On-Board MRI Guidance for Prostate Radiotherapy

Potential causes of prostate displacement include: 1) Differences in bladder and rectal filling between planning and treatment. 2) Patient setup variations. 3) Anatomical changes due to weight loss or tumor response. 4) Bowel gas or stool in the rectum. 5) Hip prosthesis artifact causing distortion in the MRI, which could mimic displacement. To verify position, use multiple MRI sequences (e.g., T2-weighted, bSSFP) and compare with the planning MRI. Check the alignment of fiducial markers if present. Use the on-board MRI to visualize the prostate and surrounding structures; if the artifact from the hip prosthesis is causing distortion, the apparent displacement may be artifactual. Also, check the patient's setup with external markers or cone-beam CT if available. Review the bladder and rectal filling; consider a repeat scan after emptying/filling. If the displacement is real, it may be due to rectal filling; a rectal balloon or dietary instructions could help.

To adapt the plan: 1) Re-optimize the plan based on the on-board MRI (online adaptive radiotherapy). This involves re-contouring the target and organs at risk on the daily MRI and generating a new plan. 2) Use a library of plans (plan-of-the-day) if available. 3) If the displacement is small and within the PTV margin, no adaptation may be needed, but with 5 mm posterior displacement, the margin may be insufficient. Challenges with hip prosthesis artifact: 1) The artifact can distort the MRI, making it difficult to accurately localize the prostate and OARs. 2) The bSSFP sequence is sensitive to off-resonance, so the artifact may be pronounced. 3) The distortion may affect dose calculation if the electron density is derived from MRI (though on MR-linac, dose is calculated on the daily MRI with bulk density override). To mitigate: 1) Use a different sequence less sensitive to susceptibility (e.g., T2-weighted fast spin echo) for contouring. 2) Apply distortion correction algorithms. 3) Use the planning CT fused with the daily MRI for dose calculation if needed. 4) If the artifact obscures the target, consider using CT-on-rails or cone-beam CT for guidance on that fraction. 5) Consult with the radiologist to ensure accurate interpretation.

For subsequent fractions: 1) Implement a bowel and bladder preparation protocol (e.g., empty rectum, full bladder) and verify compliance with a pre-treatment MRI or ultrasound. 2) Use a rectal balloon or endorectal immobilization device if tolerated. 3) Provide dietary instructions to reduce gas. 4) Consider a longer setup time to allow for bladder filling. 5) Use daily online adaptive radiotherapy to account for any displacement. 6) If the hip prosthesis artifact is consistently problematic, consider using a different MRI sequence or a combination with CT guidance. 7) Review the planning margins; if displacement is frequent, consider increasing the PTV margin, though this may increase toxicity. 8) Educate the patient on the importance of consistent bowel/bladder habits. 9) Monitor with daily imaging and adapt as needed. 10) Document the incident and review with the team.