2605 - Treatment Time Effect on Prescription Dose in the Treatment of Vestibular Schwannomas

M. G. Skinner¹, L. Weinstein², and I. Paddick³; ¹Kaiser Permanente - South San Francisco, South San Francisco, CA, United States, ²Kaiser Permanente - South San Francisco, South San Francisco, CA, ³Medical Physics Limited, Berkshire, United Kingdom

Purpose/Objective(s): Treatment time duration in stereotactic radiosurgery (SRS) has been shown in several clinical studies to affect the biological effectiveness of a given physical dose for solid, vascular, and functional targets. This occurs through sub-lethal damage repair (SLDR), a biphasic process that contains a fast and slow repair component. The linear-quadratic (LQ) model does not account for SLDR as it assumes the full dose is delivered instantly, thereby overestimating the BED delivered. We hypothesize that the resultant BED will increase for the same physical dose compared with an equivalent treatment given over a longer exposure time. Jones’ equation A9 simplification of Millars BED model is applied here to show the impact of shortened treatment times on the BED in the treatment of vestibular schwannoma (VS). Materials/

Methods: Treatment data were reviewed for 814 VS patients treated with a non-invasive stereotactic radiosurgery instrument, a frameless robotic radiosurgery system (Accuray, USA), or medical linear accelerator, from 2001 to 2023. 249 of the 814 patients that received 12 Gy in one fraction using GK Model B (76), GK Perfexion (56), CK VSI (80) or TB (37) were included in the study. GK plans used 2 - 36 isocenters (mean 4.9 (Model B); 12.9 (Perfexion)). CK plans had 56 - 219 beams with 1 - 3 fixed cones. TB plans created in Elements (Brainlab, Germany) used 3 – 5 arcs using 6FFF. Treatment data analyzed included physical prescription dose, the number of treatment isocenters (GK), beamlets (CK), or arcs (TB), the total treatment time from first beam-on to last beam-off, total beam-on time, mean inter-beam time and dose to 95% of the GTV. For a/ß = 2.47 Gy, repair half-times of 11.4 and 129.6 minutes, and a partition coefficient, x, of 0.98, the BED_2.47 was calculated and compared for each modality.
Results: Total treatment times varied from 17 - 127 min (Model B), 18 - 84 min (Perfexion), 17 - 79 min (CK) and 6 - 19 minutes (TB). Table 1 shows the range of BED_2.47 for each modality computed from Jones’ A9 and the corresponding LQ BED_2.47. BED_2.47 computed from A9 yielded widely varying BED_2.47 values. For the same physical dose of 12 Gy, TBs shorter treatment time yielded much higher mean BED_2.47, effectively delivering a “hotter” treatment than GK or CK. A9 predicts an 11 Gy dose would yield a similar mean BED_2.47 to those of GK or CK, modalities from which original VS doses were empirically derived.
Conclusion: By including the impact of SLDR during treatment in the calculation of the BED for SRS treatment plans we have demonstrated the wide variation in BED and the inadequacy of the basic LQ model in quantifying these treatments. Accounting for biphasic repair suggests a reduction in physical dose should be made for treatments on faster platforms. Abstract 2605 – Table 1

Modality	Mean A9 BED2.47 (Gy) (min - max)	A9 BED2.47 ? % (relative to GK All)	LQ BED2.47 (Gy)
GK Model B	55.2 (45.1 - 64.6)	-	70.3
GK Perfexion	54.7 (46.9 - 62.9)	-
GK All	55 (45.1 - 64.6)	-
CK VSI	56.7 (47.9 - 60.8)	+3.1
TB - 0 mm margin	68.6 (66.7 - 71.8)	+24.7
TB - 0.5 mm margin	78.8 (76.2 - 81.2)	+43.3