A quality factor to compare the dosimetry of gamma knife radiosurgery and intensity-modulated radiation therapy quantitatively as a function of target volume and shape - Technical note

The authors have developed a quality factor (QF) to compare gamma knife rad iosurgery, linear accelerator radiosurgery, and intensity-modulated radiati on therapy (IMRT) dosimetry. This QF relates the percentage of target cover ed (PTC) by the prescription radiation isodose, target volume (V-T), and en closed tissue volume, which receives greater than a particular dose (V-x): QF(x) = PTCXVT/N-x. The authors investigated target shape independent of vo lume in predicting radiosurgical complication rates. Plastic targets of a defined volume (0.2, 0.5, 1.5, and 10 cm(3)) and four increasingly complex shapes (spherical, ellipsoid, simulated arteriovenous malformation [AVM], and horseshoe) were created. Dosimetry was studied on t he Leksell GammaPlan, Adac/Pinnacle, and Nomos Corvus workstations. The dos imetry of a new 4 mm X 10-mm IMRT collimator array (the Nomos Beak) not yet validated for use in our clinical practice was studied. Particularly for larger targets, the gamma knife and IMRT Beak plans show s imilar conformality (QF assuming 15-Gy volume [QF(15)]) Particularly for sm all and round targets the gamma knife plan quality is significantly higher (QF assuming 12-Gy volume [QF(12)]). As V-T and complexity increase, the IM RT Beak QF(12) approaches that of the gamma knife. The QF(12) of gamma knife dosimetry has an inverse correlation with target shape complexity independent of V-T. At a prescription dose of 15 Gy to the target margin, the QF(12) is a confo rmality index. The 12-Gy volume (volume enclosed by 12-Gy surface/volume re ceiving at least 12 Gy) estimates the radiosurgical normal tissue complicat ion rate for AVMs. When the target is well covered, the QF(12) is inversely proportional to the complication risk and is a measure of the plan quality .