Introduction: Pattern of failure studies backed with radiobiological models show that in case of radiotherapy of macroscopic-solid tumours that if the tumour recurs it will mostly be from the gross tumour volume (GTV), not from the tumour infiltrations in the normal tissue around the GTV. Present clinical practice directs clinicians to give a homogeneous dose to the planning target volume (PTV). Due to the considerable margin around the GTV, to account for tumour inflitrations, tumour movement and set-up inaccuracy, this PTV consists mainly of normal tissue. This implies that maximum dose in radiotherapy schemes is limited by the expected normal tissue complication probability (NTCP). This also implies, due to the fact that the tumour control probability (TCP) per volume is directly related to the initial number of tumour cells (the tumour cell density and the tumour volume) that the surrounding normal tissue which includes only tumour infiltrations has in general been overdosed compared to the GTV, increasing the risk of unnecessary normal tissue complications, or that the GTV has been underdosed. Conformal therapy aims to reduce this NTCP by tailoring the treated volume (TV) to the PTV, allowing dose escalation. The choice of the PTV itself is mostly not open for discussion. The process of reducing the PTV to the clinical target volume (CTV) (this is the GTV including the region of possible tumour infiltration) as much as possible may result in an increased risk of tumour underdosage in case of unexpected treatment set-up inaccuracy. In the same way techniques to avoid irradiation of critical organs may improve treatment tolerance but may enhance treatment failure.
However, in case of radiotherapy of solid tumours the TCP can be greatly enhanced without enhancing the NTCP by additional boosting of only the GTV with a moderate dose of 10-15 Gy. This additional microboost can be given on top of the present standard irradiation, which in this case has still been given with the well accepted field settings.
Essential is that the microboost is restricted to the GTV and that the tolerance dose of normal tissues involved is not exceeded, this introduces the need for extreme on-line position verification, field tailoring and advanced 3D CT/MRI and 4D MRI delineation of the GTV. lt is essential to realize that these special positional requirements are only needed for the microboost and that missing small parts of the GTV during this microboost is not a critical parameter. On-line portal imaging with radio-opaque fiducial markers and/or external optical marker position verification combined with modern accelerators employing micro-multileaf field shaping and dynamic on-line adjustment technology can supply this accuracy for treatment of a.o. macroscopic tumours of the prostate and head and neck.
Purpose: Development and phase 1 clinical test of the technology for improving local tumour control in conformal radiotherapy of solid tumours using a fiducial marker controlled additional microboost only to the gross tumour volume.
Plan of investigation:
- To investigate patient immobilization and sophisticated absolute on-line positioning with both portal-imaging employing radio-opaque fiducial markers placed in the tumour area and external optical markers.
- The development of conformal field settings employing segmented intensity modulated fields using (micro)multi-leaf collimators.
- To investigate automatic system interrupts and on-line dynamic table and multi-leaf adjustment to guarantee field alignment.
- Dosimetric verification of the small boost fields used.
- Phase 1 clinical test of the microboost approach on tumours of the prostate and head and neck. Use of MRI/CT techniques for delineation of the GTV and evaluation of treatment.
Possible results: Improved local control without enhanced normal tissue complications of solid tumours. |
