Deriving patient-specific planning target volume for partial bladder image guided radiation therapy.

Area of Science:

• Radiation Oncology
• Medical Imaging
• Urological Cancer Treatment

Background:

• Image-guided radiation therapy (IGRT) for bladder cancer often uses large planning target volume (PTV) margins due to geometric uncertainties.
• These large margins can lead to unnecessary irradiation of surrounding healthy tissues.
• Accurate identification of the clinical target volume (CTV) is challenging in bladder cancer treatment.

Purpose of the Study:

• To describe a novel method for deriving patient-specific PTV (PS-PTV) for partial bladder IGRT.
• To evaluate the efficacy of using lipiodol and CBCT in creating PS-PTV.
• To reduce the volume of irradiated normal tissue while maintaining target coverage in bladder cancer patients.

Main Methods:

• Lipiodol was injected into the bladder wall of 12 patients prior to radiation therapy.
• Patient-specific PTV (PS-PTV) was generated by delineating CTV on 15 CBCTs, registering CBCTs to planning CT using lipiodol, creating an occupancy volume (OV) from CTVs, and expanding the OV by 3 mm.
• The PS-PTV was compared to a 20-mm population-based PTV (popPTV) for efficacy in reducing irradiated volume and ensuring coverage.

Main Results:

• The median PS-PTV (102 cm³) was substantially smaller than the median popPTV (325 cm³).
• Significant reductions in the overlapping volumes of the rectum (0.3 cm³), small bowel (5.3 cm³), and large bowel (13.0 cm³) were observed.
• The PS-PTV method achieved 100% target coverage, despite reductions in volume and margin compared to the popPTV.

Conclusions:

• The use of lipiodol and CBCT to derive PS-PTV is effective for partial bladder IGRT.
• This technique allows for significant reductions in irradiated normal tissue volume.
• The PS-PTV approach maintains adequate target coverage, improving treatment precision for bladder cancer.