Radiobiological and dosimetric analysis of daily megavoltage CT registration on adaptive radiotherapy with Helical Tomotherapy.

Pre-treatment patient repositioning in highly conformal image-guided radiation therapy modalities is a prerequisite for reducing setup uncertainties. In Helical Tomotherapy (HT) treatment, a megavoltage CT (MVCT) image is usually acquired to evaluate daily changes in the patient's internal anatomy and setup position. This MVCT image is subsequently compared to the kilovoltage CT (kVCT) study that was used for dosimetric planning, by applying a registration process. This study aims at investigating the expected effect of patient setup correction using the Hi-Art tomotherapy system by employing radiobiological measures such as the biologically effective uniform dose (D) and the complication-free tumor control probability (P(+)). A new module of the Tomotherapy software (TomoTherapy, Inc, Madison, WI) called Planned Adaptive is employed in this study. In this process the delivered dose can be calculated by using the sinogram for each delivered fraction and the registered MVCT image set that corresponds to the patient's position and anatomical distribution for that fraction. In this study, patients treated for lung, pancreas and prostate carcinomas are evaluated by this method. For each cancer type, a Helical Tomotherapy plan was developed. In each cancer case, two dose distributions were calculated using the MVCT image sets before and after the patient setup correction. The fractional dose distributions were added and renormalized to the total number of fractions planned. The dosimetric and radiobiological differences of the dose distributions with and without patient setup correction were calculated. By using common statistical measures of the dose distributions and the P(+) and D concepts and plotting the tissue response probabilities vs. D a more comprehensive comparison was performed based on radiobiological measures. For the lung cancer case, at the clinically prescribed dose levels of the dose distributions, with and without patient setup correction, the complication-free tumor control probabilities, P(+) are 48.5% and 48.9% for a D(ITV) of 53.3 Gy. The respective total control probabilities, P(B) are 56.3% and 56.5%, whereas the corresponding total complication probabilities, P(I) are 7.9% and 7.5%. For the pancreas cancer case, at the prescribed dose levels of the two dose distributions, the P(+) values are 53.7% and 45.7% for a D(ITV) of 54.7 Gy and 53.8 Gy, respectively. The respective P(B) values are 53.7% and 45.8%, whereas the corresponding P(I) values are ~0.0% and 0.1%. For the prostate cancer case, at the prescribed dose levels of the two dose distributions, the P(+) values are 10.9% for a D(ITV) of 75.2 Gy and 11.9% for a D(ITV) of 75.4 Gy, respectively. The respective P(B) values are 14.5% and 15.3%, whereas the corresponding P(I) values are 3.6% and 3.4%. Our analysis showed that the very good daily patient setup and dose delivery were very close to the intended ones. With the exception of the pancreas cancer case, the deviations observed between the dose distributions with and without patient setup correction were within ±2% in terms of P(+). In the radiobiologically optimized dose distributions, the role of patient setup correction using MVCT images could appear to be more important than in the cases of dosimetrically optimized treatment plans were the individual tissue radiosensitivities are not precisely considered.