A Comprehensive Evaluation of Single Isocenter Multiple Target SRS Plans and the Analytical Relationship between Plan Quality Indices with the Number and Volume of Targets

Approaches for Stereotactic Radiosurgery (SRS)/Stereotactic Radiotherapy (SRT) in brain metastases using different radiotherapy modalities (Feasibility study)

BACKGROUND

SRS and SRT are precise treatments for brain metastases, delivering high doses while minimizing doses to nearby organs. Modern linear accelerators enable the precise delivery of SRS/SRT using different modalities like three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT), and Rapid Arc (RA).

OBJECTIVE

This study aims to compare dosimetric differences and evaluate the effectiveness of 3DCRT, IMRT, and Rapid Arc techniques in SRS/SRT for brain metastases.

METHODS

10 patients with brain metastases, 3 patients assigned for SRT, and 7 patients for SRS. For each patient, 3 treatment plans were generated using the Eclipse treatment planning system using different treatment modalities.

RESULTS

No statistically significant differences were observed among the three techniques in the homogeneity index (HI), maximum D2%, and minimum D98% doses for the target, with a p > 0.05. The RA demonstrated a better conformity index of 1.14±0.25 than both IMRT 1.21±0.26 and 3DCRT 1.37±0.31. 3DCRT and IMRT had lower Gradient Index values compared to RA, suggesting that they achieved a better dose gradient than RA. The mean treatment time decreased by 26.2% and 10.3% for 3DCRT and RA, respectively, compared to IMRT. In organs at risk, 3DCRT had lower maximum doses than IMRT and RA, but some differences were not statistically significant. However, in the brain stem and brain tissues, RA exhibited lower maximum doses compared to IMRT and 3DCRT. Additionally, RA and IMRT had lower V15Gy, V12Gy, and V9Gy values compared to 3DCRT.

CONCLUSION

While 3D-CRT delivered lower doses to organs at risk, RA and IMRT provided better conformity and target coverage. RA effectively controlled the maximum dose and irradiated volume of normal brain tissue. Overall, these findings indicate that 3DCRT, RA, and IMRT are suitable for treating brain metastases in SRS/SRT due to their improved dose conformity and target coverage while minimizing dose to healthy tissues.

Single-Isocenter Multiple-Target SRS Planning of Five to Ten Brain Metastases Using 5 mm Multileaf Collimator: Relationship between Prescription Dose, Number and Volume of Targets

OBJECTIVE

To propose an expression relating the number and volume of targets with the prescription dose in determining normal brain volume receiving 12 Gy dose (V12) for five to ten brain metastases treated in linear accelerator-based stereotactic radiosurgery (SRS) planning. To determine the volume of tumor that can be treated within the brain tolerance dose, for different SRS prescription doses.

METHODS

Single-isocenter multiple-target (SIMT) SRS plans were devised for spherical targets that are modeled to simulate 47 tumor scenarios with varying tumor sizes and locations within the brain. Volumetric modulated arc therapy (VMAT) plans were devised using a 5-mm-leaf-width multi-leaf collimator (MLC) with high conformity and dose gradient in the Eclipse treatment planning system for the 21 Gy prescription dose with a 6FFF photon beam. The prescription dose was rescaled to 20 Gy, 18 Gy, 15 Gy and 12Gy to determine the brain V12 volume for a total of 235 SRS plans.

RESULTS

Linear correlation was observed between the number, volume and prescription dose of the tumor. The expression relating these parameters was constructed to predict the normal brain V12 volume. The maximum tumor volume that can be treated using SIMT SRS with a 5-mm MLC for 5 to 10 number of targets and for a prescription dose of 21 Gy, 20 Gy, 18 Gy and 15 Gy is determined.

CONCLUSION

Using the expression obtained, V12 volume can be calculated using the number of tumors and the total volume of tumors from the pre-planning MRI data. The prescription dose and the SRS fractionation size can be determined before radiotherapy treatment planning.