- Email: [email protected]
Low statistics positron activity reconstruction methods for proton therapy
Accepted Manuscript Low statistics positron activity reconstruction methods for proton therapy S. Muraro, N. Camarlinghi, G. Battistoni, N. Belcari, M.G. Bisogni, L. Cristoforetti, A. Del Guerra, A. Ferrari, F. Fracchiolla, A. Kraan, M. Morrocchi, R. Righetto, P. Sala, M. Schwarz, G. Sportelli, A. Topi, V. Rosso PII: DOI: Reference:
S0168-9002(18)31330-5 https://doi.org/10.1016/j.nima.2018.10.015 NIMA 61337
To appear in:
Nuclear Inst. and Methods in Physics Research, A
Received date : 20 June 2018 Revised date : 1 October 2018 Accepted date : 2 October 2018 Please cite this article as: S. Muraro, et al., Low statistics positron activity reconstruction methods for proton therapy, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript Click here to view linked References
Low statistics positron activity reconstruction methods for proton therapy S. Muraroa∗, N. Camarlinghia∗, G. Battistonib , N. Belcaria , M.G. Bisognia , L. Cristoforettic , A. Del Guerraa , A. Ferrarid , F. Fracchiollac , A. Kraana , M. Morrocchia , R. Righettoc , P. Salab , M. Schwarzc,e , G. Sportellia , A. Topi f , V. Rossoa∗∗ a
f
Department of Physics, University of Pisa and INFN, Italy; b INFN, Milano, Italy; c Proton Therapy department, Trento Hospital, Trento, Italy; d CERN, Geneva, Switzerland ; e TIFPA INFN,Trento, Italy; Department of Physical Sciences, Earth and Environment, University of Siena and INFN, Italy.
Abstract Positron emission tomography (PET) is one of the most mature techniques for monitoring in ion beam therapy. PET allows to reconstruct the β+ activity generated in the patient by the nuclear interaction of the ions. Taking advantage of the spatial correlation between positron emitters created along the ions path and the dose distribution, it is possible to perform a quality control of the treatment. Usually, to reconstruct the activity generated within the irradiated volume, standard 3D PET reconstruction techniques are implemented. In this work, we explore a new reconstruction method (Straight Forward Reconstruction) particularly useful for reconstructing activity distribution generated by mono-energetic pencil beams. The method was validated by measuring the produced activity distribution with the DoPET system. Irradiations performed with mono-energetic pencil beams on phantoms mimicking human tissues were used for this study. Both reconstruction methods reach an accuracy in the reconstruction of the activity distribution width of the order of 1.5 mm for 2 · 108 primaries. Keywords: PET, proton therapy, medical image reconstruction methods and algorithms
1
1. Introduction
16 17
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Proton beam irradiations can deliver conformal dose distri18 butions minimizing damage to healthy tissues thanks to their characteristic dose profiles. Nuclear interactions between the ions and the irradiated tissues generate β+ emitters during the 19 treatment delivery. The detection of the activity signal can be 20 used to perform the treatment monitoring by means of dedi21 cated PET prototypes. In this work, a novel algorithm, named 22 Straight Forward Reconstruction (SFR), is proposed to recon23 struct the activity width generated during the treatment. The 24 SFR was compared with the standard Maximum Likelihood 25 Expectation Maximization (MLEM) reconstruction algorithm 26 using both experimental and simulation data. The data were acquired with the DoPET system at Trento Proton Therapy Centre, whereas the simulation was performed using the FLUKA 27 ∗ Equally
contributing authors author Email address: [email protected] (V. Rossoa )
∗∗ Corresponding
Nuclear Physics B
28 29 30
Monte Carlo code [1]. The dependence of the activity distribution width uncertainty vs the number of delivered protons was studied. 2. The DoPET system DoPET is a dual-head planar PET system. Each head is composed of 9 detector modules consisting of a LYSO matrix of 23 × 23 pixels, coupled to a position sensitive photomultiplier tube model H8500 (Hamamatsu Photonics). The heads are stationary and placed in-beam at a distance of 48 cm. The acquisition is FPGA-based and works with a coincidence time window of about 3 ns. For more details see [2] . 3. Reconstruction methods 3.1. MLEM MLEM is one of the most popular algorithms used in iterative PET reconstruction [3]. More details on the implementation of October 1, 2018
33 34 35 36 37 38 39 40 41 42 43
3.2. Straight Forward Reconstruction A fast and direct event reconstruction of the activity distribution along the beam direction was developed in order to asses variations in activity range. The SFR reconstructs the annihilation position of each event by evaluating the intersection point between the Line Of Response (LOR) and the plane parallel to the detector faces and passing through the beam axis: this last information is provided by the delivery system. Each event maintains information, such as annihilation time after the β+ emitters decay and is corrected to take into account the detector acceptance. The annihilation time can be correlated with the emission position in the phantom, allowing future frequency study in selected regions.
45
[a.u.]
0.025
SFR MLEM MC with SFR
SFR MLEM MC with SFR
0.02
0.02
0.015
0.015
0.01
0.01
0.005
0.005
0
0
20
40
60
80
100
120
140
0
160
0
20
40
60
80
100
120
140
z [mm]
160
z [mm]
Figure 1: Reconstructed longitudinal profiles (beam axis). Data (109 primary protons) are reconstructed with MLEM (hatched line) and SFR (dotted line) methods. The MC FLUKA prediction (red line) is reconstructed with the SFR method. PMMA (left) and” ZEBRA” (right) phantoms. Nr protons 200
∆W50 [mm]
44
0.025
4. Monte Carlo Simulations
400
600
800
1000
102
101
Nr protons ×10
200
6
∆W50 [mm]
32
the MLEM in the DoPET system can be found in in [4].
[a.u.]
31
400
600
800
1000
×10
6
107 106 105 104
100
103
53
The experimental set-up simulation is performed with the development version of the FLUKA code reproducing the conditions of irradiations performed at the Trento Proton Therapy Centre (IT). The generated β+ activity and annihilation products in space and time are recorded. The detector geometry is implemented and the expected activity distribution is reconstructed with the same reconstruction process used for the experimental data [5].
54
5. Results
46 47 48 49 50 51 52
99
55
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
SFR Data
101
MLEM Data
100
Fluka 20
40
60
80
100
120
SFR Data MLEM Data Fluka
99
97 140
Nr Coincidence/cm
20
40
60
80
100
120
Nr Coincidence/cm
Figure 2: The ∆W50% reconstruction capability of the two methods with variable statistical conditions; PMMA (left) and ”ZEBRA” (right) phantoms.
83
84 85
56
102
98
Two kinds of targets were used for this study: a uni- 86 form PMMA (Polymethylmethacrylate) phantom and a phan- 87 tom composed of alternating brain equivalent tissue and PMMA 88 slabs of 2 cm each (referred to as ”ZEBRA”). A mono- 89 energetic beam of 130 MeV was used for all the irradiations. 90 Only data acquired for 120 s starting immediately after the end 91 of the irradiation that lasted 8 s, were used for the analysis. 92 To evaluate the reconstruction capabilities of the two methods, 93 groups of 10 samples of different statistics were created, stat- 94 ing from an acquisition of 1010 primary protons and reducing, 95 step by step, the used statistics. The activity distribution width 96 was evaluated. The errors on the activity width reconstruc- 97 tion was computed as the standard deviation over the 10 samples. The comparison of the reconstructed longitudinal profile (beam axis) is presented in Figure 1. Data are reconstructed 98 with MLEM and SFR methods. The MC FLUKA prediction 99 is reconstructed with SFR method. We characterize the activ-100 ity range with the distance from the half maximum position of101 the rising edge to the half maximum position of the distal edge102 103 (∆W50% ). The error on the determination of ∆W50% of the two104 methods with variable statistical conditions is reported in Fig-105 ure 2. For spots of 2 × 108 and 109 primary protons, the error106 is 1.5 mm and 0.5 mm, respectively. MLEM and SFR give107 108 comparable results. In the case of the PMMA and ”ZEBRA”109 phantoms irradiation presented in this study, even if the shapes110 of the activated profiles loose reliability for 108 protons the in-111 formation on the ∆W50% is preserved with an error of ∼2.5 mm. 2
6. Conclusions A new reconstruction method for PET data was developed and gives comparable results in terms of ∆W50% with respect to MLEM and FLUKA predictions. Having two different procedure to evaluate the activity range allow us to be more confident on the results. For monitoring purposes a localized geometrical information is advisable and is achievable using pencil beams. To have an error on the determination of ∆W50% lower than 1.5 mm we found that a number of protons greater than 2 × 108 is necessary both for homogeneous and heterogeneous phantoms. This number is comparable with the number of protons delivered for each cm2 in the distal energy layer of a treatment plan of 1 Gy. In view of safety margin reduction and dose escalation, this approach opens up the possibility to image guidance procedures with selected pencil beams. References [1] G. Battistoni et al;. The FLUKA code: Description and benchmarking. AIP Conf. Proc., 896:31–49, 2007. [,31(2007)]. [2] E. Fabbiani et al.;. Proton therapy treatment monitoring with in-beam pet: Investigating space and time activity distributions. NIMA, 861:71, 2017. [3] L. A. Shepp and Y. Vardi;. Maximum likelihood reconstruction for emission tomography. IEEE Transactions on Medical Imaging, 1(2):113–122, Oct 1982. [4] N. Camarlinghi et al.;. An in-beam pet system for monitoring ionbeam therapy: test on phantoms using clinical 62 mev protons. JINST, 9(04):C04005, 2014. [5] S. Muraro et al;. Proton therapy treatment monitoring with the dopet system: activity range, positron emitters evaluation and comparison with monte carlo predictions. JINST, 12(12):C12026, 2017.
Research Highlights
Highlights PET is one of the most mature techniques for monitoring in ion beam therapy A new reconstruction method (Straight Forward Reconstruction) for PET is presented Activity distribution and accuracy by pencil beams irradiation are presented
S0168-9002(18)31330-5 https://doi.org/10.1016/j.nima.2018.10.015 NIMA 61337
To appear in:
Nuclear Inst. and Methods in Physics Research, A
Received date : 20 June 2018 Revised date : 1 October 2018 Accepted date : 2 October 2018 Please cite this article as: S. Muraro, et al., Low statistics positron activity reconstruction methods for proton therapy, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.10.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript Click here to view linked References
Low statistics positron activity reconstruction methods for proton therapy S. Muraroa∗, N. Camarlinghia∗, G. Battistonib , N. Belcaria , M.G. Bisognia , L. Cristoforettic , A. Del Guerraa , A. Ferrarid , F. Fracchiollac , A. Kraana , M. Morrocchia , R. Righettoc , P. Salab , M. Schwarzc,e , G. Sportellia , A. Topi f , V. Rossoa∗∗ a
f
Department of Physics, University of Pisa and INFN, Italy; b INFN, Milano, Italy; c Proton Therapy department, Trento Hospital, Trento, Italy; d CERN, Geneva, Switzerland ; e TIFPA INFN,Trento, Italy; Department of Physical Sciences, Earth and Environment, University of Siena and INFN, Italy.
Abstract Positron emission tomography (PET) is one of the most mature techniques for monitoring in ion beam therapy. PET allows to reconstruct the β+ activity generated in the patient by the nuclear interaction of the ions. Taking advantage of the spatial correlation between positron emitters created along the ions path and the dose distribution, it is possible to perform a quality control of the treatment. Usually, to reconstruct the activity generated within the irradiated volume, standard 3D PET reconstruction techniques are implemented. In this work, we explore a new reconstruction method (Straight Forward Reconstruction) particularly useful for reconstructing activity distribution generated by mono-energetic pencil beams. The method was validated by measuring the produced activity distribution with the DoPET system. Irradiations performed with mono-energetic pencil beams on phantoms mimicking human tissues were used for this study. Both reconstruction methods reach an accuracy in the reconstruction of the activity distribution width of the order of 1.5 mm for 2 · 108 primaries. Keywords: PET, proton therapy, medical image reconstruction methods and algorithms
1
1. Introduction
16 17
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Proton beam irradiations can deliver conformal dose distri18 butions minimizing damage to healthy tissues thanks to their characteristic dose profiles. Nuclear interactions between the ions and the irradiated tissues generate β+ emitters during the 19 treatment delivery. The detection of the activity signal can be 20 used to perform the treatment monitoring by means of dedi21 cated PET prototypes. In this work, a novel algorithm, named 22 Straight Forward Reconstruction (SFR), is proposed to recon23 struct the activity width generated during the treatment. The 24 SFR was compared with the standard Maximum Likelihood 25 Expectation Maximization (MLEM) reconstruction algorithm 26 using both experimental and simulation data. The data were acquired with the DoPET system at Trento Proton Therapy Centre, whereas the simulation was performed using the FLUKA 27 ∗ Equally
contributing authors author Email address: [email protected] (V. Rossoa )
∗∗ Corresponding
Nuclear Physics B
28 29 30
Monte Carlo code [1]. The dependence of the activity distribution width uncertainty vs the number of delivered protons was studied. 2. The DoPET system DoPET is a dual-head planar PET system. Each head is composed of 9 detector modules consisting of a LYSO matrix of 23 × 23 pixels, coupled to a position sensitive photomultiplier tube model H8500 (Hamamatsu Photonics). The heads are stationary and placed in-beam at a distance of 48 cm. The acquisition is FPGA-based and works with a coincidence time window of about 3 ns. For more details see [2] . 3. Reconstruction methods 3.1. MLEM MLEM is one of the most popular algorithms used in iterative PET reconstruction [3]. More details on the implementation of October 1, 2018
33 34 35 36 37 38 39 40 41 42 43
3.2. Straight Forward Reconstruction A fast and direct event reconstruction of the activity distribution along the beam direction was developed in order to asses variations in activity range. The SFR reconstructs the annihilation position of each event by evaluating the intersection point between the Line Of Response (LOR) and the plane parallel to the detector faces and passing through the beam axis: this last information is provided by the delivery system. Each event maintains information, such as annihilation time after the β+ emitters decay and is corrected to take into account the detector acceptance. The annihilation time can be correlated with the emission position in the phantom, allowing future frequency study in selected regions.
45
[a.u.]
0.025
SFR MLEM MC with SFR
SFR MLEM MC with SFR
0.02
0.02
0.015
0.015
0.01
0.01
0.005
0.005
0
0
20
40
60
80
100
120
140
0
160
0
20
40
60
80
100
120
140
z [mm]
160
z [mm]
Figure 1: Reconstructed longitudinal profiles (beam axis). Data (109 primary protons) are reconstructed with MLEM (hatched line) and SFR (dotted line) methods. The MC FLUKA prediction (red line) is reconstructed with the SFR method. PMMA (left) and” ZEBRA” (right) phantoms. Nr protons 200
∆W50 [mm]
44
0.025
4. Monte Carlo Simulations
400
600
800
1000
102
101
Nr protons ×10
200
6
∆W50 [mm]
32
the MLEM in the DoPET system can be found in in [4].
[a.u.]
31
400
600
800
1000
×10
6
107 106 105 104
100
103
53
The experimental set-up simulation is performed with the development version of the FLUKA code reproducing the conditions of irradiations performed at the Trento Proton Therapy Centre (IT). The generated β+ activity and annihilation products in space and time are recorded. The detector geometry is implemented and the expected activity distribution is reconstructed with the same reconstruction process used for the experimental data [5].
54
5. Results
46 47 48 49 50 51 52
99
55
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
SFR Data
101
MLEM Data
100
Fluka 20
40
60
80
100
120
SFR Data MLEM Data Fluka
99
97 140
Nr Coincidence/cm
20
40
60
80
100
120
Nr Coincidence/cm
Figure 2: The ∆W50% reconstruction capability of the two methods with variable statistical conditions; PMMA (left) and ”ZEBRA” (right) phantoms.
83
84 85
56
102
98
Two kinds of targets were used for this study: a uni- 86 form PMMA (Polymethylmethacrylate) phantom and a phan- 87 tom composed of alternating brain equivalent tissue and PMMA 88 slabs of 2 cm each (referred to as ”ZEBRA”). A mono- 89 energetic beam of 130 MeV was used for all the irradiations. 90 Only data acquired for 120 s starting immediately after the end 91 of the irradiation that lasted 8 s, were used for the analysis. 92 To evaluate the reconstruction capabilities of the two methods, 93 groups of 10 samples of different statistics were created, stat- 94 ing from an acquisition of 1010 primary protons and reducing, 95 step by step, the used statistics. The activity distribution width 96 was evaluated. The errors on the activity width reconstruc- 97 tion was computed as the standard deviation over the 10 samples. The comparison of the reconstructed longitudinal profile (beam axis) is presented in Figure 1. Data are reconstructed 98 with MLEM and SFR methods. The MC FLUKA prediction 99 is reconstructed with SFR method. We characterize the activ-100 ity range with the distance from the half maximum position of101 the rising edge to the half maximum position of the distal edge102 103 (∆W50% ). The error on the determination of ∆W50% of the two104 methods with variable statistical conditions is reported in Fig-105 ure 2. For spots of 2 × 108 and 109 primary protons, the error106 is 1.5 mm and 0.5 mm, respectively. MLEM and SFR give107 108 comparable results. In the case of the PMMA and ”ZEBRA”109 phantoms irradiation presented in this study, even if the shapes110 of the activated profiles loose reliability for 108 protons the in-111 formation on the ∆W50% is preserved with an error of ∼2.5 mm. 2
6. Conclusions A new reconstruction method for PET data was developed and gives comparable results in terms of ∆W50% with respect to MLEM and FLUKA predictions. Having two different procedure to evaluate the activity range allow us to be more confident on the results. For monitoring purposes a localized geometrical information is advisable and is achievable using pencil beams. To have an error on the determination of ∆W50% lower than 1.5 mm we found that a number of protons greater than 2 × 108 is necessary both for homogeneous and heterogeneous phantoms. This number is comparable with the number of protons delivered for each cm2 in the distal energy layer of a treatment plan of 1 Gy. In view of safety margin reduction and dose escalation, this approach opens up the possibility to image guidance procedures with selected pencil beams. References [1] G. Battistoni et al;. The FLUKA code: Description and benchmarking. AIP Conf. Proc., 896:31–49, 2007. [,31(2007)]. [2] E. Fabbiani et al.;. Proton therapy treatment monitoring with in-beam pet: Investigating space and time activity distributions. NIMA, 861:71, 2017. [3] L. A. Shepp and Y. Vardi;. Maximum likelihood reconstruction for emission tomography. IEEE Transactions on Medical Imaging, 1(2):113–122, Oct 1982. [4] N. Camarlinghi et al.;. An in-beam pet system for monitoring ionbeam therapy: test on phantoms using clinical 62 mev protons. JINST, 9(04):C04005, 2014. [5] S. Muraro et al;. Proton therapy treatment monitoring with the dopet system: activity range, positron emitters evaluation and comparison with monte carlo predictions. JINST, 12(12):C12026, 2017.
Research Highlights
Highlights PET is one of the most mature techniques for monitoring in ion beam therapy A new reconstruction method (Straight Forward Reconstruction) for PET is presented Activity distribution and accuracy by pencil beams irradiation are presented