The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert

Abstract

The use of rigid carbon fiber couch inserts in radiotherapy treatment couches is a well-established method of reducing patient set-up errors associated with couch sag. Several published studies have described such inserts as radiotranslucent with negligible attenuation of the radiation field. Most of these studies were conducted with the radiation field normally incident on the couch and there appears to be no evidence in the literature of the effect of the gantry angle on the extent of beam attenuation by the carbon fiber insert alone during external beam radiotherapy. In this study we examined the magnitude of this effect over a range of posterior oblique gantry angles using a cylindrical solid water phantom containing an ionization chamber placed isocentrically. It was found that a 6 MV photon beam, field size 10 x 5 cm, was attenuated significantly as the gantry angle approached the plane of the couch, from 2% at normal incidence and reaching 9% attenuation at angle of incidence 70 degrees. This could have serious implications regarding dose to the treatment volume for treatments requiring posterior oblique angles of incidence with a possible correction factor necessary in monitor unit calculations.

Surface dose variations in 6 and 10 MV flattened and flattening filter‐free (FFF) photon beams

“…Several authors have studied the effects of carbon fiber couchtops on surface dose 6 , 31 , 32 , 33 . The proximity of the patient to the couch means that the skin sparing effect is almost completely lost and surface doses increase to 85%–95% of that seen at ${\mathrm{normald}}_{\text{max}}$, and for FFF beams a similar pattern is observed.…”

Generation and verification of QFix kVue Calypso‐compatible couch top model for a dedicated stereotactic linear accelerator with FFF beams

“…They found that the largest beam attenuation occurred at beam entry angles of about 75° from normal incidence. McCormack et al (23) found the beam attenuation of a carbon fiber couch insert on the Elekta SLi linac ranged from 2% (normal incidence) to a maximum of approximately 9% (incidence at angle of 70° from vertical). Poppe et al (24) reported the attenuation of a carbon fiber tabletop for two energies: 1) 6 MV: 2.7% for 180° gantry angle and 3.2% for 150° gantry angle, and 2) 10 MV: 2.3% for 180° gantry angle and 2.4% for 150° gantry angle.…”

Evaluating and modeling of photon beam attenuation by a standard treatment couch

“…A “simple” solution reported by McCormack et al ( 9 ) calls for a fixed posterior oblique beam, using a correction factor based on the CAX attenuation to adjust the beam's MU. However, this process may not accurately predict the treatment couch's attenuation properties.…”

“…( 1 – 3 ) A number of investigations about the attenuation properties of different treatment couches have been reported. ( 4 – 9 ) The attenuation by carbon fiber couches was found to be less than 3% at normal incidence for 6 MV photon beams, depending on the couch design. ( 7 – 9 ) McCormack et al ( 9 ) demonstrated the effect of gantry angle on attenuation by a carbon fiber couch, ranging from 2% at normal incidence to 9% at 70° incidence for 6 MV photon beams.…”

“…( 4 – 9 ) The attenuation by carbon fiber couches was found to be less than 3% at normal incidence for 6 MV photon beams, depending on the couch design. ( 7 – 9 ) McCormack et al ( 9 ) demonstrated the effect of gantry angle on attenuation by a carbon fiber couch, ranging from 2% at normal incidence to 9% at 70° incidence for 6 MV photon beams. Prior to the use of the carbon fiber, the conventionally used materials for treatment couch top were PMMA and wood, which cause more attenuation than carbon fiber.…”