COMPARISON OF MINIATURE MULTILEAF COLLIMATION (MMLC) WITH CIRCULAR COLLIMATION FOR STEREOTAXIC TREATMENT

Purpose: A prototype Miniature Multi-Leaf Collimator (MMLC) designed s pecifically for radiosurgery and small field radiotherapy has been fab ricated and evaluated at the University of Texas M. D. Anderson Cancer Center (UTMDACC). This work demonstrates the advantages of a computer -controlled MMLC vs. conventional circular collimation for the treatme nt of an irregularly shaped target volume in the brain. Methods and Ma terials: Two patient treatments were selected for this comparison from 38 intracranial tumors treated with radiosurgery at UTMDACC from 8/6/ 91 to 5/10/94. Target contours and critical structures defined for one of the patients was used to create a simulated target volume and crit ical structures in a spherical head phantom. Computer simulations were performed using traditional single isocenter treatment with a circula r collimator for a set of six arcs. The same are paths were used to co mpute the dose distribution for the MMLC and conformed beam geometries were defined using a three-dimensional (3D) treatment planning system with beam's eye view capabilities. Then, the calculated dose distribu tion for a single isocenter, conformal treatment was delivered to the spherical head phantom under static conditions by shaping the MMLC to conform the target volume shape projected as a function of couch rotat ion and gantry angle. Planar dose distributions through the target vol ume were measured using therapy verification film located in the phant om. The measurements were used to verify that the 3D treatment plannin g system was capable of simulating the MMLC technique. For the second patient with a peanut-shaped tumor, the 3D treatment planning calculat ions were used to compare dose distributions for the MMLC and for trad itional single and multiple isocenter treatments using circular collim ators. The resulting integral dose-volume histograms (DVHs) for the ta rget volume, normal brain, and critical structures for the three treat ment techniques were compared. Results: (a) Analysis of the film dosim etry data exemplified the degree of conformation of the high-dose regi on to the target shape that is possible with a computer-controlled MML C. (b) Comparison of measured and calculated dose distributions indica tes that the 3D treatment planning system can simulate the MMLC treatm ent. (c) Comparison of DVHs from the single isocenter MMLC and circula r collimator treatments shows similar coverage of the target volume wi th increased dose to the brain for circular collimation (4). Compariso n of DVHs from the single isocenter MMLC with the multiple isocenter c ircular collimator treatment approach shows a more inhomogeneous dose distribution through the target volume and increased dose to the brain for the latter. Conclusion: Dosimetry data for single isocenter treat ments using computer-controlled field shaping with a MMLC demonstrate the ability to conform the dose distribution to an irregularly shaped target volume. DVHs validated that the single isocenter MMLC treatment is preferable to both single and multiple isocenter, circular collima tor treatment because it provides a more uniform dose distribution to an irregularly shaped target volume and reduces the dose to surroundin g brain tissue for the example cases. (C) 1997 Elsevier Science Inc.