Quality assurance device for a medical linear accelerator

Details Quality assurance device for a medical linear accelerator Quality assurance device for a medical linear accelerator

2000-11-17

2003-09-02

Lee, John R. (Department: 2881)

Radiant energy

Irradiation of objects or material

Ion or electron beam irradiation

Reexamination Certificate

active

06614036

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a medical linear accelerator (Medical LINAC). More particularly, the present invention relates to a quality assurance device for ensuring the accuracy and reproducibility of the mechanical parameters of a Medical LINAC.
2. Description of the Related Art
The use of medical linear accelerators (Medical LINACs) for external beam irradiation of patients, principally for the treatment of cancerous tumors, is a well developed field. Medical LINACs have been used for this purpose since the 1940's and are common in most major hospitals around the world. The use of the Medical LINAC for stereotactic external beam irradiation, so-called stereotactic radiosurgery or stereotactic radiotherapy, has been known since around 1984.
FIG. 1
illustrates a prior art Medical LINAC in a general configuration for stereotactic or radiation therapy application designated generally by reference numeral
100
. The Medical LINAC
100
is similar to a Medical LINAC of the Varian Clinac Linear Accelerator family manufactured by Varian Medical Systems, Inc., Palo Alto, Calif. The patient's body
102
is on the Medical LINAC platform
104
, and a tumor (not shown) is identified within the patient's body
102
and placed at the intersection of the Medical LINAC axes
106
,
108
; the axis
106
being the vertical axis about which the platform
104
rotates and axis
108
being the horizontal axis about which the gantry
110
of the Medical LINAC
100
rotates.
The Medical LINAC axes
106
,
108
are aligned with the tumor by the use of cross-hairs (not shown) on a face plate
112
of the gantry
110
which are projected onto the patient's body
102
to form a cross-hair image
114
. The platform
104
is capable of rotating on a bearing within the floor
116
, as well as moving up and down on stand
118
and forwards and backwards, in order to position the tumor at the intersection of the cross-hair image
114
. Once the center of the cross-hair image
114
is aligned with the tumor, the tumor is most likely at the intersection of the Medical LINAC axes
106
,
108
.
To ensure that the tumor has been accurately positioned at the intersection of the Medical LINAC axes
106
,
108
prior to commencing treatment, three stationary lasers are used. Two lasers (only one laser
120
of the two lasers is shown by
FIG. 1
) emanate from each side wall
124
and the third laser
122
emanates from a top wall (or ceiling)
126
of the room where the Medical LINAC
100
is located within. If all three lasers are aligned with the tumor, i.e., the lasers intersect the tumor, then the tumor is at the intersection of the Medical LINAC axes
106
,
108
. If the lasers
120
,
122
are not aligned with the tumor, then the tumor is not accurately positioned at the intersection of the Medical LINAC axes
106
,
108
. Hence, the patient's body
102
is moved by moving the platform
104
, in order to place the tumor at the intersection of the Medical LINAC axes
106
,
108
.
Since the patient's body
102
needs to be moved after aligning the cross-hair image
114
with the tumor, then it may be observed that the cross-hairs are not accurately aligned with the intersection of the Medical LINAC axes
106
,
108
. However, the case may be that the cross-hairs are accurately aligned with the intersection of the Medical LINAC axes
106
,
108
, but the lasers
120
,
122
are not accurately aligned with the intersection of the Medical LINAC axes
106
,
108
.
After the laser alignment procedure, the distance of the patient skin surface to the radiation source within the Medical LINAC gantry
110
(i.e., Source-to-Surface Distance (SSD), as known in the art) is ascertained by projecting a scale onto the patient from an oblique angle. The scale is projected using a scale projector mechanically mounted to the Medical LINAC gantry
110
and a scale projector lamp. The distance to the tumor surface is read as the intersection of the scale with the projected cross-hairs. Since the scale projector can loosen and fall out of calibration, it needs to be tested and calibrated periodically. This scale projection of SSD is called an optical distance indicator (ODI).
During treatment of the patient, a beam of radiation emanates from the Medical LINAC
100
towards the tumor. Since the position of the tumor is most likely at the intersection of the two Medical LINAC axes
106
,
108
, the radiation most likely passes through the tumor.
An adjustable collimator system
130
is attached to the Medical LINAC
100
to collimate the radiation beam into a specific rectangular dimension having a length and width and defining a radiation field size. The collimator system
130
includes mechanical jaws which are typically independent and moveable so as to create the specific rectangular dimension to define the radiation field size. The angle of the collimator system, i.e., the collimator angle, as well as the angle of the Medical LINAC gantry
110
, i.e., the gantry angle, are displayed by a digital display
128
of the Medical LINAC
100
. These angles are set prior to treatment by moving the gantry
110
and collimator system
130
to positions where the respective angles indicated by the digital display
128
are within a predetermined specification. Once the respective angles are within the predetermined specification, the Medical LINAC gantry
110
and collimator system
130
are stopped from moving. The dimensions of the radiation field size are also displayed by the digital display
128
.
As described above, there are many mechanical parameters that are checked and/or set prior to commencing radiation treatment of a patient. For example, the alignment of the cross-hairs with the tumor to ensure that the tumor is at the intersection of the Medical LINAC axes
106
,
108
is checked by the use of the two lasers
120
,
122
, whereas the gantry and collimator angles are set by moving the gantry
110
and collimator system
130
and viewing their respective angles on the Medical LINAC display
128
.
Since the mechanical parameters depend on the accurate alignment and placement of various mechanical devices of the Medical LINAC
100
, the mechanical parameters tend to shift from their nominal preset values. For example, the alignment accuracy of the cross-hair image
114
depends on the cross-hairs and light source position being accurately aligned with the Medical LINAC axes
106
,
108
; the radiation field size readout depends on the accurate linkage between a position sensor (e.g., a potentiometer) of the mechanical jaws and the digital display
128
; and the ODI distance measurement readout depends on an accurate positioning of the scale projector lamp on the Medical LINAC gantry
110
.
Accordingly, like any medical instrument, the Medical LINAC
100
needs to be checked to ensure that a radiation oncology facility can accurately and reproducibly deliver the exact prescribed radiation dose by a medical professional to the tumor. For example, a medical professional needs to check whether the collimator and gantry angles are accurately set at 90.0° and 170.0°, respectively, before setting the collimator and gantry angles to the angles earlier determined for the particular patient. Also, quality control tests need to be performed on the Medical LINAC
100
to ensure the accuracy of the mechanical parameters within predetermined quality control specifications. For example, one needs to ensure that the gantry and collimator angles as indicated by the Medical LINAC display
128
are within ±0.1° and that the centering of the cross-hairs and the alignment of the lasers
120
,
122
are within ±0.5 mm.
To achieve these goals, the geometric accuracy of the radiotherapy unit must be tested and verified. It is important to at least test and verify that the following mechanical parameters are within the predetermined specifications: the ODI distance measurement readout, the gantry and collimator angles indicated by the Medical LINAC

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