Measurements of dose from secondary radiation outside a treatment field

112

Although treatment planning systems are generally thought to have poor accuracy for out‐of‐field dose calculations, little work has been done to quantify this dose calculation inaccuracy for modern treatment techniques, such as intensity‐ modulated radiation therapy (IMRT), or to understand the sources of this inaccuracy. The aim of this work is to evaluate the accuracy of out‐of‐field dose calculations by a commercial treatment planning system (TPS), Pinnacle3 v.9.0, for IMRT treatment plans. Three IMRT plans were delivered to anthropomorphic phantoms, and out‐of‐field doses were measured using thermoluminescent detectors (TLDs). The TLD‐measured dose was then compared to the TPS‐calculated dose to quantify the accuracy of TPS calculations at various distances from the field edge and out‐of‐field anatomical locations of interest (i.e., radiosensitive organs). The individual components of out‐of‐field dose (patient scatter, collimator scatter, and head leakage) were also calculated in Pinnacle and compared to Monte Carlo simulations for a 10×10 cm2 field. Our results show that the treatment planning system generally underestimated the out‐of‐field dose and that this underestimation worsened (accuracy decreased) for increasing distances from the field edge. For the three IMRT treatment plans investigated, the TPS underestimated the dose by an average of 50%. Our results also showed that collimator scatter was underestimated by the TPS near the treatment field, while all components of out‐of‐field dose were severely underestimated at greater distances from the field edge. This study highlights the limitations of commercial treatment planning systems in calculating out‐of‐field dose and provides data about the level of accuracy, or rather inaccuracy, that can be expected for modern IMRT treatments. Based on our results, use of the TPS‐reported dose could lead to an underestimation of secondary cancer induction risk, as well as poor clinical decision‐making for pregnant patients or patients with implantable cardiac pacemakers and defibrillators.PACS numbers: 87.53.Bn; 7.55.D‐

T = L + C + P

…”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Accuracy and sources of error of out‐of field dose calculations by a commercial treatment planning system for intensity‐modulated radiation therapy treatments

Huang

Followill

Wang

et al. 2013

112

“…For all measurements, the ionization chamber was placed at the level of isocenter plane at a depth of 10.0 cm in the phantom (midplane). Due to small PD depth dependence, 1 , 2 , 5 – 7 PD distributions were evaluated only at that depth. The number of monitor units was adjusted to maintain the ionization reading precision at 1%.…”

Section: Methodsmentioning

confidence: 99%

Peripheral dose distributions for a linear accelerator equipped with a secondary multileaf collimator and universal wedge

Mutic

Esthappan

Klein

2002

The American Association of Physicists in Medicine Task Group 36 (AAPM TG‐36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data does not apply to linear accelerators equipped with a multileaf collimator (MLC) and universal wedge (UW). Tertiary multileaf collimators have been shown to significantly affect PD distributions and TG‐36 reported data. Measurements were performed to evaluate PD distributions for a linear accelerator equipped with a secondary MLC, backup diaphragms, and UW. This data can be used to compliment the TG‐36 report for estimation of doses to critical structures outside primary radiation fields. For the evaluated linear accelerator, an MLC is incorporated in the upper secondary collimator jaws. Backup shielding diaphragms are located underneath the MLC. At the nominal collimator position, the MLC and the backup diaphragm provide collimation primarily in the transverse direction. Conventional, solid tungsten‐alloy jaws, located underneath the backup diaphragms, provide secondary collimation in the longitudinal direction. The universal wedge provides dose modulation in the direction of the conventional jaws. Measurements were made with an ionization chamber inserted into a 20×40×120cm3 water‐equivalent plastic phantom with the secondary collimator and MLC settings of 5×5,10×10,15×15, and 25×25cm2 with and without UW. Data was acquired along the machine's longitudinal axis for 6, 10, and 18 MV photons. Peripheral dose distributions were measured with the collimator rotated to 0° and 270° for open field measurements and to 0°, 180°, and 270° for wedged fields (IEC 1217). This allowed evaluation of peripheral dose distributions as a function of collimator rotation. Wedged fields were normalized to deliver the same dose at the depth of maximum dose on the central axis as open fields. The measured PD distributions were generally comparable to data reported by TG‐36. At distances close to the field edge (less than 30 or 40 cm), the measured PD distributions were lower when the measurement point was shielded by solid jaws than with MLC and backup diaphragm. At longer distances, this trend reversed for all energies and evaluated field sizes. However, the difference in PD distribution with collimator rotation was not large enough to warrant strategic positioning of the collimator to reduce dose to critical structures outside the primary radiation field. Because internal scatter dominates close to the field edge, wedged PD distributions were comparable to open field doses at distances closer than 30 cm. However, at distances larger than 30 cm from the field edge, wedged PD distributions were significantly grater than those for open fields due to increased contribution of leakage radiation. Increased leakage radiation is due to the increase in wedged field monitor units, which is related to a small wedge factor (0.27 to 0.29).PACS number(s): 87.53.–j, 87.66.–a

“…Since head leakage was known to be the dominant component at such a large distance from field edge, (29) the photon stray dose to the fetus can be estimated as

7000 cGy \times 3 \times 0.0005 = 105 mGy false(mSv false)

. This assumes a 0.05% head leakage,

1 cGy / μ

to the target, and an IMRT MU factor of 3.…”

Section: Discussionmentioning

confidence: 99%

Spot scanning proton therapy minimizes neutron dose in the setting of radiation therapy administered during pregnancy

Wang

Poenisch

Zhu

et al. 2016

This is a real case study to minimize the neutron dose equivalent (H) to a fetus using spot scanning proton beams with favorable beam energies and angles. Minimum neutron dose exposure to the fetus was achieved with iterative planning under the guidance of neutron H measurement. Two highly conformal treatment plans, each with three spot scanning beams, were planned to treat a 25‐year‐old pregnant female with aggressive recurrent chordoma of the base of skull who elected not to proceed with termination. Each plan was scheduled for delivery every other day for robust target coverage. Neutron H to the fetus was measured using a REM500 neutron survey meter placed at the fetus position of a patient simulating phantom. 4.1 and 44.1 44.1 μSv/fraction were measured for the two initial plans. A vertex beam with higher energy and the fetal position closer to its central axis was the cause for the plan that produced an order higher neutron H. Replacing the vertex beam with a lateral beam reduced neutron H to be comparable with the other plan. For a prescription of 70 Gy in 35 fractions, the total neutron H to the fetus was estimated to be 0.35 mSv based on final measurement in single fraction. In comparison, the passive scattering proton plan and photon plan had an estimation of 26 and 70 mSv, respectively, for this case. While radiation therapy in pregnant patients should be avoided if at all possible, our work demonstrated spot scanning beam limited the total neutron H to the fetus an order lower than the suggested 5 mSv regulation threshold. It is far superior than passive scattering beam and careful beam selection with lower energy and keeping fetus further away from beam axis are essential in minimizing the fetus neutron exposure.PACS number(s): 87.53.Bn, 87.55.D‐, 87.55.N‐