Abstract
PurposeIn stereotactic radiosurgery (SRS), prescription isodoses and resulting dose homogeneities vary widely across different platforms and clinical entities. Our goal was to investigate the physical limitations of generating dose distributions with an intended level of homogeneity in robotic SRS.MethodsTreatment plans for non-isocentric irradiation of 4 spherical phantom targets (volume 0.27–7.70 ml) and 4 clinical targets (volume 0.50–5.70 ml) were calculated using Sequential (phantom) or VOLOTM (clinical) optimizers (Accuray, Sunnyvale, CA, USA). Dose conformity, volume of 12 Gy isodose (V12Gy) as a measure for dose gradient, and treatment time were recorded for different prescribed isodose levels (PILs) and collimator settings. In addition, isocentric irradiation of phantom targets was examined, with dose homogeneity modified by using different collimator sizes.ResultsDose conformity was generally high (nCI ≤ 1.25) and varied little with PIL. For all targets and collimator sets, V12Gy was highest for PIL ≥ 80% and lowest for PIL ≤ 65%. The impact of PIL on V12Gy was highest for isocentric irradiation and lowest for clinical targets (VOLOTM optimization). The variability of V12Gy as a function of collimator selection was significantly higher than that of PIL. V12Gy and treatment time were negatively correlated. Plans utilizing a single collimator with a diameter in the range of 70–80% of the target diameter were fastest, but showed the strongest dependence on PIL.ConclusionInhomogeneous dose distributions with PIL ≤ 70% can be used to minimize dose to normal tissue. PIL ≥ 90% is associated with a marked and significant increase in off-target dose exposure. Careful selection of collimators during planning is even more important.
Highlights
Stereotactic radiosurgery (SRS) is a well-recognized and successful treatment modality for patients with malignant or benign intracranial pathologies [1,2,3,4,5,6]
The corresponding dose–volume histograms (DVHs) for dose prescriptions of 20 Gy at the 38%, 65%, 82%, and 93% isodose levels clearly show the dose-sparing effect outside the target volume with simultaneous dose escalation within the target for the lower prescribed isodose levels (PILs) associated with the smaller collimator sizes (Fig. 1b)
Our results show that for the dose plans of both algorithms, there is a similar dependency between dose conformity, volume of 12 Gy isodose (V12Gy), and treatment time on PIL and collimator selection
Summary
Stereotactic radiosurgery (SRS) is a well-recognized and successful treatment modality for patients with malignant or benign intracranial pathologies [1,2,3,4,5,6]. In SRS, very high doses are applied to the target volume in one or only a few fractions, which imposes strict requirements on minimizing the dose to normal brain tissue This is achieved by minimizing the safety margins using stereotactic head fixation or inter- and intrafractional imaging, by conforming the high doses to the shape of the target and by a steep dose fall-off at the periphery of the target volume. In many SRS systems, a set of cones with fixed circular field sizes are used for this purpose With these circular collimators, dose conformation to the target volume can either be achieved by superposition of multiple isocentric shots (‘sphere packing’) or by application of a non-isocentric beam array adapted to the target shape [13]. A more homogeneous dose distribution appears suitable for targets such as vestibular schwannoma or pituitary adenoma, as healthy tissue may be included in the target volume [12, 15,16,17,18]
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