Characterisation of dose distribution in linear accelerator-based intracranial stereotactic radiosurgery with the dynamic conformal arc technique: consideration of the optimal method for dose prescription and evaluation

Abstract

Objectives: The purpose of this study was to characterise dose distribution in linear accelerator-based intracranial stereotactic radiosurgery using the dynamic conformal arc technique, and to validate the pertinence of dose prescription to the specific percentage isodose surface (IDS). Methods: 73 plans for brain metastases were reviewed and replanned with a uniform method for target definition and treatment planning. Results: In all cases except 1 the dose prescription to the 80% IDS satisfied the criteria of the standardised prescription IDS as previously proposed. However, both of the planning target volume (PTV) coverage values for the 80% and 90% IDSs and the PTV D99 and D95 (IDS receiving at least 99% or 95% of the PTV) were inconsistent and significantly increased as a function of the PTV size. The 80% IDS for a PTV of more than 5 cm 3 achieved adequate PTV coverage without a leaf margin. The dose conformity for 80% IDS gradually worsened as the PTV increased, whereas that for the PTV D99 or D95 improved as a function of the PTV size. The addition of a leaf margin attained 100% PTV coverage for 80% IDS, while leading to a poorer dose conformity. Conclusion: The dose prescription to the specific percentage IDS does not necessarily guarantee consistent target coverage, D99 and D95, and desirable dose conformity in proportion to the target volume. The dose prescription and evaluation at the specific target coverage would therefore be preferable as an objective method in order to report the ''marginal dose'' and to clearly compare the planning parameters with those from other modalities.

Comparison of pencil beam–based homogeneous vs inhomogeneous target dose planning for stereotactic body radiotherapy of peripheral lung tumors through Monte Carlo–based recalculation

“…6,12 In brain STI using a micro-MLC, 70% IDS for target coverage exploits dosimetric advantages in a superior dose gradient outside the target and a sharper dose increase just interior to the target boundary, when compared with 70%, 80%, and 90% IDSbased plans, normalized to 100% at the isocenter. 6 Furthermore, a fixed-leaf margin setting does not necessarily guarantee consistent target coverage, and leaf margin should be individually adjusted principally according to the target volume and the delivery platform (e.g., differences in the MLC design).…”

“…6 Furthermore, a fixed-leaf margin setting does not necessarily guarantee consistent target coverage, and leaf margin should be individually adjusted principally according to the target volume and the delivery platform (e.g., differences in the MLC design). 12,15 Thus, STI is intrinsically characterized by inhomogeneous target dose distribution with steeper dose falloff outside the target, thereby ensuring sufficient target dose while minimizing normal tissue dose. 6 Accordingly, the pioneers for lung SBRT originally used lower percentage IDS, such as 65% or 80%, for target coverage, 1,16,17 whereas some practitioners prefer homogeneous target dose, a decision that is likely based on the precedent that target dose homogeneity is principally desirable for conventional radiotherapy.…”

“…An expanded PTV was generated with the addition of 415 to 20 mm margins onto the PTV to directly compute any irradiated isodose volume. 12 In addition to the PTV, the substructure doses of the PTV were further analyzed: D 98 , D 95 , mean dose (D mean ), and D 2 were calculated for the GTV, PTV, and the PTV excluding GTV (PTV-GTV), where D n means the dose encompassing at least n% of the target volumes. In most cases, the ITVs would contain a small amount of lung tissue, and the GTV was therefore chosen, instead of the ITV, to evaluate the marginal dose of the tumor bulk.…”

“…6,10,12 Comparisons of paired and unpaired variables were performed using the Wilcoxon signed-rank test and Mann-Whitney U test, respectively. Spearman's rank correlation coefficient was used to evaluate any correlations between variables.…”

Radiothérapie en conditions stéréotaxiques des métastases cérébrales

“…ainsi que sur la méthode de prescription de la dose (au centre, dans l'isodose et laquelle ?) [16][17][18][19]. Ces limites empêchent toute comparaison définitive intercentres, à la différence des séries radiochirurgicales, très homogènes dans leur méthodologie.…”