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Title: Assessment of out-of-field absorbed dose and equivalent dose in proton fields

Abstract

Purpose: In proton therapy, as in other forms of radiation therapy, scattered and secondary particles produce undesired dose outside the target volume that may increase the risk of radiation-induced secondary cancer and interact with electronic devices in the treatment room. The authors implement a Monte Carlo model of this dose deposited outside passively scattered fields and compare it to measurements, determine the out-of-field equivalent dose, and estimate the change in the dose if the same target volumes were treated with an active beam scanning technique. Methods: Measurements are done with a thimble ionization chamber and the Wellhofer MatriXX detector inside a Lucite phantom with field configurations based on the treatment of prostate cancer and medulloblastoma. The authors use a GEANT4 Monte Carlo simulation, demonstrated to agree well with measurements inside the primary field, to simulate fields delivered in the measurements. The partial contributions to the dose are separated in the simulation by particle type and origin. Results: The agreement between experiment and simulation in the out-of-field absorbed dose is within 30% at 10-20 cm from the field edge and 90% of the data agrees within 2 standard deviations. In passive scattering, the neutron contribution to the total dose dominates inmore » the region downstream of the Bragg peak (65%-80% due to internally produced neutrons) and inside the phantom at distances more than 10-15 cm from the field edge. The equivalent doses using 10 for the neutron weighting factor at the entrance to the phantom and at 20 cm from the field edge are 2.2 and 2.6 mSv/Gy for the prostate cancer and cranial medulloblastoma fields, respectively. The equivalent dose at 15-20 cm from the field edge decreases with depth in passive scattering and increases with depth in active scanning. Therefore, active scanning has smaller out-of-field equivalent dose by factors of 30-45 in the entrance region and this factor decreases with depth. Conclusions: The dose deposited immediately downstream of the primary field, in these cases, is dominated by internally produced neutrons; therefore, scattered and scanned fields may have similar risk of second cancer in this region. The authors confirm that there is a reduction in the out-of-field dose in active scanning but the effect decreases with depth. GEANT4 is suitable for simulating the dose deposited outside the primary field. The agreement with measurements is comparable to or better than the agreement reported for other implementations of Monte Carlo models. Depending on the position, the absorbed dose outside the primary field is dominated by contributions from primary protons that may or may not have scattered in the brass collimating devices. This is noteworthy as the quality factor of the low LET protons is well known and the relative dose risk in this region can thus be assessed accurately.« less

Authors:
; ; ; ; ; ;  [1];  [2];  [3];  [4]
  1. Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 (United States)
  2. (United States) and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales, 2522 (Australia)
  3. (United States)
  4. (Australia)
Publication Date:
OSTI Identifier:
22102184
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 37; Journal Issue: 1; Other Information: (c) 2010 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; 63 RADIATION, THERMAL, AND OTHER ENVIRONMENTAL POLLUTANT EFFECTS ON LIVING ORGANISMS AND BIOLOGICAL MATERIALS; BIOLOGICAL EFFECTS; BRAGG CURVE; COMPUTERIZED SIMULATION; DOSE EQUIVALENTS; IONIZATION CHAMBERS; LET; MONTE CARLO METHOD; NEOPLASMS; NEUTRONS; PHANTOMS; PROSTATE; PROTONS; QUALITY FACTOR; RADIOTHERAPY; RISK ASSESSMENT

Citation Formats

Clasie, Ben, Wroe, Andrew, Kooy, Hanne, Depauw, Nicolas, Flanz, Jay, Paganetti, Harald, Rosenfeld, Anatoly, Department of Radiation Medicine, Loma Linda University Medical Center, Loma Linda, California 92354, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114, and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales, 2522. Assessment of out-of-field absorbed dose and equivalent dose in proton fields. United States: N. p., 2010. Web. doi:10.1118/1.3271390.
Clasie, Ben, Wroe, Andrew, Kooy, Hanne, Depauw, Nicolas, Flanz, Jay, Paganetti, Harald, Rosenfeld, Anatoly, Department of Radiation Medicine, Loma Linda University Medical Center, Loma Linda, California 92354, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114, & Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales, 2522. Assessment of out-of-field absorbed dose and equivalent dose in proton fields. United States. doi:10.1118/1.3271390.
Clasie, Ben, Wroe, Andrew, Kooy, Hanne, Depauw, Nicolas, Flanz, Jay, Paganetti, Harald, Rosenfeld, Anatoly, Department of Radiation Medicine, Loma Linda University Medical Center, Loma Linda, California 92354, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114, and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales, 2522. Fri . "Assessment of out-of-field absorbed dose and equivalent dose in proton fields". United States. doi:10.1118/1.3271390.
@article{osti_22102184,
title = {Assessment of out-of-field absorbed dose and equivalent dose in proton fields},
author = {Clasie, Ben and Wroe, Andrew and Kooy, Hanne and Depauw, Nicolas and Flanz, Jay and Paganetti, Harald and Rosenfeld, Anatoly and Department of Radiation Medicine, Loma Linda University Medical Center, Loma Linda, California 92354 and Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales, 2522},
abstractNote = {Purpose: In proton therapy, as in other forms of radiation therapy, scattered and secondary particles produce undesired dose outside the target volume that may increase the risk of radiation-induced secondary cancer and interact with electronic devices in the treatment room. The authors implement a Monte Carlo model of this dose deposited outside passively scattered fields and compare it to measurements, determine the out-of-field equivalent dose, and estimate the change in the dose if the same target volumes were treated with an active beam scanning technique. Methods: Measurements are done with a thimble ionization chamber and the Wellhofer MatriXX detector inside a Lucite phantom with field configurations based on the treatment of prostate cancer and medulloblastoma. The authors use a GEANT4 Monte Carlo simulation, demonstrated to agree well with measurements inside the primary field, to simulate fields delivered in the measurements. The partial contributions to the dose are separated in the simulation by particle type and origin. Results: The agreement between experiment and simulation in the out-of-field absorbed dose is within 30% at 10-20 cm from the field edge and 90% of the data agrees within 2 standard deviations. In passive scattering, the neutron contribution to the total dose dominates in the region downstream of the Bragg peak (65%-80% due to internally produced neutrons) and inside the phantom at distances more than 10-15 cm from the field edge. The equivalent doses using 10 for the neutron weighting factor at the entrance to the phantom and at 20 cm from the field edge are 2.2 and 2.6 mSv/Gy for the prostate cancer and cranial medulloblastoma fields, respectively. The equivalent dose at 15-20 cm from the field edge decreases with depth in passive scattering and increases with depth in active scanning. Therefore, active scanning has smaller out-of-field equivalent dose by factors of 30-45 in the entrance region and this factor decreases with depth. Conclusions: The dose deposited immediately downstream of the primary field, in these cases, is dominated by internally produced neutrons; therefore, scattered and scanned fields may have similar risk of second cancer in this region. The authors confirm that there is a reduction in the out-of-field dose in active scanning but the effect decreases with depth. GEANT4 is suitable for simulating the dose deposited outside the primary field. The agreement with measurements is comparable to or better than the agreement reported for other implementations of Monte Carlo models. Depending on the position, the absorbed dose outside the primary field is dominated by contributions from primary protons that may or may not have scattered in the brass collimating devices. This is noteworthy as the quality factor of the low LET protons is well known and the relative dose risk in this region can thus be assessed accurately.},
doi = {10.1118/1.3271390},
journal = {Medical Physics},
number = 1,
volume = 37,
place = {United States},
year = {Fri Jan 15 00:00:00 EST 2010},
month = {Fri Jan 15 00:00:00 EST 2010}
}