An Analysis Method of Dose Error Caused by the Couch Tilt of CT Simulation Machine

Purpose: If the horizontal plane is inconsistent between computer tomography (CT) couch and line accelerator (LA) couch, the position of the isocenter point in the radiotherapy treatment planning system (TPS) should deviate from that of it on the accelerator couch board. Then, the actual dose distribution was different from the radiotherapy plan we designed in TPS. This paper introduces a method to evaluate the dose deviation caused by the tilt of CT couch. Methods: We calculated the isocenter points of the position coordinates both in CT couch and LA couch, and compared the dose distribution when the twopositions of isocenter points were applied in radiotherapy plan independently in TPS. The dose distribution difference of a breast radiotherapyplan was analyzed as a demonstration with this method. Results: The distance between the two isocenter positions increased with angle of CT couch. The tilt of the couch had an impact on the dose distribution, especially in larynx's maximum dose parameter. Conclusion: This method could quantitatively analyze the dose distribution deviationcaused by the tilt of CT couch plate. The results can provide a valuable suggestion for clinical medical strategy.


Introduction
In radiotherapy, the couch of computer tomography (CT) simulation machine and the couch of radiotherapy linear accelerator (LA) should be horizonta l [1,2]. Due to the aging of the CT machine, the CT couchmight tilt. Then, the horizontal plane is inconsistent between CT couch and accelerator couch. This maycause the position of the isocenter pointin the radiotherapy treatment planning system (TPS) was inconsistent with that of the isocenter point on the LA couch. Finally, the actual dose distributionmay bedeviated from the radiotherapy plan's we designed [3]. In the study, we introduced a method to evaluate this dose deviation. And, according to an example of a breast cancer radiotherapy plan, we calculated the dose difference in this method for a specific analysis.
Radiation imaging data of apatient with right breast cancer (female, 53 years old). Before treatmenta written informed consent was obtained, and the procedures was conducted in accordance with the ethical standards of the institutional ethical committee. The patient was fixed inthe vacuum mold.

Radiotherapy Procedure
The main process of radiotherapy includes: 1, Fixing the patient's body in the vacuum mold on CT couch, and the adhesive tapes with blue lines which were coincided with the laser lines were stuck on the vacuum mold [10][11][12][13][14][15], as shown in Figures 1, 2, Mark points (lead point) were stuck on thevacuum mold for indicating the center of the blue lines cross, thencarry out the CT scanning. 3, Transmitting the CT images to the TPS. 4, Determining the position of the radiotherapy isocenter point according to the three lead points of the CT images, and the radiotherapy plan was designed based on the isocenter points, as shown in Figures 2, 5, After the acceptance of the plan, the patient was setup on the LA couch for treatment, as shown in Figure 3.

Description of the Isocenter Point Position
Because the horizontal plane was inconsistent between CT couch and accelerator couch. The laser lines cannot coincide with the blue lines cross marked in the adhesive tapes when setup on the LA couch [16,17], as shown in Figure 3. The isocenter point in TPS wasbased on the center of the laser line cross which coincide with the lead point. However, on the LA couch, the isocenter point just can be locatedbased on the center of the laser line cross, which do not coincide with the blue lines cross marked (or the lead point). Thus, the position of isocenter points were different between in TPS and in LA couch. As a result, the actual dose distribution in LA was different from the plan we designed in TPS.

Analysis of Isocenter Point Position
First of all, it should be clear that the external laser lineswere horizontal both CT room and accelerator room. When the patient lying on the tilted CT couch plate, the two blue line cross points on adhesive tape (right and left) based on the external laser lines of CT room at the same level (or Point E and Point F are in a horizontal plane), as shown in Figure 3. However, when patient was lying on the accelerator couch in accelerator room, the two blue linescannot match the external laser lines. The result was that the blue lines on adhesive tape were higher on one side and lower on the other side compared with the external laser lines. If we made the blue line match with the external laser line in patient right side, the external laser line was higher than the blue line in patient left side, as shown in Figure 3. We make a geometric diagram according to Figure 3, as shown in Figure 4. Then, EF represents the couch surface level of CT. EN represents thecouch surface level of LA. Point A was shown in Figure 2. Point E represents blue lines cross point on the patient's right side. Point F represents blue lines cross point on the patient's left side in CT room. Point N represents external laser lines cross point on the patient's left side in LA room. Then, the Point B was the isocenter point in TPS, the Point C was the isocenter point on the LA couch as actual setup.
In Figure 4, assuming that Point B is taken as the origin for a plane coordinate system xBy, and the coordinatevalueof Point C in this coordinate system is (x, y). According to the geometric relationship, we can get: x=(EB+ AB* TAN(ASIN(NF/EF)))*COS(ASIN(NF/EF))* COS(ASIN(NF/EF)) -EB y=(EB+ AB* TAN(ASIN(NF/EF)))*COS(ASIN(NF/EF))*(NF/EF) The distance between Point B and Point C is: .

Analysis Method of the Different Distribution
In TPS, we named the radiotherapy plan designed in TPS as the original plan, which the isocenter point was Point B. Then, we move the isocenter point of the original plan from Point B to Point C, and the new plan was named as the simulated plan. All parameters of the two plans are the same except for the isocenter point and fields angle. All fields angle in the simulated plan is ∠CAB smaller than that in the original plan. Take a breast radiotherapy plan as an example, dose parameters difference between the two plans were compared and analyzed, including: lung, cord, clinical target volume (CTV), Planning Target Volume (PTV), bone, larynx.

Results
In TPS, EF=353mm, EB=95mm, AB=42mm and NF=4mm are measured. According to the formula, the coordinates of Point C are: x=0.46, y=1.08; ∠ CAB=0.65°. Field angles were reduced by 1 degree in the simulated plan compared with the original plan (Pinnacle 10.3 TPS only supports the modulation of the integer value of the field). The dose difference between the simulated plan and the original planned is shown in Table 1. Note: Min. is the minimum dose, in cGy; Max. is the maximum dose, in cGy; Mean. is the average dose, in cGy; PRV cord is the outline of the cord with an external expansion of 5mm. ∆Min.% is the difference percentage between the simulated plan and the original plan, and the calculation method is: (simulated plan-original plan) / original plan * 100%; the calculation methods of ∆Max.% and ∆Mean.% are in accordance with ∆Min.%.
When maintaining team values of theEF, EB and AB, if the NF changes, the relationship between the distance between Segment BC and the∠CABare shown in Table 2. x + y ∠ ∠

Discussion
Radiotherapy was based on high-value medical equipment such as CT machine and linear accelerator [18,19]. Generally, it might take many years for these devices to be updated. If an equipment fails and needs to be repaired after years of service, the maintenance might not be responded in time due to the shortage of accessories, etc. For the CT machine, the horizontal level of the CT couch is reduced due to long-term wear caused by multiple movements. If the tilt of the CTcouchcannot be corrected immediately, it was important how to evaluate the effect on dose and whether to continue radiotherapy [2,3].
In the literature, more reports were the influence of CT couch plate on the dose for the attenuation of the material to the radiation [3,20]. Although there were many maintenance reports for the CT couch plate movement failure, the study and analysis of dose error due to the CT couch tilt was rare.
In this study, we introduce a method to evaluate the effect of the CT couch tilt on the dose distribution. We think that this method could achieve the purpose of evaluating dose error due to CT couch tilt. The results can provide a valuable suggestion for clinical medical strategy: whether the radiotherapy plan should be implemented or not. For all that, it must be made clear that this method was only an auxiliary method. Correcting and repairing the CT couch immediately always be the first step.
It should be pointed out that the method assumes that the patient is treated as a rigid body and the position, volume and shape changes of the organare ignored; at the same time, it assumes that the tilt angle of the couch plate is consistent, which does not change with the moving of the couch.

Conclusion
The tilt of the CT couch leads to the change of the isocenter point position, which will eventually lead to the dose difference between the LA couch and the CT couch. In this study, we recommended a specific method to evaluate this dose difference. This method could quantitatively analyze the dose distribution deviation caused by the tilt of CT couch plate, and the results of dose analysis provide a valuable suggestion for clinical medical strategy.

Conflict of Interest
The authors declare that they have no competing interests.