Purpose
The prescribed percentage-isodose-line (PIDL) in linac-based SRS varies among institutions. For plans with similar tumor coverage and conformity index, the one with sharper dose falloff outside the tumor volume would be preferred because the probability of brain necrosis is related to the irradiated volume (for example V12Gy) outside the tumor. The aim of this study is to investigate the optimal isodose line that yields the steepest dose falloff for linac-based SRS using dynamic conformal arc technique (DCA).
Methods
30 patients with brain tumors were retrospectively studied. The MLC-based DCA was used for planning. For each patient, 5-7 plans with different PIDLs but similar conformity indices were generated. All plans were normalized such that 95% of target volume receives the prescription dose (PD). Gradient index was calculated. The plan with minimum GI was considered optimal.
Results
Optimal GI decreases (3.9 to 2.2) as target volumes increases (0.15 to 50.1cm3), and the optimal PIDL shifts to higher percentile. Median optimal PIDL is 40.0±7.2% (range 33.2-53.1%) for targets <1cm3 and 62.3±8.3% (range 44.6-72.9%) for those >1cm3. The average planned PIDL used for treatment was 83.6±3.3%. The lower optimal PIDL results in smaller V0.5PD and higher mean dose to the tumor.
Conclusion
The optimal PIDL appears to be between 50% and 75% which is lower than the commonly used PIDLs in published studies. Larger targets tend to have higher optimal PIDLs. By choosing an optimal PIDL, we could reduce the volume of irradiated normal brain while delivering higher mean dose to the tumor.