315 / 1456
Keeping things in perspective
• GTV
PTV margin
– All cells in the GTV are considered to be tumor
– P
underdosage
= P
geometrical miss
– Use margin prescription
• CTV
PTV margin
– In the CTV there is a probability of tumor cells
– P
underdosage
= P
geometrical miss
x P
presence of tumor cells
– Margin can probably be smaller
• Caveat: Tumor cell probability is needed