2257 - Patient-Based Dosimetric Analysis of Photon GRID SFRT: Insights from a Large Institutional Experience

Purpose/Objective(s):

In an effort to standardize photon GRID radiation therapy (GRID RT) prescription and planning, the Radiosurgery Society (RSS) published a consensus guideline (Zhang et al., 2020) outlining several key dosimetric parameters. Since then, studies have been published reporting dosimetric characters based on phantom-generated data. Our study, however, is one of the first and largest reports based on actual patient-generated data from a single institute.

Materials/Methods:

Dosimetric plans for 44 patients treated with GRID RT at our institution were retrospectively analyzed. Dose metrics recommended by the RSS GRID/LATTICE working group were extracted and evaluated. Key dose metrics, including peak-to-valley ratio (P/V), peak-to-peak distance (p-p), and peak width (defined by full width half maximum) were assessed at Dmax, 50% and 90% of maximal depth for each treatment energy. Additionally, global metrics such as target size, maximum depth and D10%/D90% are reported.

Results:

Of the 44 patients considered, 9 received 6MV-FFF, 34 received 10MV-FFF, and one received 18MV. Target depths increased with energy: 7.6 ± 2.4 cm (6MV), 10.8 ± 3.0 cm (10MV), and 17.8 cm (18MV). Average target doses (D50%) ranged from 7–10 Gy, increasing with energy: 732 ± 134 cGy (6MV), 755 ± 94 cGy (10MV), and 1022 cGy (18MV). The D10(%)/D90(%) ratio decreased with energy due to greater lateral penumbra, leading to lower valley doses. The highest values were observed for 6 MV (5.5 ± 1.0), decreasing for 10MV (3.6 ± 1.3) and 18MV (3.2). The peak dose and P/V ratio declined with depth due to beam lateral scatter. For 6MV, P/V dropped from 7.8 ± 1.6 at Dmax to 5.7 ± 1.3 at mid-target and 5.0 ± 1.2 at the distal edge. A similar trend was observed for 10MV from 3.7 ± 0.4 at Dmax to 3.4 ± 0.6 at mid-target and 3.0 ± 0.3 at the distal target edge and for 18MV(2.8 to 2.1). Peak width exhibited a minor increase with depth (~2–3 mm) due to beam divergence, ranging from ~1.0 cm (6MV) to ~1.3 cm (18MV). Similarly, p-p distance increased slightly (1–2 mm) at 1.46-1.58 cm (6MV), 1.36-1.44 cm (10MV) and 1.8-2.1 cm (18MV) depths.

Conclusion: Our study provides the first patient-based dosimetric analysis of photon GRID RT at our institution, which is valuable for pre-clinical modeling and future clinical planning. The data reveals valuable insights for treatment parameters across different beam energies and for the ongoing refinement of GRID RT prescription and planning

Table 1

Energy (MV)	Target Maximum Depth (cm)	Volume (cm3)	D50% (cGy)	D10(%)/D90(%)	Measured Depth (cm)	Peak to Valley	Peak Width (cm)	Peak-to-peak Distance (cm)
6	7.6 ± 2.4	205± 159	732 ± 134	5.5 ± 1.0	Dmax: 1.17 ± 0.09	7.8 ± 1.6	0.93 ± 0.10	1.46 ± 0.31
					mid: 3.82 ± 1.18	5.7 ± 1.3	0.94 ± 0.13	1.53 ± 0.37
					distal: 6.88 ± 2.13	5.0 ± 1.2	1.01 ± 0.10	1.58 ± 0.35
10	10.8 ± 3.0	450 ± 397	755 ± 94	3.6 ± 1.3	Dmax: 1.94 ± 0.39	3.7 ± 0.4	0.95 ± 0.09	1.36 ± 0.26
					mid: 5.41 ± 1.48	3.4 ± 0.6	1.02 ± 0.11	1.41 ± 0.24
					distal: 9.74 ± 2.67	3.0 ± 0.3	1.06 ± 0.11	1.44 ± 0.24
18	17.8	932	1022	3.2	Dmax: 3.82	2.8	1.20	1.81
					mid: 8.90	2.3	1.32	1.95
					distal: 16.02	2.1	1.53	2.08