Dosimetric Improvements Utilising Intensity Modulated Radiation Therapy for Patients with Glioblastoma Multiforme

Aims: The EORTC-NCI study investigating the addition of temozolomide trial to standard radiation therapy has demonstrated improved duration of survival in patients with Glioblastoma multiforme (GBM). With longer survival duration, there is the potential for latent RT morbidity, not previously seen in historical patients. This study evaluates the potential dosimetric advantages of utilising IMRT over 3D-conformal RT in such patients. Methods: 10 consecutive patients with GBM formally screened for a clinical study over a two-month period were planned and treated with IMRT utilising daily on-board imaging (OBI). The EORTC protocol dosimetric criteria and constraints were used in target delineation and planning. For each patient, a 3DCRT plan was also produced. Endpoints for dosimetric evaluation analysed related to tumour dose: mean PTV60 dose (mPTV60Dose), Conformity Index (CI); and normal tissue dose: mean normal brain dose (mBrainDose) and V40 Brain (Brainv40). IGRT endpoints were the median isocentre shifts required in 3 axes measured in one direction. The variation between the IMRT and 3DCRT dosimetric endpoints was examined using Wilcoxon analysis. Results: The 10 patients had tumours located in temporal (3), parietal (3), occipital (2) and callosal (2) regions. The median PTV and normal brain volumes were 308.1 cm and 1077.5 cm respectively. The IMRT dosimetry was significantly improved in all endpoints specifically CI (p = 0.002), mPTV60Dose (p = 0.004), mBrainDose (p = 0.002) and Brainv40 (p = 0.019). OBI directed isocentre measurements in the patient group were available for 230 treatments. The median shifts (and 95% C.I.s) were 0.1 cm vertical (0.1 0.2), 0.1 cm longitudinal (0.1 0.2) and 0.2 cm lateral (0.2 0.2). At a minimum follow-up of 2 years’ post diagnosis, the median survival of the group is 18.0 months (95% CI: 13.4 22.6 months). Conclusion: IMRT for GBM produces significant dosimetric advantages in relation to planning target volume and normal tissue dose compared with 3D conformal plans. The data also confirm the accuracy of IMRT technique for CNS with IGRT delivery utilising OBI demonstrating minimal deviation from planned to treated isocentre.