Impact of prostate focused alignment on planned pelvic lymph node dose

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Prostate cancer patients with positive lymph node margins undergo radiation therapy targeting the prostate, seminal vesicles, and lymph nodes.
• External beam therapy (EBT) for these patients involves a 45 Gy dose to the planning target volume (PTV).
• Prostate localization uses fiducial markers and portal imaging, but pelvic lymph nodes are fixed to bony anatomy, potentially causing coverage discrepancies.

Purpose of the Study:

• To evaluate the difference between planned and delivered pelvic lymph node coverage in prostate cancer patients undergoing EBT.
• To determine if pelvic lymph node motion significantly impacts radiation dose delivery.

Main Methods:

• Prostate and pelvic lymph node motions were tracked for 19 patients using kV-images and bony anatomy registration.
• Deviations were calculated and input as field shifts in Eclipse to simulate planned vs. delivered coverage.
• Dose coverage was assessed using V(100) criteria for lymph node clinical target volume (LN CTV) and lymph node planning target volume (LN PTV).

Main Results:

• A significant difference (P=0.005) was observed in planned vs. delivered dose coverage for the LN PTV, averaging 3.5%.
• 26% of patients received less than 95% dose coverage for the LN PTV based on bony anatomy registration.
• No significant difference in coverage was found for the LN CTV (P=0.53).

Conclusions:

• A significant discrepancy exists between planned and delivered radiation dose coverage for the pelvic lymph node planning target volume (LN PTV).
• Pelvic lymph node motion should be considered alongside prostate motion during patient alignment for EBT.
• Accurate patient setup is critical to ensure adequate lymph node coverage in prostate cancer radiotherapy.