ESTRO 2024 - Abstract Book

S3539

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

Purpose/Objective:

To evaluate the dose gradient from the planning target volumes (PTVs) to the spinal cord surface, among volumetric modulation arc therapy (VMAT)-based using Eclipse™ and Elements® with Cyberknife (CK)-based using Precision®. In addition, we assessed doses to PTVs, plan indices, the spinal cord, and surrounding organs at risk (OARs).

Material/Methods:

Twenty bony lesions, including five cervical (C), ten thoracic (T), and five lumbar (L) spines, were included. All patients were treated in the supine position. Five shapes of spinal lesions were contoured as the clinical target volumes (CTVs) followed the ISRC guideline. (1) PTVs were generated by adding an isotropic expansion of 2.0 mm to the CTVs. VMAT plans were created on the Varian TrueBEAM® with high-definition multi-leaf collimators, using 6 MV FFF photons and three full coplanar arcs with 358° gantry rotation (between 179° and 181°). The collimator angles were set at 10°, 350°, and 90°. Eclipse™ v16.1.0 used Acuros XB algorithm, while Elements® v3.0 employed both pencil beam (PB) and Monte Carlo (MC) algorithms. CK treatments utilized the InCise® MLC, 6 MV FFF photons and Precision® v3.3.1.2 with the MC algorithm. The PTVs were prescribed to 30 Gy in 5 fractions, with a criterion that at least 95% of volume receive 30 Gy. Dose gradient from the PTVs to the surface of the spinal cord was assessed (Figure 1). Doses delivered to the PTVs, the spinal cord, and the other OARs were compared. Plan indices including inverse Paddick conformity index (IPCI), gradient index (GI), and homogeneity index (HI) were evaluated. A Friedman’s two-way ANOVA analyzed the doses among the three TPSs (p < 0.05) and post hoc with Bonferroni's correction.

Results:

Table 1: Dosimetric data for the PTVs, plan indices, and the spinal cord in all lesions.

Absolute mean difference (95% of CI)

Precision:

Elements:

Eclipse:

Global p value

Dosimetric parameters

[p-value]

mean (range)

mean (range)

mean (range)

Precision Elements

Precision Eclipse

Elements Eclipse

Passing rate (%)

90.00 (90, 100)

100.00 (100, 100)

100.00 (100, 100)

0.135

-10.00 (-24.41, 4.41) [1.000]

-10.00 (-24.41, 4.41) [1.000]

0.00

[1.000]

Dose gradient

1.86

1.59 (1.32, 2.23)

1.57 (1.30, 2.17)

<0.001 0.27 (0.19, 0.35) [0.001]

0.29 (0.21, 0.37) [0.000]

0.02 (-0.02, 0.06) [0.618]

(1.63,

2.28)