Assessing prostate, bladder and rectal doses during image guided radiation therapy--need for plan adaptation?

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Image Guided Radiotherapy (IGRT) is crucial for precise dose delivery in localized prostate cancer treatment.
• In-room imaging (CT) allows for daily tracking of prostate, bladder, and rectum anatomy to adjust radiation therapy.
• Inter-fraction organ motion introduces dosimetric uncertainties that need evaluation.

Purpose of the Study:

• To evaluate dosimetric uncertainties caused by inter-fraction organ variation during IGRT for prostate cancer.
• To assess the accuracy of dose recomputation using daily CT scans from different imaging modalities.
• To investigate the feasibility of adaptive radiotherapy planning based on daily image feedback.

Main Methods:

• Compared kilovoltage cone beam CT (kvCBCT) on Elekta Synergy and Megavoltage CT (MVCT) on Tomotherapy Hi-ART for 10 patients.
• Performed dose recomputation using daily CT anatomy and evaluated image quality using phantoms (ComTom, Catphan 600).
• Utilized Planned Adaptive software on Tomotherapy to construct cumulative Dose Volume Histograms (DVHs) and adapt plans.

Main Results:

• Direct dose recomputation on kvCBCT was inaccurate due to CT number deviations.
• Maximum deviation to the prostate was 2.7% with kvCBCT.
• Significant daily variations observed in rectum (up to 12%) and bladder (up to 40%) doses without adaptation.
• Tomotherapy's adaptive planning with MVCT enabled delivered doses to closely match planned doses.

Conclusions:

• IGRT requires careful consideration of imaging modality accuracy for dose recomputation.
• Adaptive planning, particularly with systems like Tomotherapy using daily MVCT, is effective in managing organ variations.
• Adaptive strategies ensure accurate dose delivery and improve treatment outcomes in prostate cancer radiotherapy.