Out-of-field doses of CyberKnife in stereotactic radiotherapy of prostate cancer patients

Transcription

1 EURAMED,MELODI and EURADOS Workshop, Helsinki Out-of-field doses of CyberKnife in stereotactic radiotherapy of prostate cancer patients Jan Seppälä, Chief Physicist Tuomas Virén, Medical Physicist Kuopio University Hospital Cancer Center Kuopio, Finland

2 Out-of-field doses In radiotherapy high doses of radiation are delivered safely to the tumour. Combining both accurate in- and out-of-field dose calculation is challenging and usually the focus is to determine the in-field doses as accurately as possible. For example the Monte Carlo models used in radiotherapy are always optimized for accurate in-field dosimetry and unfortunately the out-of-field doses are and has been with less concern. However, the out-of-field doses have become more important with the increased survival of the treated cancer patients. The modeling and minimizing the incidence of second cancer has become more and more relevant. The out-of-field doses are the sum of collimator and phantom (patient) scatter and leakage radiation. The smaller the beam size becomes, the higher the demand for the out-of-field dosimetry is, since more radiation will be accumulated from collimator scatter and leakage. 2

3 Background OPERRA project: QA guidance and procedures for biobank of samples of patients exposed to medical radiation in CT examinations and radiotherapy. Validation of several biomarkers with prostate cancer patients treated with stereotactic radiotherapy of 5 x 7.25 Gy with CyberKnife system. Several biomarkers will be analysed from the samples (blood and hair). The biomarkers include free DNA and metabolomics, immunology related markers and mitochondrial DNA analyses of hair samples, mirna, phospho-atm, Raman spectroscopy and microparticles. Classical dicentric assay will be used as a reference for the biomarkers validated. A need to define the integral dose of the treated prostate cancer patients with high accuracy. Both the doses from the planning CT and actual radiotherapy treatments will be investigated. The determination of integral doses of the patients require measurements since treatment planning software are known to underestimate peripheral doses. 3

4 CyberKnife system X-ray tubes Linear accelerator Robot X-ray detectors The treatment accuracy is based on the x-ray imaging system integrated to the robotics with matching coordinates.

5 CyberKnife beam collimation With CyberKnife device the radiation beam is collimated with circular collimators (diameters of 5 60 mm). Fixed collimators Iris-collimator

6 CyberKnife treatment planning CyberKnife has only 6 MV energy Inverse optimization of the dose distributions. The plan optimization defines the optimal beam directions and the user can only define from what directions the beams are not allowed to intersect the patient. The final treatment plan consists of about 200 non-coplanar radiation beams. The treatment time with prostate cancer patients is usually about 30 minutes. 6

7 Aim Determine the integral dose (blood) and dose to organs at risk in the low dose regions in STR of prostate cancer Determine the accuracy of MultiPlan dose calculation algorithms (Monte Carlo and Raytracing) in the out-of-field region Estimate the contribution of out-of-field doses to total integral dose

8 Measurement Set Up IBA blue phantom PTV farmer ionization chamber IBA cc13 ionization chamber NE electrometer EBT3 film Glass dosimetry

9 Measurement Set Up

10 Measurements Measurement of beam profiles (scan width 460 mm) and depth doses 50 mm and 15 mm collimators Iris and Fixed To estimate head scattering/leakage, measurements were repeated with block (field size 0 mm) Point measurements with farmer chamber and NE electrometer-> central axis and 100 mm and 200 mm off axis Three different SSD distance 785 (standard in QA), 650 and 500 mm Farmer and cc13 ionization chambers (volume effect vs. sensitivity) MultiPlan Monte Carlo and Raytracing algorithms

11 Depth dose and Profiles

12 SSD 785 cm, 50 mm Iris collimator Depth 15 mm Depth 100 mm Depth 300 mm

13 SSD 785 cm, 15 mm Iris collimator Depth 15 mm Depth 100 mm Depth 300 mm

14 RT MC Out-of-Field doses Table: Difference in measured and calculated doses in central axis and out-of field region. Measurement Depth 100 mm. 50 mm 15 mm IR DCA (mm) MC 0 0.8% 1.0% 0.4% -0.9% 0.5% -1.1% RT 0 0.0% -0.1% -0.6% 0.1% 1.3% 0.6% % 32.1% 33.8% 65.6% 67.7% 74.0% % 58.1% 60.5% 89.5% 91.0% 94.5% % % % 80.5% 72.7% 69.7% % % % 70.3% 64.0% 61.0%

15 RT MC Out-of-Field doses Table: Difference in measured and calculated doses in central axis and out-of field region. Measurement Depth 100 mm. 50 mm 15 mm IR DCA (mm) MC 0 0.8% 1.0% 0.4% -0.9% 0.5% -1.1% RT 0 0.0% -0.1% -0.6% 0.1% 1.3% 0.6% % 32.1% 33.8% 65.6% 67.7% 74.0% % 58.1% 60.5% 89.5% 91.0% 94.5% % % % 80.5% 72.7% 69.7% % % % 70.3% 64.0% 61.0%

16 RT MC Out-of-Field doses Table: Difference in measured and calculated doses in central axis and out-of field region. Measurement Depth 100 mm. 50 mm 15 mm IR DCA (mm) MC 0 0.8% 1.0% 0.4% -0.9% 0.5% -1.1% RT 0 0.0% -0.1% -0.6% 0.1% 1.3% 0.6% % 32.1% 33.8% 65.6% 67.7% 74.0% % 58.1% 60.5% 89.5% 91.0% 94.5% % % % 80.5% 72.7% 69.7% % % % 70.3% 64.0% 61.0%

17 RT MC Out-of-Field doses Table: Difference in measured and calculated doses in central axis and out-of field region. Measurement Depth 100 mm. 50 mm 15 mm IR DCA (mm) MC 0 0.8% 1.0% 0.4% -0.9% 0.5% -1.1% RT 0 0.0% -0.1% -0.6% 0.1% 1.3% 0.6% % 32.1% 33.8% 65.6% 67.7% 74.0% % 58.1% 60.5% 89.5% 91.0% 94.5% % % % 80.5% 72.7% 69.7% % % % 70.3% 64.0% 61.0%

18 Integral Dose Integral dose was determined as product of dose and volume (Gy*dm^3) Integral dose was determined based on beam profiles measured with Farmer ionization chamber Volume effects were taken into account

19 Integral Dose 50 mm 205 mm - 50 mm IR collimator - SSD 785 In Filed Region Out of Field Region Dose (Gy)

20 Integral Dose Table: Difference (%) between measured and calculated (MC) integral dose 50 mm collimator 15 mm collimator Coll/ SSD (mm) Iris 12.2% 13.0% 18.1% 32.2% 36.0% 50.4% Fixed 13.9% 16.0% 19.7% 49.4% 55.2% 63.5%

21 Out of Field doses and Head Scattering

22 Evaluation of film and glass dosimetry 50 mm Iris Collimator 15 mm 15 mm Iris Collimator

23 Conlcusions Monte Carlo algorithm underestimate the out-of-field dose, and thus, the ID Raytracing more or less useless Contribution of head scattering/ leakage to measured out-of-field doses might be significant Film dosimetry seems to be feasible method for measurement of out-of-field doses

24 Future work Development of robust correction factor for integral and organ dose calculation Validation of correction factor with film (MOSFET and RPL) measurements in anthropomorphic fantom Full Monte Carlo algorithm with more accurate modelling of out-of-field areas Contribution of imaging dose in the total integral dose -> film measurements

25