Additional FDG PET-CT in week 5-6 of radiotherapy for patients with NSCLC as a means of dose escalation planning

Aims:

To detect a reduction in disease volume during radical radiotherapy for non-small cell lung cancer (NSCLC) using PET-CT and to determine whether this would facilitate dose escalation.

Methods:

Ten patients with localised inoperable NSCLC were prospectively enrolled. Each received conformally planned radiotherapy to a dose of 66Gy/33# over 6.5 weeks using 6-15MV photons and prescribed to the 100% isodose. PET-CT imaging was performed just prior to and following 50 or 60Gy. Target volume definition was performed by one senior radiation oncologist with the help of a senior radiologist and nuclear medicine physician. For all patients and at both time points CT and PET-delineated gross tumour volumes were generated (GTV_CT, GTV_PET). A composite GTV was then created (GTV_CT+PET) and 15mm added in all planes to form the planning target volume (PTV). No correction for organ movement was incorporated and no elective nodal irradiation performed. Each of the different volumes were compared before and after 50-60 Gray. Two plans were then created and compared: 78Gy delivered to the initial PTV and 66Gy to the initial PTV with a 12Gy boost to the post 50/60Gy PTV.

Results:

All patients (mean age 64 years) had stage III disease (4 IIIA and 6 IIIB). There were 4 squamous cell and 6 adeno-carcinomas. After 50/60 Gray the GTV_CT, GTV_PET, GTV_CT+PET and PTV reduced by a mean of 22%, 43%, 30% and 22% respectively. The delivery of 78 Gray to the initial PTV could have been safely achieved in 4/10 patients. Of these delivering treatment in two phases would have substantially spared normal tissue in 2 patients. In the remaining 6 patients, delivering 78 Gray to the initial PTV would have exceeded normal tissue constraints
and no benefit was seen when planned in 2 phases.

Conclusions:

The PTV, consequent on changes seen on PET-CT, reduces during a course of radical radiotherapy for NSCLC. Such a reduction permits dose escalation in a subset of patients and may lead to improved therapeutic outcomes.