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Proton Therapy Dosimetry
Published in Arash Darafsheh, Radiation Therapy Dosimetry: A Practical Handbook, 2021
Michele M. Kim, Eric S. Diffenderfer
Double scattered systems require patient-specific hardware for conformation of the dose to the target volume. The first piece of hardware is an aperture, or a brass beam-stop with a hole that is shaped to the outer projection of the target in the beam's eye view. This can also be achieved with a multileaf collimator [96]. With the use of this hardware, there can be two areas of unwanted high dose proximal to the target towards the edges of the target (thus the edges of the range compensated portion) due to the constant range modulation of the field. This is not seen in scanned beams. The treatment planning system can be used to prepare files for automatic fabrication of patient-specific, field-specific hardware such as range compensators [97]. These can be designed to have different goals, such as guaranteeing target coverage with respect to alignment errors or patient internal organ motion [98].
Medical Linear Accelerators
Published in Eric Ford, Primer on Radiation Oncology Physics, 2020
One tradition way to accomplish this is through the use of blocks poured from a low-melting point metal such as Wood’s metal (tradename Cerrobend) which has a density of 9.7 g/cm3 and a melting point of 70°C (Figure 9.1.7). The blocks can be constructed by first cutting the shape in a Styrofoam block using a heated wire cutter and tracing out the required shape from a film. The wire cutter is designed to have a divergence which exactly matches the beam. The metal is then poured in or around the Styrofoam block on a cooling table. Finally the block is mounted on a plastic tray in the head of the linac. This is a labor-intensive process, requires dedicated facilities and staff, and introduces toxic metals into the clinic. Also, importantly, it does not allow for dynamic shaping of the beam and requires that the blocks be replaced between each beam which makes the treatment delivery much more time consuming. For these reasons, poured blocks have largely been replaced by MLCs. Figure 9.1.8 shows the potential of an multileaf collimator (MLC) for beam shaping. The MLCs are thin blades of tungsten arranged one next to the other as seen in Figure 9.1.8B. This particular linac head has 80 MLC leaves (40 per side), each of which can be moved to the desired location by tiny motors. This provides the beam shaping as needed.
A History of Surface Guidance Methods in Radiation Therapy
Published in Jeremy D. P. Hoisak, Adam B. Paxton, Benjamin Waghorn, Todd Pawlicki, Surface Guided Radiation Therapy, 2020
Jeremy D. P. Hoisak, Todd Pawlicki
The drive to minimize normal tissue dose and enable a maximization of tumor dose led to the addition of shielding by blocks, cones and later, the multileaf collimator (MLC) so that the fields could conform to the shape of the target in three dimensions. With 3D conformal radiation therapy, it became both more important and more technically challenging to ensure the target was within the planned high-dose area. Modern linear accelerators equipped with MLCs together with intensity modulated radiation therapy have now made it possible to deliver radiation dose distributions that are highly conformal to the target while sparing adjacent healthy tissues. These finely modulated dose distributions require that the target be localized as accurately as possible to ensure the dose is delivered as intended. Precise and accurate localization of the target can reduce geometric uncertainties and therefore reduce the impact of treatment-limiting side effects. This is a technical problem of considerable difficulty. Many technologies have been developed to help ensure precise and accurate localization of the therapy target, including rigid immobilization devices3 (e.g., thermoplastic masks and vacuum bags), X-ray image guidance with kV4,5 or MV6,7 sources, and nonradiographic localization devices.
Radiation responses of cancer and normal cells to split dose fractions with uniform and grid fields: increasing the therapeutic ratio
Published in International Journal of Radiation Biology, 2022
Linda Joanne Rogers, Juliette Cornelia Harley, David Robert McKenzie, Natalka Suchowerska
A Varian Novalis Tx™ linear accelerator (Varian Medical Systems, Palo Alto, CA) was used to deliver a 6 MV photon beam. The field size was defined by the secondary collimators set to 20 × 17 cm. The high definition 120 micro-multileaf collimator (HDmMLC) was used to create the spatially modulated grid field. The HDmMLC fields were generated using only the inner 32 leaf pairs, which have a projected leaf width of 2.5 mm at the isocenter, while the outer leaves were kept closed and offset to minimize interleaf end leakage. Using only the inner leaves, the fields were limited to a maximum of 8 cm in one dimension, sufficient to cover the cell culture flasks to be irradiated. The HDmMLC defined grid field was created as a text file and then saved using the Shaper™ software (Varian Medical System, Palo Alto, CA). A grid pattern of open–close exposures were delivered in a single static field. The growth medium in each flask was topped up to 10 mL prior to irradiation to create sufficient radiation scattering volume (Claridge Mackonis et al. 2012; Claridge Mackonis et al. 2018). The irradiations were designed as a split dose study such that half the dose was initially delivered and the remaining dose was delivered at a given time interval later, where the time interval ranges from 2 to 120 minutes. All fields were delivered at a dose rate of 600 MU/min.
Stereotactic Body Radiation Therapy (SBRT) in Pelvic Lymph Node Oligometastases
Published in Cancer Investigation, 2020
Leonid B. Reshko, Martin K. Richardson, Kelly Spencer, Charles R. Kersh
Intensity-modulated radiotherapy and volumetric-modulated arc therapy planning techniques were considered. A gross tumor volume (GTV) was contoured and a clinical target volume was kept equivalent to the GTV. A non-uniform planning target volume of five millimeters was added to account for localization and targeting uncertainties. The radiotherapy dose was prescribed to this volume. The treatment planning techniques included non-coplanar static aperture arcs and non-coplanar static fields. Treatments were delivered using a linear accelerator with four-millimeter multileaf collimator leaf width and a robotic couch. Cone-beam CT was obtained prior to each treatment. The fractions were delivered weekly. The radiotherapy treatment technique did not undergo any major changes between 2007 and 2017. The BED was calculated by the formula: nd[1 + d/(α/β)]. Equivalent doses of 2-Gy fractions (EQD2) were calculated from the BED with the following formula: nd[(d + α/β) ÷ (2 + α/β)]. α/β is a ratio unique for a given tissue, d = dose per fraction, and n = number of fractions (26). α/β of 10 was used for our calculations (27). The specific dose constraints depended on the tumor location and were based on the prior studies and on our institutional experience (28). Radiologic imaging and clinical follow-up were performed at 3-month intervals or more frequently to assess treatment response.
Knowledge-based planning for oesophageal cancers using a model trained with plans from a different treatment planning system
Published in Acta Oncologica, 2020
Yoshihiro Ueda, Masayoshi Miyazaki, Iori Sumida, Shingo Ohira, Mikoto Tamura, Hajime Monzen, Haruhi Tsuru, Shoki Inui, Masaru Isono, Kazuhiko Ogawa, Teruki Teshima
Treatment planning was performed on two TPSs, namely, RayStation version 6.2.0 and Eclipse version 13.0. A Varian Truebeam linear accelerator equipped with a Millennium 120-leaf multileaf collimator (MLC) (Varian Medical Systems, Palo Alto, CA, USA) was used for the treatments. The procedure of treatment planning involved the following steps. First, the CT images were imported to Eclipse to define the targets and organs. The clinical target volumes (CTV) were contoured on both, ExCT and InCT sets. The typical CTV for elective nodal irradiation was based on the tumour location. In upper thoracic oesophageal cancer, the CTV generally encompassed the bilateral supraclavicular, cervical, paraesophageal, and mediastinal lymph nodes up to the tracheal bifurcation. In middle thoracic oesophageal cancer, it generally encompassed the bilateral supraclavicular, cervical, paraesophageal, mediastinal, paracardial, lesser curvature, and left gastric lymph nodes, while in lower thoracic oesophageal cancer, the CTV usually encompassed the mediastinal, paracardial, lesser curvature, left gastric, and coeliac artery lymph nodes. To define the internal target volume (ITV), both CTVs, contoured on ExCT and InCT, were combined. The planning target volumes (PTV) were delineated by adding 5-mm margins to the ITV in all directions to compensate for setup errors. The OARs, namely, the lungs, heart, and spinal cord, and the PTVs were contoured on the ExCT.