Improved lung dose calculation using tissue-maximum ratios in the Batho correction

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Abstract
The Batho power law for computing the dose within and beyond lung irradiated with small and large fields of 60Co and 6-MV X-rays was reexamined. Using slab phantoms consisting of 2 materials, agreement between calculated and measured doses was within 2% inside lung for 6-MV X-irradiation, but much poorer (9%) for 60Co irradiation. For 60Co irradiation, tissue-air ratios (TAR) were used initially in the Batho equation, while for 6-MV X-rays, tissue-maximum ratios (TMR) were used. When TMR values were substituted instead of TAR values for 60Co marked improvment by nearly 5% in the accuracy of dose calculated within lung was found. This was confirmed by numerical comparison of the Batho expression with an analytic solution of the primary and 1st-scattered radiation. The use of TMR for 60Co radiation is encouraged especially for larger radiation fields, and measured data tables for field sizes up to 50 .times. 50 cm2, and depths up to 30 cm are provided. In addition to unifying the dosimetry for all megavoltage irradiation, this approach improves the accuracy of doses calculated within lung.
Funding Information
  • Alberta Cancer Board