Maria Skłodowska-Curie Memorial Cancer Centre and Institute of Oncology, Warsaw, Poland 8th ECMP, Athens, Dosimetry audits in radiotherapy in Poland W. Bulski, K. Chełmiński, J. Rostkowska Medical Physics Department

Radiotherapy centres in Poland as of 2013: 30 radiotherapy centres

Radiotherapy equipment in Poland as of 2013: About 115 accelerators in 30 centres

For over 20 years a postal audit (IAEA methodology) has been carried out to check the TPS dose calculations in a homogenous phantom (water) in reference and various non-reference conditions.

Non-reference conditions, on axis, MLC shaped fields reference (1) „small” (2) „irregular” (5) „inverted Y” (4) „circular” (3) „irregular” +wedge (6)

MLC shaped photon beams from linear accelerators; dose calculated with TPS vs. measured with TLD. D (stated) - dose stated by the participant, D (TLD) - dose determined by the SSDL Results of the postal audit in 2012:

In the above example all results were within the ± 5% level. Only 10 out of 168 results (6%) were over ±3.5% level. This allowed us for adoption of the levels of the evaluation of the results: acceptance level ± 3,5%, and intervention level ± 5%. Such levels are adopted by the IAEA only in the case of Secondary Standard Dosimetry Laboratories (SSDL). However, all the measurements were done in a homogenous (water) phantom. In the radiotherapy Treatment Planning Systems (TPS) various calculation algorithms are used and they accuracy of dose calculations in heterogenous medium has to be verified. For this reason a heterogeneous cubic-shape phantom has been designed within a Coordinated Research Project of the IAEA.

Materials and methods The heterogeneous phantom was developed in the frame of an IAEA Coordinated Research Project. The phantom consists of frame made with polystyrene and of bone and lung inhomogeneity slabs. Special inserts allow to position TLD capsules within the polystyrene below the bone or lung material and also within the lung equivalent material. There are also inserts for positioning an ionization chamber. The comparisons were performed for a number of various TPS and for a number of various linear accelerators in radiotherapy departments in Poland.

The phantom consists of a frame made of polystyrene and inhomogeneitis - bone or lung equivalent exchangeable slabs. polystyrene bone lung

IAEA heterogeneity phantom with cassettes for TLD or films and ionization chamber cavity inserts.

Beam Radiation unit TLD set # User stated (TPS) dose [Gy] IAEA (measured) dose [Gy]* deviation relative** to IAEA mean dose [%] IAEA mean dose / User stated dose 6 MVClinac 2300CD P (Polystyrene) 2,001,990,70,99 BP (Bone) 2,001,990,31,00 LP (Lung on-axis) 2,002,04-1,61,02 LL (Lung off-axis) 2,282,222,90,97 Pencil beam algorithm, X6 MV, Warsaw, Poland EXAMPLE RESULTS OF IAEA/WHO TLD POSTAL DOSIMETRIC QUALITY AUDIT

IAEA – HETEROGENEITY PHANTOM AUDIT RESULTS FROM PILOT STUDY IN POLAND PERFORMED BY SSDL-WARSAW Irradiaton of TLD capsules Ionizing chamber measurements using dedicated inlet Irradiation conditions (same for all detectors): beam quality: 6MV field size: 6 cm x 6 cm SSD = 90 cm

Ten radiotherapy centers (of 30 total in Poland), six TPS types with alternative algorithms were examined giving 15 TPS/Algorithm/Linac combinations No.TPSAlgorithmLinac 1Panther 5.01EPSiemens Artiste 2PrecisePlan 2.16PBCElekta Synergy 3Eclipse 7.3PBCVarian Clinac MasterPlan 4PBCVarian Clinac Eclipse 8.2PBCVarian Clinac Panther 5.01CCCSiemens Artiste 7CMS XiO 4.62CCCElekta Synergy 8MasterPlan 4CCCSiemens Primus 9MasterPlan 4CCCVarian Clinac MasterPlan 4CCCElekta Synergy 11Eclipse 8.2AAAVarian Clinac Eclipse 10.0AAAVarian Clinac EX-S 13Eclipse 8.6AAAVarian Clinac Eclipse 11.0AAAVarian Clinac Monaco 3.2MCElekta Synergy PBC – Pencil Beam Convolution AAA – Analytical Anisotropic Algorithm MC – Monte-Carlo EP – fast photon effective path CCC – Collapsed Cone Convolution)

TLD – POLYSTYRENE The TLD capsules were located in the polystyrene material at 10 cm depth. The plan was normalized to 100% for 2 Gy at 10 cm depth..

TLD – BONE The TLD capsules were located under the bone tissue equivalent material at 10 cm depth. The plan was normalized to 100% for 2 Gy at 10 cm depth.

TLD – LUNG In case of lung tissue equivalent material the TLD capsules were located in two positions: in lung tissue, under lung tissue slab at 10 cm depth. The plan was normalized to 100% for 2 Gy in the point at 10 cm depth.

Results Ten Polish radiotherapy centers (of 30 in total) were audited. Six different TPSs and eleven calculation algorithms were examined. Generally, most of the results from TLD measurements were within 5% tolerance. Differences between doses calculated by TPSs and measured with TLD did not exceed 4% for bone and polystyrene equivalent materials. Under the lung equivalent material, on the beam axis the differences were lower than 5%, whereas within the lung and off the beam axis – in some cases were around 7%. For algorithms which use point kernel convolution/superposition and density variation in 3D with modeling of lateral electron and photon transport (CCC, AAA) the calculated doses were usually underestimated compared to the TLD measurements.

National audit system of TPS in Poland within the IAEA pogramme with the CIRS Phantom Ten radiotherapy centres, seven TPS systems with various algorythms. Test cases were planned and measured for 31 combinations of beams of various energies and various calculation algorythms.

IAEA SUPPORTED AUDIT OF TPS IN POLAND CC – Collapsed Cone AAA –Anisotropic Analytic Algorithm PBC – Pencil Beam Convolution FPH – Fast Photon Algorithm FFT – Fast Fourier Transform Convolution 31 combination of TPS x algorithm x beam energy were tested in 10 audited centers

IAEA SUPPORTED AUDIT OF TPS IN POLAND CC – Collapsed Cone AAA –Anisotropic Analytic Algorithm PBC – Pencil Beam Convolution other: FPH – Fast Photon Algorithm FFT – Fast Fourier Transform Convolution 31 combination of TPS x algorithm x beam energy were tested in 7 audited centers

IAEA SUPPORTED AUDIT OF TPS IN POLAND CC – Collapsed Cone AAA –Anisotropic Analytic Algorithm PBC – Pencil Beam Convolution other: FPH – Fast Photon Algorithm FFT – Fast Fourier Transform Convolution

IAEA SUPPORTED AUDIT OF TPS IN POLAND

Conclusions Over the last 20 years the postal TLD audits fulfilled their role and remain the primary and well established dosimetric audit method; The measurements allow to the detect limitations of TPS calculation algorithms. The audits performed with the use of the IAEA heterogeneous phantom seem to be an effective tool for detecting errors in radiotherapy procedures. The variety of new sophisticated heterogenous phantoms allow for thorough testing of TPS performance, for detecting computing algorithm’s limitations and corrections in treatment planning procedures. New irradiation modalities (tomotherapy, CyberKnife, etc.) require specific methods of dosimetric audits which are now being elaborated in a number of radiotherapy centres.

Maria Skłodowska-Curie Memorial Cancer Centre and Institute of Oncology, Warsaw, Poland 8th ECMP, Athens, Thank you for your attention W. Bulski, K. Chełmiński, J. Rostkowska Medical Physics Department