Does Dmax (Depth of dose maximum) depend on chamber? If we measured Dmax (Depth of dose maximum)using cylindrical chamber for specific electron energy (10 MeV) with field size (10x10) and then using parallel plate chamber, does Dmax differ?
The ability to measure Dmax correctly will depend on chamber shape and volume. A cylindrical chamber will present the most problems. The difficulty is establishing the correct scatter geometry. An extrapolation chamber is the best method. Extrapolation chambers are easily constructed for beams of !0 MeV.
I guess the question refers to the PDD measurements in water. If correctly measured (i.e. taking into account the differences in the effective point of measurement for each chamber used), then the Dmax should have the same value. For cylindrical chambers, however, measuring in the effective point of measurement correctly is a bit tricky, so this might lead to some differences between the measurements with different chambers. These differences, however, if all other parameters are set up correctly, should be within 1%, I think.
Radu A. Vasilache Yes, PDD in water phantom. I need to know cylindrical chamber vs parallel plate chamber. Does both chamber gives same Dmax or not? what if i used large volume parallel plate chamber (PPC40) and small volume cylindrical chamber (0.13cc) ? Joseph L Alvarez Mostafa Hassanzadeh
The smaller the cylindrical chamber the better. Electron beams are easily perturbed. 10 MeV electrons will have Dmax between 2 and 3 cm. The accuracy will be affected by the total geometry. Wall thickness, central electrode, probe size and material. Multiple measurements with small increments in position and orientation can indicate geometry effects and location of Dmax.
As already stated in the previous answer, the value of Dmax is an intrinsic value of the PDD curve and, therefore, does not depend on the detector. However, its correct determination may be challenging from the experimental point of view. I may suggest to perform the measurement with different detectors, including diodes, that generally have a better spatial resolution and are less tricky respect to the effective point of measurement. May I ask what is the linac under test?
Normally for electrons one should use the Advanced Markus chamber (or equivalent), for measuring the PDDs. If you insist on using a cylindrical chamber, try the 3D PinPoint (or equivalent). If the cylindrical chamber does not have an homogeneous 3D response (i.e. it has different response for axial and radial irradiation) , than you need to measure, with the same chamber, both in axial and in radial presentation and then deduce the Dmax from the two. If correctly done, in the end the Dmax obtained with the parallel plane chambers and the cylindrical chambers should be close enough. But it take some work.
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