Use of plastic phantoms for relative dosimetry

The use of plastic phantoms is strongly discouraged as, in general, they are responsible for discrepancies in the determination of absorbed dose. Plastic phantoms should not be used for reference dosimetry in proton beams since the required water to plastic fluence correction factors, h_pl, are not known. Nevertheless, when accurate chamber positioning in water is not possible or when no waterproof chamber is available, their use is permitted for the measurement of depth dose distribution for low energy proton beams (approximately below 100 MeV). In this case, the dosimeter reading at each plastic depth should be scaled using the fluence scaling factor h_pl. It is assumed that h_plhas constant value of unity at all depths.

The criteria determining the choice of plastic materials are discussed in Section 4.2.3. The density of the plastic, ρ_pl, should be measured for the batch of plastic in use rather than using a nominal value for the plastic type. Each

measurement depth in plastic z_pl(expressed in g/cm²) must also be scaled to give the corresponding depth in water z_wby

z_w= z_plc_plg/cm²(z_plin g/cm²) (42)

where c_plis a depth scaling factor. For proton beams, c_plcan be calculated, to a good approximation, as the ratio of csda ranges (in g/cm²) [118] in water and in plastic. The depth scaling factor c_pl has a value of 0.974 for PMMA and 0.981 for clear TABLE 32. ESTIMATED RELATIVE STANDARD UNCERTAINTY^a OF D_w,Q AT THE REFERENCE DEPTH IN WATER AND FOR A CLINICAL PROTON BEAM, BASED ON A CHAMBER CALIBRATION IN ⁶⁰Co GAMMA RADIATION

Physical quantity or procedure

Relative standard uncertainty (%) User chamber type: Cylindrical Plane parallel

Step 1: Standards laboratory SSDL^b SSDL^b

N_D,wcalibration of secondary standard at PSDL 0.5 0.5

Long term stability of secondary standard 0.1 0.1

N_D,wcalibration of the user dosimeter at the 0.4 0.4

standards laboratory

Combined uncertainty in step 1 0.6 0.6

Step 2: User proton beam

Long term stability of the user dosimeter 0.3 0.4

Establishment of reference conditions 0.4 0.4

Dosimeter reading M_Qrelative to beam monitor 0.6 0.6

Correction for influence quantities k_i 0.4 0.5

Beam quality correction, k_Q 1.7 2.0

Combined uncertainty in step 2 1.9 2.2

Combined standard uncertainty in D_w,Q(steps 1 + 2) 2.0 2.3

aSee the ISO Guide for the expression of uncertainty [32], or Appendix IV. The estimates given in the table should be considered typical values; these may vary depending on the uncertainty quoted by standards laboratories for calibration factors and on the experimental uncertainty at the user’s institution.

bIf the calibration of the user dosimeter is performed at a PSDL then the combined standard uncertainty in step 1 is lower. The combined standard uncertainty in D_wshould be adjusted accordingly.

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polystyrene. The procedure given in Section 10.6.1 should be followed to generate central axis depth dose distributions from the measure depth ionization distributions.

If a plastic phantom is used to measure the beam quality index, the measured quantity is the residual range in the plastic, R_res,pl. The residual range, R_res, in water is also obtained using the scaling Eq. (42).

10.7. ESTIMATED UNCERTAINTY IN THE DETERMINATION OF ABSORBED DOSE TO WATER UNDER REFERENCE CONDITIONS The uncertainties associated with the physical quantities and procedures involved in the determination of the absorbed dose to water in the user proton beam can be divided into two steps. Step 1 considers uncertainties up to the calibration of the user chamber in terms of N_D,w at a standards laboratory. Step 2 deals with the subsequent calibration of the user proton beam using this chamber and includes the uncerainty of k_Qas well as that associated with measurements at the reference depth in a water phantom. Estimates of the uncertainties in these two steps are given in Table 32, yielding a combined standard uncerainty of 2% and 2.3% for the determi-nation of the absorbed dose to water in a clinical proton beam with a cylindrical and plane-parallel ionization chamber, respectively. Details on the uncertainty estimates for the various physical parameters entering in the calculation of k_Q are given in Appendix II.

10.8. WORKSHEET

Determination of the absorbed dose to water in a proton beam

User: Date:

1. Radiation treatment unit and reference conditions for D_w,Qdetermination

Proton therapy unit: Nominal energy: MeV

Nominal dose rate: MU/min Practical range, R_p: g/cm²

Reference phantom: water Width of the SOBP: g/cm²

Reference field size: cm ×cm Reference SSD: cm

Reference depth, z_ref: g/cm² Beam quality, Q(R_res): g/cm²

2. Ionization chamber and electrometer

Ionization chamber model: Serial No.: Type: ❏ cyl ❏pp

Chamber wall/window material: thickness = g/cm²

Waterproof sleeve/cover material: thickness = g/cm²

Phantom window material: thickness = g/cm²

Absorbed dose to water calibration factor N_D,w= ❏ Gy/nC ❏ Gy/rdg

3. Dosimeter reading^aand correction for influence quantities

Uncorrected dosimeter reading at V₁and user polarity: ❏ nC ❏ rdg

Corresponding accelerator monitor units: MU

Ratio of dosimeter reading and monitor units: M₁= ❏ nC/MU ❏ rdg/MU (i) Pressure P: __________ kPa Temperature T : _______ °C Rel. humidity (if known): ____ %

(ii) Electrometer calibration factor^bk_elec: ❏ nC/rdg ❏ dimensionless k_elec=

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(iv) Recombination correction (two voltage method)

Polarizing voltages: V_l(normal) = ____________ V V₂(reduced) = _______________ V Readings^dat each V: M₁= ___________________ M₂= _______________________

Voltage ratio V₁/V₂= __________________________ Ratio of readings M₁/M₂= ___________

Use Table 9 for a beam of type: ❏ pulsed ❏pulsed-scanned

a₀= ____________ a₁= ____________ a₂= ____________

_____________^e,f

Corrected dosimeter reading at the voltage V₁:

M_Q= M₁k_TPk_eleck_polk_s= ❏ nC/MU ❏ rdg/MU

4. Absorbed dose rate to water at the reference depth, z_ref Beam quality correction factor for user quality Q: k_Q=

taken from ❏ Table 31 ❏ Other, specify:

Absorbed dose calibration of monitor at z_ref:

D_w,Q(z_ref) = M_QN_D,w,k_Q= Gy/MU

aAll readings should be checked for leakage and corrected if necessary.

bIf the electrometer is not calibrated separately, set k_elec= 1.

cM in the denominator of k_poldenotes reading at the user polarity. Preferably, each reading in the equation should be the average of the ratios of M (or M₊or M_–) to the reading of an external monitor M_em. It is assumed that the calibration laboratory has performed a polarity correction. Otherwise, k_polis deter-mined according to:

rdg at +V₁for quality Q_o: M₊= ___________ rdg at –V₁for quality Q_o: M_–= ____________

dStrictly, readings should be corrected for polarity effect (average with both polarities). Preferably, each reading in the equation should be the average of the ratios of M₁or M₂to the reading of an external monitor M_em.

eIt is assumed that the calibration laboratory has performed a recombination correction. Otherwise, the factor k′_s= k_s/k_s,Qoshould be used instead of k_s. When Q_ois ⁶⁰Co, k_s,Qo(at the calibration laboratory) will normally be close to unity and the effect of not using this equation will be negligible in most cases.

f