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

Sufficient skin dose needs to be delivered by a radiotherapy chest wall tre atment regimen to ensure the probability of a near surface tumor recurrence is minimized. To simulate a chest wall treatment a hemicylindrical solid w ater phantom of 7.5 cm radius was irradiated with 6 MV x-rays using 20x20 c m(2) and 10x20 cm(2) fields at 100 cm source surface distance (SSD) to the base of the phantom. A surface dose profile was obtained from 0 to 180 degr ees, in 10 degrees increments around the circumference of the phantom. Dosi metry 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 degrees beam entry position and 58% at the 90 degrees o blique beam position. Superficial dose results were also obtained using ext ra thin thermoluminescent dosimeters (TLD) (effective depth 0.14 mm) of 30% at 0 degrees, 57% at 90 degrees, and a metal oxide semiconductor field eff ect transistor (MOSFET) detector (effective depth 0.5 mm) of 43% at 0 degre es, 62% at 90 degrees. Because the differences in measured superficial dose s were significant and beyond those related to experimental error, these di fferences are assumed to be mostly attributable to the effective depth of m easurement of each detector. We numerically simulated a bolus on/bolus off technique and found we could increase the coverage to the shin. Using an al ternate "bolus on," "bolus off" regimen, the skin would receive 36.8 Gy at 0 degrees incidence and 46.4 Gy at 90 degrees incidence for a prescribed mi dpoint dose of 50 Gy. From this work it is evident that, as the circumferen ce of the phantom is traversed the SSD increases and hence there is an inve rse square fluence fall-off, this is more than offset by the increase in sk in dose due to surface curvature to a plateau at about 90 degrees. Beyond t his angle it is assumed that beam attenuation through the phantom and inver se square fall-off is causing the surface dose to reduce. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)00107-3].