Skin dose from radiotherapy X-ray beams: The influence of energy

Abstract

SUM MARYSkin-sparing properties of megavoltage photon beams are compromised by electron contamination. Higher energy beams do not necessarily produce lower surface and basal cell layer doses due to this electron contamination. For a 5xScm field size the surface doses for 6 MVp and 18MVp X-ray beams are 10% and 7 % of their respective maxima. However, a t a field size of 4Ox40cm t h e percentage surface dose is 42% for both 6 M V p and 18 M V p beams. The introduction of beam modifying devices such as block trays can further reduce the skin-sparing advantages of high energy photon beams. Using a 10mm perspex block tray, t h e surface doses for 6 M V p and 18MVp beams with a Sx5cm field size are 10% and 8%, respectively. A t 4Ox40cm, surface doses are 61 % and 63% for 6 M V p and 18MVp beams, respectively. This trend is followed a t the basal cell layer depth. A t a depth of 1 mm, 18MVp beam doses are always a t least 5% smaller than 6 M V p doses for t h e same depth a t all field sizes when normalized to their respective Dmax values. Results have shown that higher energy photon beams produce a negligible reduction of t h e delivered dose to the basal cell layer (0.1 mm). Only a small increase in skin sparing is seen a t t h e dermal layer (1 mm), which can be negated by the increased exit dose from an opposing field.

Skin dose during radiotherapy: a summary and general estimation technique

“…Universally, the surface dose increases linearly with field size. While many studies have found a reduced surface dose at higher MV energies, ( 23 , 26 , 29 , 32 ) others have not, particularly for large field sizes, ( 11 , 31 , 33 – 35 ) as demonstrated with the low‐energy and high‐energy best‐fit lines in Fig. 1.…”

“…The relative increase is negligible for a field, for a field, and for a field. ( 11 , 13 , 22 , 31 , 35 , 38 , 39 )…”

“…The International Commission on Radiological Protection recommends assessing the skin dose at a depth of 0.07 mm (basal layer), ( 9 , 10 ) while the dermal layer may be assessed at 1.0 mm. ( 11 ) The dose to the basal layer (0.07 mm) is often used interchangeably with surface dose. ( 12 – 14 )…”

Investigation of surface and buildup region doses for 6 MV high energy photon beams in the presence of a thermoplastic mask

“…The use of beam modifiers such as blocks, Perspex trays, and immobilization devices causes an increase in the electron contamination of photons, which alleviates the skin-sparing effect of high energy photon beams. The dose in these depths is also related to the energy of the beam, source to skin distance (SSD), field size and obliquity of the beam (2) .…”

Analytical Consideration of Surface Dose and Kerma for Megavoltage Photon Beams in Clinical Radiation Therapy

“…With an increasing energy of these radiations, the penetration power of photons and secondary electrons will be increased and as a result the point of maximum dose (d_max) is placed in more depth (Apipunyasopon et al, 2013).Depending on the amount of energy, dose build up region occurs for x-rays megavoltage in the first depths of the entrance surface of phantom or skin. Dose build up region are a causes the skin sparing effect in megavoltage beams and makes them suitable for delivering the lethal dose to deep tumors as well as reaching the lowest dose in the surface layers of the skin (Butson et al, 1997;Ravikumar and Ravichandran, 2000). On the other hand, low-energy photons and electron contamination causes an increase in the dose of skin, and loss or decrease in the skin sparing effect in the treatment with megavoltage beams (Ravikumar and Ravichandran, 2000).…”