TLD measurements of absorbed dose were made in the anthropomorphic
phantom to determine the out-of-field stray radiation dose relative to the
in-field therapeutic radiation dose. The linear accelerators used in this study
were calibrated according to the American Association of Physicists in Medicine
Task Group 51 calibration protocol (
Almond
et al 1999) by a medical physicist certified by
the American Board of Radiology. Additionally, the outputs of the linear
accelerators were verified to within 2% on the days of phantom irradiation. In
the measurement at MD Anderson, there were 72 TLD locations in organs and
tissues throughout the phantom outside of the field or near the field edge. In
the measurement at AUBMC, there were 209 TLD locations: 184 were in organs and
tissues throughout the phantom outside of the field or near the field edge, 20
were attached to the surface of the phantom to measure skin dose, 3 were at
in-field calculation points to verify delivery of the prescribed dose, and 2
were kept outside of the treatment room to quantify background radiation at the
facility. A list that maps the TLD locations to organs at risk for
radiation-induced cancer was provided by the phantom manufacturer, which
included brain, eyes, thyroid, bone marrow, esophagus, thymus, breasts, lungs,
liver, pancreas, gall bladder, spleen, stomach, kidneys, adrenals, intestine,
prostate, bladder, and testicles.
The dosimeters were composed of lithium fluoride TLD-100 powder capsules
(Quantaflux Radiological Services, San Jose, California). This TLD is suitable
for measuring out-of-field photon dose in 6-MV radiotherapy (Kry
et al 2007 (
link), Scarboro
et al 2010 (
link)). The
TLDs were calibrated and read by certified technicians at the Radiological
Physics Center (MD Anderson, Houston, Texas), which includes an accredited
dosimetry calibration laboratory.
The absorbed doses at all TLD locations were collected from the TLD
measurements and from the TPS calculations at points corresponding to the TLD
locations and compared. The values of
D were reported relative
to the prescribed therapeutic dose of 30.6 Gy, or
D/
DRx (in cGy/Gy), for the
treatment plan.
The overall uncertainty of the absorbed dose measured by each in-field
TLD was 2.5% (
Kirby et al1992). We assumed the uncertainty in the in-field dose calculation
would be approximately 2% on the basis of validation measurements obtained at
the time of commissioning of each linear accelerator. We did not account for the
possibility of systematic uncertainty in the absorbed dose measured by each
outof-field TLD because of the non-uniformity of photon spectral fluence in that
region (see section 4), however, we would estimate this uncertainty to be less
than 12% (
Kry et al2006,
Olko et al2006,
Scarboro et al2011). No information about the uncertainty in the out-of-field dose
calculations by the TPSs was available from the manufacturers.
Taddei P.J., Jalbout W., Howell R.M., Khater N., Geara F., Homann K, & Newhauser W.D. (2013). Analytical model for out-of-field dose in photon craniospinal irradiation. Physics in medicine and biology, 58(21), 7463-7479.