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For a megavoltage X-ray beam, the surface absorbed dose is below the subsurface maximum. Photons generate energetic secondary electrons, mainly through Compton scattering; the downstream electron fluence initially builds up until approximate charged-particle equilibrium is established. Beyond this buildup region, decreasing primary photon fluence, scattering and geometric spreading make the depth-dose curve fall gradually. A penetrating photon beam has a substantial exit dose.

A nearly monoenergetic proton beam deposits a moderate entrance dose, then a sharp Bragg peak near the end of its range. As nonrelativistic protons slow, their electronic stopping power rises approximately as $1/v^2$ over the relevant part of the track. Once the protons stop, the dose falls steeply. Range straggling and energy spread broaden the peak; nuclear interactions add a smaller nonideal tail, so an exact hard cutoff is an idealization. Superposing appropriate incident energies produces a spread-out Bragg peak covering an extended target.^[ref]

The label MV for an X-ray treatment beam specifies the nominal accelerating potential of the electron accelerator. Its bremsstrahlung beam is a broad photon energy spectrum with an endpoint of order $eV$, not a monoenergetic beam whose individual photons all carry the quoted numerical energy. Proton therapy beam energies can instead be specified directly in MeV per incident proton.

A water phantom is a water-filled reference object used to measure how a radiation field deposits energy under specified, reproducible conditions. In radiation dosimetry, a detector can be positioned at controlled depths and lateral locations inside the water. The measured quantity is related to absorbed dose in that reference medium; the tank is not a complete anatomical model of a patient.^[ref]

The surrounding medium matters physically. Photons and charged particles can scatter, attenuate and transfer energy before it is collected locally. An empty holder, a small isolated water sample and a large water tank need not produce identical fields at a detector. The named IAEA edition treats both water as reference material and the margins of scattering material around a measurement region. A water-equivalent material must be compared in relevant absorption and scattering properties, not identified by similar bulk density alone.^[ref]

A practical apparatus combines a tank, a positionable ionization chamber or another characterized detector, a readout and a way to define the water surface. For example, PTW's MP1 description specifies one-dimensional detector travel, levelling supports and fill marks. Its depth-positioning mechanism supports a depth-dose curve; one mechanical coordinate is not itself a full three-dimensional scan.^[ref]

An original position-error model shows why the reference coordinates matter. At a fixed depth, stipulate a lateral absorbed dose profileA $1\mathrm{mm}$ displacement from $x=20\mathrm{mm}$ changes the dose ratio toThe same displacement from the centre gives $D(1)/D(0)\simeq 0.99875$. Thus the same absolute positioning error can produce about $5\%$ variation on a gradient and only $0.125\%$ at the centre of this stipulated profile. These are mathematical examples, not measured beam characteristics or equipment accuracy claims.

An ionization chamber also samples a finite sensitive volume, so detector volume averaging can differ from a point value of $D(x)$. Converting its collected electric charge to absorbed dose requires the applicable measurement calibration and measurement model, as described in medical physicist. Changing the tank position does not remove a detector's intrinsic spatial response.

Comparisons should therefore identify the water surface, detector reference point, irradiation geometry, water conditions and background treatment. The IAEA discussion of waterproof sleeves and chamber positioning illustrates why the detector and its surrounding hardware are part of the measurement configuration.^[ref]