Quality Assurance of accelerators; the technologists responsibility

Transcription

1 Quality Assurance of accelerators; the technologists responsibility Christa Timmermans Radiation Technologist, Erasmus MC- Daniel den Hoed, Rotterdam, The Netherlands EORTC-ROG RT technologists Section November 6, 2008 Overview Morning check procedure of an Elekta accelerator Pre-treatment verification of IMRT planning The contribution of the technologist and the medical Physicist in QA Future plans 1

2 Morning check procedure on an Elekta accelerator Warming up of the accelerator Different energy s Warming up of the kv-tube (XVI) Universal Control Phantom Leaf check procedure (EPID* based) * TheraView-NT (Cablon Medical) Universal Control Phantom Developed in the Radiation department of Rotterdam Used for: Isocenter verification (lasers) Field shape XVI, isocenter verification Translation verification 2

3 UCP Field shape Isocenter/ lasers Translation Blue Green Red XVI isocenter verification UCP: XVI isocenter verification Make an image of the UCP Daily image Automatic Images Match Reference image < ±1.5 mm 3

4 Leaf check procedure Procedure is split up in two morning checks: Leaf position verification Deviations of the leaf position always inside the PTV, implication in terms of cold or hot spots Field flatness & output verification With step & shoot IMRT many small segments with few MU are given, the field flatness and output of the linac have to be ok Recording static field and IMRT field Automatic QA analyses Leaf position verification Profile example for leafpair (leaf 31) Addition segments will give a profile Tolerance: 0.5 mm ( 20%) 4

5 Automatic leaf position verification Field flatness & Output verification Field flatness Output Tolerance: 3% 5

6 Pre-treatment verification QA IMRT-planning (R&O 2006: Zijtveld) Fluence ok? Calculated leafposition and movement ok? Transfer IMRT plan ok? Accelerator ok? Pre-treatment verification with EPID of IMRT-planning (Approval pre-treatment before 4th fraction) Recording all treatment fields at original gantry position Recording rectangular field (stability accelerator/ EPID) Difference analysis: Measurement vs Prediction Linac beam EPID IMRT pre-treatment verification: gamma analysis Gamma index (3 mm / 3 %) prediction measured PDI (EPID-detected) 3% 3 mm measurement difference Gamma index 1 pretreatment is accepted predicted PDI 6

7 EPID dosimetry (example) Red position in the gamma index for leaf 23. Difference due to segment limitation of the X-block The pre-treatment will be approved by the Physicist The workflow of the technologist and the medical physicist The technologist: - Logistics on the accelerator (when is time available for pre-treatment) - Imaging the treatment fields on the accelerator - Inform the physicist that the pre-treatment is performed Positive: Noticing the treatment before start The physicist: - Evaluation of results - Manual analysis of rejected beams - Approval or rejection of pre-treatment verification 7

8 The technologist point of view Intensification of their expertise Complex and new radiation techniques Better notion for what is important in QA and IMRT What can go wrong in dose delivery by IMRT Multidisciplinary cooperation of technologist and physicist The Physicist: final responsibility Future plans Training for the technologist in QA Education for expert technologists starting Automatic difference analyses Activities move over to the technologist QA the technologist responsibility? Maybe for the expert technologists it will worked out in the future 8

9 Thank you for your attention EPID dosimetry 2D dose verification, pretreatment measurement After normalisation 9

10 Pre-treatment verification Measured EPI IMRT fluency IMRT fluency Measured Doses profile predicted Doses profile 2D matching: Difference Analyse images Gamma evaluation Morning check Pre-treatment procedure Technologist vs Physicist Future plans 10