« Previous | Contents | Next »
Listen
4. Incident description
4.1 Referral
4.1 Lisa Norris, who at the time of this incident was 15 years of age, was referred to the BOC by a consultant clinical oncologist on 13 th September 2005 for a course of radiation treatment for a relatively rare brain tumour. Referral was by completion of a Treatment Booking Form (form number BOC 00017). The consultant clinical oncologist prescribed a dose of radiation of 35 Grays* to a treatment volume to include the whole of the central nervous system ( CNS); to be delivered in 20 equal fractions of 1.75 Grays, to be followed by 19.8 Grays targeted on the tumour itself in 11 equal fractions of 1.8 Grays.
4.2 Regarding formal duties under the IR(ME) Regulations, the referring Oncologist was therefore acting as both 'referrer' and 'practitioner'.
4.2 Treatment Planning for patient Lisa Norris
4.2.1 General treatment planning provisions
4.3 Treatment planning in cancer radiotherapy is the process whereby patient information is gathered and assimilated, for example by CT scanning, and is used to plan the precise manner in which the dose of radiation prescribed by the clinical oncologist will be delivered. The people who undertake these tasks in UK radiotherapy centres are most commonly in staff categories identified as 'therapeutic radiographers', 'medical physicists' 'medical technical officers' ( MTO) and 'medical dosimetrists'.
4.4 The job title 'Therapeutic Radiographer' is a 'protected title', limited to those individuals who are registered as such with the UK Health Professions Council ( HPC). Medical physicists can also register with the HPC under the protected title of 'Clinical Scientist'. MTOs are not HPC registered but the Institute of Physics and Engineering in Medicine ( IPEM) operates a 'Voluntary Register for Clinical Technologists' for which MTOs are eligible. The term 'medical dosimetrist' is relatively new in the UK and tends to cover radiographers and MTOs who work in treatment planning. However, it is not yet well defined.
4.5 Staffing provisions for radiotherapy treatment planning vary across UK centres. For example, at some centres the bulk of treatment planning is carried out by therapeutic radiographers. At the BOC, all but the simplest of treatment planning is carried out by medical physicists, some of whom (in general the more senior) are HPC registered and by MTOs, some of whom are on the IPEM voluntary register. The division of planning duties between staff in these two groups is mainly in accordance with training and experience rather than by staff category, though responsibilities for checking of treatment plans fall normally to the more senior of the medical physicists. (See also Paragraph 6.6 and Annex 6.) Some of the simplest treatment plans are, however, prepared by the radiographers themselves.
* The 'Gray' is the international unit of 'absorbed dose' that quantifies the amount of energy that is deposited in the tissue of the body by the radiation. One Gray is equal to a radiation absorbed dose of one Joule of energy per kilogram of tissue. This report also quotes absorbed dose in units of centiGrays, where a centiGray is one hundredth of a Gray.
4.6 The majority of planning, scheduling and delivery of radiotherapy treatment at the BOC is managed using a computer software package called Varis. This comprises a number of individual software modules which deal with different aspects of treatment planning and delivery. The paragraphs that follow describe the general features of the Varis system relevant to this Report (including the features of the BOC's latest version, Varis 7) and consider specific aspects of its use at the BOC. This includes consideration of the nature of the error that was made. Section 5 of this report then goes on to consider the circumstances that contributed to the error.
4.7 Within the Varis system, data from the Treatment Booking Form referred to in Paragraph 4.1 is entered into the software modules called Patient Manager and RTChart by staff in the BOC's Booking Office. These data can include the patient's personal details and the treatment parameters prescribed by the clinical oncologist. The software module that has been used for treatment planning at the BOC since 2003 is called Eclipse. The output from the Eclipse module is in the form of a 'Treatment Plan Report' which contains detailed information on the beam energy, size and machine settings which determine the position of the radiation fields to be delivered by the linear accelerator (Linac) and the setting, in Monitor Units ( MU), that is required on the Linac to deliver the total dose for each treatment field. Several treatment fields may be used to deliver a complete treatment fraction. 'Monitor Units' is the term used for the reading that arises from the monitor on the radiation delivery unit (the Linac) that measures the total output of radiation from the delivery unit during an exposure. The radiographer sets the Linac to terminate the exposure when the pre-set number of MU have been delivered. The MU setting is therefore critical in achieving the correct dose.
4.8 Prior to the introduction of the latest version, Varis 7, in May 2005, a decision had been taken at the BOC to use the Eclipse planning system as a stand-alone module within the overall architecture of Varis for a number of operational and technical reasons. After the upgrade in May 2005 to Varis 7, a decision was taken at the BOC to integrate the Eclipse module more fully with the other Varis software modules. Following this change, the data entered in the Patient Manager and RTChart modules, as described in Paragraph 4.7, could be transferred electronically to the forms that were completed within Eclipse by the treatment planners using information gathered during the treatment simulation or the CT scanning phase. In addition, the information contained in the Treatment Plan Report created by Eclipse could then be transferred electronically back to the RTChart module. RTChart supports the electronic verification of these treatment parameters by the radiographer and is part of the final check to confirm that these parameters are correct before being transferred to the Linac for delivery of the treatment. It also allows the parameters that were intended and the actual Monitor Units delivered at treatment to be reviewed.
4.9 The initial radiation treatment plan that was prescribed for Miss Norris was a 'whole CNS plan'. This comprises separate treatment plans for the radiation exposure of the head and of the spine, generally referred to as the 'head fields' and 'spine fields'. In this case, the treatment plan for the spine was further divided into lower and upper spine fields.
4.10 Because of the complexity of whole CNS treatment plans, management of treatment planning considered that electronic data transfer of the Treatment Plan Report from Eclipse to RTChart was not appropriate for this procedure. The main source of complexity lies in the spine fields. The system that was used prior to the introduction of Varis 7 was therefore retained, for both the head and the spine fields, whereby the relevant information from the Treatment Plan Report is transferred manually to the BOC's 'Medulla Planning Forms'.
4.11 The Medulla Planning Form is then passed to the treatment radiographers to provide them with the information required for final monitor unit calculations and treatment delivery.
4.12 Whole CNS treatments of a similar type are performed about six times per year in the BOC. (The total number of new treatment plans at the BOC is advised as being between 4500 and 5000 per year.)
4.13 The bulk of the treatment planning for Miss Norris was carried out by a BOC treatment planner referred to in this report as Planner B during the period 16 th to 19 th December 2005.
4.2.2 Spine fields
4.14 Referring firstly to the spine fields, the treatment plan produced initially by Planner B was checked by Principal Planner A, who identified errors in the design of the compensators (shaped wax blocks used to eliminate dose non-uniformity resulting from surface contour irregularities). The compensator designs were amended and the plan was then passed to Senior Planner C, who checked the compensators on the spine fields, looked over the plan, and signed it off.
4.15 As a result of this checking process, the spine fields were planned and delivered correctly and no further consideration is given in this report to this aspect of the treatment, except to note that, (i) errors were made by Planner B and (ii) Dr Martin's incident report notes that this was considered the most difficult part of the planning process and speculates that the attention of the treatment planners who carried out the checks may have been distracted by the initial errors in this part of the whole CNS plan.
4.2.3 Head fields
4.16 Within Varis7, the Eclipse treatment planning module allows the user to open the patient file created within Patient Manager and then to choose whether to import electronically the relevant data on the prescribed radiation dose (the 'Physician's Intent') entered previously within the RTChart module (Paragraph 4.7). For the head fields (but not for the spine fields) this option was selected by the treatment planner and the data that was transferred electronically to the Eclipse treatment planning module included the planned total radiation dose and the number of fractions prescribed for Miss Norris by the clinical oncologist. (Both Principal Planner A and Planner B were involved at this stage in the planning process but their recollections could not clarify which of them made the (critical) choice to include the prescribed radiation dose in the treatment planning process.) For reasons that remain unclear, at some point in this process, the number of treatment fractions was changed from 20 to 21. (This change was not in any way a cause of the incident.) The remainder of the planning process for the head fields was carried out by Planner B.
4.17 The Eclipse treatment planning module then used the values that were transferred electronically for the total radiation dose and the number of fractions to determine the 'Daily Total Dose' (calculated on the basis of 21 fractions as 167 centiGrays rather than 175 centiGrays) and the setting required for the linear accelerator to deliver this radiation dose to the head in each of the daily treatment sessions. The calculated setting was 91 monitor units.
4.18 Under their ISO 9000 quality system, controlled document number WI.14.01.01 ( Annex 1 to this report) is the BOC's written procedure for 'Medulla Planning'. It includes an instruction to the treatment planner (on completion of the planning process) to ' Write up plan on Medulla Planning Form FM.14.013 or FM.14.014 as appropriate.'
4.19 In this case, the Medulla Planning Form used was FM.14.014, for two spine fields. FM.14.014 is also a controlled document under the provisions of the BOC's quality system. A blank copy of the version in use in December 2005, which is dated 11 th August 1998, is appended to this report as Annex 2.
4.20 The table at the foot of Page 1 on Medulla Planning Form FM.14.014 requires the planner to enter calculated values of the 'Output'. The 'Output' is the intended number of daily MU from the delivery unit 'normalized **' to 100 centiGrays ( i.e. expressed in units of MU per 100 centiGrays). In this case, the normalized 'Output' should have been calculated (for 21 rather than 20 fractions) as 91 MU divided by 167 (centiGrays) and multiplied by 100 to give a required 'Output' of 54 MU per 100 centiGrays.
4.21 In this case, however, the treatment planner (Planner B) omitted the normalization procedure and erroneously entered an 'Output' figure of 91 MU per 100 centiGrays on the Medulla Planning Form instead of the correct figure of 54. This was the critical error and it was not identified by the more senior treatment planners who checked the plan.
** The term 'Normalization', as used in this report, is the process whereby a parameter is multiplied or divided by a number so that it can be expressed in more convenient or standardized units. For example, the recommended weight of fertilizer to be applied to a lawn might be normalized to a unit of area such as 'grams per square metre'. The actual amount needed is then determined by multiplying this normalized figure by the area of the actual lawn in question.
4.3 Treatment Delivery
4.22 The Medulla Planning form for Miss Norris, containing the erroneous 'Output' figure was passed to the treatment radiographers to provide them with the information required for calculating monitor units for treatment delivery.
4.23BOC Work Instruction WI 13.26.06 (issued in October 2005 to update WI 13.67, Version 4, dated May 2002 and appended to this report as Annex 3), sets out the procedure to be followed by radiography staff during the pre-treatment processes for 'Medulloblastoma Calculations'. This requires that the number of 'Daily MU' should be calculated by multiplying the intended 'Daily Total Dose' by the 'Output' which (see Paragraph 4.20) is obtained from the Medulla Planning Form that has been completed by the treatment planner.
4.24 In this case, the radiographer, working in accordance with WI 13.26.06, used the erroneous entry (91 MU per 100 centiGrays) and calculated the daily number of monitor units, for the 20 dose fractions prescribed, as:
Daily MU = ((175 x 91) / 100) = 159 MU.
4.25 The correct calculation should have been:
Daily MU = ((175 x 54) / 100) = 94.5 MU.
4.26 The Daily Total Dose equivalent to 159 MU from both the (right lateral and left lateral) head fields is 2.92 Grays (cf. the prescribed dose of 1.75 Grays) and this is the dose that was delivered in each of 19 fractions before the error was discovered. The total dose to Miss Norris was therefore 55.5 Grays (19 x 2.92 Grays), which is 58% higher than the intended total dose of 35 Grays.
4.27 Following discovery of the error, the prescribed, targeted treatment for the tumour region of 19.8 Grays in 11 equal fractions of 1.8 Grays that was scheduled to follow the whole CNS treatment was abandoned on the instruction of the clinical oncologist.
« Previous | Contents | Next »