Method for calculating an optimal arc angle of dynamic arc radiotherapy by volume-based algorithms

Abstract

This invention provides a method applied for the new dynamic arc radiotherapy treatment planning to calculate an optimal arc angle. With this invention, an operator without rich experience is able to reach the expected low dose in lungs easily and quickly. This invention can not only estimate the distribution of low radiation dose in lungs but also reduce the shortcomings like consumption of time and inaccuracy caused by manual trial and error.

Claims

1. A method for rapidly calculating an optimal arc angle of a computer radiation treatment plan for an intrathoracic cancer receiving volumetric modulated arc therapy, comprising: substituting a value set comprising a thoracic transverse diameter, a transverse diameter of a radiotherapy planning target volume and a longitudinal length of the radiotherapy planning target volume in a medical image including a thoracic cavity into a restricted volume radius algorithm to calculate a radius of a first unilateral restricted volume and a radius of a second unilateral restricted volume; substituting the radius of the first unilateral restricted volume, the radius of the second unilateral restricted volume and a desired value of low radiation dose (5 Gy) in lungs V.sub.5 into a restricted volume algorithm to obtain the first unilateral restricted volume; substituting the radius of the first unilateral restricted volume, the radius of the second unilateral restricted volume and the desired value of low radiation dose (5 Gy) in lungs V.sub.5 into the restricted volume algorithm to obtain the second unilateral restricted volume; substituting the first unilateral restricted volume and the second unilateral restricted volume into a volume-based algorithm respectively to obtain a first unilateral restricted angle and a second unilateral restricted angle; and subtracting the first unilateral restricted angle and the second unilateral restricted angle from 360 degrees leaves the optimal arc angle.

2. The method of claim 1, wherein the optimal arc angle is an angle that a gantry of a radiotherapy equipment rotates around an isocenter, a centroid of the radiotherapy planning target, to radiate.

3. The method of claim 1, wherein the medical image including a thoracic cavity is a computed tomography or a magnetic resonance image.

4. The method of claim 1, wherein the radiotherapy planning target is an intrathoracic tumor.

5. The method of claim 4, wherein the intrathoracic tumor is an esophageal cancer, lung cancer, or any combination thereof.

6. The method of claim 1, wherein the thoracic transverse diameter, the radiotherapy planning target volume transverse diameter and the longitudinal length of the radiotherapy planning target are measured by Euclid distance.

7. The method of claim 1, wherein the first unilateral restricted volume is a non-irradiated volume in a unilateral lung, and the second unilateral restricted volume is a non-irradiated volume in the other unilateral lung.

8. The method of claim 1, wherein the restricted volume radius algorithm is $R=\frac{T-E-4}{2};$ wherein R is the radius of the first unilateral restricted volume or the radius of the second unilateral restricted volume, T is the thoracic transverse diameter and E is the transverse diameter of the radiotherapy planning target volume.

9. The method of claim 1, wherein the restricted volume algorithm is V.sub.RES+V.sub.OR=V(1−V.sub.5); wherein V.sub.RES is the first unilateral restricted volume or the second unilateral restricted volume, V.sub.O is a unilateral out-of-field volume or the other unilateral out-of-field volume, V is a unilateral lung volume or the other unilateral lung volume, and V.sub.5 is the desired value of low radiation dose in lungs.

10. The method of claim 1, wherein the desired value of low radiation dose in lungs V.sub.5 is 55% or less.

11. The method of claim 9, wherein the unilateral out-of-field volume is a unilateral lung volume not included in the radiation field in medical image, and the other unilateral out-of-field volume is the other unilateral lung volume not included in radiation field.

12. The method of claim 1, wherein the volume-based algorithm is, $V_{\mathrm{RES}}=\pi R^2\frac{{\theta }_{RES}}{360°}\left(Lt+4\right)$ wherein V.sub.RES is the first unilateral restricted volume or the second unilateral restricted volume, R is the radius of the first unilateral restricted volume or the radius of the second unilateral restricted volume, θ.sub.RES is the first unilateral restricted angle and the second unilateral restricted angle, and Lt is the longitudinal length of the radiotherapy planning target of the radiotherapy planning target volume.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flowchart of one embodiment of the present invention for calculating an optimal arc angle by a volume-based algorithm.

(2) FIG. 2 displays one embodiment of the present invention for segmenting the volume of lungs in computed tomography images. FIG. 2A-2C are the cross-sectional, coronal and sagittal sections of the computed tomography images; FIG. 2D-2F are the cross-sectional, coronal and sagittal sections of segmented computed tomography images according to the gray scale value distribution obtained by the threshold method, and the segmented lung region is marked as pink.

(3) FIG. 3 shows one embodiment of the present invention for labeling the different regions in a thoracic medical image, and the FIG. 3A and 3B are cross-sectional and sagittal section images respectively. In FIG. 3A, the light blue circle is a clinical target, the red circle indicates a radiotherapy planning target (tumor), the blue circle is lungs, the green circle is a heart, the pink circle is a spinal cord and the yellow circle is a region expanded outward from the radiotherapy planning target (tumor) for 2 cm; in FIG. 3B, the red region is the radiotherapy planning target (tumor) volume (PTV). A width of the tumor (F), a longitudinal length of the tumor (Lt) and a width of the thorax (T) are measured by the Euclid distance and substituted into an algorithm for obtaining a radius of a restricted volume (R).

(4) FIG. 4 illustrates the definition of each parameter in the volume-based algorithm of the present invention. In the FIG. 4A, cross-sectional image, the light blue circle is a clinical target volume, the red region is a radiotherapy planning target (tumor) volume (PTV), the blue circle is volume of lungs, the purple region is a restricted volume of the right lung (V.sub.RESR), the yellow region is a restricted volume of the left lung (V.sub.RESL), the green circle is volume of a heart, the pink circle is a spinal cord volume; furthermore, a non-radiated volume (V.sub.NR) is the sum of the restricted volume and the volume of the out-of-field lungs (V.sub.OW); and the radiated volume is the black region of the lungs. In the FIG. 4B, the green region is volume of the right out-of-field lungs (V.sub.OR), the dark blue region is volume of the left out-of-field lungs (V.sub.OL), the red region is the radiotherapy planning target (tumor) volume (PTV), the purple region is the restricted volume of the right lung (V.sub.RESR), the yellow region is a restricted volume of the left lung (V.sub.RESL), and the green circle is heart volume.

(5) FIG. 5 illustrates the definition of the arc angle (θ.sub.A) and the restricted angel (θ.sub.RES). The arc angle is an angle that a gantry of a radiotherapy equipment rotates around an isocenter, the radiotherapy planning target (PTV) centroid, to radiate. And the remaining angle, named the restricted angel, is the angle that the radiation is restricted. The arc angle plus the restricted angle equals complete 360 degrees. Normally, if θ.sub.A is less than 60 degrees, a patient will not be suggested to have an arc radiotherapy treatment. As a result, θ.sub.A is from 60 degrees to 360 degrees, and θ.sub.RES is from 0 degree to 300 degrees. A volume of left lung (V.sub.L), a volume of right lung (V.sub.R), a length of the tumor (Lt) and a radius of a restricted volume (R) are used for calculating a volume of the left out-of-field lungs (V.sub.OL), a volume of the right out-of-field lungs (V.sub.OR), and the restricted volume of the right and left lung (V.sub.RESR and V.sub.RESL). In the final step defining a desired value of low radiation dose in lungs V.sub.5, substituting the above values into the following formula:

(6) $$\begin{gathered} V_{RESR}+V_{OR}=V_R\times \left(1-\mathrm{desired}{V}_5\right), \\ {V}_{RESL}+V_{OL}=V_L\times \left(1-\mathrm{desired}{V}_5\right), \\ {V}_{RESR}=\pi R^2\frac{{\theta }_{RESR}}{360°}\left(\mathrm{Lt}+4\right), \\ {V}_{RESL}=\pi R^2\frac{{\theta }_{RESL}}{360°}\left(\mathrm{Lt}+4\right), \\ {\theta }_A+0_{RESL}+{\theta }_{RESR}=360°, \end{gathered}$$
to calculate the restricted angles θ.sub.RESL and θ.sub.RESR needed for V.sub.RESR and V.sub.RESL calculation. And as a result, 360 degrees minus θ.sub.RESL and θ.sub.RESR leaves an optimal arc angle θ.sub.A.

(7) FIG. 6 is a dose distribution diagram and a dose-volume histogram (DVH) of a volumetric modulated arc therapy (VMAT) adopting the present invention, the volume-based algorithms. FIG. 6A and 6B exhibit the dose distribution and DVH of various organs in thoracic cavity without applying an optimal arc angle and dose limit; on the other hand, FIG. 6C and 6D display the dose distribution and DVH of various organs in thoracic cavity when an optimal arc angle is applied with no dose limit. Furthermore, FIG. 6E and 6F show the dose distribution and DVH of various organs in thoracic cavity when an optimal arc angle is applied with a dose limit. Compared to the DVH without applying optimal arc angle, it is obvious that the desired value of low radiation dose in lungs, desired V.sub.5, in the DVH of adapting an optimal angle (θ.sub.A) drops to 55%, and if it is set with a dose limit, V.sub.5 value of lung can be reduced to 45%.

DETAILED DESCRIPTION OF THE INVENTION

(8) Unless further defined, the technical terms and scientific terms used in the specification are the definitions that are generally known to those of ordinary skill in the art.

(9) The present invention is further illustrated by the following embodiments, which are intended to be illustrative only and no to limit the scope of the invention.

(10) In certain embodiments of the present invention, the radiotherapy target is esophageal cancer, and the location of the tumor can be at upper, middle and lower segments, and left or right side of the esophagus. The esophageal cancer generally locates in the center of the thoracic cavity, and the low dose radiation tends to affect the surrounding organs, such as lungs, heart, liver, thyroid, etc. Therefore, the embodiment demonstrates the diversity of the volume-based algorithms of the present invention.

EXAMPLE 1

A Method for Calculating an Optimal Arc Angle

(11) A method using volume-based algorithms of the present invention for calculating an optimal arc angle is demonstrated in this example, and a schematic flow thereof is shown in FIG. 1. The processing steps and numerical calculation procedures included in the method are detailed as follows.

Segmenting Lung Volumes in Computed Tomography Images

(12) Medical images are composed of a plurality of pixels with different grayscale values. Therefore, a threshold method is used for image segmentation based on the distribution of the image grayscale value, and the different feature thresholds are set by the Otsu method. If the values of pixels match the set thresholds, the pixels are retained; however, if it does not match, the pixels are removed. By means of this, the boundary between the silhouette of the lungs and the air in the lungs is defined, so that the segmenting volume step of the lungs is accomplished (as shown in FIG. 2).

Defining and Calculating Parameters of Tumors and Organs in Computed Tomography Images

(13) In this embodiment, the medical image is a thoracic cavity computed tomography image. In its transverse section (FIG. 3A) and the sagittal section (FIG. 3B), a thoracic transverse diameter (T), a transverse diameter of radiotherapy planning target (E) and a longitudinal length of radiotherapy planning target (Lt) are measured and defined by Euclid distance, and then these parameters are substituted into a restricted volume radius algorithm to calculate a radius of restricted volume (R). In this embodiment, the restricted volume is a lung volume; wherein the restricted volume radius algorithm is

(14) $R=\frac{T-E-4}{2},$
wherein R is the radius of the unilateral restricted volume or the radius of the other unilateral restricted volume, T is the thoracic transverse diameter and E is a transverse diameter of radiotherapy planning target volume.

Defining a Restricted Volume (V.SUB.RES.) and a Non-Radiated Volume (V.SUB.NR.)

(15) In this embodiment, the medical image is a thoracic cavity computed tomography image. According to the field range of a radiation therapy (as shown in FIG. 4), the total lung volume (V) can be divided into the out-of-field lungs volume (V.sub.OW), a restricted volume (V.sub.RES) and a radiated lung volume. The out-of-field lungs volume plus the restricted volume equals a non-radiated volume (V.sub.NR), and the restricted volume can be further divided into a restricted volume of the right lung (V.sub.RESR) and the restricted volume of the left lung (V.sub.RES).

Calculating an Optimal Arc Angle

(16) After segmenting the lung volume, a series of algorithms is used to calculate an optimal arc angle, comprising the following steps: first, setting a desired value of low radiation dose in lungs V.sub.5; substituting the value into an algorithm, V.sub.RES+V.sub.OR=V (1−V.sub.5), to obtain a V.sub.RES value, wherein the values of V.sub.OR and V can be obtained from a current radiotherapy plan software; substituting the V.sub.RES value into another algorithm to calculate a θ.sub.RES value, wherein the θ.sub.RES value is an unilateral restricted angle (for instance, a θ.sub.RESL), and the other unilateral restricted angle (for instance, a θ.sub.RESR) can be obtained from the same step as well; addition of the unilateral restricted angle, the other unilateral restricted angle and an optimal arc angle (θ.sub.A) is 360 degrees as a complete circle.

EXAMPLE 2

The Dose Correlation Between an Optimal Arc Angel and a Lung V.SUB.5 .Value in an Esophageal Cancer Treatment Plan

(17) The optimal arc angle obtained from using the method of the present invention is applied to a computer dynamic arc radiotherapy program and thereby compared with a general radiotherapy program to see the difference of the dose distribution in the critical organs. In the general radiotherapy program without the optimal arc angle, the lung V.sub.5 value is as high as about 90% (as shown as the light blue curve in FIG. 6B); after applying with the optimal arc angle, the lung V.sub.5 value is decreased significantly to 55% (as shown as the light blue curve in FIG. 6D) and the received dose of PTV still remains at an equal level (as shown as the red curve in FIG. 6D); if a minimum dose limit of constraint is given, the lung V.sub.5 value can be further lowered to 45% (as shown as the light blue curve in FIG. 6F).

(18) Therefore, the present invention provides an optimal arc angle for quickly optimizing lung dose, V5, in the new dynamic radiation therapy planning system, thereby the excessive time consumption and artificial error caused by repeated tests can be reduced effectively and the optimal arc angle and lung low dose radiation distribution are calculated accurately. Also, the optimal arc angle obtained by the present invention can be used in a radiation therapy planning system immediately, thereby an inexperienced operator can achieve the desired lung dose distribution quickly without repeated tests.