Field matching considerations in craniospinal irradiation.

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Craniospinal axis radiotherapy commonly uses parallel-opposed lateral cranial fields adjacent to a posterior spinal field.
• Treatment protocols often require cranial field rotation to match diverging spinal fields, creating a junction in the mid-sagittal plane.
• Some centers implement gaps between fields to prevent spinal cord overdosing.

Purpose of the Study:

• To investigate the dose distribution in the junction region of divergence-compensated radiotherapy fields with varying separations (overlap to gap).
• To evaluate the impact of field gapping and feathering techniques on dose uniformity in craniospinal radiotherapy.

Main Methods:

• Calculated composite dose profiles and isodose maps for 60Co and 4 MV photon beams.
• Simulated field separations ranging from 0.5 cm overlap to a 1 cm gap.
• Assessed static and moving junction (feathering) techniques.

Main Results:

• Appropriate field matching without gaps results in smooth dose variation across the junction.
• Gapping divergence-compensated fields leads to detrimental underdosage throughout the treatment volume.
• Spinal cord myelopathy dose is unlikely to be exceeded in leukemia patients with proper setup; it may occur in medulloblastoma patients with significant field overlap.

Conclusions:

• Field gapping in craniospinal radiotherapy compromises dose uniformity and is generally detrimental.
• Proper field matching and localization are essential for accurate dose delivery.
• Feathering may offer a safety margin for medulloblastoma but is unnecessary for leukemia, emphasizing the need for quality assurance and precise patient setup.