A retrospective evaluation of inter-fraction motion in prostate cancer patients with involved nodes receiving prostate and pelvic ± para-aortic nodal irradiation

Introduction

The inter-fraction motion of pelvic ± para-aortic (PA) nodal volumes in prostate cancer patients with involved nodes is yet to be quantified and the optimal IGRT strategy for these patients is currently unknown.

Methods

A single-centre retrospective evaluation was performed investigating inter-fraction motion in the prostate and involved nodal volumes of patients receiving pelvic ± PA nodal irradiation. Patients were selected for inclusion if they; were undergoing prostate + pelvic node +/- PA node radiation for prostate cancer with involved lymph nodes and had received daily online CBCT scans. The planning CT and subsequent CBCT images from each treatment fraction were selected for analysis.

Results

Out of 567 CBCTs, from 20 patients, there were no incidences where independent lymph node displacement was >0.5 cm from planning volumes. Lymph node motion relative to prostate was >0.5 cm on 15 occasions out of 567 scans (2.6%). Where the difference between prostate and lymph nodes was >0.5 cm, this was always a result of the rectum causing variation in the prostate position, not a change in nodal position.

Discussion

These results suggests that there is limited independent displacement between the involved pelvic ± PA nodal volumes and bony anatomy. Therefore, bony anatomy could be used as a lymph node match surrogate for prostate patients receiving nodal irradiation for active disease. The results also suggest additional emphasis should be placed on bowel preparation in these patients to reduce the risk of geographical miss.

Conclusion

In conclusion, the results of this evaluation suggests that there is limited independent displacement between the involved pelvic ± PA nodal volumes and bony anatomy. This provides evidence to trials investigating the role of pelvic ± PA nodal irradiation to ensure appropriate margins and IGRT strategies are used when investigating this further.