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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.