Modern radiotherapy pursues the three elements of “accuracy”, “shape” and “effectiveness”: “accuracy” refers to accurate positioning, which is as important as targeting, off-target means that the enemy will be released to hurt friends, which will definitely lead to radiotherapy failure; “shape” emphasizes the right shape, which is to make the high-dose area consistent with the target area in three-dimensional direction with the idea of surgery; “effectiveness” refers to the flexible application of the linear-quadratic (L-Q) model of radiobiological effects in order to improve the efficacy of treatment. The “efficacy” refers to the flexible application of the linear-quadratic (L-Q) model of radiobiological effects to improve the efficacy. Based on more than two years of clinical practice, we propose the following points
We would like to discuss with our colleagues:
1, CT positioning: Before scanning, the target area should be initially determined on the analog positioning machine and adjusted and fixed by the negative pressure bag, and the body surface marker (rolled into a Φ5×20mm column with medical tape) should be placed during CT scanning, and the two body surface positioning points targeted by the positioning pointer should be tattooed into small crosses with a tattoo device, and the depth of the positioning pointer and the scale values of the left and right channels should be noted. The layer spacing of the lesion area is 3-5 mm, and the maximum layer spacing of the non-vital non-lesion area can be 10 mm,
The scanning range should be long enough (20-30 cm) so that the non-coplanar incoming rays do not exceed the reconstructed area.
2.Pre-training the patient to breathe: take small, fast, shallow light breaths during CT scanning and treatment, and add a restrictive pressure plate for breathing movements to minimize the range of motion of the diaphragm and internal organs.
3. Posture management: The patient’s feeding and water intake, urination and defecation, stretching state of tight shirt and pants, and the pumping hardness of negative pressure bag, etc., should be controlled after the patient’s breathing.
The patient’s food and water intake, urination and defecation, the stretching state of the tight liner and the pumping hardness of the negative pressure bag should be requested as needed before CT positioning, and each treatment should be consistent with the CT scan. For air
The negative pressure bag with poor air tightness should be checked and pumped regularly to maintain the hardness, and in case of serious air leakage, the shape of the negative pressure bag can be restored according to the depth of the positioning pointer and the grain cross to avoid the huge cost of redoing the CT positioning and treatment plan.
4, the choice of non-coplanar bed angle: In 3DTPS, priority should be given to the use of 0 ° bed angle as much as possible to set up a conformal fixed field or more arc, because this bed angle can be selected more incidence near, vertical, rack can turn down the arc or field. In addition, the non-co-planar bed angle should not be too large, in the range of ± 50 ° has been sufficient, otherwise it is easy to incidence oblique, depth, beyond
out of the reconstruction range, etc.
5.Flexible choice of collimator and projection techniques according to the conformability:
(1) For small lesions close to the circle, a cylindrical collimator or a manual collimator can be used to form a circular field
(1) For small lesions close to the circle, a cylindrical collimator or manual collimator can be used to form a circular field with multi-arc non-coplanar rotational irradiation, which can obtain both conformal, large dose distribution gradient, the ideal dose of the same shape as the knife cut
distribution.
(2) For flat ellipsoidal, columnar and other near-rectangular cross-section of the target area, the primary tungsten gate of the gas pedal can be used
(jaw) as a beam collimator, by changing the length, width, small head angle, selection of projection cross-section for each single arc (or field)
The beam collimator is used as a primary tungsten gate (jaw) for beam collimation. The positioning accuracy, fixation method, and conformality of the body part are better than those of the body part because of the visceral movement.
The range of expansion should be larger, often 5-8 mm, because the body has visceral movement, positioning accuracy, fixation method and conformality are worse than that of the head.
The use of rotation or non-coplanar projection with the fixed field can still make the normal tissue low and dispersed, which can improve the focal ratio and dose gradient and obtain a better three-dimensional conformality.
(3) For the target area with very irregular shape, manual MLC or manual casting conformal block must be used, but then generally can not rotate, must choose more fixed field (such as more than 6), otherwise, although the beam cross-section two-dimensional conformal, but because the beam axial very unconformable, not three-dimensional conformal and high-gradient dose distribution, often polygonal or petal-shaped dose distribution, low focal skin ratio, is still not ideal 3DCRT.
(4) If there is automatic (or dynamic) MLC, any target area shape can be achieved multi-arc non-coplanar dynamic rotation conformal irradiation, is the most ideal, advanced multi-leaf collimator.
6, to ensure that the size, direction and shape of the template is accurate: the size, direction and shape of the printed template should be actually measured and calculated to the isocentric plane and the known number of controls, and marked head, foot, left and right direction, printed in duplicate, leaving a copy in the treatment room for comparison, to prevent mistakes in direction and correspondence.
7, the processing of the template: in order to make the processing accurate, easy and save materials, can be used instead of about 1mm thick thin Plexiglas plate, with hot wire cutting machine red hot electric wire directly cut, with a permanent pad in the positioning module on a substrate, the positioning pin will spin high to the thin template into the MLC blade. Some people also use extra hard foam that can be cut by the hot wire cutter, but because it is opaque, the shape of the conformal light field cannot be checked, and the template must be withdrawn after clamping the MLC blade. If an electric wire saw can be purchased, the cutting of the template can be made more automatic and faster. In the treatment plan setting
If the shape of the MLC blade is found to affect the conformal advancement, the small head can be rotated by an appropriate angle in advance.
8. Application principles of fractionation, time-reduction, incremental and L-Q modes: Radiotherapists may refer to the following conditions for fractionation according to the location of lesions, pathological characteristics, early and late stages, age and physique, comprehensive treatment, etc:
(1) General: small and limited lesions, no external invasion, early clinical staging (often a small number of cases), according to α/β value and L-Q formula or table, most of them can be divided into 5-9 times, 9-6 Gy each time or even larger, every other day or even once a day, to achieve the purpose of less fractionation and more dose (according to the biological effect dose, usually 10-20 Gy) and shorten the total treatment time.
(2) Combined with general external irradiation: Most patients can have 30-40Gy of moderate external irradiation first, and then 3DCFSRT (three-dimensional conformal fractionated stereotactic radiotherapy) as a supplementary dose of field reduction, 3-6 times, 8-6Gy each time, every other day or even once a day, depending on the lesion’s discretion.
(3) Advanced age, frailty or special importance: Strictly 3DCFSRT-type precise radiotherapy (PT) can be adopted in the treatment technique, and the conventional fractionation of once a day or five times a week can be used in the fractionation mode, with 10-20 Gy added to the total amount, and the plan should be repositioned and modified once in the treatment. This can obtain good results with good efficacy and very mild side effects, but the workload is larger and the cost of treatment is higher. There are such cases and patterns at home and abroad.
After years of clinical practice, the above summary points have proved to be practical and effective, and are conducive to the optimization and improvement of the three elements of accuracy, conformability and efficacy.