The required number of treatment imaging days for an effective off-line correction of systematic errors in conformal radiotherapy of prostate cancer - a radiobiological analysis

Background and purpose: To use radiobiological modelling to estimate the nu mber of initial days of treatment imaging required to pin most of the benef it from off-line correction of systematic errors in the conformal radiation therapy of prostate cancer. Materials and Methods: Treatment plans based on the anatomical information of a representative patient were generated assuming that the patient is tre ated with a multi leaf collimator (MLC) four-field technique and a total is ocentre dose of 72 Gy delivered in 36 daily fractions. Target position vari ations between fractions were simulated from standard deviations of measure d data found in the literature. Off-line correction of systematic errors wa s assumed to be performed only once based on the measured errors during the initial days of treatment. The turnout control probability (TCP) was calcu lated using the Webb and Nahum model. Results: Simulation of daily variations in the target position predicted a marked reduction in TCP if the planning target volume (PTV) margin was smal ler than 4 mm (TCP decreased by 3.4% for 2 mm margin). The systematic compo nents of target position variations had greater effect on the TCP than the random components. Off-line correction of estimated systematic errors reduc ed the decrease in TCP due to target daily displacements, nevertheless, the resulting TCP levels for small margins were still less than the TCP level obtained with the use of an adequate PTV margin of similar to 10 mm. The ma gnitude of gain in TCP expected from the correction depended on the number of treatment imaging days used for the correction and the PTV mar.-in appli ed. Gains of 2.5% in TCP were estimated from correction of systematic error s performed after 6 initial days of treatment imaging for a 2 mm PTV margin . The effect of various possible magnitudes of systematic and random compon ents on the gain in TCP expected from correction and on the number of imagi ng days required was also investigated. Conclusions: Daily variations of target position markedly reduced the TCP i f small margins were used. Off-line correction of systematic errors can onl y partly compensate for these TCP reductions. The adequate number of treatm ent imaging days required for systematic error correction depends on the ma gnitude of the random component compared with the systematic component, and on the size of PTV margin used. For random components equal to or smaller than the systematic component, 3 consecutive treatment imaging days are est imated to be sufficient to min most of the benefit from correction for curr ent clinically used margins (6-10 mm), otherwise more days are required. (C ) 2001 Elsevier Science Ireland Ltd. All rights reserved.