Dosimetric verification of biologically adapted IMRT.

Purpose: To investigate the delivery of biologically adapted high resolution intensity modulated radiotherapy (IMRT) to an anthropomorphic phantom, using dosimetric and radiobiologic measures.

Methods: A compartment based 3D hypoxia map of a highly heterogeneous tumor was imported into the CT planning basis of an anthropomorphic phantom. Biologically adapted IMRT was planned according to the corresponding 3D dose prescription map with elevated dose to the hypoxic regions. Three treatment fractions were delivered to the phantom by means of a conventional linear accelerator equipped with a high resolution micro multileaf collimator (mMLC). EDR2 radiographic films were positioned in two planes of the phantom during irradiation. Software was developed to analyze dose distributions from the prescription, dose plan, and films. Dose distributions were scored within each of four radiobiologic tumor compartments. The treatment effect in each tumor compartment was estimated as the equivalent uniform dose (EUD). A conventional gamma analysis was utilized for quantitative comparisons of planned and delivered dose distributions.

Results: The planned and delivered dose maps qualitatively resembled the prescribed dose map. The gamma analyzes showed that, on average for all films, more than 95% of the pixels within the tumor passed the 3%/2 mm criteria. For compartments with increasing degree of hypoxia and thereby increased prescribed dose, the planned and delivered EUDs were severely reduced compared to the prescription. The prescribed compartmental doses were met only for the most oxic compartment. The mean tumor dose as measured by the films was 6.6% lower than the corresponding planned dose.

Conclusions: Biologically adapted radiotherapy may be delivered with high precision according to the dose plan. However, the large reduction in compartmental EUD values from prescribed to planned treatment indicated lower tumor effect than expected for such a treatment.