# SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields

## Abstract

Purpose: It is common practice to tabulate dosimetric data like output factors, scatter factors and detector signal correction factors for a set of square fields. In order to get the data for an arbitrary field, it is mapped to an equivalent square, having the same scatter as the field of interest. For rectangular fields both, tabulated data and empiric formula exist. We tested the applicability of such rules for very small fields. Methods: Using the Monte-Carlo method (EGSnrc-doseRZ), the dose to a point in 10cm depth in water was calculated for cylindrical impinging fluence distributions. Radii were from 0.5mm to 11.5mm with 1mm thickness of the rings. Different photon energies were investigated. With these data a matrix was constructed assigning the amount of dose to the field center to each matrix element. By summing up the elements belonging to a certain field, the dose for an arbitrary point in 10cm depth could be determined. This was done for rectangles up to 21mm side length. Comparing the dose to square field results, equivalent squares could be assigned. The results were compared to using the geometrical mean and the 4Xperimeter/area rule. Results: For side length differences less than 2mm, the difference betweenmore »

- Authors:

- University of Wuerzburg, Wuerzburg (Germany)

- Publication Date:

- OSTI Identifier:
- 22649004

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; CYLINDRICAL CONFIGURATION; MATRICES; MONTE CARLO METHOD; PHOTONS; RADIATION DOSES; RADIOTHERAPY

### Citation Formats

```
Sauer, OA, Wegener, S, and Exner, F.
```*SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields*. United States: N. p., 2016.
Web. doi:10.1118/1.4956593.

```
Sauer, OA, Wegener, S, & Exner, F.
```*SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields*. United States. doi:10.1118/1.4956593.

```
Sauer, OA, Wegener, S, and Exner, F. Wed .
"SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields". United States.
doi:10.1118/1.4956593.
```

```
@article{osti_22649004,
```

title = {SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields},

author = {Sauer, OA and Wegener, S and Exner, F},

abstractNote = {Purpose: It is common practice to tabulate dosimetric data like output factors, scatter factors and detector signal correction factors for a set of square fields. In order to get the data for an arbitrary field, it is mapped to an equivalent square, having the same scatter as the field of interest. For rectangular fields both, tabulated data and empiric formula exist. We tested the applicability of such rules for very small fields. Methods: Using the Monte-Carlo method (EGSnrc-doseRZ), the dose to a point in 10cm depth in water was calculated for cylindrical impinging fluence distributions. Radii were from 0.5mm to 11.5mm with 1mm thickness of the rings. Different photon energies were investigated. With these data a matrix was constructed assigning the amount of dose to the field center to each matrix element. By summing up the elements belonging to a certain field, the dose for an arbitrary point in 10cm depth could be determined. This was done for rectangles up to 21mm side length. Comparing the dose to square field results, equivalent squares could be assigned. The results were compared to using the geometrical mean and the 4Xperimeter/area rule. Results: For side length differences less than 2mm, the difference between all methods was in general less than 0.2mm. For more elongated fields, relevant differences of more than 1mm and up to 3mm for the fields investigated occurred. The mean square side length calculated from both empiric formulas fitted much better, deviating hardly more than 1mm and for the very elongated fields only. Conclusion: For small rectangular photon fields, deviating only moderately from square both investigated empiric methods are sufficiently accurate. As the deviations often differ regarding their sign, using the mean improves the accuracy and the useable elongation range. For ratios larger than 2, Monte-Carlo generated data are recommended. SW is funded by Deutsche Forschungsgemeinschaft (SA481/10-1)},

doi = {10.1118/1.4956593},

journal = {Medical Physics},

number = 6,

volume = 43,

place = {United States},

year = {Wed Jun 15 00:00:00 EDT 2016},

month = {Wed Jun 15 00:00:00 EDT 2016}

}