Alterations in normal liver doses due to organ motion

Area of Science:

• Radiation Oncology
• Medical Physics
• Image-Guided Therapy

Background:

• Accurate radiation dose calculation is crucial for effective cancer treatment.
• Static computed tomography (CT) scans may not reflect actual patient anatomy and motion during treatment.
• Patient setup variations and organ motion (e.g., breathing) introduce uncertainties in delivered radiation dose.

Purpose of the Study:

• To evaluate the clinical significance of dose calculation differences between static CT and motion-compensated methods for intrahepatic lesions.
• To assess the impact of patient setup variations and breathing motion on liver normal tissue complication probability (NTCP).

Main Methods:

• Dose calculations for 40 patients treated with conformal therapy were re-evaluated.
• A geometric convolution approach was used, incorporating 3D probability distributions for setup uncertainties.
• A 1D distribution function modeled breathing-induced motion in the inferior-superior direction.

Main Results:

• Clinical target volume (CTV) dose was generally adequate, meeting planning target volume (PTV) dose in most cases.
• Significant changes in liver NTCP were observed, with increases for lower liver tumors and decreases for upper liver tumors.
• The average magnitude of change in liver NTCP was 3.9%, with required prescription dose adjustments averaging 1.9 Gy.

Conclusions:

• The planning target volume (PTV) concept adequately ensures clinical target volume (CTV) coverage.
• Doses to normal liver tissue are inaccurately modeled when patient-related geometric uncertainties are not included in treatment planning.