Absorption ratio of treatment couch and effect on surface and build-up region doses

Abstract

Aim

In this study, at different fields, energies and gantry angles, treatment couch and rails dose absorption ratio and treatment couch effect on surface and build-up region doses were examined.

Background

It is assumed that radiation attenuation is minimal because the carbon fiber couches have low density and it is not generally accounted for during treatment planning. Consequently, it leads to a major dosimetric mistake.

Materials and methods

Solid water phantom was used for relative dose measurement. The measurements were done using a Farmer ion chamber with 0.6[[ce:hsp sp="0.25"/]]cc volume and a parallel plane ion chamber starting from surface with 1[[ce:hsp sp="0.25"/]]mm depth intervals at 10[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]10[[ce:hsp sp="0.25"/]]cm² field, SSD 100[[ce:hsp sp="0.25"/]]cm. Measurements were taken for situations where the beams intersect the couch and couch rails.

Results

Dose absorption ratio of carbon fiber couch obtained at gantry angle of 180° was 1.52%, 0.69%, 0.33% and 0.25% at different field sizes for 6[[ce:hsp sp="0.25"/]]MV. For 15[[ce:hsp sp="0.25"/]]MV, this ratio was 0.95%, 0.27%, 0.20% and 0.05%. The absorption ratio is between 3.4% and 1.22% when the beams intersect with couch rails. The couch effect increased surface dose from 14% to 70% for 6[[ce:hsp sp="0.25"/]]MV and from 11.34% to 53.03% for 15[[ce:hsp sp="0.25"/]]MV.

Conclusions

The results showed that the carbon fiber couch increased surface dose during posterior irradiation. Therefore, the skin-sparing effect of the high energy beams was decreased. If the effect of couch is not considered, it may cause significant differences at dose which reaches the patient and may cause tissue problems such as erythema.