Session Item

Aim of this study was to assess the Tumour Control Probability (TCP) of a personalized Dose Painting By Contour (DPBC) strategy based on the synergistic exploitation of the tumour clonogenic cell information derived from FDG PET images and the tumour oxygen distribution derived from FMISO PET images.

Thirty H&N cancer patients imaged with FDG and FMISO PET/CT before radiochemotherapy were analysed. FMISO PET images were converted into maps of oxygen partial pressure (pO₂) using a non-linear conversion function of radiotracer uptake previously derived. A dose distribution at voxel level considering the heterogeneity in radiosensitivity related to the oxygenation and an heterogenous distribution of clonogenic cells in the CTV was determined. The tumour clonogens information was retrieved from FDG PET images by applying a linear conversion of the radiotracer uptake, having as an anchor point the tumour cell density carrying capacity set to correspond to the maximum uptake level retrieved from the patient dataset. The CTV was segmented into hypoxic target volume (HTV), GTV-HTV and CTV-GTV. A dose escalation strategy consisting of uniform doses applied to the segmented targets (dose painting by contour, DPBC) aiming at 95% of TCP in the CTV was employed.

Automated photon treatment plans were made in RayStation (v10, RaySearchLaboratories) with ±3mm setup errors (7 scenarios) for minimax robust optimization. The unsupervised planning strategy used the same objective functions for the whole patient population and a total dose delivered in 35 fractions with an integrated boost. A dosimetric evaluation of the treatment plans, accounting for target coverage and constraints for the organs at risk, was followed by an assessment of the TCP in the targets accounting for the dose distribution in the nominal plan and the radiosensitivity maps derived from combined FMISO and FDG PET information (Figure 1).

Prescribed doses expressed as equivalent doses in 2 Gy per fraction (EQD2) for D_50% were 82±4 Gy in the HTV, 72±3 Gy in GTV-HTV, and 68±2 Gy in CTV-GTV. 97% of the plans were clinically feasible considering brain stem, spinal cord, and mandible constraints. Parotids could be spared in only 57% of the cases, since they were inside the PTV, and target coverage was prioritized. The TCP accounting for the planned dose distribution and image-derived radiosensitivity maps was higher than 95% control for the nominal plan in all the targets for 87% of the cases (Figure 2). For four remaining cases lower TCP values, seemingly attributable to voxels receiving low doses at the target periphery, were however observed requiring further investigation and potential a posteriori adjustment.

Automated treatment planning with integrated boost targeting hypoxia and considering a heterogeneous density of clonogens in the tumour based on functional imaging has shown clinical feasibility and high tumour control prediction.