Perturbation of electron beam doses as a function of SSD due to the use of shielding blocks on the Clinac-18

Thomadsen, Bruce R.; Lloyd, W. L.; Geijn, J. Van de; Paliwal, Bhudatt R.; Cheng, Pocheng

doi:10.1118/1.594999

Cited by 6 publications

(3 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This method represents an alternative approach to the effective SSD method, using the same set of measured data. Both methods are consistent with the suggestion of Thomadsen et al (1981) to measure any output at an extended distance, as they both provide a methodology for storing and using the data.…”

Section: Correction Of Outputsupporting

confidence: 73%

Clinical electron‐beam dosimetry: Report of AAPM Radiation Therapy Committee Task Group No. 25

et al. 1991

View full text Add to dashboard Cite

Section: Correction Of Outputsupporting

confidence: 73%

Clinical electron‐beam dosimetry: Report of AAPM Radiation Therapy Committee Task Group No. 25

et al. 1991

View full text Add to dashboard Cite

“…For a given applicator, there is a strong dependence of SSD eff on insert size. 109 Investigators have shown that SSD eff is only weakly dependent on the applicator 40,110 so tables of SSD eff versus insert size are clinically adequate. In fact, Sharma and Johnson 111 suggested that a single SSD eff for each energy can be used for field sizes larger than 10 × 10 cm 2 .…”

Section: B2d Effectivementioning

confidence: 99%

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71

et al. 2014

View full text Add to dashboard Cite

A protocol is presented for the calculation of monitor units (MU) for photon and electron beams, delivered with and without beam modifiers, for constant source-surface distance (SSD) and source-axis distance (SAD) setups. This protocol was written by Task Group 71 of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol defines the nomenclature for the dosimetric quantities used in these calculations, along with instructions for their determination and measurement. Calculations are made using the dose per MU under normalization conditions, D 0 , that is determined for each user's photon and electron beams. For electron beams, the depth of normalization is taken to be the depth of maximum dose along the central axis for the same field incident on a water phantom at the same SSD, where D 0 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D 0 ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of d m , with D 0 = 1 cGy/MU, although both systems are acceptable within the current protocol. For photon beams, the formalism includes the use of blocked fields, physical or dynamic wedges, and (static) multileaf collimation. No formalism is provided for intensity modulated radiation therapy calculations, although some general considerations and a review of current calculation techniques are included. For electron beams, the formalism provides for calculations at the standard and extended SSDs using either an effective SSD or an air-gap correction factor. Example tables and problems are included to illustrate the basic concepts within the presented formalism.

show abstract

“…Physical limitations in the treatment set-up mean that high-energy electron therapy may be given at extended source-surface distances (SSD) (Thomadsen et al 1981). Irregularities of the skin surface may also create air gaps or minor areas of the field at extended SSD, which affect the absorbed dose, although the SSD effect may be minor compared with that of surface scatter.…”

Section: Introductionmentioning

confidence: 99%

Variation in calculated effective source - surface distances with depth

Ostwald

Kron²

1996

Phys. Med. Biol.

View full text Add to dashboard Cite

Effective source-surface distances (ESSD) are assessed at the depth of maximum dose in electron beams. This study investigated the variation of the ESSD with the depth of measurement. The dose was measured with the range of SSDs 100-130 cm, using a water-equivalent parallel-plate ion chamber in solid water. ESSDs were calculated for electron beams in the energy range 4-20 MeV and were found to vary with depth. The surface ESSD varied from 68 cm for 4 MeV to 82 cm for 16 MeV, but increased with depth to a maximum value, which was found at approximately half the practical range (Rp), at 0.3Rp for 4 MeV and at 0.6Rp for 20 MeV. Beyond this depth the ESSD decreased towards the end of the practical range. Without an electron applicator, the ESSD was higher at the surface. For smaller field sizes, the depth of the maximum ESSD increased towards Rp, and ESSD values increased. The 20 MeV beam in the 6 cm x 6 cm2 field showed a difference of 31 cm between the surface ESSD and the maximum ESSD. The ESSD calculated at the maximum dose depth (Dmax) may be used with reasonable accuracy for calculation of the dose in the therapeutic range, except at larger SSDs or when high-energy beams are used in small fields Depth-dose distributions under these conditions should be compared with measured results.

show abstract

Perturbation of electron beam doses as a function of SSD due to the use of shielding blocks on the Clinac-18

Cited by 6 publications

References 0 publications

Clinical electron‐beam dosimetry: Report of AAPM Radiation Therapy Committee Task Group No. 25

Clinical electron‐beam dosimetry: Report of AAPM Radiation Therapy Committee Task Group No. 25

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71

Variation in calculated effective source - surface distances with depth

Contact Info

Product

Resources

About