A Monte Carlo model for calculating out‐of‐field dose from a Varian beam

The growing use of cone‐beam computed tomography (CBCT) for IGRT has increased concerns over the additional radiation dose to patients. The in‐field dose of IGRT and the peripheral dose (PD) from kilovoltage CBCT (KV‐CBCT) imaging have been well quantified. The purpose of this work is to evaluate the peripheral dose from megavoltage CBCT (MV‐CBCT) imaging for nasopharyngeal carcinoma IGRT, to determine the correlation of peripheral dose with MU protocol and imaging field size, and to estimate out‐of‐field organ‐at‐risk (OAR) dose delivered to patients. Measurements of peripheral MV‐CBCT doses were made with a 0.65 cm3 ionization chamber placed inside in a specially designed phantom at various depths and distances from the imaging field edges. The peripheral dose at reference point inside the phantom was measured with the same ionization chamber to investigate the linearity between MUs used for MV‐CBCT imaging and the PD. The peripheral surface doses at the anterior, lateral, and posterior of the phantom at various distances from the imaging field edge were also measured with thermoluminescent dosimeters (TLDs). Seven nasopharyngeal carcinoma patients were selected and scanned before treatment with head–neck protocol, and the peripheral surface doses were measured with TLDs placed on the anterior, lateral, and posterior surfaces at the axial plane of 15 cm distance from the field edge. The measured peripheral doses data in the phantom were utilized to estimate the peripheral OAR dose. Peripheral dose from MV‐CBCT imaging increased with increasing number of MUs used for imaging protocol and with increasing the imaging field size. The measured peripheral doses in the phantom decreased as distance from the imaging field edges increased. PD also decreased as the depth from the phantom surface increased. For the patient PD measurements, the anterior, lateral, and posterior surface doses of 15 cm distance from the field edge were 2.84×10−2, 1.01×10−2, and 0.78×10−2 cGy/MU, respectively. The lens, thyroid, breast, and ovary and testicle, which are outside the treatment and imaging fields, were estimated to receive peripheral OAR doses from MV‐CBCT imaging of 42.4×10−2, 11.9×10−2, 1.4×10−2, 1.0×10−2, and 0.5×10−2 cGy/MU, respectively. In conclusion, MV‐CBCT generates a peripheral dose beyond the edge of the MV‐CBCT scanning field that is of a similar order of magnitude to the peripheral dose from kV‐CBCT imaging. In clinic, using the smallest number of MUs allowable and reducing MV‐CBCT scanning field size without compromising acquired image quality is an effective method of reducing the peripheral OAR dose received by patients.PACS number: 89

Section: Discussioncontrasting

confidence: 74%

Peripheral dose from megavoltage cone‐beam CT imaging for nasopharyngeal carcinoma image‐guided radiation therapy

Jia

Wang

et al. 2012

“…This affects the accuracy of film dose measurements in the periphery of the field, where low‐energy scatter is more prevalent (see, for example, Chofor et al and by Kry et al) 21, 22. Because complete information on the photon spectrum of the 5 cm diameter calibration field was not available at the time of calibration, we have determined an EBT3 calibration curve in a 10 × 10 cm 2 field on central axis and subsequently made an allowance for the EBT3 under‐response in the periphery of the calibration field in our uncertainty budget.…”

Section: Methodsmentioning

confidence: 99%

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

Kupfer

Lehmann

Butler³

et al. 2017

PurposeThis study investigates a large‐area plane‐parallel ionization chamber (LAC) for measurements of dose‐area product in water (DAP w) in megavoltage (MV) photon fields.MethodsUniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high‐resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm2 field were acquired. A 6 MV calibration coefficient, ND ,w, LAC, was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP w. Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed.ResultsElectrode separation is uniform and agrees with specifications. Volume‐averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off‐axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm2 field. ND ,w, LAC was 163.7 mGy cm−2 nC −1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.ConclusionsThe LAC requires only small correction factors for DAP w measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over‐response. The accuracy of relative point‐dose measurements in the field's periphery is an important limiting factor for the accuracy of DAP w measurements.

“…The effective square field size was 11 cm for the primary treatment plans and 8 cm for the boost treatment plans. Similar data for peripheral dose distributions are also available from other sources 1, 6, 11, 12. For TomoTherapy peripheral dose estimates, perhaps the best available reference is the 2013 paper by Lissner et al; fig.…”

Section: Methodsmentioning

confidence: 96%

“…The measurements from the three TLDs were averaged together for each point of interest for each treatment plan. While out‐of‐field dose has minimal dependence on depth, the superficial dose is markedly higher than the rest of the depth–dose curve; the bolus was used to position the TLDs beyond this superficial region 11. In addition to the TLDs, a Farmer‐type ionization chamber placed in the center of the 30 cm slabs of phantom material (depth of 15 cm) at the longitudinal level of the patient's umbilicus integrated charge over the course of each irradiation.…”

Section: Methodsmentioning

confidence: 99%

“…While prior reports have provided estimates of peripheral dose for various combinations of beam energy and geometry, these reports often apply to a prior generation of treatment delivery system 1, 4, 5, 6, 10, 11, 12, 13, 14. The present manuscript details fetal dose measurements on current‐generation treatment delivery systems and summarizes the anticipated risks to the patient's fetus using published guidance.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Radiation treatment planning and delivery strategies for a pregnant brain tumor patient

Labby

Barraclough

Bayliss

et al. 2018

The management of a pregnant patient in radiation oncology is an infrequent event requiring careful consideration by both the physician and physicist. The aim of this manuscript was to highlight treatment planning techniques and detail measurements of fetal dose for a pregnant patient recently requiring treatment for a brain cancer. A 27‐year‐old woman was treated during gestational weeks 19–25 for a resected grade 3 astrocytoma to 50.4 Gy in 28 fractions, followed by an additional 9 Gy boost in five fractions. Four potential plans were developed for the patient: a 6 MV 3D‐conformal treatment plan with enhanced dynamic wedges, a 6 MV step‐and‐shoot (SnS) intensity‐modulated radiation therapy (IMRT) plan, an unflattened 6 MV SnS IMRT plan, and an Accuray TomoTherapy HDA helical IMRT treatment plan. All treatment plans used strategies to reduce peripheral dose. Fetal dose was estimated for each treatment plan using available literature references, and measurements were made using thermoluminescent dosimeters (TLDs) and an ionization chamber with an anthropomorphic phantom. TLD measurements from a full‐course radiation delivery ranged from 1.0 to 1.6 cGy for the 3D‐conformal treatment plan, from 1.0 to 1.5 cGy for the 6 MV SnS IMRT plan, from 0.6 to 1.0 cGy for the unflattened 6 MV SnS IMRT plan, and from 1.9 to 2.6 cGy for the TomoTherapy treatment plan. The unflattened 6 MV SnS IMRT treatment plan was selected for treatment for this particular patient, though the fetal doses from all treatment plans were deemed acceptable. The cumulative dose to the patient's unshielded fetus is estimated to be 1.0 cGy at most. The planning technique and distance between the treatment target and fetus both contributed to this relatively low fetal dose. Relevant treatment planning strategies and treatment delivery considerations are discussed to aid radiation oncologists and medical physicists in the management of pregnant patients.