DOSIMETRIC IMPACT ON BLADDER AND RECTUM BY INDUCING COUCH SHIFTS IN ALL THREE AXES OF PROSTATE VMAT_SIB TREATMENT PLANS.

Abstract

Purpose:

Accurate patient positioning is essential in prostate radiotherapy because modern volumetric modulated arc therapy (VMAT) produces highly conformal dose distributions with steep dose gradients. Residual setup uncertainties may compromise target coverage and alter the dose delivered to surrounding organs at risk (OARs). This study evaluated the dosimetric impact of translational couch shifts ranging from 1 to 5 mm along the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions on target coverage and OAR doses in prostate VMAT plans.

Materials and Methods:

Ten patients with locally advanced prostate cancer (T3–T4N0M0) treated with radical VMAT using the simultaneous integrated boost (SIB) technique were retrospectively included. Clinically approved treatment plans generated in Eclipse Treatment Planning System (Version 18.1) and delivered on a Varian TrueBeam linear accelerator were selected for analysis. Systematic translational setup uncertainties of 1–5 mm were introduced independently in the LR, SI, and AP directions while maintaining the original beam parameters and multileaf collimator fluence. A total of 300 recalculated treatment plans were generated. Dose-volume histogram (DVH) parameters were analyzed to evaluate changes in planning target volume (PTV) coverage, conformity index, and dosimetric indices for the bladder, rectum, and femoral heads. All dosimetric data were recorded in Microsoft Excel (Microsoft Corporation, Redmond, WA, USA). Descriptive statistical analysis was performed to quantify the variation in target coverage and OAR doses resulting from translational setup uncertainties ranging from 1 mm to 5 mm in the LR, SI, and AP directions.

Results:

Increasing setup uncertainty produced a progressive reduction in target coverage in all three directions. The largest deterioration occurred following AP displacement, where the deviation in PTV70 coverage reached 9.28% at a 5 mm shift. LR shifts produced the greatest increase in femoral head, whereas SI and AP shifts resulted in increase in bladder and rectum dose-volume parameters. Bladder dose-volume parameters demonstrated a directional dependence. During LR shifts, only modest increases were observed, with V40Gy, V50Gy and V60Gy increasing by 3.28%, 5.13%, and 2.94%, respectively, at 5 mm couch displacement. Rectal dosimetry exhibited greater sensitivity to translational uncertainties than bladder dosimetry. LR shifts resulted in gradual increases in rectal dose-volume parameters, with rectal V60Gy increasing from 1.62% at 1 mm to 10.84% at 5 mm. Mean rectal dose increased from 0.19% to 2.78% over the same range of displacement.

Conclusion:

Induced setup errors significantly influence target dose coverage and OAR dosimetry in prostate VMAT. AP displacement has the greatest impact on target coverage, bladder and rectum doses whereas LR displacement predominantly affects femoral head. Dosimetric deviations increased with the magnitude of positional error, emphasizing the importance of accurate patient positioning, daily image guidance, and consistent bladder and rectal preparation. These findings support maintaining setup reproducibility within 3 mm to preserve target coverage and minimize unnecessary irradiation of adjacent normal tissues