Assessment of dose inhomogeneity at target level by in vivo dosimetry: can the recommended 5% accuracy in the dose delivered to the target volume be fulfilled in daily practice?

Abstract

The inhomogeneity of the dose delivered to the target volume due to irregular body surface and tissue densities remains in many cases unknown, since the dose distribution is calculated for most radiation treatments in only one transverse section and assuming the patient to be water equivalent. In the present study, the transmission and the target absorbed dose homogeneity is assessed for 11 head-and-neck cancer treatments by in vivo measurements with silicon diodes. Besides the dose to the specification point, the dose delivered to 2-4 off-axis points in the midline sagittal plane is estimated from entrance and exit dose measurements. Simultaneously made portal films allow to identify the anatomical structures passed by the beam before reaching the exit diode. The mean deviation from the expected transmission is -6.8% for bone, +6% for air cavities and -2.5% for soft tissue. At the midplane, the mean deviations from the expected target dose are respectively -3.5%, +2.3% and -1.9%. The deviations from the prescribed dose are larger than 5% in 12 out of the 39 target points. The accuracy requirement in target dose delivery of plus or minus 5%, as proposed by ICRU, cannot be fulfilled in 7 out of the 11 patients and is mostly due to irregular body contour and tissue densities. As only a limited number of points are considered, the inhomogeneity in the dose delivered throughout the whole irradiated volume is underestimated as is illustrated from the exit dose profiles obtained from the portal image.(ABSTRACT TRUNCATED AT 250 WORDS)