Effect of prescription isodose line on tissue sparing in linear accelerator-based stereotactic radiosurgery treating multiple brain metastases using dynamic conformal arcs

Abstract

Purpose: Linear accelerator-based stereotactic radiosurgery (SRS) has become a mainstay for simultaneous management of multiple intracranial targets. Recent improvements in treatment planning systems (TPS) have enabled treatment of multiple brain metastases using dynamic conformal arcs (DCA) and a single treatment isocenter. However, as the volume of healthy tissue receiving at least 12 Gy (V12) is linked to the probability of developing radionecrosis, balancing target coverage while minimizing V12 is a critical factor affecting SRS plan quality. Current TPS allow users to adjust various parameters influencing plan optimization. The purpose of this work is to quantify the effect of negative margins on V12 for cranial SRS plans managing multiple brain metastases.

Methods: Using the Brainlab Elements v3.0 TPS (Brainlab, Munich, Germany), we calculated V10, V12, V15, monitor units, and conformity index for seventeen SRS plans treating 2-10 metastases on our Elekta Versa HD (Elekta, Stockholm, Sweden) linear accelerator. We compared plans optimized using 70%-90% prescription isodose lines (IDL) in 5% increments.

Results: Irrespective of the number of treated metastases, optimization at a lower prescription IDL reduced V10, V12, and V15 and increased MU compared to the 90% IDL (p < 0.01). However, comparing the 70% and 75% IDL optimizations, there was little difference in tissue sparing. The conformity index showed no consistent trends at different IDLs due to a significant spread in case data.

Conclusion: For our plans treating up to 10 metastases, diminishing returns for tissue sparing at IDLs below 80% paired with increasing treatment MU and dosimetric hot spot made optimization at lower IDLs less favorable. In our clinic, after consulting with a physician, it was determined that optimization at the 80% IDL achieved the best balance of V12, treatment MU, and maximum dose. Clinics implementing LINAC-based SRS programs may consider using similar evaluations to develop their own clinical protocols.

Keywords: external beam; stereotactic radiosurgery; treatment planning.

FIGURE 1

Side‐by‐side comparison of optimization using the 90% prescription isodose line (IDL) and 70% IDL. At the lower IDL, collimator leaves close the aperture and partially obscure the target in the beam's eye view, creating the “negative margin.”

FIGURE 2

Results of 70−90% prescription IDL optimizations for plans treating two metastases. (a) Volume of brain tissue receiving specified dose for each IDL optimization for a representative case treating two metastases. (b) Comparison of V10 for cases treating two metastases optimized at 70−90% prescription IDLs. (c) Comparison of V12 for cases treating two metastases optimized at 70−90% prescription IDLs. (d) Comparison of V15 for cases treating two metastases optimized at 70−90% prescription IDLs (NS: Not statistically significant, *p < 0.05, **p < 0.01, ***p < 0.001).

FIGURE 3

Comparison of dose distributions and dose‐volume histogram (DVH) for 70% (solid lines) and 90% (dotted lines) prescription isodose line optimizations for a plan treating two metastases.

FIGURE 4

Results of 70−90% prescription IDL optimizations for plans treating four to six metastases. (a) Volume of brain tissue receiving specified dose for each IDL optimization for a representative case treating six metastases. (b) Comparison of V10 for cases treating four to six metastases optimized at 70−90% prescription IDLs. (c) Comparison of V12 for cases treating four to six metastases optimized at 70−90% prescription IDLs. (d) Comparison of V15 for cases treating four to six metastases optimized at 70−90% prescription IDLs (NS: Not statistically significant, *p < 0.05, **p < 0.01, ***p < 0.001)​.

FIGURE 5

Comparison of dose distributions and dose‐volume histogram (DVH) for 70% (solid lines) and 90% (dotted lines) prescription isodose line optimizations for a plan treating four metastases.​

FIGURE 6

Results of 70−90% prescription IDL optimizations for plans treating 7−10 metastases. (a) Volume of brain tissue receiving specified dose for each IDL optimization for a representative case treating eight metastases. (b) Comparison of V10 for cases treating 7−10 metastases optimized at 70−90% prescription IDLs. (c) Comparison of V12 for cases treating 7−10 metastases optimized at 70−90% prescription IDLs. (d) Comparison of V15 for cases treating 7−10 metastases optimized at 70−90% prescription IDLs (NS: Not statistically significant, *p < 0.05, **p < 0.01, ***p < 0.001)​.

FIGURE 7

Comparison of dose distributions and dose‐volume histogram (DVH) for 70% (solid lines) and 90% (dotted lines) prescription isodose line optimizations for a plan treating six metastases.

FIGURE 8

Comparison of treatment monitor units (MU) for plans optimized at the 70−90% IDLs. (a) Results for plans treating two metastases. (b) Results for plans treating four to six metastases. (b) Results for plans treating 7−10 metastases (NS: Not statistically significant, *p < 0.05, **p < 0.01, ***p < 0.001)​.

FIGURE 9

Comparison of conformity index (CI) for plans optimized at the 70−90% IDLs. (a) Results for plans treating two metastases. (b) Results for plans treating four to six metastases. (b) Results for plans treating 7−10 metastases (NS: Not statistically significant, *p < 0.05, **p < 0.01, ***p < 0.001).