Sources of electron contamination for the Clinac‐35 25‐MV photon beam

Petti, Paula L.; Goodman, M.S.; Sisterson, J. M.; Biggs, Peter J.; Gabriel, T.A.; Mohan, R.

doi:10.1118/1.595348

Cited by 76 publications

(60 citation statements)

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“…For short SSDs, 80 -100 cm, the electrons generated in air contribute little to the dose near the surface compared to those originating in the accelerator head [12][13][14]. At longer SSDs, the relative contribution from electrons generated in air increases with increasing irradiated air volume and ultimately dominates as the head-generated electrons are lost [14].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Doses near the surface during total-body irradiation with 15 MV X-rays

Kassaee¹,

Xiao²,

Bloch³

et al. 2001

Int. J. Cancer

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SUMMARY An extended source-surface distance (SSD) is often employed in total-body irradiation (TBI) to obtain fields sufficiently large to encompass the total body. It is clinically desirable to administer a uniform dose to all tissues, including the surface, especially to counteract the skin-sparing of high-energy X-rays. The electrons generated from the air contribute more to the dose near the surface at extended distances than at regular SSDs. However, if further increase of the dose at shallow depths is desired a spoiler can be employed. Monte Carlo simulations were used to study these effects for the electron component of the beam and the parameters needed for the pencil-beam dose calculation of this component. Measurements were performed to confirm these studies. The energy spectrum of electrons generated in air is independent of SSD. All the electrons in the beam, however, originate from the spoiler when one is used. The spectrum of these electrons is nearly independent of the SSD. The pencil-beam dose kernels were calculated using the derived electron spectra. To calculate doses in the buildup region with and without a spoiler, we use one pencil-beam kernel for the electrons generated in air and one for the electrons generated from the spoiler. Measured depth-dose data agreed with the calculation results.

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Section: Introductionmentioning

confidence: 99%

“…At longer SSDs, the relative contribution from electrons generated in air increases with increasing irradiated air volume and ultimately dominates as the head-generated electrons are lost [14].…”

Section: Introductionmentioning

confidence: 99%

Doses near the surface during total-body irradiation with 15 MV X-rays

Kassaee¹,

Xiao²,

Bloch³

et al. 2001

Int. J. Cancer

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“…Dose at the surface is primarily due to electron contamination from the flattening filter, beam modifiers and air. The magnitude of the surface dose depends on the field size, angle of beam incidence, air gap and the use of beam modifiers (Biggs and Ling 1979, Gerbi et al 1987, Lamb and Blake 1998, Lopez Medina et al 2005, Petti et al 1983a, 1983b, Yang et al 2004, Zhu and Palta 1998.…”

Section: Introductionmentioning

confidence: 99%

Assessment of skin dose for breast chest wall radiotherapy as a function of bolus material

et al. 2008

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Skin dose assessment for chest wall radiotherapy is important to ensure sufficient dose to the surface target volume without excessive skin reaction. This study quantified changes to the surface doses as a function of bolus material for conventional and intensity modulated radiation therapy (IMRT) tangential fields. Three types of bolus materials (2 mm solid, 2 mm fine mesh and 3.2 mm large mesh Aquaplast) were compared with Superflab. Surface dose measurements were performed using an Attix parallel plate chamber in a flat solid water phantom at 0• , 45• and 70• incident angles. Over-response correction factors were applied to the Attix chamber results for different incident angles. Surface dose measurements on an anthropomorphic phantom were done using a thermoluminescent dosimeter extrapolation method. Dose characteristics of Superflab and solid Aquaplast were within 2% of solid water material. No significant differences (within 3%) in the surface dose were found between conventional and IMRT tangential techniques. The bolus effect was large for chest wall tangential radiotherapy, with up to an 82% increase using 2 mm fine mesh Aquaplast. The dosimetric effect of different Aquaplast materials has been quantified in this work. These materials can be used to create a custom bolus with potentially better reproducibility of placement.

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“…, (7) , (17) Head‐scattered photons emanate primarily from sources upstream from the secondary collimators (flattening filter and primary collimator) (18) . For any point in the measurement plane, the surface‐contaminating radiation dose is primarily a function of the amount of exposed flattening filter viewable through the collimators (11) .…”

Section: Introductionmentioning

confidence: 99%

“…For any point in the measurement plane, the surface‐contaminating radiation dose is primarily a function of the amount of exposed flattening filter viewable through the collimators (11) . Although forward peaked, the spectrum of contaminating electrons has a finite angular spread (7) . The result is a Gaussian‐type distribution of dose near the surface, falling off away from central axis and extending outside of the open field (6) …”

Section: Introductionmentioning

confidence: 99%

Surface buildup dose dependence on photon field delivery technique for IMRT

Yokoyama

Roberson

Litzenberg

et al. 2004

J Applied Clin Med Phys

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The more complex delivery techniques required for implementation of intensity‐modulated radiotherapy (IMRT) based on inverse planning optimization have changed the relationship between dose at depth and dose at buildup regions near the surface. Surface buildup dose is dependent on electron contamination primarily from the unblocked view of the flattening filter and secondarily from air and collimation systems. To evaluate the impact of beam segmentation on buildup dose, measurements were performed with 10×10 cm2 fields, which were delivered with 3 static 3.5×10 cm2 or 3×10 cm2 strips, 5 static 2×10 cm2 strips, 10 static 1×10 cm2 strips, and 1.1×10 cm2 dynamic delivery, compared with a 10×10 cm2 open field. Measurements were performed in water and Solid Water using parallel plate chambers, a stereotactic diode, and thermoluminescent dosimeters (TLDs) for a 6 MV X‐ray beam. Depth doses at 2 mm depth (relative to dose at 10 cm depth) were lower by 6%, 7%, 11%, and 10% for the above field delivery techniques, respectively, compared to the open field. These differences are most influenced by differences in multileaf collimator (MLC) transmission contributing to the useful beam. An example IMRT field was also studied to assess variations due to delivery technique (static vs. dynamic) and intensity level. Buildup dose is weakly dependent on the multileaf delivery technique for efficient IMRT fields.PACS numbers: 87.53.‐j, 87.53.Dq

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Sources of electron contamination for the Clinac‐35 25‐MV photon beam

Cited by 76 publications

References 0 publications

Doses near the surface during total-body irradiation with 15 MV X-rays

Doses near the surface during total-body irradiation with 15 MV X-rays

Assessment of skin dose for breast chest wall radiotherapy as a function of bolus material

Surface buildup dose dependence on photon field delivery technique for IMRT

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