Estimation of set-up errors and planning target volume margin in craniospinal irradiation: A prospective study

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Planning

Background:

• Cranio-spinal irradiation (CSI) planning is complex due to large target volumes and numerous organs at risk (OARs).
• Assessing set-up errors and daily shifts during CSI is crucial for accurate radiation delivery.
• Cone-beam computed tomography (CBCT) is utilized for image guidance in radiotherapy.

Purpose of the Study:

• To assess set-up errors and daily shifts during CSI using CBCT.
• To calculate the planning target volume (PTV) margins for CSI based on observed errors.
• To determine the necessity of differential PTV margins for different anatomical regions.

Main Methods:

• CSI was planned using volumetric modulated arc technique (VMAT).
• Daily CBCT imaging was acquired for the initial 3 days, followed by twice-weekly imaging.
• Systematic (Σ) and random errors (σ) were calculated, and PTV margins were determined using Van Herk's formula (2.5Σ + 0.7σ).

Main Results:

• In the absence of daily imaging, isotropic PTV margins of 8 mm, 12 mm, and 15 mm were required for the brain, upper spine, and lower spine, respectively.
• Set-up inaccuracies were observed to increase towards the lower spine.
• Analysis included 20 medulloblastoma patients.

Conclusions:

• A uniform PTV margin for CSI risks target misses or OAR overexposure due to increasing set-up inaccuracies towards the lower spine.
• Differential PTV margins are necessary for accurate CSI delivery.
• Daily image guidance, such as CBCT, can reduce PTV margins, thereby minimizing radiation exposure to surrounding normal tissues.