Physics Bucharest 2017

Influence of margins on volume

• Third-power relationship between radius of a sphere and volume (4/3π r )

• GTV, 2 cm diameter, volume 4.2 cm 3 Add 1 cm to • CTV, 4 cm diameter, volume 33.5 cm 3 Add 1 cm to • PTV, 6 cm diameter, volume 113 cm 3 • •

• Small reduction in margin (5mm) yields a 50% reduction in volume

• The volume of the outer layer equals the volume of the core of the orange

PTV

CTV

GTV

Verellen D, Ridder MD, Linthout N, Tournel K, Soete G, Storme G: Innovations in image-guided radiotherapy. Nat Rev Cancer 2007, 7(12):949-960.