Session Item

Large anatomical changes may occur during RT of lung cancer potentially leading to over-dosage of organs at risk (OAR) or decrease in tumour dose. The daily delivered dose can be calculated based on CBCT scans used for patient positioning. We report on actual delivered dose, that may be calculated real time, in lung cancer patients included in the NARLAL2 dose escalation trial.

We investigated 12 patients included in the NARLAL2 trial. Patients were randomized between the standard arm homogeneously delivering 66Gy in 33fractions (fx) and the experimental arm delivering heterogeneous dose escalation. Dose escalation was driven by the GTV part with highest FDG-PET uptake, where the dose was escalated up to mean doses of 95Gy/33fx for primary tumour (GTV-T), and 74 Gy/33fx for malignant lymph nodes (GTV-N). The escalation dose is limited in favour of OAR constraints being D1cc <70Gy for oesophagus, <74Gy for heart, trachea and bronchi and <45Gy for spinal cord. In this study, only the escalated plan was investigated regardless of randomization result. Patients were set up according to the GTV-T position on the daily CBCTs. Deviations <2mm and <5mm were allowed for GTV-T and GTV-N, respectively. CTV-PTV margins were 4mm for T and 7mm for N. For each fx, the RTTs evaluated if deviations above tolerance was seen and after 3 consecutive fx, the patients were referred for rescanning.

Contours delineated on planCT (pCT) were deformably propagated to each CBCT using the online registration (MIM Software). Dose was calculated for the CBCTs based on stoichiometric calibration curves yielding mean deviations for the mean dose of 0.2%±0.7%[Holm, Acta Oncol. 2021]. Dose to 95% (D95) of PTV-T and PTV-N and D1cc to OAR were analysed. D95<62.7Gy (95% of 66Gy) should result in plan adaptation. Acceptable doses for OAR were D1cc<74Gy to oesophagus, D1cc<78Gy to heart, trachea and bronchi, D0.05cc<50Gy to spinal cord. Dose parameters were compared between pCT and daily CBCTs.

In all patients, D95>62.7Gy for PTV-T in agreement with daily setup on GTV-T. For PTV-N, D95<62.7Gy in four patients (Fig 1) due to shifts of the GTV-N position. Fig 2 illustrates shift in the position between GTV-T and GTV-N resulting in underdosage for patient 9. The patient received an adapted plan after 11 fx to restore PTV-N coverage. For the OAR, overdosage was seen for oesophagus and bronchi in two patients, heart in one patient, and spinal cord in one patient (Fig 1). No overdosage was seen for trachea. OAR overdosage originating from GTV-T shrinkage is shown for patient 10 in Fig 2. Additionally, D1cc is illustrated for each fx.

Heterogeneous dose escalation in lung cancer patients, may lead to overdose of OAR due to anatomical changes during the seven weeks of radiotherapy. The overdosage may be mitigated by daily dose calculation based on CBCT used for setup followed by plan adaption. The described method can be used for real time dose calculation improving the online treatment process.