Measurements of the photon and neutron dose delivered to organs outside the radiation beams for 3DCRT and IMRT radiotherapy A. Kowalik 1, W. Jackowiak 2, J. Malicki 1,3, M. Skórska 1, M. Adamczyk 1, E. Konstanty 1, T. Piotrowski 1,3, K. Polaczek – Grelik 4 1. Dep. of Medical Physics, Greater Poland Cancer Centre, Garbary 15, Poznan, Poland, phone , 2. Dep. Of Radiotherapy,` Greater Poland Cancer Centre, Garbary 15, Poznan, Poland 3. Dep. of Electroradiology, University of Medical Sciences, Poznan, Poland 4. Dep. of Medical Physics, University of Silesia, Uniwersytecka 4, Katowice, Poland Introduction: Nowadays, there are many modern techniques of radiotherapy: IMRT (Intensity Modulated Radiotherapy), Volumetric Modulated Arc Therapy (VMAT), Stereotactic Radiation Therapy (SRS) etc. Rapid development of technology in radiotherapy introduced tools that allow more homogenous dose in the target but may cause dose increase outside the target. The IMRT technique provides more homogenous dose in the target than 3DCRT but the dose delivered to organs at risk during treatment can be bigger than during 3DCRT delivery. Estimation of doses outside the PTV from all sources is necessary to assess total stochastic risks and thus provide adequate justification of the exposures as required by International Commission on Radiological Protection (ICRP). Materials and methods: The doses in organ at risk (OAR) outside the target: thyroid, lung, bladder and testes were measured in the anthropomorphic Alderson phantom using thermoluminescence detectors TLD 100 ( 6 Li (7.5%) and 7 Li (92.5%); (Harshaw Chemical Company). Subsequently, the prostate (Clinical Target Volume: CTV1), nodes (CTV2) and organs at risk (bladder, rectum, femoral heads, lung, thyroid and testes) were contoured. The target consists of the whole pelvic lymph nodes and the prostate gland with seminal vesicles. In the next stage three treatment plans were calculated for three alternative techniques: 3DCRT, IMRT and Tomotherapy. For organs which were inside or close to the treatment fields the doses were read from Treatment Planning System (TPS), i.e. rectum, bladder and testes. However, the readout of the doses for the thyroid and lung from the treatment planning system was impossible because commercial systems do not calculate doses at such a long distances outside the target volume. The thickness of each layer in the Alderson phantom is 2.5 cm and the distance between dosimetric points is 3 cm which allows for an accurate determination of the doses in chosen organs. Additionally, the neutron fluence rate [cm- 2s-1] at chosen points inside the same phantom was measured in terms of neutron activation analysis (NAA) with the use of gold which have 0.5 cm of diameter and the mean surface density of g/cm3. Purpose: To check the doses from photon beam irradiation of prostate cancer and from scattered neutrons in organs at risk (OARs) for static fields and IMRT. Conclusions: However for IMRT technique the doses were lower only for the bladder compared with 3DCRT. The surprise for IMRT was that for this technique the doses were lower only for the bladder compared with 3DCRT. The present investigation shows that photoneutron dose (resulting from the use of high-energy X-ray beam) constitutes about 0.5% of the therapeutic dose prescribed in PTV. Literature: 1. Al-Ghamdi H., Fazal-ur-Rehman, Al-Jarallah M.I., Maalej N., Photoneutron intensity with field size around radiotherapy linear accelerator 18-MeV X-ray beam, Radiation Measurements 43: S495– S499, Chibani O., Ma Ch-M.Ch., Photonuclear dose calculations for high-energy photon beams from Siemens and Varian linacs, Medical Physics 30(8): 1990–2000, Francois P., Beurtheret C., DutreixA., Calculation of the dose delivered to organs outside the radiation beams, Medical Physics, 15(6), , Howell R.M., Scarboro S.B., Kry S.F., Yaldo D.Z., Accuracy of out- of-field dose calculations by a commercial treatment planning system, Physics in Medicine and Biology, Dec7; 55(23): , Hall EJ, Wuu C: Radiation-induced second cancers: the impact of 3D-CRT and IMRT, International Journal of Radiation Oncology, Biology and Physics, 56:83-88, Harrison RM, Wilkinson M, Shemilt A, Rawlings DJ, Moore M, Lecomber AR. Estimating second cancer risk following radiotherapy: organ doses from prostate radiotherapy and concomitant exposures, Biomedizinishe Technik, 50(Suppl. vol 1 Part 1):768–9, Results: The doses for particular organs measured during 3DCRT treatment were: thyroid gland – 0.62Gy, lung – 0.99Gy, bladder – 80.61Gy, testes – 4.38Gy; during IMRT treatment: thyroid gland – 2.88Gy, lung – 4.78Gy, bladder – 53.75Gy, testes – 6.48Gy. In the case where the neutron dose was measured, the neutron dose (resulting in the use of high-energy X-ray beam) constituted about 0.5% of the therapeutic dose prescribed in PTV. The further from the field edge the higher the contribution of this secondary radiation dose (from 8% to ~45%). localization IMRT3DCRTTomotherapy mean dose [Gy] thyroid 2.88 ± ± ± lung 4.76 ± ± ± bladder ± ± ± testes 6.48 ± ± ± Localization Effective dose [mSv/Gy] Total neutron effective dose normalized to 76 Gy in PC [mSv] Mean normalized dose [mSv] in organ Slow neutrons Fast neutrons Pelvis – right side ± ± ± Pelvis – left side ± ± ± Right lung – the corner ± ± ± Left lung – the corner ± ± ± Right lung – the center ± ± ± Left lung – the center ± ± ± Right lung – the top ± ± ± Left lung – the top ± ± ± Right eye ± ± ± Left eye ± ± ± a)b) Fig. 1. Tomotherapy (a), 3DCRT (b) and IMRT (c) plan for anthropomorphic phantom for prostate with nodes case. Table 2. The results of measurements out of the field of irradiation for 3DCRT, IMRT and Tomotherapy. Table 3. The effective neutron dose estimated out of the field of irradiation for IMRT prostate radiotherapy. Figure 3. The distribution of neutron fluence during prostate IMRT normalized to 1 Gy of dose in PTV on the left side (a) and on the right side (b) of the anthropomorphic phantom. The slow ( ■ ) and the fast ( ■ ) components are separated, showing the decrease of total neutron fluence when the distance from the isocentre is increased. a) b)c)