Impact of variable RBE in prostate cancer IMPT planning on normal tissue complications

Area of Science:

• Radiation Oncology
• Medical Physics
• Cancer Treatment

Background:

• Proton therapy offers potential advantages in dose distribution for cancer treatment.
• Relative biological effectiveness (RBE) in proton therapy is complex and can vary.
• Accurate assessment of normal tissue complication probabilities (NTCPs) is crucial for optimizing treatment plans.

Purpose of the Study:

• To investigate the impact of variable RBE on NTCPs in prostate intensity-modulated proton therapy (IMPT).
• To compare NTCPs calculated using fixed RBE (1.1) versus variable RBEs.
• To assess the clinical implications of RBE variations on organs at risk (OARs) surrounding prostate tumors.

Main Methods:

• Utilized a linear function of dose-averaged linear-energy-transfer (LETd) to determine variable RBEs.
• Generated IMPT plans with both fixed (1.1) and variable RBEs for twelve prostate cancer patients.
• Calculated NTCPs for bladder and rectal walls using the Lyman-Kutcher-Burman (LKB) Probit model.

Main Results:

• Variable RBE plans resulted in higher NTCP values for the bladder wall (average 5.7% increase) and rectal wall (average 4.6% increase) compared to fixed RBE plans.
• Maximum NTCP differences reached 9.5%, particularly in regions with steep dose gradients.
• These increases in NTCP indicate a higher risk of normal tissue complications with variable RBE.

Conclusions:

• Incorporating variable RBE in proton therapy planning increases NTCPs in OARs.
• Accurate accounting for RBE variations is essential for optimizing treatment outcomes and minimizing toxicity.
• This study underscores the need for precise RBE modeling in clinical proton therapy.