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Chapter 21 Ionizing Radiation: Radiotherapy
Published in B H Brown, R H Smallwood, D C Barber, P V Lawford, D R Hose, Medical Physics and Biomedical Engineering, 2017
Research into the nature of fundamental particles has resulted in megavoltage x-ray generators that can be used in therapy. The use of ionizing radiation in therapy depends on the fact that tumour cells are more susceptible to radiation damage than normal cells. The radiation used is usually either x-rays (produced by decelerating electrons) or ϒ-rays from radioactive materials. The effects of x- and ϒ-ray photons on tissue are the same, and no distinction will be made between them.
Analysis of the bias induced by voxel and unstructured mesh Monte Carlo models for the MCNP6 code in orthovoltage applications
Published in Radiation Effects and Defects in Solids, 2019
Lorenzo Isolan, Marco De Pietri, Mauro Iori, Andrea Botti, Elisabetta Cagni, Marco Sumini
Simulation based on Monte Carlo methods are a current standard in the Treatment Planning Systems (TPS) of most Mega-Voltage photon external beams and in many dosimetry applications (1–3) in the radiotherapy field. However, this approach is not common for lower energies treatments such as orthovoltage and surface radiotherapy applications. These therapies are still widely used especially for nonmelanoma skin lesions, and they are effective for Basal Cell Carcinoma (BCC) and Squamous Cell Carcinoma (SCC) in terms of tumor local control and cosmetic outcomes (4–17). Furthermore, their applications are expected to increase with an aging population and an increasing number of poor-surgical candidates (18). Indeed, while complications related to surgery are much more likely, superficial and orthovoltage X-ray therapies are well-tolerated, simple and low-cost substitutes (18). As it is well known, superficial (Superficial X-ray Therapy, SXT) and orthovoltage (Deep X-ray Therapy, DXT) radiotherapy utilize low energy ionizing radiation to treat cancer and other conditions that occur either on or close to the skin surface. SXT utilizes X-ray energies between 40 and 150 kV, having a treatment range up to 3–4 mm, while DXT utilizes 150–300 kV X-rays penetrating at a useful depth of 3–4 cm. The shallow penetrating power of both SXT and DXT means that they are often superior to megavoltage external beam radiation for the treatment of superficial lesions if the patient head is involved. There are also some advantages related to eye shielding, compared with other superficial technique such as electrons (19). Relatively high absorption of these low energy X-rays in bones also means that orthovoltage treatment could also be well suited to the palliative treatment of painful bony metastases in shallow regions such as the ribs and the sternum. Both these modalities are an excellent noninvasive alternative to surgery for skin cancer in sensitive locations, such as the folds of the nose, the eyelids or the ears. However, one of the most important disadvantages of orthovoltage RT is a higher differential dose absorbed in bone and cartilage vs. soft tissue (20). As also indicated by AAPM TG-61 report (21), a more accurate dose calculation system would allow the estimation of the dose to these tissues and could improve the clinical evaluation. In the clinical practice, dose mapping delivery is actually based mainly on look-up tables, even if some Monte Carlo-based analyses (for instance with EGS4 or MCNPX (22)) have been performed using simple computational phantoms. Instead, modern radiotherapy treatment planning systems have been designed for dose calculations of megavoltage X-ray and electron beams (23). Such algorithms (i.e. pencil beam, convolution/superposition) rely on target electron density information from the CT data and dose kernels and are applicable primarily when the Compton effect is the dominant interaction process. Investigations of using convolution/superposition treatment planning system for predicting kilovoltage X-ray beam dosimetry showed differences up to 145% in the region around bone inhomogeneities between TLD measured and computed doses (19).