In my last post “KV/MV Homo-Source Dual-Beam Accelerator: the new round of competition in IGRT about to begin“, I have introduced the progress of this new approach, which is also reflected that IGRT is gearing up on the road. The new created Homo-Source Dual-Beam (HSDB) accelerator technology has already been much concerned, as one of the key points in IGRT research, by RT device experts. In this post, I’d attempt to expand application of HSDB accelerator, combining with the theory of Dual-Source CT (DSCT), to a full new concept of Multi-Source Multi-Beam (MSMB), which could be a further developing to the Tomotherapy.

Your valuable opinions and comments are welcomed very much.

In order to name easily, the short names used in this article are presented below:

• HSDB:   Homo-Source Dual-Beam
• MSMB:   Multi-Source Multi-Beam
• DSCT:   Dual Source Computerized Tomography
• DSMV:   Dual Source MV Delivery (Treatment)

1. Dual-Source Tri-Beam system

Based on Tomotherapy prototype published in 1993, if the original KV imaging system is reserved, but that linac is to be replaced by a new HSDB accelerator, it will become a so-called Dual-Source Tri-Beam system, consisting of 2 KV beams + 1 MV beam.

The principle is illustrated below.

Tomotherapy: 2KV 1MV

There are total three beams in this Dual-Source Tri-Beam system, in which two beams are for KV imaging. And these two KV beams are placed orthogonally each other, so that identified essentially with the structure of Siemens’ Dual-Source CT (DSCT).

 Schematic diagram of DSCT

If the DSCT technology is integrated seamlessly into the Tomotherapy IGRT system, it will contribute the advanced features of DSCT to the imaging behavior of Tomotherapy in a natural agreement. For example:

• Considerably speedup in Data Acquisition / increased temporal resolution.

According to the theory of DSCT, its technology of “dual-beam dual-sector imaging” will improve the temporal resolution by four times. At present, the tomotherapy gantry can rotate at fastest of 10 sec per round. The temporal resolution of single-sector data acquisition can be improved to 2.5 sec, upon the condition of unchanged rotation speed of the gantry. It can access the projection data in full 180 degree, so long as the gantry rotates 90 degree. This feature will benefit to improve the imaging quality on chest and abdomen, through the reduction of artifacts in respiration.

If the accelerator runs upon C-band or X-band, the weight of accelerator equipped on the gantry can be greatly lost, the inertia of rotation will be decreased dramatically, and it will speed up the gantry’s rotation. The temporal resolution will be better, therefore, and be more conducive to the image quality on chest and abdomen.

• The radiation dose for imaging is decreased by 50 percent.

By the same principle, if the temporal resolution was improved by four times, during dual-beam imaging, the radiation dose is reduced doubled.

• “Dual-energy subtraction” imaging.

If KVp was set to 180KV for KV output in a KV/MV HSDB accelerator, the KVp is set to 80~100KV in that independence KV tube. (It needs to adjust the current appropriately to ensure the output energy). The subtracted image, that acquired by Dual-energy CT scan, can be used to separate anatomically into skeleton and soft tissue respectively. It should be good to correct the positioning and registration fusion in accordance with the bony mark shown clearly on the skeleton image. And soft tissue image will be beneficial to check the change of tumor in morphology during the treatment, which can be used in adaptive plan.

• Prolong the magnetron life

The life of the magnetron in a 2KV+1MV Dual-Source Tri-Beam system can be prolonged effectively than that of a 1KV+1MV HSDB device. Because the scan time is reduced by half when in KV output mode in a HSDB linac, and the magnetron’s high-voltage load time can be shortened.

2. “Dual-Source Quadri-Beam” system

If replace the independent KV tube in a Dual-Source Tri-Beam system with a HSDB linac, the entire system in which including two HSDB linacs is so-called Dual-Source Quadri-Beam system. This is a full new concept of Multi-Source Multi-Beam (MSMB) IGRT system. Illustrated below.

Tomotherapy 2KV 2MV Multi-Source Multi-Beam system

In this MSMB system, the KV / MV energy can be set in different, such as, KV can be set to 180 KV/80KV respectively, and MV energy will be 6MV and 15MV either.

The principle of KV imaging in this system is still similar to the theory of DSCT cited above, will not get into details any more.

Tomotherapy treatment works on the principle of “Reverse Delivery of CT projection reconstruction“, which is based on the mathematical theory of Radon Transformation. The principle of Tomotherapy delivery (treatment) is established together with the theory of CT imaging on a common mathematical principle, but just on reversed direction each other. Therefore, theoretically, the development of basic principle of CT will impact and introduce directly on the improvement of technology used by Tomotherapy delivery. (see my post “Thinking after reading 2”)

In this Dual-Source Quadri-Beam system, the structure of two MV orthogonal beams will have features came from DSCT. In this article, it is called as Dual-Source MV Delivery (DSMV).

The features are assumed as follows:

• Increase speed greatly of the treatment.

It now only needs the gantry to rotate half a circle to complete the delivery of the treatment, which was a full circle before, by using the high-speed binary MLC. The treatment speeds up double, under the condition of remaining maximum speed of gantry rotation. In practice, when the gantry rotates to the semi-circle, the couch top can be moved forward one step (not necessarily a pitch, changeable in accordance to the treatment planning).

• Dosage output of a single accelerator reduces to half.

For example, the dose rate of the original accelerator was required in the 600 MU/min, it now can be down to 300 MU/min, if use the new DSMV technology, and ensure to exposure 600 MU/min @ isocenter. This will help greatly to reduce specifications of the accelerator, gun, target, and the microwave source. In addition, this kind of configuration usually adapts to C-band or X-band. It will realize production much easier when lower the specifications. Furthermore, it assumes that using DSMV technology could reduce dose distribution in the normal body tissue. Its significant value will be detailed in the Conclusion section behind.

• Optimize dose distribution

During the gantry rotating, the system can select the proper energy automatically in order to achieve the optimized dose distribution in the body by controlling switch on/off these two accelerators in real time according to the location and distance between dual-accelerators and tumor target volume.

There is an alternative design in using this “Dual-Source Quadri-Beam” system structure. It is called as Dual Slice Helical system. That is, the two rotating planes that contained linacs and detectors parallel to each other. This design will be good at speeding up the treatment doubled by dual-slice delivery, but short on useless of the advanced features of the DSCT.

Moreover, the current released Topotherapy technology can be used to reduce the demand of Dual-Slice Helical Treatment in clinic.

However, the HSDB accelerator can be attached a new designed KV beam steering, in order to expand the KV projection’s width corresponding to the width of dual-row detector. And, it can move the position of the detectors to interdigitated position,  sharing the middle row each other, as shown blow. Can it result to same effect of DSCT? Requires further study to verify this kind of design.

Dual row interdigitated detector 

3. Conclusion

A new concept of Multi-Source Multi-Beam (MSMB) IGRT is introduced briefly in this article.

Thanks to the development of HSDB accelerator, we can design and construct more powerful IGRT system in unprecedented convenience. With the assistance of utilizing C-band/X-band accelerator technology, some achievements on medical imaging can be used in radiotherapy.

Obviously, it is much difficult to build a Dual-Source Quadri-Beam IGRT system than the Dual-Source Tri-Beam system. Leaving aside the difficult of realization, it is discouraged to evaluate its outlook from the view of economic cost, while combining two sets of linac into one machine. How will you choose, however, between US$20M~30M for a proton system and US$5M~8M for a Dual-Source Quadri-Beam IGRT System?

In addition, it is worth to study an important task, that is, whether the dose distribution in body (normal tissue) is dropped down, when using this method of Dual-Source MV  (DSMV) treatment to reduce the output dose?  How much does it improve? Can we extend the conclusion of “reduced dose” from the theory of DSCT to reduced 50% dose from DSMV? This study has too meaningful.

If the result of study supports this assumption, it will greatly support the conventional (S/C/X-band) accelerator radiotherapy, and will reduce future dependence on proton system and its high investment, as well as the patient’s expense!

As mentioned above, a Dual-Source Quadri-Beam system can be built by using two sets of orthogonal HSDB accelerator. As an alternative scheme to technology and cost, two Cobalt 60 sources positioning orthogonally can be equipped onto the gantry in DSCT. This becomes the so-called Quadsource Quadbeam system. The advantage of this structure is to greatly reduce the complexity and cost of the system, and in favor of widely application in clinical.

The new concept of Multi-Source Multi-Beam IGRT is based on the innovated HSDB accelerator and integrated with the theory of DSCT. The concept also introduces the so-called DSMV. It needs to further investigate to the value described in this article.

I believe, however, the theory of DSCT will indicate doubtless a brilliant future of MSMB IGRT system! Because radiotherapy is returning to imaging!

 

Note:   This post is also compiled to a pdf file, you can download this file by click:
Download: MSMB.pdf