Intensity modulated radiotherapy for skull base meningioma

Abstract

Meningioma is a primary central nervous tumour (CNS) affecting mostly adults. Skull base meningiomas cause morbidity (and occasionally mortality) by compressing adjacent critical organs, and the aim of treatment is to optimise and preserve CNS function. Radiotherapy is an effective treatment for meningioma, with doses of 50-54Gy in conventional fractionation schedules resulting in long-term local control rates of 80-90%. However, local recurrence is unacceptably high in aggressive histological subtypes (WHO G2 and G3), and these patients have a poor outcome following standard dose regimens. In recent years Intensity-Modulated Radiotherapy (IMRT) has been developed, and can deliver highly conformal dose distributions with sharp dose gradients, making dose escalation a possibility for many tumours, including those located in the skull base. The aim of this thesis was to address two issues pertaining to the role of radiotherapy for skull base meningioma. Firstly the goal was to investigate the potential role of dose escalation above 54Gy for meningioma and secondly, to assess the feasibility/practicality of delivering high doses to the skull base region. Due to the long duration of follow up required to document treatment response in this condition, an assessment of clinical endpoints (i.e. local control, survival) was not within the scope of this work, and will not be presented. Chapter One consists of a literature review of meningioma and its management, including details about Intensity-Modulated Radiotherapy (IMRT), as a background to support the proceeding work. The role of dose escalation is addressed in Chapter Two, in which a retrospective review of a large cohort of patients in our department is presented. This data is discussed with reference to other series, and confirms that there is a role for dose escalation to at least 60Gy for G2 and G3 meningiomas. The role of IMRT to deliver this dose is presented in Chapter Three. Using a retrospective CT database, a series of planning studies were performed in which cases were re-planned using IMRT, investigating the impact of parameters such as MLC width and number/position of fixed beams etc on dosimetry. Plans were compared using both physical and biological parameters and the results analysed for differences to identify an optimal planning solution. The results show that not only is it possible to deliver 60Gy to skull base meningiomas safely using fixed field IMRT, a statistically significant and practical planning class solution has been identified. Chapter Four presents the results of a planning study comparing the role of another form of highly conformal radiotherapy, helical tomotherapy (HT). A comparison of IMRT and HT plan dosimetry showed that the two treatment systems are equivalent in their ability to escalate dose to 60Gy in the skull base, with HT demonstrating a clear advantage with regard to reduced monitor units and ‘on-beam’ treatment times, making this form of IMRT the preferred option if available. After considering of the results of this body of work within the clinical context, the following can be concluded; dose escalation to 60Gy or more is indicated for high-grade meningioma to maximise CNS function preservation. This dose can be achieved with IMRT using fixed field linear accelerator based or helical arc (HT, VMAT) technology using published planning class solutions. Therefore, all appropriate patients should be considered for dose escalation and have access to these technologies, even if it means referral to a metropolitan centre. Furthermore, in this climate of rapid radiotherapy technology development, planning studies to asses the dosimetric and practical aspects of emerging techniques as described here, are vital to ensure not only that the best modality available is being utilised, but also that it is being operated to its optimal capacity.