Optimization in external beam radiotherapy

» What level of accuracy is needed in safe clinical practice in EBRT?

A very high level of accuracy is needed.

Within acceptable ranges of good practice and equipment performance, the prescribed absorbed dose at the prescribed beam quality should be delivered to the planning target volume (PTV) while doses to other tissues and organs are minimized. In practice, this means that a very high level of accuracy should be striven for at all steps of the treatment chain in external beam radiotherapy.

When considering the percentage change in response for tumour control or normal tissue damage for a 1% change in dose (normalized dose response gradient), the figures are uncertain and depend on tumour site or normal tissue reaction being monitored. A range of 0.5% to 5.0% decrease in probability for tumour control with a 1% decrease in dose has been indicated, with corresponding values for normal tissue complications of around 2-3%.

A small deviation in absorbed dose of only a few per cent from the intended one might thus mean a reduced probability for tumour control or an increased probability for normal tissue complications. Since there are many steps in the treatment chain, each step should be kept very accurate.

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  • BRAHME, A., CHAVAUDRA, J., LANDBERG, T., McCULLOUGH, E.C., NüSSLIN, F., RAWLINSON, J.A., SVENSSON, G., SVENSSON, H., Accuracy Requirements and Quality Assurance of External Beam Therapy with Photons and Electrons, Acta. Oncol. Suppl. 1 (1988). 
  • MIJNHEER, B.J., The clinical basis for dosimetric accuracy in radiotherapy, In: Radiation Incidents. Br. J. Radiol. (1996) 16-20. 
  • DISCHE, S., SAUNDERS, M.I., WILLIAMS, C., HOPKINS, A., AIRD, E., Precision in reporting the dose given in a course of radiotherapy. Radiother. Oncol. 29 (1993) 287-293. 

» How do I safely optimize the volume that I want to target with curative EBRT?

By identifying, quantifying, reducing and taking into account all geometric variations in the treatment chain.

When a tissue volume is judged to be likely to contain tumour cells, and a decision has been taken to attempt to cure the patient using radiotherapy, the targeted volume should be optimized. The optimization process is crucial to the potential success of the treatment, and should ensure that this targeted tissue volume has a high probability of being included in the volume receiving a high absorbed dose (treated volume) at all times, while surrounding healthy organs are excluded from significantly high dose (irradiated volume) as much as possible.

The process of optimization of external beam radiotherapy should take into account geometric variations in the location of the target and organs at risk throughout the treatment chain by creating a planning target volume (PTV) and one or several planning organ at risk volumes (PRVs). The geometric variations in the whole radiation treatment chain (such as organ motion and patient set-up variations) all contribute to the size of the margin around the tissue volume where a high dose is required. The identification and quantification of these variations is necessary for the creation of a precise and safe margin.

Modern conformal radiation therapy should not only aim to conform the treated volume to the PTV, i.e. increase the isodose sculpting ability in treatment planning and execution, but also to conform the PTV to the clinical target volume, i.e. reduce geometric variations in the treatment chain, and to conform the clinical target volume to the gross tumour volume, i.e. improve clinical visualization methods of tumour cells. Once the variation components are known and minimized, they have to be combined to an overall margin to ensure the inclusion of tumour cells in the treated volume.

» How should I approach quality control (QC) in EBRT?

By studying and following the relevant local, national and international guidelines on QC.

According to the International Organization for Standardization (ISO), QC is the regulatory process through which the actual quality performance is measured, compared with existing standards and finally the actions necessary to keep or regain conformance with the standard. It should be emphasized that in addition to controlling the performance of radiotherapy equipment, it is vital to also control the performance of radiotherapy processes.

The actual parameters to control, and how and when this should be done might be nationally regulated. There might also be recommendations by professional organizations, international bodies or local hospital rules. You need to find out what directly applies in your hospital and country.

Thorough acceptance tests and commissioning procedures must be carried out before new equipment is taken into clinical use. QC is then applied to ensure that relevant parameters of the equipment maintain conformation with what has been found at these acceptance tests and commissioning measurements. The IAEA is developing guidelines and recommendations for the setting up of national programmes in radiotherapy dosimetry and for the physical and technical aspects of QC in radiotherapy. Overall, QC should be involved in all steps of the radiotherapy process in order to maintain confidence in the continued quality of the patients’ treatment. It is also important that results from all tests are adequately documented.

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» Is 1 gray in my clinic equal to 1 gray in other clinics?

You ensure this by measuring absorbed dose according to an established protocol within a national/international framework.

To ensure that the absorbed dose is equivalent in different clinics, and thus that experience and evidence in dose versus effect can be shared between clinics, it is necessary for experts in radiation oncology physics to determine the absorbed dose according to established protocols. Some countries have national protocols, while others follow international protocols such as the IAEA code of practice.

Equipment for this dose determination (e.g. ion chambers, electrometers) should be regularly calibrated in such a way that the calibration can be traced to a primary standard dosimetry laboratory (PSDL) through secondary standard dosimetry laboratories (SSDL), e.g. the IAEA/WHO network of SSDLs. Since these laboratories compare results, 1 gray (Gy) at one site will be very close to 1 Gy at another site. Regulatory authorities should establish the calibration frequency, where applicable.

Dose quality audits and follow-up programmes are powerful tools to ensure that there is a continued consistency in dose determination. The IAEA/WHO provide an independent quality audit of radiation dose delivery in radiotherapy hospitals.

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