The Usefulness of Adaptative Radiotherapy in Prostate Cancer: How, When, and Who?

Area of Science:

• Radiation oncology
• Medical physics
• Image analysis

Background:

• Adaptive radiotherapy (ART) aims to adjust treatment based on anatomical changes during radiation therapy.
• Accurate assessment of dose delivered to organs at risk, like the rectum, is crucial for safe and effective treatment.
• Deformable image registration (DIR) offers a method to compare planned and delivered doses by accounting for anatomical variations.

Purpose of the Study:

• To develop and validate an offline adaptive replanning protocol using DIR.
• To identify early dosimetric changes in rectal dose during the initial fractions of prostate cancer radiotherapy.
• To establish criteria for initiating adaptive replanning based on predicted rectal dose constraints.

Main Methods:

• Prospective acquisition of 240 images (24 planning CT, 216 CBCT) from 24 prostate cancer patients.
• Application of DIR to correlate planned CT (pCT) doses with doses on kilovoltage cone-beam CT (CBCT) images.
• Calculation of rectal dose (D50) and comparison with the dose limitation (R50) to assess compliance.

Main Results:

• Significant variations in rectal volume were observed in early fractions (1, 3, 5).
• A strong correlation was found between accumulated rectal D50 in early fractions and non-compliance with the R50 constraint.
• A D50 variation rate greater than 1% in fraction 3 reliably predicts exceeding the rectal dose constraint.

Conclusions:

• The developed DIR-based offline ART protocol effectively identifies patients at risk of rectal dose escalation.
• Early detection of rectal dose variations in fraction 3 can guide timely adaptive replanning decisions.
• This approach enhances the safety and efficacy of radiotherapy for prostate cancer by minimizing rectal toxicity.