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ACDS - Level III Audits

Audit Index:

LEVEL III AUDITS
Wedged Asymmetric Inhomogeneity: Lung (WAIL)

Definition

The ACDS Level III audit determines absorbed dose to water delivered to selected points within an anthropomorphic phantom. This is an end-to-end audit where the phantom undergoes all steps within the radiotherapy treatment chain.

The Level III audit includes a number of optional modalities, which can be completed by the facility to reflect their clinical practice:

Custom CIRS thoral phantom in place  Figure 1: Custom CIRS thoral phantom
  • 3DCRT
  • IMRT (in trial)
  • VMAT (in trial)
  • IMRT FFF (in trial)
  • VMAT FFF (in trial)

Dosimetry measurements are made in a custom CIRS IMRT Thorax phantom, using a ‘TomoElectrometer’ multi-channel electrometer (Standard Imaging), with Farmer type and CC13 ionisation chambers as the primary detectors. Ion chambers are calibrated by the national primary standards laboratory at ARPANSA. As recommended by TRS-398, the ACDS uses ion chamber calibration factors determined in high-energy beams of similar quality (referred to as 'Directly measured').

Audit Cases

For the 3DCRT modality there are 4 cases in the audit, with the option of repeating these cases with multiple beam models. The 3DCRT modality consists of reference and wedged beams, measured with and without lung inhomogeneity. A schematic of the cases is shown in Figure 2.

Diagram showing four views of the phantom Figure 2: 3DCRD Cases 1-4

Cases 5-8 consist of complex target volumes for inclusion in the IMRT and VMAT modalities. The facility has the option to include as many IMRT and/or VMAT beam models in the audit as applicable to their clinical practice. Cases 5-8 are repeated for the IMRT FFF and VMAT FFF modalities.

Diagram showing square box with red 'H' in the middle  Figure 3: The 'Chair' test

 

Case 5 – The ‘Chair’ Test
The chair test is an adaptation of the test described by Van Esch et al. (1), where a chair-like fluence (Fig. 3) is delivered by dynamic MLC movement. The test aims to separate the effects of leaf transmission from dosimetric leaf separation in a single test.

 

 

 

Case 6 & 7 – The C-Shape
The C-Shape target volume has been adapted from AAPM: TG119 (2), a horseshoe shaped target volume surrounding a central avoidance structure. Two treatment plans for the C-Shape are required; with and without inhomogeneities (Fig 4).

Diagram showing to views of the phantom showing a horseshoe shaped target surrounding a central avoidance structure   Figure 4: C-Shape with and without lung inhomogeneities

Diagram showing two adjacent target structures - box with green rectangle and small pint triange on top  Figure 5: The Complex Case

 

Case 8 – The Complex Case
The ACDS derived the ‘complex’ case from elements of IMRT/VMAT practice observed in the clinic. The complex case (Fig.5) consists of two adjacent target structures, with varying dose objectives, and an exclusion sphere fully encompassed by the higher dose target.

 

 

Audit Outcome

The audit results are determined ‘point-by-point’ for each case using a percentage deviated of the facility stated dose (planned) from the ACDS determined dose (measured). An overall audit outcome is determined for each modality, which is equal to the lowest recorded result.

‘CT’ Dataset (RT Structures)

ACDS Audit Level III Documentation

References

  1. Van Esch A, Bohsung J, Sorvari P, Tenhunen M, Paiusco M, Iori M, et al. Acceptance tests and quality control (QC) procedures for the clinical implementation of intensity modulated radiotherapy (IMRT) using inverse planning and the sliding window technique: experience from five radiotherapy departments. Radiotherapy and Oncology 2002;65(1):53-70.
  2. Ezzell GA, Galvin JM, Low D, Palta JR, Rosen I, Sharpe MB, et al. Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee. Medical physics 2003; 30(8):2089-115.
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