Reference dosimetry on TomoTherapy: an addendum to the 1990 UK MV dosimetry code of practice

Abstract

The current UK code of practice for high-energy photon therapy dosimetry (Lillicrap et al 1990 Phys. Med. Biol. 35 1355-60) gives instructions for measuring absorbed dose to water under reference conditions for megavoltage photons. The reference conditions and the index used to specify beam quality require that a machine be able to set a 10 cm × 10 cm field at the point of measurement. TomoTherapy machines have a maximum collimator setting of 5 cm × 40 cm at a source to axis distance of 85 cm, making it impossible for users of these machines to follow the code. This addendum addresses the specification of reference irradiation geometries, the choice of ionization chambers and the determination of dosimetry corrections, the derivation of absorbed dose to water calibration factors and choice of appropriate chamber correction factors, for carrying out reference dosimetry measurements on TomoTherapy machines. The preferred secondary standard chamber remains the NE2611 chamber, which with its associated secondary standard electrometer, is calibrated at the NPL through the standard calibration service for MV photon beams produced on linear accelerators with conventional flattening filters. Procedures are given for the derivation of a beam quality index specific to the TomoTherapy beam that can be used in the determination of a calibration coefficient for the secondary standard chamber from its calibration certificate provided by the NPL. The recommended method of transfer from secondary standard to field instrument is in a static beam, at a depth of 5 cm, by sequential substitution or by simultaneous side by side irradiation in either a water phantom or a water-equivalent solid phantom. Guidance is given on the use of a field instrument in reference fields.

Combining tissue‐phantom ratios to provide a beam‐quality specifier for flattening filter free photon beams

“…The difference in L /ρ water air at the same TPR 20,10 found in this study is close to the largest chamber specific and field dependent beam-quality correction factor for a lightly filtered beam measured at NPL (0.6% for a 4 MV lightly filtered beam). 22 The average difference between Monte Carlo calculated and TRS-398 predicted L /ρ water air was only 0.3% for flattening filter free beams in our study (RMSD 0.0028). This small deviation can be further improved by using an additional specifier, TPR 10,5 , based on which a simple bilinear equation provides a more accurate prediction regardless of beam filtration in the investigated energy range.…”

“…More recently, an addendum to the UK dosimetry code of practice for TomoTherapy reference dosimetry has been published. 22 For the determination of the beam-quality specifier for this unit, Palmans' 24 revised version of the method proposed by Sauer 23 is used to correct the TPR 20,10 value measured in nonstandard conditions. In that protocol, correction is made for the field-size only and not for the beam quality.…”

Detector dose response in megavoltage small photon beams. I. Theoretical concepts

“…These new techniques use beams that do not comply with conventional reference dosimetry protocols and therefore have been given the designation nonstandard, denoting either small fields or modulated photon beams. In 2008, the international IAEA-AAPM working group on small and composite field dosimetry proposed a new formalism for nonstandard beams, 3 which was adopted in dosimetry protocols for helical therapy 4,5 and which will be used in a future protocol addressing small photon fields.…”

Comments on the TRS‐483 protocol on small field dosimetry

“…Similar discrepancies [ Fig. 1(b)] exist between recommended microDiamond k f clin ;f msr Q clin ;Q msr factors for CyberKnife compared to the data from a large collaborative study involving [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] institutions and several microDiamonds. [5][6][7] The error bars are shown if such data exist in the publications.…”

“…For the Tomotherapy unit, there are no data for the Exradin A1SL chamber which are universally used for output calibration and therefore its absence is a major practical obstacle for the adoption of the TRS-483 protocol. On the other hand, IPEM CoP 26 for Tomotherapy provides this value as unity for Exradin A1SL ion chamber. Other difficulty is TRS-483 recommendation of phantom dimension (30 9 30 9 30 cm 3 (as listed in table 10) which is physically not possible to fit in the Tomotherapy bore.…”