ABSTRACT. The prognosis of patients with glioblastoma (GBM) remains poor, and the use of hyperfractionation or dose escalation beyond 60 Gy has not conferred any survival benefit. More recently, hypofractionated radiotherapy (HFRT) has been employed as a novel approach for achieving dose escalation, with interesting results. We present here a systematic overview of the role and development of HFRT as a possible therapeutic strategy in patients with GBM. We searched the PubMed database for studies published since 1990 that reported on the tolerance, safety and survival outcomes after HFRT. These studies reported on the paradox of improved survival in patients developing central radionecrosis within the high-dose volume. Most series reported no significant increase in early or late toxicity, except for one study that reported visual loss in one patient at 7 months after treatment. More recently, studies of HFRT combined with concurrent temozolomide (TMZ) reported a trend towards improved survival compared with historical controls, with a few studies reporting a median survival of approximately 20 months. The interpretation of data from the above studies is limited by the heterogeneities of patient population and the significant variation in the range of employed dose schedules. However, high-dose HFRT using intensity-modulated radiotherapy appears to be a safe and feasible therapeutic option. There is a suggestion of improved outcomes on combining HFRT with TMZ, which warrants further investigation in a randomised trial.

Carbon fibre couch inserts are widely used in external beam radiotherapy to provide rigid and lightweight patient support. Carbon fibre is often perceived to be essentially radiotranslucent implying that it does not interfere with the radiation beam. However, there is evidence in the literature which suggests that this perception may not be appropriate, particularly at oblique angles of incidence. Furthermore, there is evidence indicating that the use of carbon fibre significantly reduces the skin sparing effect. In this study, the radiation attenuation and surface dose enhancement characteristics of the carbon fibre insert for the Varian ExactTM couch have been investigated. It was found that attenuation increased significantly with increasing angle of incidence, resulting in in-phantom dose reductions of up to 6% at 6 MV and 4% at 15 MV. It has been shown that it is possible to model couch attenuation on a commercial treatment planning system (Elekta CMS XiO) by including the carbon fibre insert in the planning computed tomography (CT) dataset. Finally, the carbon fibre insert was found to significantly increase skin dose to the patient. The skin dose was approximately three times as large when the couch insert was added to 6 and 15 MV photon beams. However, even with this substantial increase it is highly unlikely that the skin tolerance dose will be exceeded.

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Treatment plan quality was investigated to determine whether the total monitor units (MUs) could be reduced by using the MU objective function within the Eclipse TM planning system (Varian Medical Systems) for stereotactic ablative body radiotherapy (SABR) lung plans. The plans were produced using RapidArc ® (Varian's volumetric modulated arc therapy) technique. This study used physical measurements to assess the deliverability and quality of the resultant plans on the treatment machine. Ten patients (dose prescriptions ranging from 6.25 to 18 Gy per fraction) were re-planned retrospectively using objective limit MUs to approximately 50% of the clinical plan (CP) MUs. A number of parameters including dose volume histogram, conformity index, gradient measure, total MUs, treatment time, mean leaf gap, small aperture score (SAS) and gamma (2%/2 mm) were compared. To assess deliverability, plans were measured on a treatment machine using the MatriXX Array and the Compass TM system (IBA Dosimetry). Whilst the quality of plans with and without MU objective function was not statistically different, the number of MUs and overall treatment time was reduced by 23% and 1.71 min, respectively. Mean leaf gap increased by 22.3%, SAS reduced by 49% and normalised MU (MU/cGy) reduced by 22.0%. Importantly, verification and improvements in plan deliverability were measured. The MU objective method showed an overall improvement in plan quality and deliverability; no plans were statistically worse. By reducing the number of MUs for treatment the delivery time is reduced thereby decreasing the time the patient is on the radiotherapy couch. Our institution now uses this technique for clinical lung SABR patients routinely.