Patient dose calculation systems

Question 1

Why do we do independent dose calculations? (3 marks)

Answer 1

• legally required to do so
• checking the TPS is correct
• can be factor based or beam model based

Question 2

How are factor based calculations carried out? (4 marks)

Answer 2

• assume the patient is a homogeneous rectangle
• identify the differences from reference conditions
• correct for those differences
• can use the ESTRO formulism

Question 3

Explain the ESTRO formulism for isocentric treatments? (5 marks)

Answer 3

D(z,c) = D_R x U x O(c) x V(z,c)/V(z_ref, c_ref) x T(z,c)

where:
D_R = dose per MU under ref conds
U = MUs
O(c) = head scatter correction
V = scatter volume ratio
TPR = distance correction

Question 4

What are the two main issues and solutions with using the ESTRO formalism? (4 marks)

Answer 4

• volume scatter ratio is not easily defined, use phantom scatter ratio instead
• beam size is defined at the surface, use isocentric method and define beam size at isocentre

Question 5

What is the practical version of the ESTRO formalism? (5 marks)

Answer 5

D(z,c) = D_R x U x S_p(z,c) x S_c(z,c) x TPR(z,c)