Sep 29

PQA 06 - Genitourinary Cancer, Gynecological Cancer, and Health Care Access and Engagement

3749 - Interfraction Motion of the Retroverted Uterus during Radiotherapy for Cervical Cancer

02:15pm - 03:30pm ET

Poster Hall - Exhibit Hall A

Screen: 24

Presenter(s)

Elena Riggenbach, MD - Radiation Medicine Program, University Health Network and Princess Margaret Cancer Centre, Toronto, ON

E. Riggenbach^1,2, J. M. Croke^1,2, J. Lukovic^1,2, M. Milosevic^1,2, J. Xie^1,2, K. Han^1,2, and T. Tadic^1,3; ¹Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada, ²Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada, ³Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada

Purpose/Objective(s):

Uterine motion during pelvic radiotherapy for cervical cancer can be substantial, and guidelines recommend inclusion of the entire uterus with generous margins to account for this variability. However, the anatomical variation of the retroverted uterus is poorly studied and may further increase positional uncertainty. We hypothesize that cervical cancer patients with a retroverted uterus can exhibit significant interfraction motion, particularly at the fundus. This study aimed to quantify interfraction motion patterns of the retroverted uterus.

Materials/Methods:

A total of 334 cone-beam CT (CBCT) scans from 14 patients with a retroverted uterus who underwent definitive chemoradiation with individualized ITV and PTV margins (according to EMBRACE II) between 2018 and 2025 for stage IB2–IIIC1 cervical cancer were retrospectively analyzed. On each scan, uterine position was classified as retroverted/-flexed, anteverted/-flexed, or neutral. Bladder, rectum and uterocervix were manually delineated on the planning CT and all CBCTs, as well as uterine fundus, isthmus and cervical os points of interest (POI). Deformable image registration was used to quantify uterocervix motion. The correlation between POI motion and bladder/rectum was assessed by Spearman’s ?. P values were adjusted for multiple comparisons using the Benjamini-Hochberg method.

Results:

Of the 14 patients, 79% showed a change in uterine position from retroverted/retroflexed to a neutral position, and 57% to an anteverted/anteflexed orientation during treatment. Day-to-day POI motion was nonuniform (Table 1), with the largest movement in the fundus (median 3.7 cm). On average, the total volume occupied by the uterocervix over treatment was 2.3 times the planning volume (range 1.2–3.2). A volumetric miss >1 cc of the uterocervix occurred in 31% and 7% of the fractions relative to the individualized ITV and PTV, respectively. Bladder and rectal volumes significantly correlated with POI motion in all directions, with the strongest association between increasing bladder volume and superior fundal displacement (? = 0.55, p < 0.0001).

Conclusion:

Most cervical cancer patients with a retroverted uterus at diagnosis experience a change in uterine orientation during radiotherapy. The uterocervix exhibits large, nonuniform interfraction motion relative to the bony pelvis. Rigid setup corrections and individualized ITV are insufficient to achieve full coverage. Uterine retroversion may present a potential selection criterion for online adaptive radiotherapy, to avoid excessive margin expansion and normal tissue irradiation.

POI	Median (range) day-to-day point motion, cm
POI	Left-Right	Anterior-Posterior	Superior-Inferior
Fundus	1.8 (1.1–2.4)	3.7 (1.1–7.8)	2.4 (1.4–4.5)
Isthmus	1.2 (0.7–2.0)	1.9 (1.3–3.1)	1.8 (0.9–2.7)
External Os	1.2 (0.7–2.3)	1.9 (1.1–3.3)	1.7 (1.1–2.2)