Contact radiotheraphyMonte Carlo dosimetry for the Papillon P50 contact radiotherapy and IORT device
Section snippets
Materials and methods
Two systems of dosimetry are in common use for devices with an accelerating potential in the tens of kilovolts range and are described in detail in [2], [3], [8]. Both methods are based around an air kerma calibration and determine the dose at the surface of a water phantom.
The chamber factor [2], [3] has been determined experimentally [9], [10]. We performed similar measurements in air and then at the surface of a water phantom according to the IPEMB Code of practice [2], [3]. Setting the two
Results
The water-to-air mass energy-absorption coefficient ratio is relatively insensitive to the photon spectrum, varying on the level of tenths of a percent over the tens of kV range. The value found using the Penelope physics models implemented in Geant 4 was 1.022 with a statistical uncertainty of approximately 0.05%.
The calculated backscatter factor is shown in Fig. 1 for two of the applicators, the 22 mm and the 25 mm applicators. As the backscatter factor applies to an abstract point at the
Discussion
The mass energy absorption coefficient ratios were 0.2–0.3% smaller than those quoted in [2], [3]. Clinically a 0.3% difference is not significant. The backscatter factor calculated by the Monte Carlo was greater by approximately 1% than would be found by using the HVL, FSD and field-size based tables in [2], [3]. Assuming the locally measured HVL of 0.64 mm Al, then the backscatter factor according to [4], [7] is approximately 1.069, compared to our 1.082 for the 25 mm applicator and 1.068
Conclusions
We find that the dosimetry system used for other kV treatments is applicable to the Papillon P50. The mass energy-absorption coefficient ratio is consistent with previous values although this value is not very sensitive to beam quality or set-up conditions. The backscatter factor has been found to be 1.076 + 0.001 for a 22 mm applicator, at 2.9 mm FSD. Similarly the chamber factor was found to be consistent with the 1.06 currently recommended, albeit with large experimental errors. A full Monte
References (14)
- et al.
Geant4—a simulation toolkit
NIM A
(2003) - et al.
Contact Radiotherapy using a 50 kV X-ray system: evaluation of relative dose distribution with the Monte Carlo code PENELOPE and comparison with measurements
Radiat Phys Chem
(2012) - et al.
Renaissance of contact X-ray therapy for treating rectal cancer
Expert Rev Med Devices
(2011) - et al.
The IPEMB code of practice for the determination of absorbed dose for X-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL; 10–300 kV generating potential
Phys Med Biol
(1996) - et al.
Addendum to the IPEMB code of practice for the determination of absorbed dose for X-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL)
Phys Med Biol
(2005) Dependence of the photon backscatter factor for water on source-to-phantom distance and irradiation field size
Phys Med Biol
(1990)- et al.
Memorandum from the Institute of Physical Sciences in Medicine: back-scatter and F-factors for low- and medium-energy X-ray beams in radiotherapy
Br J Radiol
(1991)
Cited by (6)
Improving radiotherapy through medical physics developments
2015, Radiotherapy and OncologyThe on-going quest for treatment precision and conformality in radiotherapy
2013, Radiotherapy and OncologyProton and other heavy charged-particle beams
2021, Handbook of Radiotherapy Physics: Theory and Practice, Second Edition, Two Volume SetRelative dose measurements
2021, Handbook of Radiotherapy Physics: Theory and Practice, Second Edition, Two Volume SetPresent state and issues in IORT Physics
2017, Radiation OncologyElectronic brachytherapy-current status and future directions
2015, British Journal of Radiology