Measurement of radiotherapy x-ray skin dose on a chest wall phantom

Authors

K. Y. Quach, Illawarra Cancer Care Centre, Wollongong Hospital
J. Morales, University of Wollongong
Martin Butson, University of WollongongFollow
Anatoly B. Rosenfeld, University of WollongongFollow
Peter E. Metcalfe, Illawarra Cancer Care Centre, Wollongong HospitalFollow

RIS ID

21597

Publication Details

This article was originally published as Quach, KY, Morales, J, Butson, MJ, Rosenfeld, AB and Metcalfe, PE, Measurement of radiotherapy x-ray skin dose on a chest wall phantom, Medical Physics, 27, 2000, 1676. Original American Institute of Physics journal available here.

Abstract

Sufficient skin dose needs to be delivered by a radiotherapy chest wall treatment regimen to ensure the probability of a near surface tumor recurrence is minimized. To simulate a chest wall treatment a hemicylindrical solid water phantom of 7.5 cm radius was irradiated with 6 MV x-rays using 20×20 cm² and 10×20 cm² fields at 100 cm source surface distance (SSD) to the base of the phantom. A surface dose profile was obtained from 0 to 180°, in 10° increments around the circumference of the phantom. Dosimetry results obtained from radiochromic film (effective depth of 0.17 mm) were used in the investigation, the superficial doses were found to be 28% (of D_max) at the 0° beam entry position and 58% at the 90° oblique beam position. Superficial dose results were also obtained using extra thin thermoluminescent dosimeters (TLD) (effective depth 0.14 mm) of 30% at 0°, 57% at 90°, and a metal oxide semiconductor field effect transistor (MOSFET) detector (effective depth 0.5 mm) of 43% at 0°, 62% at 90°. Because the differences in measured superficial doses were significant and beyond those related to experimental error, these differences are assumed to be mostly attributable to the effective depth of measurement of each detector. We numerically simulated a bolus on/bolus off technique and found we could increase the coverage to the skin. Using an alternate “bolus on,” “bolus off” regimen, the skin would receive 36.8 Gy at 0° incidence and 46.4 Gy at 90° incidence for a prescribed midpoint dose of 50 Gy. From this work it is evident that, as the circumference of the phantom is traversed the SSD increases and hence there is an inverse square fluence fall-off, this is more than offset by the increase in skin dose due to surface curvature to a plateau at about 90°. Beyond this angle it is assumed that beam attenuation through the phantom and inverse square fall-off is causing the surface dose to reduce.