Download Radiotherapy treatment planning

Radiotherapy treatment planning
RAD Magazine, 41, 480, 29-30
Hayley James
Consultant clinical scientist, head of radiotherapy physics
Ipswich Hospital NHS Trust
email: [email protected]
Introduction
In radical radiotherapy treatment we aim to
accurately deliver a lethal dose of radiation to
tumour cells while sparing any surrounding normal tissues. Radiotherapy treatment planning is
the process undertaken to determine the most
appropriate way to deliver the radiation in order
to meet the clinical prescription.
Rapid advances in technology and computing, particularly
in relation to imaging, beam modelling and dose calculation
for treatment planning, and advances in our radiotherapy
delivery systems, have led to significant advances in our
ability to conform differing radiation dose levels to ever more
complex target volumes, with more efficient sparing of the
sensitive normal tissues and organs at risk. This in turn
can lead to the possibility of dose escalation, a reduction in
the toxicity associated with the radiotherapy treatment and
ultimately improved outcomes for patients.
While the systems available to perform accurate treatment planning have evolved over time, the requirements
remain the same:
• A form of patient model and the means to accurately
delineate target volumes and other critical or normal tissue structures within that patient model
• A beam model to simulate what happens when radiation
beams are applied to the patient
• A means of accurately calculating the resulting dose
within the patient and determining the treatment
machine parameters required to deliver that dose
• A means of visualising and evaluating the dose distributions within the patient.
Tumour delineation
Accurate localisation and delineation of gross tumour volumes (GTV) and clinical target volumes (CTV) is vital in
modern radiotherapy. In forward planning for 3D conformal
treatments, the user determines the optimum beam configuration, scoring the resultant dose distributions with reference to how well the dose conforms to the defined structures.
In inverse planning for intensity modulated radiotherapy
(IMRT) and volumetric modulated arc therapy (VMAT) the
delineated target volumes and organs at risk are a critical
part of the prescription. The voxels within every defined volume can form part of the cost function associated with the
dose optimisation process, which in turn determines the
required treatment fields. Figure 1 shows a typical structure set delineated for radical radiotherapy to the retromolar
area. The structure set includes CTV and organ at risk volumes.
Modern computerised treatment planning systems enable
the user to visualise and reconstruct 3D and 4D anatomical
datasets from a variety of sources. These datasets can provide geometrical and functional information to aid the localisation and delineation of tumour volumes and normal tissue
structure sets. CT scans of the patient in the treatment position provide both anatomical information and tissue densities for dose calculation purposes. There is often poor
contrast between gross tumour and normal tissues which
can lead to inaccuracies in delineation. Contrast enhancement can improve visualisation of the anatomy in the CT
scan, however consideration has to be given to how the presence of contrast influences the Hounsfield units used to calculate tissue densities and hence the dose calculation itself.
MRI scans can provide superior soft tissue anatomical information when compared with CT. For example, an MRI scan
of the prostate gives a more accurate means of defining the
anatomical borders of the GTV. Figure 2 shows a planning
CT scan of a prostate patient with artificial hip fused with
an MRI scan to aid delineation of the prostate.
Positron emission tomography (PET) has an increasing
role in radiotherapy for a number of clinical sites such as
in head and neck cancer, rectal cancer and cervical cancer.
The use of F-18 fluorodeoxyglucose (FDG) PETCT has been
shown to be superior to CT alone and MRI for staging disease, determining nodal involvement and detecting the presence of distant metastases. PETCT can also provide
biological and metabolic information about a tumour. In nonsmall cell lung cancer (NSCLC) F18 FDG PETCT can be
used to localise the metabolically active tumour enabling
the radiation to be targeted to these areas with the potential
for dose escalation and better sparing of normal tissues.
When using other forms of imaging to aid tumour localisation in CT treatment planning careful consideration must
be given to patient positioning and registration of the different image datasets. Image registration tools embedded
within many advanced treatment planning and virtual simulation systems enable these different datasets and any
structure sets delineated on them to be fused with or without deformable registration, however the results must be
carefully reviewed by the prescribing clinician on a slice by
slice basis.
Dose calculation
An accurate means of calculating the dose within a patient
is another vital part of the treatment planning process.
Changes in dose can lead to changes in tumour control probability (TCP) and have an impact on normal tissue complication probabilities (NTCP). While direct Monte Carlo
simulation methods remain the most accurate means for
dose computation, calculation times are long and can impact
on patient throughput in the clinical setting. Model-based
algorithms (type b) have replaced measurement-based algorithms (type a) in the majority of commercially available
treatment planning systems for photon treatments. These
convolution algorithms model the energy fluence from the
primary photon interaction (terma – total energy released
per unit mass) and combine it with a matrix of the dose distribution from the resulting scattered photons and electrons
(convolution kernel). The convolution kernels can be determined either by direct measurement or Monte Carlo methods. When commissioning treatment planning systems, the
Monte Carlo generated primary energy fluence and convolution kernels can be adjusted to ensure they accurately
model measured depth doses and beam profiles.
Convolution-superposition algorithms include a correction
for radiological path length and so take account of differing
electron densities of tissue relative to water. This is particularly important when the radiation fields pass through low
density lung tissue or high density tissues such as bone.
These algorithms are able to model electron scattering
within these inhomogeneities more accurately than type a
algorithms.
Treatment plan evaluation
Treatment planning systems enable the clinician and planner to visually evaluate planned dose distributions within
the patient model by means of a full 3D isodose distribution
and dose volume histograms (DVH). Volumetric and dosimetric data such as conformity and homogeneity indices can
also give an indication of how closely a plan meets the original clinical prescription and desired dose and dose volume
constraints. In a clinical setting it is these tools that are
used to determine the suitability of a particular plan and
to compare different plans optimised to meet the same prescription and to quantify the quality of individual plans.
Figure 3 shows a representation of the dose distribution
and dose volume histogram for a VMAT plan for radical
radiotherapy to the tonsil. Radiobiological indices such as
complication-free tumour control probability and biologically
effective uniform dose can also provide a further means of
assessing the efficacy of a treatment plan by consideration
of the dose response characteristics of the tumour and normal tissue volumes to give a more realistic measure of clinical outcomes. Predicting tumour control and normal tissue
complication probabilities can give a more accurate and complete assessment of the quality of a treatment plan
compared with dosimetric measures by accounting for variations in the radiosensitivity of the tumour and normal
tissue volumes within the patient. Currently, however, there
is only limited scope within commercially available treatment planning systems for using TCP and NTCP as part of
overall plan evaluation.
of the treatment planning process in advanced radiotherapy
delivery. While advances in technology and computational
methods are rapid, there are limiting factors within the clinical setting when adopting these advances. Auto-segmentation tools within treatment planning systems may speed up
delineation processes but a full evaluation of the resulting
structure sets is of paramount importance before treatment
plan optimisation can commence. The most accurate dose
optimisation and calculation processes still take significant
amounts of time to complete and may not be practical when
considering more adaptive radiotherapy planning that takes
into account changes in patient anatomy throughout the
course of treatment. Compromises need to be considered at
each step of the treatment planning process as we continue
to improve clinical outcomes for patients.
Further reading
International Atomic Energy Agency IAEA. October 2008. The Role of PET/CT
in Radiation Treatment planning for Cancer Patient Treatment. IAEA-TecDoc1603.
Lee P, Kupelian P, Czernin J, Ghosh P. Current concepts in F18 FDG
PET/CT-based radiation therapy planning for lung cancer. Frontiers in
Oncology 2012;2:71.
Lu L. Dose calculation algorithms in external beam photon radiation therapy.
Int J Cancer Ther Oncol 2013;1(2):01025.
Asnaashri K, Nodehi M R, Mahdavi S R, Gholami S, Khosravi H R.
Dosimetric comparison of different inhomogeneity correction algorithms for
external beam photon calculations. J Med Phys 2013;38(2):74-81.
Mavroidis P. Clinical implementation of radiobiological measures in treatment
planning. Why has it taken so long? Int J Cancer Ther Oncol 2013;1(1):01019.
Conclusions
Accuracy in localisation and specification of tumour volumes
and delineation of normal tissue structures, accuracy in calculation of dose distributions within the patient and a comprehensive means of plan evaluation are all key elements
Figure 2
Figure 1
Delineated structure set for radical radiotherapy to
the retromolar area forming part of the clinical prescription for IMRT/VMAT treatment planning.
CT planning scan registered and blended with MRI
scan for prostate patient with artificial hip. MRI
scan shows anatomical borders of prostate more
clearly than CT.
Figure 3
Dose colour wash and
dose volume histogram
resulting from VMAT
plan for radical radiotherapy to the tonsil.