16 2011
IMAGING & ONCOLOGY
each of these areas. The Quality Assurance (QA) subgroup has written recommendations on the commissioning of SBRT which details: • Dosimetry requirements • Immobilisation methods • Assessment of tumour motion methods • Treatment planning techniques • Types of algorithms which should be used • Linac QA • Image guidance • Plan delivery techniques
All of these areas are also covered in the AAPM task report7 .
The lack of funding for a nationwide QA programme is a significant issue and the consortium is currently exploring avenues where a QA programme could be established, outside a trial setting.
It is recommended that each centre should measure the systematic and random errors relating to their own systems of immobilisation and image guidance before using any new technique8
. This should be undertaken before any new tumour site is treated using SBRT.
Traditionally, lung SBRT was delivered using fixed frames or vacuum immobilisation devices with diaphragmatic pressure to reduce breathing motion4
. With the introduction of four-
dimensional computed tomography planning and 3D volumetric image guidance on linear accelerators, it is now possible to deliver SBRT without rigid frames with greater accuracy9
.
Locally we have evaluated our standard lung immobilisation equipment using cone beam CT data from our Varian OBI linacs and have quantified the pre and post treatment uncertainties. The post treatment systematic ∑ and random σ errors were less than 2mm in all directions (table 1) representing the residual displacement error, plus the intrafraction patient movement. These are the important values as we use an on-line imaging protocol with an action level of 2mm, and support the GTV to PTV (gross tumour volume to planning treatment volume) margins of 5mm. This gives confidence in the immobilisation equipment and the treatment accuracy.
Similar results have been shown by a number of UK centres using standard immobilisation; and they are equivalent to values reported by centres using vacuum bag or frame devices.
PHYSICS (PLANNING) ISSUES Although the field sizes used are smaller than for conventional lung radiotherapy, they are not generally small enough to prove a challenge for most modern treatment planning systems (unlike SRS in the cranial setting). However, the combination of relatively small
5 0 1 2 3 4 Centre Figure 2 Time taken at different UK centres for stages in the process for delivering SBRT. 5 6 7 10
Set-up error Mean SD
∑ setup σ setup
Pre-treatment CBCT (mm) VERT -0.2 2.7 1.8 3.0
LONG -0.9 3
4.1 3.3
LAT
-0.4 3
3.3 3.2
Post-treatment CBCT (mm) VERT 0.8 1.4 0.9 1.7
LONG 0.2 1.1 0.6 1.3
Table 1 Pre- and post-treatment systematic and random error margins (in mm)
LAT 0.5 1.4 0.9 1.8
25 20 15
1# treat verification planning outlining Scanner
Schedule
SBRT (5 x 11 Gy) SBRT (3 x 18 Gy)
Standard (20 x 2.75 Gy)
In-room time (min)
24 24 12
No. of
hospital visits 5
3 20
Total time (min)
120 72
240 Table 2 Summary of in-room time and hospital visits for SBRT treatment and standard treatment.
Hours
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