Prostate or bone? Comparing the efficacy of image guidance surrogates for pelvis and prostate radiotherapy using accumulated delivered dose.

Introduction: This study assessed the impact of dosimetry to both the target and normal tissue when either bony anatomy (BA) or prostate (PRO) was used as surrogates for image guidance for pelvis and prostate radiotherapy using a dose accumulation process.

Methods: Thirty patients who were prescribed 50-54Gy to the pelvic lymph nodes (PLN) and 78Gy to the prostate/seminal vesicles were included. Daily acquired CBCTs were rigidly registered to the CT using BA and PRO to simulate two different treatment positions. The accumulated delivered dose (D) of PLN, prostate, bladder and rectum for each surrogate were compared with the planned dose. Deviation from the planned dose (ΔD) of >5% was considered clinically significant.

Results: Prostate was displaced from bony anatomy by > 5 mm in 96/755 fractions (12.7%). Deviation between the mean D and the planned dose for PLN and prostate was <2% when either BA or PRO was used. No significant deviation from planned dose was observed for bladder (p > 0.2). In contrary, D for rectum D was significantly greater than the planned dose when BA was used (Mean ΔD = 6%). When examining individual patient, deviation from the planned dose for rectum D was clinically significant for 18 patients for BA (Range: 5-21%) and only 8 patients for PRO (Range: 5-8%).

Conclusions: The use of either BA or PRO for image guidance could deliver dose to PLN and prostate with minimal deviation from the plan using existing PTV margins. However, deviation for rectum was greater when BA was used.