Advantage of Cutting Field IMRT (CFIMRT)

IMRT                                      CFIMRT

-Comparison with IMRT (Intensity Modulated Radiation Therapy)-


MLC-based Intensity modulated radiotherapy (IMRT)

Description of basic theory of IMRT using clay

 1. Optimization

The optimal radiation distribution is assumed on a computer to the target of complicated form.

2. Making intensity map

Intensity map is made at each ports. Each port needs intensity modulated beam, the inside of one field can deliver only uniform irradiation at one irradiation. So if intensity modulated beam is needed, 'segmentation' is required.

3. Segmentation

In order to deliver intensity modulated beam at each directions, segmentation is needed.

4. Deliver IMRT

The original dose distribution is theoretically reproducible if segments are compounded in each directions. But there are many complicated problems in these processes.

Please see GIF anime about MLC moving of DMLC-IMRT ( Conventional IMRT )

The demerits of IMRT will be described below. CFIMRT conquers the demerits of IMRT.

Basic theory of CFIMRT

@CFIMRT is the radiation therapy which combined the rectangular rotational irradiation with the fixed multi-ports irradiations. Usually, it treats combining about six conventional sized radiation field without small subsegments. Although naming as IMRT in fact, it is only in the combination of fundamental radiotherapy. If it is said why whether IMRT was named, since CFIMRT shows almost equivalent dose distribution to IMRT, and intensity modulation is added by fiexed multiports in the radiation field of rectangular rotational irradiation field. So, in a pure meaning, it is considered a kind of IMRT. The following logo mark shows the feature.

Inverse plan Vs forward plan

IMRT by insurance institution in USA is needed using optimization program by Inverse Plan, and to combination at least four or more subsegments in each direction. In fact is difficulty also there. Although it calculates so that homogenous irradiation may be made in accordance with the form of a neoplasm, and a part is made, man sets up various paramters. However, if it is the region difficult to treat, these segments will become smaller and smaller, such as a jigsaw puzzle. Constructing, if segments become smaller takes long time (prolongation of treatment time). It may be lost when segments are too small (difficulty of QA). If the done puzzle is not made well, you have to retry (extension of plannig time) to set intensity level, or beam arrangement. This is a part of problem of IMRT.

 If it compares and says, please consider another language translation software. Although surprising translation was made the first stage, surely it is the software which can be used recently. But its intention does not get across to a computer too. As know-how, it may change and change into language intelligible for a computer purposely. However, the last has only corrected personally. IMRT will be compared to translation software with it very difficult to correct by a human power.        

For example, please think that it is the neighborhood edge of a neoplasm. Radiation decides to glare from a top to the bottom. When a plan is made by IMRT, as shown in the left figure, it becomes the quality of near edge straightly faithful exactly (it notes consisting small segments.). The difference in a color shows the difference of radiation dose. However, it may be better to be the slope of radiation gradient to a slight degree in man's body (such as neigber the pancreas,or the duodenum or the stomach). Now, it is next to impossible to make by IMRT. On the other hand, since it is needed under the category of the usual radiotherapy and he also arranges a part greatly (forward plan), processing of edge can do CFIMRT by human pawer. Fine tuning while looking at the completed result . Most clinical examples are adding fine tuning altogether. Even when it is merely small, you treat patients by do your best! CFIMRT is the proposal of the way and it is one of a slogan. 

The difficult point of IMRT

Photon beam is a kind of light.

Photon beam has the same characteristic as light. As long as there is no big gravity etc., light runs straightly, without bending. However, please consider solar sunlight.

I am sorry to be a slightly strange picture. Please consider a left picture to be a time of light passing through a square frame from a light source. Does a light hit downward squarely distinctly? As for some portions of a frame, light falls. When the same thing passes through MLC (probably rate variable iris diaphragm) with radiation, the intensity of radiation falls by the side. Moreover, light spreads in the range conversely larger than the portion of an original frame. In short, in edge of the form of the frame which lets light pass, without becoming the strength of an original light, light becomes somewhat weak, fades and is carried out. It is the characteristic of edge at the time of this being photon irradiation. (right figure)

In IMRT, many small parts (segments) with such characteristics are compounded.

Please think that such three parts were compounded.

For example, it becomes like this. Probably, you also understand a thing to be tuned finely in order to obtain a scheduled dose distribution in the target position. If an iris diaphragm of MLC enters into an irradiation field, difficulty will come out in this way. When many parts blend, optimization is impossible in human's trial and error any longer. Furthermore, since various things are in passage courses of radiation, such as air and a bone, in a human body, more and more complicated. Therefore, irradiation Nouchi has many small parts like IMRT, and in performing the irradiation method which has an iris diaphragm of MLC innumerably in PTV, an optimization program called Inverse Plan is needed. However, Inverse Plan needs the exact dose of every small segments.

Difficulty of a small irradiation field

Let's consider once again the parts (segments) used by IMRT here.

 Thus, in the case of a thin irradiation field, as explained also like the point, a dose falls off near MLC. When determining the exposure dose when irradiating radiation in fact, after energy is fixed, it is necessary to decide on the irradiation time per area corresponding to beforehand. Usually, the amount of ionization of the radiation of the air of 10cm distance is measured from an beam outlet, the factor of the irradiation time for every machine of the is measured in advance. Usually measure the parameter to the 4cm square. However, the data of a smaller irradiation field is required of IMRT. In fact, this measurement itself has a very difficult thing. In the irradiation fields of the shape of a thin slit, since the way of dotage is conspicuous, I think that verification is also very difficult and it can imagine that it is hard to use. This has also been discussed since before as difficulty of IMRT.

 The more familiar one is well reminded only for the principle of IMRT of a block and the piece of a jigsaw puzzle. However, the edges of each beam are not sharp! Yes, these are fuzzy. Please think that the parts (segments) of IMRT are the aggregate of a beams with fuzzy edge.

  I think that it turns out that it is very difficult to make it uniform in three dimensions combining these. For this reason, in IMRT, importance is attached to QA. If it is inattentive and carries out dependent on a machine or a treatment planning system, it will not be avoided that an unexpected high dose and a low dose portion occur.

  CFIMRT is one of the slogan.  which should usually combine irradiation to the last, according to usage, I think that it may run on and on to the combination of a fine irradiation field like IMRT. I myself am controlling myself not to put an iris diaphragm of MLC into irradiation field as much as possible. It is, after considering the characteristic of the above radiation. I ask you again!

 Do you want to carry out complexed radiotherapy, even if you can treat patients by combination with conventional method?

application at an abdominal lesions

Probably, one of the greatest merits of CFIMRT is comparatively easy for application at an abdominal lesions. Basically small subsegments are not used in CFIMRT, if a margin is fully processed, it is applicable also at abdominal lesion with organs movement. for exsample, if you want to treat abdominal lymphnodes, please try to plan such manner.

You must not think sever about each ports angles, but you should set some ports predominantly backside. Perhapes you see highly conformal irradiation plan! You have to do on your responsibility about clinical use, but if patients treated such irradiation pattern, they wil never complaint about radiation sickness or diarrhea.

Instead of Electron Beam

A case of skin meta of nasal cancer (post op.)

This patient underwent the operation of the cancer of a nasal cavity previously. Her suffered skin metastasis recently. I treat her by 4MV photon.

This plan was made such as weight and directions.

last modification date: 2014/08/15