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 20ϫ20 cm 2 and 10ϫ20 cm 2 fields at 100 cm source surface distance ͑SSD͒ 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 mm͒ 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 ͑TLD͒ ͑effective depth 0.14 mm͒ of 30% at 0°, 57% at 90°, and a metal oxide semiconductor field effect transistor ͑MOSFET͒ detector ͑effective depth 0.5 mm͒ 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.

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The authors conclude that the new PTW 60019 microDiamond detector is generally suitable for relative dosimetry in small 6 MV SRS beams for a Novalis Trilogy linear equipped with circular cones.

This work shows that for very small fields such as 4.0 mm cone sizes, a measureable difference can be seen in the relative output factor based on the circular ROI and the size of the area of analysis using radiochromic film dosimetry. The authors recommend to scan the Gafchromic EBT3 film at a resolution of 1200 dpi for cone sizes less than 7.5 mm and to utilize an extrapolation technique for the output factor measurements of very small field dosimetry.