Level II audits

The Level II audit is a diagnostic test of the Treatment Planning System performance. The audit is delivered to a rectilinear solid water phantom, with lung inhomogeneity in some cases, and measured using a 2D ion chamber array. The Level II audit includes 3D Conformal, Intensity Modulated Radiation Therapy and Volumetric Modulated Radiation Therapy treatment deliveries. The Level II audit is conducted once every four years.

Definition

The ACDS Level II audit determines absorbed dose to water, for selected mega-voltage photon beams, within a simple phantom geometry. This is an audit of the beam models within a treatment planning system, where the phantom CT is supplied to the facility for treatment planning and delivery.

The 2D array in solid water/lung phantom
Figure 1: The 2D array in solid water/lung phantom

The Level II audit includes a number of modalities:

  • 3DCRT
  • IMRT
  • VMAT
  • IMRT FFF
  • VMAT FFF

Dosimetry measurements are made in a custom phantom of CIRS solid water, using a 2D ionisation chamber array as a primary detector and supporting measurements with CC13 ionisation chambers. The 2D array is calibrated against a Farmer type ionisation chamber, which is traceable to the primary standard at ARPANSA.

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Audit Cases

For the 3DCRT modality there are 12 cases in the audit, with the option of repeating these cases with multiple beam models. The 3DCRT modality consists of non-reference and wedged beams, measured with and without lung inhomogeneities. Figure 2 shows a schematic of some of the audit cases included in the 3DCRT modality.

Graphic showing a selection (6) of 3DCRD cases
Figure 2: Selection of 3DCRT cases in the Level II audit

Cases 13-16 consist of complex target volumes and diagnostic tests for 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 13-16 are repeated for the IMRT FFF and VMAT FFF modalities.

Case 13 – 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.

Graphic showing square box with red 'H' in the centre
Figure 3: 2D coronal plane of the Chair test

Case 14 & 15 – 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).

Graphic showing two square boxes, box showing blue C-shape with red dot in the middle
Figure 4: Transverse view of the C-Shape with and without inhomogeneities

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Case 16 – The 4L

The ACDS prescribed 4L test is an adaptation of a typical Monaco Treatment Planning System (IMPAC Medical Systems Inc., Elekta AB, Sweden) commissioning test (3), which consists of 4 nested ‘L’ shaped MLC patterns. The abutting regions of the MLCs between each ‘L’ shape allow assessment of the dosimetric leaf gap and inter leaf leakage, whilst the continuously blocked region shows the leaf transmission. An example of a 2D measured coronal plane of the 4L plan is shown in Figure 5.

Heat graph showing measured 2D coronal plane of the FI
Figure 5: Measured 2D coronal plane of the 4L

Audit outcome

The outcome for an individual case is determined using the measured 2D dose map.

For 3DCRT, the audit results are determined for each case using the dose variation between the Facility Stated Dose (planned dose) and the ACDS Measured Dose. A measurement case/plane is considered passed at the Optimal Level if the maximum absolute variation of the facility stated plan dose from the ACDS measured dose is within 3.3% for all assessed points. It is considered passed at the Action Level if the maximum absolute variation is between 3.3% and 5%. It is considered Out of Tolerance if the maximum absolute variation is outside 5%.

For IMRT and IMRT FFF, gamma criteria of 3%/3mm relative to 2Gy, with dose <20% suppressed, are assessed across the entire measurement plane for each case.

An overall Audit Outcome for each modality is determined, which is equal to the worst case outcome for each modality.

General audit pass criteria
  Pass
(Optimal level)
Pass
(Action aevel)
Out of tolerance Reported not scored
(RNS)
3DCRT < 3.3%* > 3.3% and < 5% > 5% Any deliveries which are restricted for clinical use, repeated measurements, alternative scoring options and supporting measurements
IMRT & IMRT FFF γ < 1 at 3%/3mm+
for > 97.5% points
γ < 1 at 3%/3mm
for > 90% and < 97.5% points
γ < 1 at 3%/3mm
< 90% points

*% dose variation = (Planned dose - ACDS measured dose) / ACDS measured dose
+ Gamma criteria relative to 2Gy, across entire CT dataset, with dose < 20% suppressed

CT datasets

ACDS audit Level II 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.
[3] IMPAC Medical Systems I. Monaco Technical Reference: Post Modeling Adjustment of MLC Parameters. 2013.