Megavoltage x-ray skin dose variation with an angle using grid carbon fibre couch tops

Abstract

It is well known that a skin dose from high-energy x-ray radiation varies with the angle of beam incidence or the presence of a radiotherapy linear accelerator couch top material. This note investigates changes produced to the skin dose from a Varian carbon fibre grid couch top at differing angles of incidence for 6 MV x-rays as is often the case clinically. Results have shown that the skin dose can easily be measured using an EBT Gafchromic film whereby the delivered skin dose can be quantified to a high level of spatial resolution, not easily achieved with other skin dose detectors. Results have shown a significant increase in the skin dose specifically at the point of a cross-sectional carbon fibre grid. Values in % of the skin dose increased from approximately 27% (an open area within a 10 cm x 10 cm field) up to 55% (same field size) at the centre of the carbon fibre mesh strip (0 degrees incidence). This is compared to 19% of the skin dose for an open field of a 10 cm x 10 cm beam without the couch material present. At larger angles similar effects occur with values changing from 52% to 75% (60 degrees , 10 cm x 10 cm) in the open area and under the grid, respectively. This produces a wave effect for the skin dose. The average skin dose magnitude increases with the angle of incidence of the beam, ranging from 37.5% to 66% from 0 degrees to 60 degrees (10 x 10 cm), respectively. The symmetric wave nature of the skin dose profile skews to deliver an increased dose on the posterior side of the carbon fibre grid as the angle of incidence increases. Simulated fractional dose delivery on a phantom has shown that over 30 fractions the wave nature of the delivered skin dose is minimized due to the random nature of most patient positioning on the treatment couch. However, some variations are still present as the ratio of the open to grid area is approximately 4:1 and the dose spread is not necessarily completely averaged during a typical fractionated radiotherapy treatment. As such, if the treatment type results in a more rigorously positioned patient on the treatment couch, the wave nature of skin dose delivery may need to be taken into account.

A validation of carbon fiber imaging couch top modeling in two radiation therapy treatment planning systems: Philips Pinnacle3 and BrainLAB iPlan RT Dose

“…It is important to assess this loss in skin sparing because lack thereof could result in side effects such as induced erythema, moist desquamation and permanent hair loss [8]. The loss of skin sparing from megavoltage photons when using CF has been documented in the literature for different couch designs [32-35]. The early work by Meara and Langmack [8] noted an increase in build up with respect to Dmax of 47-56% through different carbon fiber combinations compared to 17.8% with no CF for 6 MV photon.…”

“…The early work by Meara and Langmack [8] noted an increase in build up with respect to Dmax of 47-56% through different carbon fiber combinations compared to 17.8% with no CF for 6 MV photon. Butson et al [35] reported that for the Varian Exact TM tabletop, the maximum skin dose (defined at a depth of 0.15 mm), for a 10 cm × 10 cm 6MV photon field was 55% compared to 19% for open field.…”

Increased beam attenuation and surface dose by different couch inserts of treatment tables used in megavoltage radiotherapy

“…Our corresponding measured attenuations were 1.9% and 2.7%. Butson et al ( 8 ) used GAFCHROMIC EBT films to measure the increase in skin dose with a Varian carbon fiber grid couchtop. With a direct posterior 6 MV beam and a field size of $10\times 10\hspace{0.17em}{\text{cm}}^2$, the skin dose increased from 27% with no buildup to 55%.…”

“…The magnitude of skin dose will increase with the angle of incidence of the beam. ( 8 ) That is, the presented values are the best‐case scenarios with respect to skin doses when treating through the couchtop. The reader should also keep in mind that, in this study, we only investigated the couch inserts and not any immobilization devices or possible couch rails which also contribute to the beam attenuation.…”

Evaluation of the magnitude of EBT Gafchromic film polarization effects

“…Specifically EBT Gafchromic has been utilized for high energy and low energy radiotherapy / diagnostic dosimetry [7][8][9][10][11][12][13][14] . Some of its characteristics which have defined its usefulness include high spatial resolution 15 and near energy independence 16 .…”