Out-of-field dose measurements in a water phantom using different radiotherapy modalities

Abstract

This investigation focused on the characterization of the lateral dose fall-off following the irradiation of the target with photons, protons and carbon ions. A water phantom was irradiated with a rectangular field using photons, passively delivered protons as well as scanned protons and carbon ions. The lateral dose profile in the depth of the maximum dose was measured using an ion chamber, a diamond detector and thermoluminescence detectors TLD-600 and TLD-700. The yield of thermal neutrons was estimated for all radiation types while their complete spectrum was measured with bubble detectors during the irradiation with photons. The peripheral dose delivered by photons is significantly higher compared to both protons and carbon ions and exceeds the latter by up to two orders of magnitude at distances greater than 50 mm from the field. The comparison of passive and active delivery techniques for protons shows that, for the chosen rectangular target shape, the former has a sharper penumbra whereas the latter has a lower dose in the far-out-of-field region. When comparing scanning treatments, carbon ions present a sharper dose fall-off than protons close to the target but increasing peripheral dose with increasing incident energy. For photon irradiation, the contribution to the out-of-field dose of photoneutrons appears to be of the same order of magnitude as the scattered primary beam. Charged particles show a clear supremacy over x-rays in achieving a higher dose conformality around the target and in sparing the healthy tissue from unnecessary radiation exposure. The out-of-field dose for x-rays increases with increasing beam energy because of the production of biologically harmful neutrons.

A simple and fast physics-based analytical method to calculate therapeutic and stray doses from external beam, megavoltage x-ray therapy

“…Figure 8 plots data from Stovall et al along with measured and calculated dose values for the same field size from this work. Error bars for measured data from Kaderka et al (2012) represent one standard deviation as reported in their study. Error bars for Stovall et al (1995) represent one standard deviation estimated based on subsequently published uncertainty in out-of-field TLD measurements for a 6 MV photon beam (Kry et al , 2007).…”

“…The measurement conditions for all training data are listed in Table 1. Water-tank measurements used to train this model were taken from a study by Kaderka et al (2012) that were performed on an electron linac (Elekta SL25, Stockholm) at Universitätsklinikum (KGU) in Frankfurt, Germany operated at 6 MV. Kaderka et al measured photon doses using a diamond detector (60003 PTW, Freiburg) in a water tank (PTW, Freiburg).…”

“…Doses were measured in the cross-plane direction for a 10×10 cm 2 field with an SSD of 100 cm and at a depth of 1.5 cm in water. Measured data include error bars representing one standard deviation, as reported by Kaderka et al (2012).…”

“…Plot of the total absorbed dose for a 5×5 cm 2 treatment field at a depth of 10 cm normalized to 1 mGy/Gy on the CAX at a depth of maximum dose measured with a diamond detector in the in-plane direction a water-box phantom (Kaderka, 2012), measured with TLDs in a water-equivalent phantom (Stovall et al , 1995), calculated by two different commercial treatment planning systems (Pinnacle, Philips Healthcare, Andover, MA and Eclipse, Varian Medical Systems, Inc., Palo Alto, CA), and calculated by the analytical model proposed in this work.…”

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

“…2). Although it has been shown in several studies that there is very little depth‐dependence for out‐of‐field dose measurements taken at a constant distance from the field edge, ( 22 , 23 ) TPS‐calculated out‐of‐field doses do show depth dependence. ( 10 ) Since measurement and calculation locations were not taken at a uniform depth in the anthropomorphic phantoms, this nonexponential behavior in the TPS‐calculated doses is likely attributable to this depth‐dependence.…”