Current through Register Vol. 24, December 20, 2024
(1)
The equipment used in therapeutic x-ray installations shall be as follows:
(a) The tube housing shall be of therapeutic
type. Contact therapy machines shall meet the additional requirement that the
leakage radiation at 2 inches from the surface of the housing not exceed 0.1
R/h.
(b) Permanent diaphragms or
cones used for collimating the useful beam shall afford the same degree of
protection as the tube housing. Adjustable or removable beam-defining
diaphragms or cones shall transmit not more than 5% of the useful beam obtained
at the maximum possible kilovoltage and with maximum treatment
filter.
(c) The filter system shall
be so arranged as to minimize the possibility of error in filter selection and
alignment. The filter slot shall be so constructed that the radiation escaping
through it does not produce an exposure exceeding 1 roentgen per hour at 1
meter, or, if the radiation from the slot is accessible to the patient, 30
roentgens per hour at 2 inches from the external opening. Each removable filter
shall be marked with its thickness and material.
(d) A filter indication system shall be used
on all therapy machines using changeable filters. It shall indicate from the
control panel the presence or absence of any filter and it shall be designed to
permit easy recognition of the filter in place.
(e) The x-ray tube shall be so mounted that
it cannot turn or slide with respect to the aperture. A mark on the housing
should show the location of the focal spot.
(f) Means shall be provided to immobilize the
tube housing during stationary portal treatment.
(g) There shall be on the control panel an
easily discernible indicator which will give positive information as to whether
or not the x-ray tube is energized.
(h) A suitable exposure control device shall
be provided to terminate the exposure after a preset exposure or dose limit. It
shall be designed to preserve its accumulated response in the event of
equipment failure during patient treatment. If a timer is used, it should
permit accurate presetting and determination of exposure times as short as 1
second.
(i) Unless it is possible
to bring the x-ray exposure rate to the prescribed value within 5 seconds after
the x-ray "on" switch is energized, the tube housing on machines operating
below 500 kVp shall be fitted with an "ON-OFF" shutter operated from the
control panel and of lead equivalent not less than that of the tube housing.
The "ON-OFF" positions of the shutter shall be indicated at the control
panel.
(j) Mechanical or electrical
stops shall be provided on x-ray machines capable of operating at 150 kVp or
above to insure that the useful beam is oriented only toward primary
barriers.
(k) Interlocks shall be
provided for x-ray therapy equipment capable of operating above 75 kVp so that
when any door to the treatment room is opened, either the machine will be shut
off automatically or the radiation level within the room will be reduced to an
average of not more than 2 mR/hr and a maximum of 10 mR/hr at a distance of 1
meter in any direction from the source. After such a shutoff or reduction in
exposure rate, it shall be possible to restore the machine to full operation
only from the control panel.
(l)
The x-ray control circuit shall be so designed that it is not possible to
energize the x-ray tube without resetting the x-ray "on" switch at the control
panel.
(m) X-ray therapy machines
shall be provided with a locking device to prevent unauthorized use.
(n) When a beam interceptor is provided, it
shall transmit not more than 0.1% of the useful beam under any operating
conditions. It shall also reduce by the same factor the radiation scattered by
the patient through an angle up to 30 E from the central ray.
(o) When the relationship between the beam
interceptor and the useful beam is not permanently fixed, mechanical or
electrical stops shall be provided to insure that the beam is oriented only
toward primary barriers.
(p)
Special consideration shall be given to the safety design of x-ray machines
with electron beam extraction capability such as linear accelerators.
(2) Therapeutic x-ray machines
shall be structurally shielded as follows:
(a) All wall, floor, and ceiling areas that
can be struck by the useful beam, plus a border of 1 foot, shall be provided
with primary protective barriers.
(b) All wall, floor, and ceiling areas that,
because of restrictions in the orientation of the useful beam, cannot be struck
by the useful beam shall be provided with secondary protective
barriers.
(c) With equipment
capable of operation above 75 kVp, the control station shall be outside the
treatment room.
(3) The
operating procedures applicable to therapeutic x-ray installations shall be as
follows:
(a) A radiation protection survey of
all new installations and existing installations not previously surveyed shall
be made by, or under the direction of, a qualified expert. A resurvey shall be
made after every change in equipment, workload, or operating conditions which
might significantly increase the probability of persons receiving more than the
MPD. If, as a result of a radiation survey, supplementary shielding is
installed, another survey shall be made in order to confirm the adequacy of the
shielding after the modification.
(b) The qualified expert shall report his
findings in writing, including recommendations for any required corrective
measures, to the person in charge of the installation, a copy of which shall be
sent to the department. The report shall indicate if a further survey is
necessary after corrections have been made.
(c) The installation shall be operated in
compliance with any limitations indicated by the protection survey.
(d) An x-ray therapy machine shall be
calibrated by a qualified expert before use for the treatment of
patients.
(e) X-ray therapy
equipment capable of operating above 75 kVp shall not be operated routinely
until the radiation safety of the installation has been established.
(f) Both the control panel and the patient
shall be kept under observation during exposure.
(g) When a patient must be held in position
for radiation therapy, mechanical supporting or restraining devices shall be
used.
(h) No person other than the
patient shall be in the treatment room where the tube is operated at potentials
exceeding 75 kVp.
(i) If the x-ray
tube of a contact therapy machine is hand-held during irradiation, the operator
shall wear protective gloves and apron. A cap of at least 0.5 mm lead
equivalent shall cover the aperture window of the tube housing of such
apparatus when the apparatus is not being used.
(j) Lead, lead rubber, lead foil, etc., used
for limiting the field, shall not transmit more than 5% of the useful beam (see
Table I).
(k) Notwithstanding ARM
37.14.721, personnel monitoring
shall be required for all persons who enter a controlled area.
TABLE I
Thickness of lead required to reduce useful beam to
5%*
Beam Quality
|
Required Lead Thickness millimeters |
Potential |
Half Value Layer millimeters |
60 kVp
100 kVp
100 kVp
100 kVp
140 kVp
200 kVp
250 kVp
|
1.2 Al
1.0 Al
2.0 Al
3.0 Al
0.5 Cu
1.0 Cu
3.0 Cu
|
0.10
0.16
0.25
0.35
0.7
1.0
1.7
|
400 kVp
1000 kVp
|
4.0 Cu
3.2 Pb
|
2.3
20.5
|
2000 kVp
2000 kVcp
3000 kVcp
6000 kV
8000 kV
Cobalt 60
|
6.0 Pb
14.5 Pb
16.2 Pb
17.0 Pb
15.5 Pb
10.4 Pb
|
43.0
63.0
70.0
74.0
67.0
47.0
|
*Approximate values for broad beams. The third column refers
to lead or to the required equivalent lead thickness of lead-containing
materials (e.g., lead, rubber, lead glass, etc.).
Sec.
75-3-201 and
75-3-202,
MCA; IMP, Sec.
75-3-201,
75-3-202 and
75-3-204,
MCA;