Treatment Field Design and Planning Technique PDF

Treatment Field Design and Planning Technique By Dr: Suzy Fawzy Gohar Assistant professor of clinical oncology  After the anatomic model of the patient has been established, the next major step in the planning process is to use the planning system to create a set of beams to be used for pla...

Treatment Field Design and Planning Technique By Dr: Suzy Fawzy Gohar Assistant professor of clinical oncology  After the anatomic model of the patient has been established, the next major step in the planning process is to use the planning system to create a set of beams to be used for planning. This collection of beams, usually known as a “plan,” can be created using standard protocols (“treat all prostates with a four-field box of conformal fields”) or designed based on the specific anatomy of the case.  Basic decisions on beam technique are typically made very early in the planning process, using experience and/or site-specific protocols. Typically, these decisions include picking the energy and number of beams, the basic orientations for the beams, and the type of beam shaping or intensity modulation to be used. Beam Definition Single beam arrangement: Treatments with photon beams using only a single beam are generally not commonly used. The following criteria of acceptability may be used for a single field treatment: ✓(a) the dose distribution within the tumor volume is reasonably uniform (e.g., within ±5%), ✓(b) the maximum dose to the tissues in the beam is not excessive (e.g., not more than 110% of the prescribed dose), ✓(c) normal critical structures in the beam do not receive doses near or beyond tolerance. ✓Whereas single fields of superficial x-rays are routinely used for treating skin cancers that are confined to a depth of a few millimeters, single megavoltage beams are used only in rare cases for which a combination of beams is either technically difficult or results in unnecessary or excessive irradiation of the normal tissues.  Examples of a few areas that use single megavoltage beams include the ✓Supraclavicular nodes ✓Internal mammary nodes (anterior field) ✓Spinal cord metastases (posterior field). Although the dose distribution is not ideal, the single-field technique in these cases results in simplicity of setup without violating the above criteria of.  Major disadvantage is proper coverage of deep-seated tumors by the required dose without causing very high-dose depositions in superficial tissues. This problem can be solved by using multiple beam treatments. Parallel-Opposed Beams The simplest combination of two fields is a pair of fields directed along the same axis from opposite sides of the treatment volume.  The advantages of the parallel opposed fields are: ✓ The simplicity and reproducibility of setup ✓ Homogeneous dose to the tumor ✓Less chance of geometric miss (compared with angled beams), given that the field size is large enough to provide adequate lateral coverage of the tumor volume. If the separation is not too great for the available beam energy, then a dose range of 7 to 5% is perhaps just achievable.  Disadvantages: ✓the excessive dose to normal tissues and critical organs above and below the tumor. ✓The high dose regions may present a problem at wide separations ✓the hour-glass shaped contour at about the 90–95% level may result in unacceptably low doses at the midplane of the volume. Clinical applications include large pelvis volumes and whole brain treatments. Tangential Beams Radiotherapy of the breast and chest wall (post mastectomy) is usually carried out using two tangential fields. This ensures that the beams do not diverge into the lung, which would result in decrease in lung dose. Wedges will usually be required to produce a uniform PTV dose by compensating for the ‘missing tissue’ and the presence of lung. Wedged pairs For superficial PTVs that extend quite deeply, the wedged pair can produce the optimum dose distribution. Two beams with wedges are used to achieve a homogeneous dose distribution inside an otherwise trapezoid or diamond-shaped irradiated area which, if treated without wedges, would have resulted in very high doses in the inner part of the volume (at the intersection of both beams) and low doses at the periphery of the area.  Highly wedged beams are required to maintain a uniform dose across the depth of the PTV. A greater hinge angle tends to produce a more uniform dose gradient and may be necessary for deeper PTVs but is likely to increase normal tissue dose adjacent to the PTV. This is useful in superficial lesions with angled surfaces (e.g., maxillary sinus, laryngeal or thyroid lesions).  The exit doses from a two-field plan can be significant for critical structures such as the mouth or cord in parotid treatments and sometimes a lightly weighted third field can be beneficial if the relative weighting of the two fields cannot be adjusted. Multiple Coplanar Beams For PTVs lying deeper in the patient, three or more fields with paths intersecting at the PTV usually offer the best dose distribution for the PTV while avoiding excessive dose to normal tissue. The field arrangement is designed to ensure that the ‘treated volume’ conforms as closely as possible to the PTV while complying with critical structure constraints and minimizing the volume of normal tissue irradiated, the ‘irradiated volume’ as defined in ICRU 50. 1. Four-field techniques: In the classical four-field technique, there are two pairs of parallel-opposed beams incident in such a way that each is at a right angle to its neighboring beam, and this arrangement produces a box-shaped region of tissue receiving the prescribed dose. This type of beam geometry is helpful in areas where the opposing surfaces are relatively parallel and the region to be irradiated lies sufficiently deep or is near or at the center of the inter-surface distance such as in the pelvis (for cancers of the prostate, bladder or uterus).  There is another similar arrangement of beams called four-field crossfire, in which the angle between each beam is less than 90° and resultant irradiated volume takes the shape of a diamond. This technique is used if more anteroposterior coverage is required rather than lateral extension or some critical normal structure is present lateral to the target volume which requires sparing from beam portal. 2. Three-field technique: This technique is like the four-field technique and is generally preferred in areas which are closer to the surface or where the tumor is in area with lot of surrounding critical structures and thus to avoid more beams passing through these tissues such as in the rectum, esophagus or in some cases lung. Wedges are used in the two beams which are angulated at each other to compensate for the dose gradient in the third beam Multiple Noncoplanar Beams Usually, patients receive their irradiation with coplanar beams, i.e., the beams move around a single axis. Sometimes this is not suitable to get a required dose distribution or to avoid entry or exit doses to important structures. In such cases, the beams can be positioned in a noncoplanar distribution, i.e. the axis of movement of the gantry head will be different. Non-coplanar beams arise from non-standard couch angles coupled with gantry angulations.  This type of arrangement is frequently used in treating CNS tumors as well as in providing direct perineal irradiation. ✓It is important to ensure that the axis of gantry rotation is freely available to prevent any collision between the gantry and patient/ couch. ✓The exit planes of these beams should be designed such that there is no exit through the length of the body to decrease whole body integral dose (e.g. True vertex beams are not used as the exit beam is through the whole body). Position Of The Isocenter  The isocenter is termed as the point in space through which the central rays of the radiation beams pass. The mechanical isocenter is the point in space about which the linear accelerator and couch rotate The radiation isocenter is the point where the radiation beams intersect if the gantry, collimator or couch is rotated.  The first step in preparing a plan is to decide on the position of the isocenter. ✓ the center of the PTV might appear to be the best location. ✓in practice, it may be preferable to set the isocenter to predefined skin marks which have been set at the time of CT scanning. ✓The skin marks will have been chosen to lie on a stable skin location, perhaps at a standard anatomical point or height above the couch and avoiding steep body contour gradients. This results in a simpler and safer set-up and the fields will then be aligned to the PTV by asymmetric collimator settings.  Another strategy is to select the isocenter position based on the internal anatomy of either the PTV or adjacent critical structures. This can minimize the dose to critical structures by eliminating beam divergence without complicating the field arrangement. Examples are breast treatments without the beams diverging into the lung or abutting nodal fields, or brain treatments avoiding beam edges diverging into critical structures, such as the lens.  Another aspect regarding choice of internal anatomy relates to the dose computation algorithm and placement of the isocenter in solid material and not in an air cavity in order to improve the dose prediction accuracy at that point for checking. Choice Of Beam Energy  The beam energy is chosen by considering the depth of d max and the penetration properties of the beam. Low energy beams, e.g., 4–6 MV are suited to more superficial volumes, e.g., head and neck PTVs Deep pelvic PTVs will require 10–20 MV. Beam energies above 15 MV do not give a great benefit to the planner, but their frequent use on a linac can introduce radiation protection problems due to neutron production.  The build-up region is a significant feature of the MV beam in that it gives ‘skin sparing’ allowing doses that would severely damage the skin to be delivered deep into the patient. However, a PTV which extends from near the surface to a depth requiring MV photons may require bolus to be added to the skin. By placing this tissue equivalent material on the skin, the build-up region is shifted into the bolus and the maximum dose of the beam is delivered to on or near the skin. Identification Of Reference Point  ICRU Reference Point The target dose should be specified and recorded at what is called the ICRU reference point. A point located at the center (or central part) of the PTV generally fulfills ICRU requirements and is recommended as the ICRU Reference Point When possible, at the intersection of the beam axes  This point should satisfy the following general criteria: 1. The point should be selected so that the dose at this point is clinically relevant and representative of the dose throughout the PTV. 2. The point should be easy to define in a clear and unambiguous way. 3. The point should be selected where the dose can be accurately calculated. 4. The point should not lie in the penumbra region or where there is a steep dose gradient.  For a single beam, the target absorbed dose should be specified on the central axis of the beam placed within the PTV. For parallel opposed, equally weighted beams, the point of target dose specification should be on the central axis midway between the beam entrances.  For parallel opposed, unequally weighted beams, the target dose should be specified on the central axis placed within the PTV. For any other arrangement of two or more intersecting beams, the point of target dose specification should be at the intersection of the central axes of the beams placed within the PTV.