Skin dose estimation for various beam modifiers and source-to-surface distances for 6MV photons

Abstract

The purpose of this study was to learn the skin dose estimation for various beam modifiers at various source-to-surface distances (SSDs) for a 6 MV photon. Surface and buildup region doses were measured with an acrylic slab phantom and Markus 0.055 cc parallel plate (PP) ionization chamber. Measurements were carried out for open fields, motorized wedge fields, acrylic block tray fields ranging from 3 x 3 cm(2) to 30 x 30 cm(2). Twenty-five percent of the field was blocked with a cerrobend block and a Multileaf collimator (MLC). The effect of the blocks on the skin dose was measured for a 20 x 20 cm(2) field size, at 80 cm, 100 cm and 120 cm SSD. During the use of isocentric treatments, whereby the tumor is positioned at 100 cm from the source, depending on the depth of the tumor and size of the patient, the SSD can vary from 80 cm to 100 cm. To achieve a larger field size, the SSD can also be extended up to 120 cm at times. The skin dose increased as field size increased. The skin dose for the open 10 x10 cm(2) field was 15.5%, 14.8% and 15.5% at 80 cm, 100 cm and 120 cm SSDs, respectively. The skin dose due to a motorized 60 degrees wedge for the 10 x 10 cm(2) field was 9.9%, 9.5%, and 9.5% at 80 cm, 100 cm and 120 cm SSDs. The skin dose due to acrylic block tray, of thickness 1.0 cm for a 10 x 10 cm(2) field was 27.0%, 17.2% and 16.1% at 80, 100 and 120 cm SSD respectively. Due to the use of an acrylic block tray, the surface dose was increased for all field sizes at the above three SSDs and the percentage skin dose was more dominant at the lower SSD and larger field size. The skin dose for a 30 x 30 cm(2) field size at 80 cm SSD was 38.3% and it was 70.4% for the open and acrylic block tray fields, respectively. The skin doses for motorized wedge fields were lower than for open fields. The effect of SSDs on the surface dose for motorized 60 degrees wedge fields was not significant for a small field size (difference was less than 1% up to a 15 x 15 cm(2) field size), but for a larger field (field size more than 15 x 15 cm(2)), the difference in a percentage skin dose was significant. The skin dose for the open field was more than that for the MLC blocked field and lower than that for the acrylic blocked tray field. The block was 25% of the 20 x 20 cm(2) open field. Skin doses were increased as the SSD decreased and were dominant for larger field sizes. The surface dose was weakly dependent on the MLC block.

Keywords: Acrylic block Tray; Block; Field size; Motorized 60° wedge; Multileaf collimator; Percentage skin dose; Surface dose; source-to-surface distance.

Figure 1

Experimental setup

Figure 2

Diagram of the blocked field

Figure 3

Comparison of percentage skin dose for open fields vs. motorized 60° wedge and acrylic block tray fields at 80 cm SSD

Figure 4

Comparison of percentage skin dose for open fields vs. motorized 60° wedge and acrylic block tray fields at 100 cm SSD

Figure 5

Comparison of percentage skin dose for open fields vs. motorized 60° wedge and acrylic block tray fields at 120 cm SSD

Figure 6

Comparsion of percentage skin dose at 100 cm SSD vs. 80 cm and 120 cm SSD for open fields

Figure 7

Comparision of percentage skin dose at 100 cm SSD vs. 80 cm and 120 cm SSD for motorized 60° wedge fields

Figure 8

Comparison of percentage skin dose at 100 cm SSD vs. 80 cm and 120 cm SSD for acrylic block tray fields

Figure 9

Comparison of percentage skin dose 20 × 20 cm2 open field vs. 25% blocked field with tray and MLC for various SSDs

Figure 10

Percentage dose buildup curves for 6 MV at various source-tosurface distance for 10 × 10 cm2 open field

Figure 11

Percentage dose buildup curves for 6 MV photon at various source-to-surface distance for 10 × 10 cm2 wedge fields

Figure 12

Percentage dose buildup curves for 6 MV photons at various source-to-surface distance for 10 × 10 cm2 field with block tray