Quantification of uncertainties in conventional plan evaluation methods in Intensity Modulated Radiation Therapy.

Purpose: In Intensity Modulated Radiation Therapy (IMRT) dose distributions tend to be more complex and heterogeneous because of the modulated fluences in each beamlet of every single beam. These dose-volume (DV) parameters derived from the dose volume histogram (DVH) are physical quantities, thought to correlate with the biological response of the tissues. The aim of this study was to quantify the uncertainty of physical dose metrics to predict clinical outcomes of radiotherapy.

Methods: The radiobiological estimates such as tumor control probability (TCP) and Normal Tissue Complication Probability (NTCP) were made for a cohort of 40 cancer patients (10 brain;19 head & neck;11 cervix) using the DV parameters. Statistical analysis was performed to determine the correlation of physical plan quality indicators with radiobiological estimates.

Results: The correlation between conformity index (CI) and TCP was found to be good and the dosimetric parameters for optic nerves, optic chiasm, brain stem, normal brain and parotids correlated well with the NTCP estimates. A follow up study (median duration 18 months) was also performed. There was no grade 3 or 4 normal tissue complications observed. Local tumor control was found to be higher in brain (90%) and pelvic cases (95%), whereas a decline of 70% was noted with head & neck cancer cases.

Conclusions: The equivalent uniform dose (EUD) concept of radiobiological model used in the software determines TCP and NTCP values which can predict outcomes precisely using DV data in the voxel level. The uncertainty of using physical dose metrics for plan evaluation is quantified with the statistical analysis. Radiobiological evaluation is helpful in ranking the rival treatment plans also.