt h of 10 mm. Use of the 1.7 mm leaf width micro-multileaf collimator allowed one to decrease the median volume of bladder and rectum within the prescri ption isodose surface by 48% and 39%, respectively, compared to the multile af collimator with a leaf width of 10 mm. Conclusions: For most lesions treated with radiosurgery, the use of a micro -multileaf collimator with a leaf width of 1.7-370 mm at isocenter and 3-5 static fields allows one to meet the Radiation Therapy Oncology Group guide lines for treatment planning. Both planning and treatment are relatively st raightforward with a micromultileaf collimator, allowing for efficient trea tment of non-spherical targets with either stereotactic radiosurgery or fra ctionated stereotactic radiotherapy. When the clinical target volume consis ts of the prostate gland, micro-multileaf collimators with a minimum leaf w idth of 1.7-3.0 mm allow one to spare more bladder and rectum than one can with a multileaf collimator that has a IO-mm leaf width based on an analysi s of PITV ratios, isodose distributions, and dose volume histograms. (C) 19 99 Elsevier Science Inc.