Multi-Institutional Dosimetric Evaluation of Modern Day Stereotactic Radiosurgery (SRS) Treatment Options for Multiple Brain Metastases

Abstract

Purpose/Objectives: There are several popular treatment options currently available for stereotactic radiosurgery (SRS) of multiple brain metastases: ⁶⁰Co sources and cone collimators around a spherical geometry (GammaKnife), multi-aperture dynamic conformal arcs on a linac (BrainLab Elements™ v1.5), and volumetric arc therapy on a linac (VMAT) calculated with either the conventional optimizer or with the Varian HyperArc™ solution. This study aimed to dosimetrically compare and evaluate the differences among these treatment options in terms of dose conformity to the tumor as well as dose sparing to the surrounding normal tissues. Methods and Materials: Sixteen patients and a total of 112 metastases were analyzed. Five plans were generated per patient: GammaKnife, Elements, HyperArc-VMAT, and two Manual-VMAT plans to evaluate different treatment planning styles. Manual-VMAT plans were generated by different institutions according to their own clinical planning standards. The following dosimetric parameters were extracted: RTOG and Paddick conformity indices, gradient index, total volume of brain receiving 12Gy, 6Gy, and 3Gy, and maximum doses to surrounding organs. The Wilcoxon signed rank test was applied to evaluate statistically significant differences (p < 0.05). Results: For targets ≤ 1 cm, GammaKnife, HyperArc-VMAT and both Manual-VMAT plans achieved comparable conformity indices, all superior to Elements. However, GammaKnife resulted in the lowest gradient indices at these target sizes. HyperArc-VMAT performed similarly to GammaKnife for V_12Gy parameters. For targets ≥ 1 cm, HyperArc-VMAT and Manual-VMAT plans resulted in superior conformity vs. GammaKnife and Elements. All SRS plans achieved clinically acceptable organs-at-risk dose constraints. Beam-on times were significantly longer for GammaKnife. Manual-VMAT_A and Elements resulted in shorter delivery times relative to Manual-VMAT_B and HyperArc-VMAT. Conclusion: The study revealed that Manual-VMAT and HyperArc-VMAT are capable of achieving similar low dose brain spillage and conformity as GammaKnife, while significantly minimizing beam-on time. For targets smaller than 1 cm in diameter, GammaKnife still resulted in superior gradient indices. The quality of the two sets of Manual-VMAT plans varied greatly based on planner and optimization constraint settings, whereas HyperArc-VMAT performed dosimetrically superior to the two Manual-VMAT plans.

Keywords: conformity index; dynamic conformal arcs; multiple brain metastases; stereotactic radiosurgery; volumetric arc therapy.

Figure 1

Conformity index results for both RTOG and Paddick definitions displayed as box plots per SRS plan type, divided into five separate target size diameter bins.

Figure 2

Gradient Index (GI), V12_Gy per target (defined as the volume of 12Gy delivered to the surrounding brain tissue contributed only from that individual target), and V12_Gy-TV(defined as the total volume of brain receiving 12Gy per target excluding the target volume) results displayed as box plots per SRS plan type, divided into five separate target size diameter bins.

Figure 3

Box plot results per SRS plan type for the following dosimetric parameters across all patients: the total volume of brain receiving 12Gy, 6Gy, and 3Gy (V_12Gy, V_6Gy, V_3Gy) and the mean dose to the brain excluding the targets (Brain mean dose).

Figure 4

Box plot results per SRS plan type for the following dosimetric parameters across all patients: the maximum dose to the brainstem (D_max Brainstem), maximum dose to the left eye and optic nerve (D_max L Eye and D_max L ON), maximum dose to the right eye and optic nerve (D_max R Eye and D_max R ON), and maximum dose to the optic chiasm (D_max OC).

Figure 5

Axial, coronal, and sagittal cuts of patient #15 selected to visually demonstrate differences in dose distribution among the five different SRS plans.

Figure 6

Spider plot graphically comparing the studied SRS techniques across the different categories of dosimetry and efficiency. SRS treatment modalities were ranked relative to each other per specified category on a scale of 1 through 4.