3185 - CBCT-Based Online Adaptive Therapy and Target Coverage In Prostate SBRT with Interfraction Anatomic Variation

Purpose/Objective(s):

To evaluate interfraction anatomic variation and dosimetric impact of online adaptive radiation therapy (ART) for prostate stereotactic body radiation therapy (SBRT) delivered on the Varian Ethos platform.

Materials/Methods:

Five patients with localized prostate cancer were treated with 40 Gy in 5 fractions to the clinical target volume (CTV) on Varian Ethos platform. For each fraction, cone-beam CT (CBCT)-based online adaptation was performed. Volume variations (cc) for CTV, bladder, rectum, and bowel were quantified as standard deviation from the reference plan across fractions. Target coverage and organs at risk (OARs) dose were assessed by comparing CTV D95% and D99% for adaptive plans (ADP) and scheduled plans (SCH, recalculated on daily anatomy without re-optimization) relative to the original reference plan. Differences were reported as percent change from reference.

Results:

Interfraction volumetric variation was observed in both targets and organs at risk. The standard deviation of CTV volume ranged from 1.4–5.1 cc across patients. OAR variation was more pronounced, particularly for bladder (7.4–128.1 cc) and bowel (30.2–159.7 cc), with rectal variation up to 21.0 cc. Adaptive replanning maintained CTV coverage comparable to the reference plan. For ADP, mean CTV D95% differences ranged from 0.0% to +0.2%, and D99% ranged from –0.8% to +1.3%. In contrast, SCH plans demonstrated systematic undercoverage: D95% reductions ranged from –0.2% to –2.8%, and D99% reductions ranged from –0.1% to –2.8%. Overall, adaptive plans consistently restored target coverage relative to scheduled recalculation on daily anatomy. Dose to the OARs varied with higher and lower doses than those in the original reference plan observed from either ADP or SCH plans. Nevertheless, most of them still met the planning dose constraints.

Conclusion:

Significant interfraction anatomic variability occurred in prostate SBRT, particularly for bladder and bowel volumes. Online ART on the Varian Ethos platform effectively compensated for these changes, preserving CTV coverage with acceptable dose to the OARs of concern compared with non-adapted plans. These findings support the dosimetric robustness and clinical value of CBCT-based online adaptive therapy for hypofractionated prostate treatment.

Table: Dose change (%) for target (CTV) and some OARs in Adaptive Plan and Scheduled Plan as mean value vs base value in Reference Plan

Adaptive Plan		Pt 1	Pt 2	Pt 3	Pt 4	Pt 5	Min	Max	Mean
CTV D95%		0.2	0.1	0.0	0.2	0.1	0.0	0.2	0.1
CTV D99%		1.2	-0.4	1.3	-0.1	-0.8	-0.8	1.3	0.2
Rectum D1cc		0.4	-5.4	-7.1	-1.2	-0.7	-7.1	0.4	-2.8
Bladder D0.03cc		0.8	-1.3	0.0	-0.3	-2.4	-2.4	0.8	-0.7
Scheduled Plan		Pt 1	Pt 2	Pt 3	Pt 4	Pt 5	Min	Max	Mean
CTV D95%		-1.3	-1.1	-1.5	-0.2	-2.8	-2.8	-0.2	-1.4
CTV D99%		-1.3	-1.8	-1.8	-0.1	-1.1	-1.8	-0.1	-1.2
Rectum D1cc		2.9	-3.6	-5.9	2.3	-0.4	-5.9	2.9	-0.9
Bladder D0.03cc		-0.2	-5.0	-1.2	0.0	-3.9	-5.0	0.0	-2.1