What factors affect total dose at dmax in parallel-opposed fields?

[hkabc]

Dear members,
I am frustrated by the Raphex exam questions. there are many I cannot solved. And the answer section is very brief for most questions.

T 10. The total dose delivered at depth dmax, from a pair of parallel-opposed fields, expressed as a percentage of the total dose at mid-plane:
A. Decreases as photon energy increases.
B. Decreases as field size increases
C. Increases as patient thickness increases.
D. Is slightly less for an SSD setup than for an SAD setup.
E. All of the above.

T 11 . When treating a lung volume with off-cord parallel opposed-oblique fields, 10 MV photons
would be preferable to 6 MV photons because:
A. 6 MV would give inadequate dose in the build-up region.
B. Lung corrections are more accurate with higher energy.
C. The total dose at dmax is less with 10 MV
D. The cord dose is less with 10 MV

ANSWERS:
T10. E Any factor that increases the PDD will decrease the total dose at dm,,x, compared with the total dose at midplane. Treating at SSD rather than SAD gives a slightly higher PDD.
T 11 . C For parallel-opposed fields, the total dose at dm,,X, as a percentage of the dose at the isocenter, is lower for higher photon energy.


Could anyone elaborate, please?

 

[Asim Riaz]

For T10, the answer is that all of the factors mentioned, such as increasing photon energy, field size, and patient thickness, will lead to an increase in the PDD (percentage depth dose). This increase in PDD will cause a decrease in the total dose at dmax, compared to the total dose at midplane. Treating at SSD rather than SAD will give a slightly higher PDD, which leads to a slight decrease in the total dose at dmax compared to that of an SAD setup.For T11, the answer is that for parallel-opposed fields, the total dose at dmax, as a percentage of the dose at the isocenter, is lower for higher photon energy. This means that using 10 MV photons, instead of 6 MV, would result in a lower total dose at dmax. This is because the PDD is lower for higher photon energies, leading to a decrease in the total dose at dmax compared to the total dose at the isocenter.