Obtaining Oblique Technique Source-to-Skin Distances for Irregular Field (Clarkson) Calculations: The Mayo Off-Axis Distance Indicator

Significant dose inhomogeneities may exist between the supraclavicular fossa (SCF) and the internal mammary chain (IMC) regions in the irregular L-shaped (hockey stick) field associated with breast cancer treatments. This dose inhomogeneity exists, in part, because of a positive air gap in the SCF and a negative air gap in the IMC locations. Independent of treatment technique, [i.e., whether anterior-posterior (AP) or oblique fields are used], accurate source-to-skin distance (SSD) values for the SCF, IMC, and axilla are necessary when doing an irregular field (Clarkson) dose calculation. However, when an oblique technique is used to treat the hockey stick field, obtaining non-central-axis SSDs is not as straightforward as when an AP technique is employed. The “Mayo Off-axis Distance Indicator” was constructed to slide into the blocking tray slot of the simulator or treatment machine. This mechanical measuring device provides quick and accurate SSD measurements for non-central-axis points under either AP or, more importantly, oblique treatment conditions.