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Annex 1: BOC quality system document number WI.14.01.01, written procedures for 'Medulla Planning'.
BEATSON ONCOLOGY CENTRE - QA CONTROLLED DOCUMENT
WI.14.01.01
Medulla Planning
Scope: Planning of medulloblastoma or whole CNS treatments by Radiotherapy Physics Treatment Planning Staff.
Documentation:
Medulla Data Tables (DA.15.037, DA.15.038, DA.15.039, DA.15.054, DA.15.055, DA. 15.056, DA. 15.075, DA.15.088)
Medulla Planning Form - 1 Spine Field - (FM. 14.013)
Medulla Planning Form - 2 Spine Fields - (FM. 14.014)
Medulla Head Fields on SLs - Asymmetric Settings (FM. 14.006) Manual of Sample Plans
Set-up Description:
Whole CNS treatments are usually given for medulloblastoma. The patient lies prone on a special `medulla mattress' and has a BDS.
The brain is treated with lateral opposing beams. The central axis is located at the outer canthus of the eye and asymmetric collimator settings are used. Eyes and mouth are shielded with customized alloy blocks.
The spine is treated from above using one or two beams, at standard or extended SSD as necessary. When using a single spine field, the gantry is set to zero, and the lateral head fields have a collimator rotation to match the divergence of the spine field. When using an upper (Sup) and a lower (Inf) spine field, the upper field is treated with a gantry of zero but the gantry for the lower spine field is angled to Superior (by an angle equal to the sum of the divergences of the two spine fields) so that a geometrical match of the beams is obtained. The lateral head fields have a collimator rotation to match the divergence of the upper spine field.
Compensators are manufactured for spine fields if required to achieve a range of dose of less than or equal to 5%.
Planning Methods:
1. Simulation.
Screen for head fields, centre on outer canthus, ball bearing and washer markers at outer canthii. Asymmetric settings: Sup, Ant and Post borders to cover head; Inf border at junction of C4/C5. Physicist estimates the collimator rotation required to match to spine field. This is based on approximation of spine length, choice of one or two spine fields, SSD. Collimator should be at 180° +/-tilt. Tilt is seldom less than 8° and must not exceed 10°. Mark Inf border (the junction) on Post surface of BDS.
Place medulla marker strip on midline over head and spine, with an identified marker at the junction. Radiograph is taken with 100cm to midline and FFD of 150cm. Other head radiograph is taken. Radiotherapist marks shielding on radiographs.
Overlapping lateral radiographs of the spine are taken (100cm to midline, 150cm S SD). Collimator set at zero, field defining wire set to about 6cm, to provide reference horizontal, line. Inf border is at S2/S3. Mark on radiograph, project back to skin surface (allowing for beam divergence). Using objects from marker strip for guidance, transfer this position onto patient's skin surface for tattooing.
2. Planning.
Use form Medulla Head Fields on SLs - Asymmetric Settings (FM. 14.006) to convert Simulator settings for the head -fields to those required on the SL75/5 treatment units. Calculate the effective head size and hence the equivalent square at isocentre and at entry point. Measure the separation of the BDS at the centre of the head. Using the entry point equivalent square calculate the midplane depth dose using tabulated data plus a correction for change in SSD. Output factor (with plain tray) is based on the isocentre equivalent square at 100cm SSD and so percentage depth doses also have to be increased by an inverse square law factor to account for this.
As the head fields are isocentric, the T.A.D. at midplane is normalised ie set to 100%. The percentage depth dose figure becomes the normalisation factor.
Radiographs are sent to the Mould Room for manufacture of the shielding blocks and templates for Simulator checks.
Join the radiographs of the spine together. Draw position of spine field(s) and Anterior edge of spinal cord. Select representative points on spinal cord. Demagnify depth, effective SSD and off-axis distance (at depth of COM). From Medulla Data Tables (DA.15.037, DA.15.038, DA. 15.039, DA.15.054, DA.15.055, DA.15.056, DA.15.075, DA.15.088) determine central axis depth doses, off-axis correction factors and SSD correction factors and hence determine % depth dose at each point on the spinal cord.
If the range of dose exceeds 5%, a wax compensator is required. Calculate the percentage dose reduction required at each point to bring the dose down to the lowest value. The wax reduces the dose by 3.8% per cm (at 5MV). Hence the required thickness of wax can be determined for each point. Note: allow for the attenuation of the 3mm thick perspex plate on which the wax is mounted. Draw profile of wax on paper. Scale the off-axis distances from 5cm deep in the patient to the tray distance (69.7cm). Two copies of this are sent to the Mould Room. The wax should normally be 6cm wide.
Calculate the maximum subcutaneous dose. This usually occurs where no wax is required. It is equal to 100% multiplied by the off=axis correction factor multiplied by the effective S SD factor. This is then reduced by 1.14% (to allow for 3mm of perspex).
Write up plan on Medulla Planning Form FM. 14.013 or FM. 14.014 as appropriate.. Scribe position of junction on Post surface of BDS.
3. Treatment.
To minimise the effects and likelihood of dose inhomogeneity at the junctions, field sizes are adjusted twice during treatment.
With a single spine field, its length is increased by l cm. and the centre moved 0.5 cm to Sup each time ie the junction point moves I cm to Sup. The Inf border of the head field (X2 jaw) moves I cm to Sup each time.
With 2 spine fields, the length of the upper spine field is increased by 2cm (centre stays fixed). The lower spine field length is reduced by I cm and its centre moved 0.5cm to Inf. Head fields are adjusted as above.
Set-up relies on the accurate alignment of the X-ray field with the light field. Radiation Technology Staff should check these parameters for the field dimensions being used, using X-ray film. If match is acceptable, treatment is set up by matching Inf edge of head fields with Sup edge of (upper) spine field at the scribe mark on the BDS. If there is a mismatch, and recalibration of the machine is not possible, then place a piece of black tape of appropriate thickness at the junction: set the head field to one edge of the tape and the spine field to the other edge. `Mismatches' of less than 2mm (both fields combined) can be ignored. With 2 spine fields, the Inf border of the upper spine field is marked on the skin, and the Sup border of the lower spine field is matched to this point. Again, black tape can be used for any mismatch. Inf border of (lower) spine field should always be set to the tattoo at S2/S3. Small adjustments to field size may be made on a daily basis to achieve this. Compensators should be marked as follows: For a single spine field, mark Inf field border. For two spine fields, mark the Inf field border on the lower spine compensator. Mark the Sup field border on the upper spine compensator. NB: this mark has to be moved each time the junction is moved on the BDS.
Notes:
With a lower spine field, the wax compensator needs to be bolted onto a drilled tray and so slots have to be made in the perspex plate for the bolts.
Sometimes the lower spine requires to be `spade shaped'. This has to be defined from a PA radiograph. Pb is fixed to the compensator plate with the wax.
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