3082 - Quantification of Six-Dimensional Setup Errors and Margin Optimization in Pelvic Radiotherapy Using Cone-Beam CT Image Guidance: A Prospective Observational Study

Purpose/Objective(s):

Modern intensity-modulated radiotherapy (IMRT) demands sub-millimeter precision in target localization, making geometric accuracy critical. In pelvic malignancies, substantial organ motion and daily anatomic variability introduce translational and rotational setup uncertainties that may compromise dose conformity. Image-Guided Radiotherapy (IGRT) with six-dimensional cone-beam computed tomography (6D-CBCT) allows correction of both translational and rotational deviations, potentially enabling smaller planning target volume (PTV) margins without compromising coverage.

Objectives of the study was (1) quantify systematic and random setup errors using 6D-CBCT in patients receiving definitive pelvic radiotherapy, and (2) determine optimal PTV margins through Van Herk’s margin formulation to enhance treatment precision

Materials/Methods:

A prospective single-institution observational study was conducted in 79 patients with biopsy-proven locally advanced pelvic malignancies (cervix, rectum, anal canal, endometrium, bladder, prostate) treated with definitive IMRT. Thrice-weekly 6D-CBCT was performed to record translational (X, Y, Z) and rotational (yaw, pitch, roll) errors. Rotational deviations were transformed into translational equivalents using Euler’s rotation matrix to compute total geometric uncertainty. Systematic (S) and random (s) errors were derived, and corresponding PTV margins were calculated using the Van Herk formula (M = 2.5S + 0.7s).

Results:

The mean age was 56.6 ± 11.9 years; female predominance (63.3%) reflected a high proportion of cervical cancer cases. Comorbidities were present in 41% of patients, predominantly hypertension (29.1%) and diabetes (16.4%). Translational setup errors were smaller in the lateral (X: 0.48 cm) and longitudinal (Z: 0.49 cm) axes, with slightly larger deviation anteroposteriorly (Y: 0.58 cm). Mean rotational errors were yaw 2.48°, pitch 2.38°, and roll 2.08°. Random errors exceeded systematic errors due to intra-fractional organ motion and variable reproducibility of bladder/rectal filling. Application of 6D CBCT reduced overall PTV margins to 6–9 mm, safely lower than traditional 15–20 mm recommendations, enabling improved target conformity and decreased organ-at-risk dose exposure.

Conclusion:

This prospective study demonstrates that thrice-weekly 6D-CBCT guidance effectively quantifies and corrects six-dimensional setup deviations in pelvic radiotherapy. The incorporation of rotational corrections and Van Herk-based margin estimation permits substantial margin reduction while maintaining dosimetric safety. Institutional-specific PTV margins of ˜ 0.5–0.6 cm were validated, representing a pragmatic balance between geometric accuracy and toxicity reduction. These findings support the integration of 6D CBCT-based IGRT as a standard tool for precision assurance and margin optimization in pelvic radiotherapy.