X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2000-08-07
2001-10-09
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C702S179000
Reexamination Certificate
active
06301329
ABSTRACT:
This invention relates to radiation modelling apparatus and methods for radiation modelling.
It is often desirable to know to a good approximation the characteristics of a radiation beam. Such a beam may be emitted for treatment purposes from a linear accelerator or may represent leakage from a radioactive source. One application where the beam characteristics must be known in considerable detail is in radiotherapy treatment planning.
The purpose of radical radiotherapy treatment is to deliver a lethal dose of radiation to tumour cells while keeping the radiation dose delivered to normal cells as low as possible. To accomplish this several beams of radiation are aimed into a patient from a variety of different directions, as shown schematically in
FIG. 1A
of the accompanying drawings. Energy is deposited into tissue lying along the path of the beam and thus a much greater amount of energy is deposited in the region where all the beams overlap, as shown, for example, in the dose contours of FIG.
1
B.
Radiotherapy treatment planning aims to ensure that this beam overlap region encompasses only tumour cells, and that a high amount of energy is deposited uniformly throughout the tumour region as a radiation dose. Radiotherapy treatment planning has other secondary aims. It is important to ensure that particularly radiosensitive tissues receive a dose lower than a threshold where permanent disability or debilitating side effects may occur, and that the total dose delivered be kept to a minimum. In
FIG. 1A
a schematic cross section of the skull of a radiotherapy patient receiving treatment is shown. The patient is lying on a treatment couch
30
. The patient has a brain tumour
20
represented by an area enclosed by thick line
25
. Geometrical edges of three radiation beams
45
a
,
45
b
and
45
c
used to treat the tumour
20
are shown being emitted from a linear accelerator treatment head
40
which is rotated around the centre of the tumour into three different positions
40
a
,
40
b
and
40
c
. These three positions
40
a
40
b
and
40
c
, together with the width, length and relative weight of the beams are chosen to ensure an even dose distribution across the tumour and avoid radiosensitive areas like the patient's eyes
50
. A high dose area
60
where all the radiation beams
45
a
45
b
45
c
overlap is indicated by hatching and is constructed to coincide with the tumour
20
.
FIG. 1B
shows a dose distribution predicted (by modelling) to be produced by the three radiation beams
45
a
45
b
and
45
c
. Isodose lines
70
indicate the relative dose to different parts of the patient's brain. These isodose lines
70
vary depending on the properties of radiation beams
45
a
,
45
b
and
45
c
. They are used to determine whether the dose received by the tumour
20
, the patient's normal tissue and the patient's eyes
50
is clinically acceptable. For example, according to well established clinical guidelines the dose across the tumour should be uniform to within ±5%. This is clearly not achieved in FIG.
1
B. The plan shown in
FIG. 1B
would be refined, for example by changing the position, weighting and field widths of the radiation beams
45
a
,
45
b
and
45
c
, until this and other clinical objectives were met.
The amount of radiation to be received by the patient is prescribed with consideration of the treatment plan. If the treatment plan indicates that the dose to sensitive areas is too high shielding blocks will be used; if, on the other hand the dose to the skin is too low wax may be placed around the treatment site. It is, therefore, vital that the treatment plan is as accurate as reasonably possible. In order for the planner to try out treatment options the plan must also be complete within a reasonable period of tire, which is usually less than one hour. It is this compromise that the various types of treatment planning system address.
The difficulty of reconciling these clinical objectives is exacerbated as many types of tissue show damage from radiation only at a very late point in the treatment, when the dose that they may have received is already sufficient to cause necrosis and tissue failure. This is particularly true in the head and neck region where a high dose is needed for cure, but critical structures such as the spinal cord lie within or close to the high dose region. The delicate balance of chance of cure against the chance of late tissue necrosis is known as the therapeutic ratio and defined as “the ratio of probability of irradicating tumour within the irradiated volume to the probability of causing severe late damage to normal tissue”.
The relationship between dose and normal tissue injury and tumour cure is shown schematically in
FIG. 2
of the accompanying drawings. This graph shows the probability of an effect on the y axis versus the integral dose on the x axis. It may be seen that due to fractionation (dividing the treatment into many small sessions) and other radiobiological reasons the tumour cells are more susceptible to radiation than normal tissue—the curve
170
for tumour cells lies to the left of the curve
180
for normal tissue. Ideally a dose
200
is given such that there is a large probability
170
for tumour cure but a small probability
180
for normal tissue necrosis—i.e. a vertical line
190
drawn between the two curves is at a maximum. It may be seen that if the dose
200
is moved slightly to the left the probability of tumour cure drops rapidly while if it is moved slightly to the right the probability of normal tissue necrosis rises rapidly. A small error on the treatment plan could therefore greatly reduce the chance of cure or greatly increase the chance of normal tissue necrosis, and the resultant undesirable side effects such as blindness or paralysis.
Due to the increase in normal tissue necrosis the total dose delivered must be kept as low as possible. Physicians therefore generally prescribe the minimum dose to the tumour area that will achieve a high probability of killing the tumour. The dose variation across the tumour area depends on the characteristics of the beams and so an inaccurate model of beam characteristics may result in a false impression of uniformity across tumour volume in the treatment plan while in fact some areas are receiving less dose, enabling the tumour to survive. This may result in the death of the patient. On the other hand there may be areas of the patient which receive higher doses than indicated by the treatment plan, resulting in necrosis of the tissue and, for example, possible failure of the spinal cord. Any improvement, therefore in the modelling of beam characteristics that gives rise to a more accurate treatment plan could have significant clinical implications.
Beam characteristics vary with many different parameters. The dose profile of a radiation beam from a linear accelerator, for example, has more of a rectangular shape than that formed by a Cobalt
60
radiation source. Once the beam is formed it undergoes flattening, collimation and possibly shaping with wedges or multi-leaf collimators. The beam interacts with these devices, generating secondary scattered electrons which will influence the dose distribution in the patient. Such beam characteristics are highly individual, with even two machines of the same design generating beams with different characteristics.
Cancer affects many people, a significant proportion of whom will need radical radiotherapy treatment. This is usually given on an outpatient basis, with the patient returning perhaps every day until treatment is complete. To collect sufficient expertise and cutting edge equipment to optimally treat patients with life-threatening tumours is expensive. Oncology centres are therefore extremely busy and to maintain patient throughput must be able to plan a patient's treatment within a short time such as one hour. The planning process may involve several iterations as the physicist views the dose distribution and changes some beam parameters to improve the dose distributio
Ho Allen C.
Kim Robert H.
Renner , Otto, Boisselle & Sklar, LLP
The University of Southampton
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