Investigating Setup Error Differences between Conventional and Ultra-Hypofractionated Whole Pelvis Radiation Therapy:...

Area of Science:

• Radiation Oncology
• Medical Physics
• Prostate Cancer Treatment

Background:

• The HOPE trial (NCT04197141) evaluated conventional versus ultra-hypofractionated whole pelvis radiotherapy for high-risk prostate cancer patients receiving brachytherapy boost.
• Setup errors for prostate (CTVp) and nodal (CTVn) clinical target volumes were investigated within this randomized controlled trial.

Purpose of the Study:

• To compare setup errors between conventional and ultra-hypofractionated radiotherapy regimens in prostate cancer patients.
• To determine appropriate planning target volume (PTV) margins based on quantified setup uncertainties for both treatment arms.

Main Methods:

• Fifty-four patients (28 ultra-hypofractionated, 26 conventional) were analyzed from a single institution.
• Fiducial markers in the prostate enabled daily cone-beam CT (CBCT) matching; nodal errors were assessed via bone matching.
• Asymmetric PTV margins were calculated using the van Herk formalism to account for measured setup errors.

Main Results:

• Statistically significant reductions in 3D setup errors were observed for CTVp in the ultra-hypofractionated arm (1.2 mm) versus conventional (1.4 mm).
• No significant difference in CTVn setup errors was found between the two arms (2.9 mm vs. 2.8 mm).
• Required PTV margins varied by direction, with slightly larger margins needed for nodal CTVs, particularly in the ultra-hypofractionated arm.

Conclusions:

• A 6-mm PTV margin for CTVn appears adequate, but CTVp margins might be reducible in ultra-hypofractionated regimens if other errors are controlled.
• Smaller prostate setup errors in the ultra-hypofractionated arm may relate to longer image matching times or increased use of 6-DOF couches.
• Centers must independently assess treatment uncertainties, as differences were observed between arms despite standardized protocols.