Accounting for treatment delays when treating highly proliferative tumours

Clinical Implementation of Adaptive Helical Tomotherapy: A Unique Approach to Image-Guided Intensity Modulated Radiotherapy

Image-guided IMRT is a revolutionary concept whose clinical implementation is rapidly evolving. Methods of executing beam intensity modulation have included individually designed compensators, static multi-leaf collimators (MLC), dynamic MLC, and sequential (serial) tomotherapy. We have developed helical tomotherapy as an innovative solution to overcome some of the limitations of other IMRT systems. The unique physical design of helical tomotherapy allows the realization of the concepts of adaptive radiotherapy and conformal avoidance. In principle, these advances should improve normal tissue sparing and permit dose reconstruction and verification, thereby allowing significant biologically effective dose escalation.

Recent radiobiological findings can be translated into altered fractionation schemes that aim to improve the local control and long-term survival. This strategy is being tested at the University of Wisconsin using helical tomotherapy with its highly precise delivery and verification system along with meticulous and practical forms of immobilization. Innovative techniques such optical guidance, respiratory gating, and ultrasound assessments are being designed and tailored for helical tomotherapy use. The intrinsic capability of helical tomotherapy for megavoltage CT (MVCT) imaging for IMRT image-guidance is being optimized.

The unique features of helical tomotherapy might allow implementation of image-guided IMRT that was previously impossible or impractical. Here we review the technological, physical, and radiobiological rationale for the ongoing and upcoming clinical trials that will use image-guided IMRT in the form of helical tomotherapy; and we describe our plans for testing our hypotheses in a rigorous prospective fashion.

Comparison of intensity modulated radiotherapy and dynamic three-dimensional conformal radiotherapy with regard to dose distribution and sparing of organs at risk

Dose escalation to the target while sparing the organs at risk near the lesion has been difficult over the last decade. However, recent radiotherapy techniques can deliver more sophisticated doses to the target. This study evaluated whether intensity modulated radiotherapy can deliver more homogeneous and conformal doses to the target than dynamic three-dimensional conformal radiotherapy while sparing organs at risk near the lesion in 13 patients with central nervous system tumors and other tumors around the central nervous system. Dynamic three-dimensional conformal radiotherapy and intensity modulated radiotherapy plans were calculated and dose distributions were compared for all patients with regard to the planning target volume and organs at risk. The plan of intensity modulated radiotherapy was significantly superior to that of dynamic three-dimensional conformal radiotherapy in target dose conformity (p = 0.0006) and organs at risk sparing (p = 0.0257). Intensity modulated radiotherapy could deliver more homogeneous and conformal doses to the target than dynamic three-dimensional conformal radiotherapy with sparing organs at risk near the lesion and may improve local control of radioresistant tumors via dose escalation.

The use of the linear quadratic model in radiotherapy: a review

To be able to predict the impact of any radiotherapy treatment the physics of radiation interactions and the expected biological effect for any radiotherapy treatment situation (dose, fractionation, modality) must be both understood and modelled. This review considers the current use and accuracy of the linear quadratic model which can be used to consider the variation in tissue response with fraction size. Cell kill following radiation damage results from damage to the DNA which can take a variety of forms. In many cases the linear quadratic model is used to estimate the relative impact for different situations especially clinical studies relating to fraction size. This is mainly undertaken using parameters derived from the linear quadratic model such as biological effective dose and standard effective dose. The model has also been adapted to consider the effect of overall treatment time, repair during treatment (as occurs for brachytherapy treatments) and other situations. There are some concerns over its use, mainly in the small dose ranges (both total low doses and low doses per fraction) where studies have shown its inaccuracy. In other situations however it does appear to provide a reasonable estimate of relative clinical effect. As with all models, however results should never be considered out of clinical context.