Abutment region dosimetry for serial tomotherapy

Purpose: A commercial intensity modulated radiation therapy system (Corvus, NOMOS Corp.) is presently used in our clinic to generate optimized dose di stributions delivered using a proprietary dynamic multileaf collimator (DML C) (MIMiC) composed of 20 opposed leaf pairs. On our accelerator (Clinac 60 0C/D, Varian Associates, Inc.) each MIMiC leaf projects to either 1.00 x 0. 84 or 1.00 x 1.70 cm(2) (depending on the treatment plan and termed 1 cm or 7 cm mode, respectively). The MIMiC is used to deliver serial (axial) tomo therapy treatment plans, in which the beam is delivered to a nearly cylindr ical volume as the DMLC is rotated about the patient. For longer targets, t he patient is moved (indexed) between treatments a distance corresponding t o the projected leaf width, The treatment relies on precise indexing and a method was developed to measure the precision of indexing devices. A treatm ent planning study of the dosimetric effects of incorrect patient indexing and concluded that a dose heterogeneity of 10% mm(-1) resulted, Because the results may be sensitive to the dose model accuracy, we conducted a measur ement-based investigation of the consequences of incorrect indexing using o ur accelerator, Although the indexing pro provides an accurate field abutme nt along the isocenter, due to beam divergence, hot and cold spots,,ill be produced below and above isocenter, respectively, when less than 300 degree s arcs a ere used, A preliminary study recently determined that for a 290 d egrees rotation in 1 cm mode, 15% cold and 7% hot spots were delivered to 7 cm above and below isocenter, respectively. This study completes the earli er work by investigating the dose heterogeneity as a function of position r elative to the axis of rotation, are length, and leaf width, The influence of random daily patient positioning errors is also investigated, Methods and Materials: Treatment plans were generated using 8.0 cm diameter cylindrical target volumes within a homogeneous rectilinear film phantom, The plans included both 1 and 2 cm mode, optimized for 300 degrees, 240 deg rees, and 180 degrees gantry rotations. Coronal-oriented films were irradia ted throughout the target volumes and scanned using a laser film digitizer, The central target irradiated in 1 cm mode was also used to investigate th e effects of incorrect couch indexing, Results: The dose error as a function of couch index error was 25% mm(-1), significantly greater than previously reported, The clinically provided ind exing system yielded 0.10 mm indexing precision. The intrinsic dose distrib utions indicated that more heterogeneous dose distributions resulted from t he use of smaller gantry angle ranges and larger leaf projections, Using 30 0 degrees gantry angle and 1 cm mode yielded 7% hot and 15% cold spots 7 cm below and above isocenter, respectively. When a 180 degrees gantry angle w as used, the values changed to 22% hot and 27% fold spots for the same loca tions. The heterogeneities for the 2 cm mode were 70% greater than the corr esponding 1 cm values. Conclusions: While serial tomotherapy is used to deliver highly conformal d ose distributions, significant dosimetric factors must be considered before treatment. The patient must be immobilized during treatment to avoid dose heterogeneities caused by incorrect indexing due to patient movement. Even under ideal conditions, beam divergence can cause significant abutment-regi on dose heterogeneities. The use of larger gantry angle ranges, smaller lea f widths, and appropriate locations of the gantry rotation axis can minimiz e these effects. (C) 1999 Elsevier Science Inc.