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R ADIDTHERAPY

aO~~~~~~~ ELSEVIER Radiotherapy and Oncology 36 (1995) 235-245

Quality assurance in the CHART clinical trial

E.G.A. Aird*, C. Williams, G.T.M. Mott, S. Dische, M.I. Saunders

Marie Curie Research Wing for Oncology, Mount Vernon Hospital, Rickmansworth Road, Northwood, Middlesex HA6 2RN, UK

Received 4 February 1995; revision received 8 June 1995; accepted 15 June 1995

Abstract

As part of the clinical trial of CHART (continuous hyperfractionated accelerated radiotherapy) a quality assurance programme was included. The technical part of this - which is reported in this paper - is a series of tests designed to check all aspects of treatment planning and delivery. The results of visits to the 13 participating centres - and repeat visits to some of these centres - are discussed. The main areas tested were as follows. The linear accelerator: mechanical, and optical, scales and indicators; radiation field size; flatness and symmetry. Dosimetry: output; wedge factor; beam energy; phantom measurements against a plan calculated by the centre. Simulator: mechanical, optical scales and indicators. The results show these ccntres work within the tolerances chosen for most parameters. Flatness, wedge factor and energy were areas of weakness in some ccntres. This must be Partly the cause of the spread of phantom measurements which, after removal of variations in output, still range from -7 to +6% between calculated and measured values.

Keywords: CHART, Quality assurance; Tolerance; Phantom measures

1. Introduction

Continuous hyperfractionated accelerated radio- Therapy (CHART) in the form of a total of 36 fractions, three fractions per day over a period of 12 days without break, was introduced at Mount Vernon Hospital in 1985 for the treatment of tumours of the head and neck and bronchus. Following a pilot study of patients, a ran- dom&d clinical trial was started under the auspices of the CHART Steering Committee. The aim of the study is to compare routine radical fractionated radiotherapy, viz: 66 Gy for head and neck tumours and 60 Gy for car- cinoma of the bronchus given daily for 5 days per week over 6 weeks, with CHART giving a dose of 54 Gy (prescribed at the centre point) in 36 fractions over 12 days. Thirteen centres enter patients into the trial, including three from outside the UK. Clinical data are col- lected and managed at the CHART trial office in Cambridge. The CHART Steering Committee, under the chairmanship of Ann Barrett, agreed at the outset that a quality assurance (QA) progrannne was essential

l Corresponding author.

to the good conduct of the trial as already found in US and Europe [ 1,2]. A quality assurance team was formed from staff at Mount Vernon Hospital involved with the delivery of radiotherapy: a radiotherapist, a radiographer, an engineer and a physicist. It was envisaged that this group would have three functions:

l to give information concerning the implementation of the protocols; l as an auditing group of patient data and planning details; l to perform tests on equipment used to plan and treat patients.

It is this latter function which is the main subject of this paper.

The main aim of this QA programme is to ensure that all patients in all centres contributing to the CHART trial receive treatment which meets the standard approv- ed by the CHART Steering Committee, viz:

(i) machine parameters within typical standards accepted internationally;

0167-8140/95/$09.50 0 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0167-8140(95)01598-B

236 E.G.A. Aird et al. /Radiotherapy and Oncology 36 (1995) 235-245

(ii) that doses delivered to all points within the target should be within 3.5% of calculated value (with the ad- ditional criterion that there should be no more than a 5% gradient across these small target volumes).

If a clinical trial has a QA programme of this nature as part of it and yields a positive result, the approach can be more readily reproduced elsewhere. Radiotherapy trials of this size in Europe and North America have long had QA systems to add credence to their results, but until now funding has not been available for such studies in the UK.

2. Materials and methods

To achieve this aim the QA group devised a series of mechanical and radiation tests for all treatment machines and simulators to be audited. In addition, anatomical phantoms were made from tissue equivalent plastics to simulate the parts of the body treated in the CHART trial. These were designed to accept small ionisation chambers to allow measurement of dose at the tumour site to be made at each centre. A cross- sectional outline of the phantoms, including inhomogeneities and target volume, was sent to the centre at least 2 weeks prior to the visit. The centre was asked to plan the treatment as it would for a patient, calculate the monitor units and, on the day of the visit, set up the phantom for treatment. The visiting team then measured the dose within the phantom at each measuring point for each beam.

The rationale adopted by the group was to devise a set of tests which could be completed in a one day visit, including the phantom measurements. The tests should ef- fectively check all parameters in routine use for acceptance of a machine. Various documents - including BSI (3) - were used for guidance in identifying what to test and the tolerance for each parameter (see Appendix 1). Following several months of development and field trials the final protocol now contains 43 items for testing. Space does not permit publication of the full protocol here, but some of the tests are discussed in detail or in outline.

2.1. The equipment

The criteria for the equipment needed to carry out the tests were governed by the accuracy of testing required and the fact that this equipment would have to be transported from centre to centre by car. These criteria included:

l a scanning system to fit any linear accelerator head; l a digital measuring system for precise positioning and measurements; l an automatic and remote drive by computer;

l the ability to record on disc and hard COPY;

l a robust assembly to withstand the stress of transpor- tation; l a mounting jig that does not distort when the gantry

is rotated.

The Bioengineering Department at Mount Vernon Hos- pital designed, constructed and assembled various modules using apparatus from several commercial manufacturers. A suitable jig was supplied by Graytech (Graytech Ltd., Unit 4, Coombelands Business Park, Addlestone, Surrey, KTl5 IJR, UK).

The scanning movement, electrometers, ionisation chambers, computer interface and software were pur- chased from Physikalisch-Technische Werkstaetten (Dr Pychlau GMBH, Loerracher Strasse 7, D-791 15 Freiburg, Germany).

The scanning movement allows positioning to an accuracy of 0.1 mm with a hand pendant containing both the movement keys and a readout of ion chamber position (a key feature of the optical vs. X-ray test (see Section 2.2). The ion chamber was used in Water Equiv- alent Plastic (WEP; as defined by D. White in Ref. [4]) phantom material. It was decided that all flatness and uniformity scans should be performed to produce results which mimicked those of a water tank in terms of the amount of scatter. This can be achieved with a WEP scanning block with sufficient volume to produce enough scatter without overburdening the scanner motor with the weight of material. It was found that a scanning block of 12.5 x 12.5 cm and 14 cm in depth produced profile scans which were identical with those in water over the flattened width (80% of field size) at 10 cm depth over the energy range 6-15 MV (see Sec- tion 2.2). This block is constructed from sheets of various thicknesses which allowed field size profiles to be measured at the appropriate depth.

The complexity of this apparatus requires a number of cables to be laid and connections to be made. This task has been simplified by assembling the dosemeters, controls and interface into one crate and placing this in- side the treatment room, care being taken to ensure it is outside the path of the primary beam at all gantry angles. The drive cables to the scanner and the ion chamber cables. are then connected to this crate and therefore the only cable to come outside the room is the RS232 link to the computer. In many treatment rooms this cable can be drawn through the ‘rat-hole’ used for dosimetry cables.

2.2. An outline of the procedure (linear accelerators)

These tests included: l accuracy of rotation of collimators (using light beam) - an essential test which if not correct (max. deviation

E.G.A. Aird et al. /Radiotherapy and Oncology 36 (1995) 235-245 237

2 mm) brought the programme to a halt until corrected, since it impacts on many of the other tests; l accuracy of front pointer and back pointer (mechanical and optical); l a check of gantry angles 0” and 90” using plumb bob and water level (6 m of water-filled tubing to check lev- els across the width of a treatment room); l a check of collimator angle using a spirit level; l laser alignment using water level.

Radiation flatness and symmetry The scanning block is mounted so that the surface of

the block is at 90 cm from the focus and the ion chamber is placed at 100 cm. A field size of 30 x 30 cm is set and the block aligned with the centre of the field. Scans of the two major axes, denoted here as A-B and G-T (A-B is at right angles to the main axis of the accelerator, G-T is in line with that axis), are made for gantry angles of O”, 90’ and 270”.

The measured dose is recorded, relative to a reference dose from an ion chamber mounted on the jig close to the exit portal of the treatment unit, at 39 points across the field. The standard parameters for flatness and symmetry are automatically calculated from each scan. The condition for flatness is fullfilled if the variation in dose within the flattened region (80% of field size) does not exceed f 3% of the mean of the maximum and minimum observed. The condition for symmetry is fulllilled if the ratio between the values of dose measured for each pair of symmetrical points shall fall between 0.97 and 1.03 within the flattened region [5].

Radiation field size In order to measure field size according to the stan-

dard definition, the chamber position was not altered, but the thickness of WEP above the chamber was reduced to provide build-up for the particular energy of the photon beam: 1.5 cm for 6 MV, 2 cm for 8 MV. It was decided to concentrate these tests on the pair of collimators which are most used clinically to define the width of the field at gantry 90” or 270”, since these will be most influenced by gravity when the gantry is rotated. Thus the length of the field was only measured at gantry angle of O”, but field width was measured for O”, 90” and 270”. Because of time restriction, only a 10 x lo-cm field was measured at first, but as the team became more efficient at the series of tests it was decided to extend the measurements to include fields of 5 x 5 cm and 20 x 20 cm to check for linearity and off-set problems with field size settings. Penumbra (as measured with a 5-mm diameter ion chamber) could also be derived from these measurements very reproducibly as the distance between the 80 and 20% dose values. The values measured by this method are ap- proximately 2 mm greater than the penumbra measured with film.

X-ray field vs. optical field The Mount Vernon Hospital team was not satisfied

with the conventional film methods for checking the agreement of the X-ray field with the optical field because the reproducibility is poor, it is very difficult to carry out the test at lateral gantry angles, and film pro- cessing is time consuming, requiring a dark room. The team found that the combined use of the motor- controlled scanner, with hand pendant read-out and control, together with a small ion chamber formed a very suitable and versatile method for checking field agreement. The ion chamber is placed in a special white- faced plate in which the chamber’s effective measuring point is about 2 mm below the surface of the plate and marked by a fine, black cross. The surface is set to be at 100 cm from the focal spot. A build-up block can be added to provide full electronic equilibrium. The ion chamber and plate can be motor driven to the edge of the beam using the hand pendant control and the measure of dose in this position compared with the dose received at the centre of the beam (taken to be 100% in each situation); the distance moved and the percentage dose received are then recorded for each edge of the beam. The correction for the distance of the effective measurement point is negligible. There is still some sub- jectivity in this method. The reproducibility on a single occasion for one observer is about ho.2 mm. To reproduce the clinical accuracy at an individual centre a radiographer from the centre is asked to perform this part of the test with the visiting team. The results can be transformed from percentage dose into distance errors by making use of the previously measured penumbra.

Energy check (in terms ofpercentage depth dose at 10 cm depth)

With the shortness of time available it was thought impractical to perform more than a basic tissue- phantom-ratio (TPR) type test. To save time this was combined with the calibration of the dosemeter (against the treatment machine’s dosemeter) to be used in the phantom measurements. The chamber is placed at 5 cm depth in WEP with the surface of the block at 100 cm FSD. The dose is recorded for a fixed number of monitor units. This is repeated with the addition of a further 10 cm thickness of WEP. The ratio of these readings is used to find the percentage depth dose on a 10 x lo-cm field at 10 cm depth using a calibration curve obtained from Mount Vernon Hospital data. This method proved to be applicable at other centres.

Wedge attenuation This is the ratio of dose on the central axis of the

beam at depth of d,, with and without the wedge. After long discussion it was concluded that it is important to study the deviation of wedge attenuation with gantry angle. This view was reinforced following visits

238 E.G.A. Aird et al. /Radiotherapy and Oncology 36 (199.5) 235-245

Reference mark

I I 1 1 1 I

cm

Fig. I. A line drawing of the head phantom showing the target volume and measuring point.

to several radiotherapy centres where problems were en- countered due to change in wedge position with gantry angle. The wedge attenuation was compared with the gantry in its lateral position and for both collimator angles (wedge thick end up and down) with the mean value of wedge attenuation at gantry 0”, i.e., the mean

of the values at the two measured collimator angles. The wedge attenuation at gantry 0’ (the wedge factor) was also compared with the value measured by the centre, on the understanding that centres measure this factor at different depths and therefore there may not be precise agreement.

output

For completeness, the absolute absorbed dose was measured using a UK standard protocol [6] with a Farmer dosemeter calibrated against a secondary standard dosemeter at Mount Vernon Hospital replacing water with WEP (which had been tested to show that there was less than 0.5% difference). Pressure and tem- perature were provided by the staff at each Centre.

2.3. Simulator Tests The treatment simulators were tested by a series of

checks of indicators, isocentric rotations, optical and X- ray field size and couch scales. Detailed testing of image intensifiers was not performed as it was considered to be too time consuming.

2.4. Phantoms

Construction Phantoms of the bronchus and the head and neck

regions were constructed from tissue equivalent materials. Lung, bone and water equivalent plastics, based on those described in ICRU 1989 [4] were manufactured by St Bartholomew’s Hospital. The cross-section chosen is shown in Figs. 1 and 2. The target volumes were chosen as reduced volumes (phase II) according to the CHART

Reference mark

Fig. 2. A line drawing of the bronchus phantom showing the target volume and measuring points.


protocol. Cylindrical holes were drilled at five different points within the target volume to accept the PTW small volume chamber type M233642. Holes were also drilled within the spinal cord (point 6) and in a contralateral position (point 7).

Planning The precise outline with anatomical structures and

target volume were entered into the treatment planning computer (RTPLAN) at Mount Vernon Hospital, so that exact copies of the cross-section could be mailed to participating centres. The planning department at each hospital was asked to plan this as the small volume or phase II volume of a CHART patient, with the same conditions as given in the protocol; i.e., with no more than a 5% gradient across the target volume and a dose limit to the cord of 42% of the prescribed centre point dose, assuming the large or phase I volume gave the full prescribed tumour dose to the spinal cord. Density values for bone and lung were given to centres, but they were asked to correct for inhomogeneities in their usual way.

Measurements These were made at every measuring point for every

field for each phantom to enable an analysis of the accuracy of dose delivery to be undertaken. However, there is not space in this paper to present a full analysis of all these measurements.

3. Results

Since two of the 10 UK departments had two separate centres, there were 12 centres visited in the UK, two in Sweden and one in Germany. A second visit has been made to UK centres in order to confirm the findings of the first visit and extend the bronchus phantom measurements to off axis. A number of other centres outside the CHART trial have now been visited both in the UK and in Sweden: the results from these centres will be the subject of a further report. All the centres visited had several accelerators with at least one simulator and one planning system. In general the com- plete QA programme was carried out only on the accelerator, simulator and planning system used for the CHART patients. A list of the equipment on which measurements were made is given in Table 1, together with a summary of the approximate numbers of tests and radiation measurements made.

3.1. Accelerators

Scales and indicators The criteria set were:

l for angles: 0.5” for mechanical indication of angle and

Table I

(a) Accelerators Number Manufacturer Type Energy Wedge

Philips 6 SL75/5 4-6 MV 2 Manual

4 Universal 1 SL75/14 8 MV Universal 1 SL75/20 8 MV Universal

Varian 3 6/100 6MV Manual 1 21/100 6MV Manual

Rad. dynamics 1 Dynaray 4 4MV Manual I BBC 6MV Manual

CGR 1 Neptune 6MV Manual

00 pu SYSW 7 RTPLAN or TARGET 7 Others: Theraplan, Picker, Westminster, Bristol, Nodecrest, Sidos U, Helax

(c) Nader of measurements Scales indicator Flatness and symmetry Field size X-ray vs. optics Energy Absorbed dose Wedge factor Phantom: all dose measurements

270 162 108 108 54 27

135 2079

1” for digital (since often these indicators do not have a decimal point); l for distance tolerances: 2 mm.

All centres complied with these requirements, except one where there was a deviation of 3 mm in the front pointer distance indicator and the gantry isocentre.

Radiation field uniformity The mean values for flatness and symmetry are shown

in Table 2. With the criteria for minor deviation set at greater than 1.06 for flatness and 1.03 for symmetry, it

Table 2 Uniformity

Mean values * 1 SD

Flaw AB (transverse scan) 1.047 f 0.011 GT (longitudinal scan) 1.050 f 0.015

splmeby: AB 1.014 f 1.008 GT 1.015 l 0.010

No significant change in any of parameters with gantry angle.


Table 3

Mean * SD (mm) Range (mm)

(a) X-ray field size Lower Jaws Gantry

O0 90”

270"

Upper Jaws

100.0 l 1.4 97.9-101.7 99.9 l 1.2 99.9 f 1.3

99.9 l 1.1 98.8-101.2

(b) Optical field size

Lower jaws 99.5 f 1.1 98.3-102.1

Upper jaws 99.6 zt 1.6 97.8-103.0

Difference between X-ray end optical field edges (may be either positive or negative)

Lower jaws 0.8 f 0.6 O-2.6

Upper jaws 0.6 zt 0.6 O-2.3

(c) Width of beam profile from 80%20% (penumbra width)

Lower jaws 6.0 + 0.5 5.2-7.1

Upper jaws 7.0 l 1.4 5.6-10.1

was found that 17% of all measurements (26/156) exceeded the flatness criterion, but no centre exceeded the criterion for symmetry. No centre showed a major deviation (classed as 1.12 for flatness and 1.06 for symmetry) for flatness or symmetry. The flatness did not change significantly at any centre with gantry angle.

Field size (X-ray and optical 10 x lo-cm field results

only) The mean value for the X-ray field size for all centres

is shown in Table 3a. The jaws generally used to treat the field width were studied in more detail and the results are shown for the mean value with change in gantry angle. There was no significant change with gantry angle. However, there was a suggestion that the field becomes slightly asymmetric as the jaws drop under gravity, though the light field remains centred.

The mean value for optical field for all centres is also shown in Table 3b. There is a larger range of values for the optical field, with centres exceeding 2 mm deviation. The difference between optical and radiation field sizes is also tabulated (there were three centres exceeding 2 mm deviation).

Penumbra The mean values for the penumbras of the lower and

upper jaws are shown separately (Table 3~).

Energy The mean difference between the measured value of

percentage depth dose (%DD) for the 10 x lo-cm field at 10 cm depth and that from the centres’ charts is 0.6 f 0.6%. Four centres at the first visit showed a deviation greater than 1% (two of these deviations were

14 I

n 1st VISIT

l 2nd VISIT

I

4

2

-4 -3 -2 -1 0 1 2 3

PERCENTAGE DIFFERENCE

Fig. 3. The range of percent deviation of wedge attenuation for all measurements made. The percent difference is that between the wedge attenuation measured at the lateral gantry angles at that measured at gantry 0’. First and second visit values are shown.

on 4-MV linear accelerators which were 5-MV machines deliberately set to run at the lower value of 4 MV). On the second visit only two centres showed a deviation greater than 1%.

Wedge attenuation

The primary aim of this test was to look for any change in wedge attenuation with change in gantry angle. The histogram (Fig. 3) shows all the percentage deviations of wedge attenuation at gantry angles of 90” and 270”. Several of the large values (greater than 3%) were corrected by the time of the second visit. These were due mainly to loose mounting of heavy wedges within their slides.

output The range of deviations from the indicated output was

-2.5 to +0.3% with a mean and standard deviation of -1.0 i 1.2%. This systematic difference is possibly at- tributable to the fact that about 60% of UK centres have changed their secondary standard calibration to absorbed dose, whereas Mount Vernon Hospital has stayed with air kerma.

3.2. Simulators

In general, the simulators performed well. The only significant difference between measured and indicators were the following:

Gantry mechanical angle indicator. > 0.5” difference, five centres; 0.6-l” difference, six centres.

Collimator mechanical angle indicator. 0.6- 1” difference, six centres; 1.1-2” difference, one centre.

Field size. small field, >2 mm difference, two centres. Large field, > 2 mm difference three centres; ~3 mm difference, two centres


With hindsight, the checking of the imaging performance of the image intensifier would have made a valuable addition to the programme since many staff were unhappy about the quality of the image in their centre.

3.3. Phan tom.9

Treatment planning Since each centre had the freedom to plan its own in-

dividual treatment to lit the target volume, it was considered useful to analyse the resulting plans. This has been done for size and direction of the chosen beams.

For the bronchus plan, all centres planned isocentrically and nine out of 15 centres used a three-field technique with parallel opposed wedged fields topped up with a third field (plain or wedged) entering through the lung (see Fig. 4); only one centre used a wedged pair of fields. Five of the centres used parallel opposed (or with a slight twist to compensate for divergence along the back field edge) fields. For the head and neck plan all centres except one planned the treatment isocentrically and the majority used an angled pair of fields. However, the alternatives of either a parallel opposed pair of fields or three-field plan were also used (Fig. 5).

Phantom measurements All the UK centres have been visited twice, but at this

time the Swedish and German centres have had only one visit. Most of the results discussed here are therefore based on measurements made at the first visit. It is specifically stated when the second visit results are involved. The aims of these phantom measurements were:

l to determine the dose given to the intersection point and its variation from 150 cGy; l to determine the dose at other points and compare it with the intended dose; l to compare the spinal cord dose with the stated dose;

- Typical beam layouts

Fig. 4. Schematic diagrams showing the two types of treatment plan used for the bronchus phantom.

Typical beam layouts

Fig. 5. Schematic diagrams showing the three types of treatment plan used for the head phantom.

l to observe the range of spinal cord doses; l to observe the range of dose within the target volume.

Some of the details of these results will form a separate paper. The essential results are reported here.

Table 4 shows the mean dose values for all the measuring points for the two phantoms together with the standard deviations to indicate the spread of results. The means within the target volume are all close to 150 cGy except for point 1 in the bronchus. This slightly lower value appears to be a property of the phantom. The range of values of dose in both phantoms are shown in Fig. 6a for point 1 and the average of points 2-5. The differences between calculated and measured doses are shown in Fig. 6b for all measuring points in both phantoms (in one centre the head and neck phantom has not been measured).

The distribution of spinal cord doses is shown in Fig. 7 for both bronchus and head and neck phantom. The mean values of both calculated and measured dose are shown in Table 5. The difference between measured and calculated values appears to be significantly different, with the measured value greater than the calculated value. This could be due partly to the use of a finite volume ion chamber measuring in the penumbra region of some of the applied fields.

Table 4 Point dose values, measured (mean dose (cGy))

Bronchus phantom Head and neck

Point I 148.3 * 3.6 150.1 f 3.2 Points 2-5 150.5 I 3.2 150.6 f 2.8 Point 6 (spinal cord) 37.7 f 11.3 40.3 f 20.1 Point 7 (contralateral) 8.9 A 3.1 21.9 f 16.9


a BRONCHUS PHANTOM DOSE DISTRIBUTION

6

5

t 4

53 r lt 2

1

0

144 147 150 153 157

Dose (cGy1

60

50

p 40

ti 30 :

20

10

0

6

5 aPoint 1

n PointZ-5

k 4

5 g3

i 2

1

0 1

144 147 150 153 157

Dose IcGvl

b

14

HEAD PHANTOM - DOSE DIFFERENCE

r- 12

10

z 58

$8

B 4 :I 2

0 I

-7.5 -8 -4.5 -3 -1.5 0 1.5 3 4.5

DOSE DIFFERENCE cGy

BRONCHUS PHANTOM - DOSE DIFFERENCE

20 r 18

18

14

c 12 i?i 2 10 2 8 E 6

4

2

0 9 9 Y ‘? Y 0 ” m ” (D ? ? 7 *

DOSE DIFFERENCE cGy

HEAD PHANTOM DOSE DISTRIBUTION

Fig. 6. (a) The distribution of dose values in both phantoms; point 1 values and the average values of points 2-5. (b) The difference between measured dose values and calculated dose values for both head phantom and bronchus phantom (all points).

SPINAL CORD DOSE

q Bronchus

Fig. 7. Dose values measured in the spinal cord for both bronchus and head phantom at each centre (arbitrarily placed on x-axis; incomplete data for centres 8 and 11.

Comparison of first and second visits A second visit was made to UK centres. A similar set

of measurements was made. The mean values of dose to point 1 (on axis) are shown in Table 6 for bronchus phantom and show a small improvement in both mean value and spread of values. An average value of the dose difference between all measured and calculated values was made by determining the RMS error at each visit. Again, an improvement is seen.

In the bronchus phantom, measurements were made at 5 cm off-axis as well as on-axis, which, with the length of field specified to be 14 cm, was 2 cm from the edge of the beam. This was achieved by moving the phantom relative to the beam, so that measurements were made in the identical section to avoid any discrepancies due to outline difference or anatomy.

The mean dose values for on and off axis are shown in Table 6. For each measuring point in the volume the percentage difference between on and off axis plans is shown in Fig. 8. It will be seen that this difference varies from 0 to -5%, which mainly reflects the flatness profile of the particular linear accelerator used.

Table 5 Spinal cord dose, calculated vs. measured (mean doses (cGy))

Bronchus

Measured Calculated

Head and neck

Measured Calculated

37.6 f 11.3 34.3 f 12.4 40.3 ziz 20.7 37.3 f 20.4 Difference: 3.3 * 1.3 3.0 * 1.4 (Paired p < 0.025 p c 0.025) mean + SE)


Table 6 Bronchus phantom dose measurement

1st visit 2nd visit

On-axis Off-axis

Mean dose WY)

148.3 f 3.6 149.6 + 2.2 147.1 f 3.4

RMS difference 2.4 m

1.3

4. Discdon

The performance of all the linear accelerators measured was generally good. The main weaknesses were with externally mounted wedges and with lack of flatness (as demonstrated by the off-axis measurements) due to the fact that the curvature of the profile towards the edge of the beam is not accurately reflected in the computed dose data.

Dosimetrically most centres met the criteria for the CHART trial. Deviations close to the limit for point 1 and for the spinal cord measurements were corrected by the second visit. The planning of the treatments was generally good and the doses within the volume agreed reasonably well with calculated values. The distribution of differences is similar to that found in other QA pro- grammes reported in the literature [2,7]. Two centres showed marginal deviations resulting from wedge pro-

blems. Spinal cord doses were very variable depending on the field size and treatment technique selected, but there were three centres where the measured dose value at the spinal cord reached the specified limit. Generally the measured cord dose values were greater than the calculated values, probably due to the difficulties of calculating and measuring precisely the dose in the penumbra region.

There appear to be no systematic differences between calculated and measured percentage doses (relative to point 1). This analysis has been extended by considering each field separately (not part of this paper) and even the field through the lung does not demonstrate any problems. This may be good luck in the choice of measuring points, but this data demonstrates that the relatively simple algorithms used in most of these planning systems are adequate for lung correction within this type of treatment plan when working with the overall accuracies demonstrated here.

The slight improvements between first and second visits are not statistically significant when all the data is taken as a whole, but in individual centres minor deviation in data and machine problems, such as wedge mountings, were seen to have been corrected by the second visit.

The overall encouraging results of this QA programme may be attributed to the fact that CHART trial participants are mainly large centres which are often involved in multicentre trials.

Our system is simple to use and the results are very

OFF-AXIS DOSE DIFFERENCE

1 2 3 4 5 6 7 8 9 10 11 12 13 14

0

-1

8 g -2 G r 1 .- -3

HPoint P g HPoint 2

:! -4

ii z

-5

h

q Point 3

q Point 4

n Point 5

Centre

Fig. 8. The percent difference between on- and off-axis measured dose values for each point in the volume at every centre (arbitrarily placed on x-axis).

244 E.G. A. Aird et al. /Radiotherapy and Oncology 36 (1995) 235-245

reproducible (Appendix 2). The implementation of QA in radiotherapy has become vitally important in recent years. Often, as has been demonstrated here, a clinical trial has led the way to the general benefit of all patients receiving radiotherapy. By pursuing QA in the first year of the clinical trial, the standard of treatment was set and any later uncertainties when analysing the results were avoided. Wariness at each centre visited was replaced by active co-operation and satisfaction with the high standards that could be achieved and maintained. In addition, these visits gave an opportunity for mutual exchange of ideas.

Acknowledgements

Thanks to all our colleagues at the centres involved in the CHART trial: Beatson Oncology Centre, Glasgow, UK, Bristol Oncology Centre, UK; Clatterbridge Hospital, Merseyside, UK; Cookeridge Hospital, Leeds, UK; Mount Vernon Hospital, Northwood, UK; Nottingham General Hospital, Nottingham, UK, Royal Marsden Hospital, London, UK, St Marys Hospital, Portsmouth, UK; Velindre Hospital, Cardiff, UK; Weston Park Hospital, Sheffield, UK, County Hospital Ryhov, Jiinkoping, Sweden; University Hospital, Ume& Sweden; Radiologische Klinik, Dresden, Germany; and to the Medical Research Council together with the Department of Health (England & Wales) for funding this project.

A-1

Test DtSTiptiOIl Acceptable pA;on deviation (up to and (greater than)*

equal to) Lhwar aeeekrator LA1 optical crosswire LA2 Front pointer

Back pointer LA3 Distance indicator LA4 90° Alignment LA5 270” Alignment LA6 Gantry O0

Mech. Digital

LA7 Couch Ram Vertical LA8 Gantry 90°

Mech. Digital

LA9 Gantry 270“

LA10 Collimator 0’ Mech. Digital

LAll-16

LAl7-21

LA22-25 LA26

LA27

Flatness and Symmetry X-ray field size and Penumbra Optics vs. X-ray Energy output Wedge factor

2mm 2mm 3mm 2nlm 2mm 2mm

0.50 1” 2mm

0.5”

iPs” 1”

0.5” 1”

6% 3% lmnl 8mm 2mm 1% 2% 2%

3mm 3mm 5mm 3mm 3mm 3mm

10 20 3mm

10 2”

12% 6% 2mm 12 mm 3mm 2% 3% 3%

LA28

LA29

LA30

shlulIator s31 S32

s33

s34

s35

S36 s37 S38 s39 MO

Wedge attenuation at Gantry 90° + 270° Bronchus phantom dose measurement Head phantom dose measurement

Laser alignment Gantry angle

Mech. Digital

Collimator angle Mech. Digital

X-ray field size (50 and 100 mm) 200mm Optical field size (50 and 100 mm) 200mm Centre of X-ray (collimator rotation) Centre of Optical (collimator rotation) Range Finder Isccentre PSS rotation PSS lateral PSS longitudinal

3%

3.5%

3.5%

1”

0.5O 1”

0.5O I0

lmm 2mm

lmm 2mm

lmm

lmm lnim 2mm 1” 2mm 2mm

5%

1%

7%

2”

1” 2”

I” 2”

2mm 3mm

2mm 3mm

2mm

2mm 2mm 3mm 2” 3mm 3mm

pThe values for minor deviation are between these two limits).

Appendix 2: Reprodwibility of key parameters

Flatness/Symmetry *0.5% Radiation Field Size to.1 mm Penumbra Lto.1 mm Optical tield size l 0.2 mm Energy l O.l% Wedge Attention l 0.2%

111

121

[31

141

I51

161

(71

Hanson, W.F., Shalek, R.J., Kennedy, P. Dosimetry quality assurance in the US from the experience of the Radiological Physics Center. SSDL Newsletter No. 30, IAEA Vienna, April 1991. Johansson, K.A., Horiot, J.C., Van Dam, J., Depinoy, D., Sentmac, I. and Sembo, G. Quality assurance control in the EORTC cooperative group of radiotherapy. 2. Dosimetric intercomparison. Radiother. Oncol. 7: 169-279, 1986. BS 5724 Section 3.1 Supplement 1. Methods of declaring functional performance characteristics of medical electron accelerators in the range 1 MeV to 50 MeV. Supplement I. Guide to functional performance values. British Standards Institute, London, 1990. ICRU Report No. 44. Tissue Substitutes in Radiation Dosimetry and Measurement 1989. International Electrotechnical Commission Medical electrical equipment - medical electron accelerators in the range 1 MeV to 50 MeV - guidelines for functional performance characteristics (IEC 977). IEC, Geneva, 1989. Standard Protocol. Revised Code of Practice for the Dosimetry of 2 to 35 MV X-rays and of Caesium-137 and Cobalt-60 gamma ray beams. Phys. Med. Biol. 28: 1097-I 104, 1983. Thwaites, D.I., Williams, J.R., Aird, E.G., Klevenhagen, S.C. and Williams, P.C. A dosimetric intercomparison of megavoltage beam in UK radiotherapy centres. Phys. Med. Biol. 37: 445-461, 1992.

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