Out-of-field photon and neutron dose equivalents from step-and-shoot intensity-modulated radiation therapy.
OBJECTIVE
To measure the photon and neutron out-of-treatment-field dose equivalents to various organs from different treatment strategies (conventional vs. intensity-modulated radiation therapy [IMRT]) at different treatment energies and delivered by different accelerators.


METHODS
Independent measurements were made of the photon and neutron out-of-field dose equivalents resulting from one conventional and six IMRT treatments for prostate cancer. The conventional treatment used an 18-MV beam from a Clinac 2100; the IMRT treatments used 6-MV, 10-MV, 15-MV, and 18-MV beams from a Varian Clinac 2100 accelerator and 6-MV and 15-MV beams from a Siemens Primus accelerator. Photon doses were measured with thermoluminescent dosimeters in a Rando phantom, and neutron fluence was measured with gold foils. Dose equivalents to the colon, liver, stomach, lung, esophagus, thyroid, and active bone marrow were determined for each treatment approach.


RESULTS
For each treatment approach, the relationship between dose equivalent per MU, distance from the treatment field, and depth in the patient was examined. Photon dose equivalents decreased approximately exponentially with distance from the treatment field. Neutron dose equivalents were independent of distance from the treatment field and decreased with increasing tissue depth. Neutrons were a significant contributor to the out-of field dose equivalent for beam energies > or =15 MV.


CONCLUSIONS
Out-of-field photon and neutron dose equivalents can be estimated to any point in a patient undergoing a similar treatment approach from the distance of that point to the central axis and from the tissue depth. This information is useful in determining the dose to critical structures and in evaluating the risk of associated carcinogenesis.
Kry
Stephen F
SF
Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
Salehpour
Mohammad
M
Followill
David S
DS
Stovall
Marilyn
M
Kuban
Deborah A
DA
White
R Allen
RA
Rosen
Isaac I
II
D000465:Algorithms; D001853:Bone Marrow; D003106:Colon; D004947:Esophagus; D006801:Humans; D008099:Liver; D008168:Lung; D008297:Male; D009502:Neutrons; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation / Q000379:methods*; D012542:Scattering, Radiation; D013270:Stomach; D013819:Thermoluminescent Dosimetry / Q000379:methods; D013961:Thyroid Gland