BEAM SHAPING FOR CONFORMAL FRACTIONATED STEREOTAXIC RADIOTHERAPY - A MODELING STUDY

Purpose: The patient population treated with fractionated stereotactic radiotherapy (SRT) is significantly different than that treated with stereotactic radiosurgery (SRS). Generally, lesions treated with SRT a re larger, less spherical, and located within critical regions of the central nervous system; hence, they offer new challenges to the treatm ent planner. Here a simple, cost effective, beam shaping system has be en evaluated relative to both circular collimators and an ideal dynami cally conforming system for effectiveness in providing conformal thera py for these lesions. Methods and Materials: We have modeled a simple system for conformal are therapy using four independent jaws. The jaw positions and collimator angle are changed between arcs but held fixed for the duration of each are. Eleven previously treated SRT cases hav e been replanned using this system. The rectangular jaw plans were the n compared to the original treatment plans which used circular collima tors. The plans were evaluated with respect to tissue sparing at 100%, 80%, 50%, and 20% of the prescription dose. A plan was also done for each tumor in which the beam aperture was continuously conformed to th e beams eye view projection of the tumor. This was used as an ideal st andard for conformal therapy in the absence of fluence modulation. Res ults: For tumors with a maximum extent of over 3.5 cm the rectangular jaw plans reduced the mean volume of healthy tissue involved at the pr escription dose by 57% relative to the circular collimator plans. The ideal conformal plans offered no significant further improvement at th e prescription dose. The relative advantage of the rectangular jaw pla ns decreased at lower isodoses so that at 20% of the prescription dose tissue involvement for the rectangular jaw plans was equivalent to th at for the circular collimator plans. At these isodoses the ideal conf ormal plans gave substantially better tissue sparing. Conclusion: A si mple and economical field shaping device has been shown to provide all of the beam shaping advantage of a hypothetical ideal dynamically con forming system at the prescription level. This system may be immediate ly implemented in the clinic. It offers a substantial advantage over t he currently used circular collimators in the high dose region with eq uivalent performance in the low dose region. (C) 1997 Elsevier Science Inc.