Chul Hee Min

Development of array‐type prompt gamma measurement system for in vivo range verification in proton therapy

PURPOSE In vivo range verification is one of the most important parts of proton therapy to fully utilize its benefits delivering high radiation dose to tumor, while sparing the normal tissue with the so-called Bragg peak. Currently, however, range verification method is not used in clinics. The purpose of the present study is to optimize and evaluate the configuration of an array-type prompt gamma measurement system on determining distal dose edge for in vivo range verification of proton therapy. METHODS To effectively measure the prompt gammas against the background gammas, the Monte Carlo simulations with the MCNPX code were employed in optimizing the configuration of the measurement system, and the Monte Carlo method was also used to understand the effect of the background gammas, mainly neutron capture gammas, in the measured gamma distribution. To reduce the effect of the background gammas, the optimized energy window of 4-10 MeV in measuring the prompt gammas was employed. A parameterized source was used to maximize computation speed in the optimization study. A simplified test measurement system, using only one detector moving from one measurement location to the next, was constructed and applied to therapeutic proton beams of 80-220 MeV. For accurate determination of the distal dose edge, the sigmoidal curve-fitting method was applied to the measured distributions of the prompt gammas, and then, the location of the half-value between the maximum and minimum value in the curve-fitting was determined as the distal dose edge and compared with the beam range assessed by the proton dose distribution. RESULTS The parameterized source term employed in optimization process improved the calculation speed by up to ∼300 times. The optimization study indicates that an array-type measurement system with 3, 2, 2, and 150 mm for scintillator thickness, slit width, septal thickness, and slit length, respectively, can effectively measure the prompt gamma distributions minimizing the contribution of background gammas. The present results show that a few hundred counts of prompt gammas can be easily obtained by measuring 10 s at each measurement location for proton beams of ∼4 nA. The distal dose edges determined by the prompt gamma distribution are 5.45, 14.73, and 27.74 cm for proton beams of 5.17 (80 MeV), 14.99 (150 MeV), and 27.38 (220 MeV) cm, respectively. CONCLUSIONS The results show that the array-type measurement system can measure prompt gamma distributions from a therapeutic proton beam within a short measurement time, and that the distal dose edge can be determined within a few millimeters of error without using any sophisticated analysis.

Proton radiography and proton computed tomography based on time-resolved dose measurements

We present a proof of principle study of proton radiography and proton computed tomography (pCT) based on time-resolved dose measurements. We used a prototype, two-dimensional, diode-array detector capable of fast dose rate measurements, to acquire proton radiographic images expressed directly in water equivalent path length (WEPL). The technique is based on the time dependence of the dose distribution delivered by a proton beam traversing a range modulator wheel in passive scattering proton therapy systems. The dose rate produced in the medium by such a system is periodic and has a unique pattern in time at each point along the beam path and thus encodes the WEPL. By measuring the time dose pattern at the point of interest, the WEPL to this point can be decoded. If one measures the time–dose patterns at points on a plane behind the patient for a beam with sufficient energy to penetrate the patient, the obtained 2D distribution of the WEPL forms an image. The technique requires only a 2D dosimeter array and it uses only the clinical beam for a fraction of second with negligible dose to patient. We first evaluated the accuracy of the technique in determining the WEPL for static phantoms aiming at beam range verification of the brain fields of medulloblastoma patients. Accurate beam ranges for these fields can significantly reduce the dose to the cranial skin of the patient and thus the risk of permanent alopecia. Second, we investigated the potential features of the technique for real-time imaging of a moving phantom. Real-time tumor tracking by proton radiography could provide more accurate validations of tumor motion models due to the more sensitive dependence of proton beam on tissue density compared to x-rays. Our radiographic technique is rapid (~100 ms) and simultaneous over the whole field, it can image mobile tumors without the problem of interplay effect inherently challenging for methods based on pencil beams. Third, we present the reconstructed pCT images of a cylindrical phantom containing inserts of different materials. As for all conventional pCT systems, the method illustrated in this work produces tomographic images that are potentially more accurate than x-ray CT in providing maps of proton relative stopping power (RSP) in the patient without the need for converting x-ray Hounsfield units to proton RSP. All phantom tests produced reasonable results, given the currently limited spatial and time resolution of the prototype detector. The dose required to produce one radiographic image, with the current settings, is ~0.7 cGy. Finally, we discuss a series of techniques to improve the resolution and accuracy of radiographic and tomographic images for the future development of a full-scale detector.

Assessing the Clinical Impact of Approximations in Analytical Dose Calculations for Proton Therapy

PURPOSE To assess the impact of approximations in current analytical dose calculation methods (ADCs) on tumor control probability (TCP) in proton therapy. METHODS Dose distributions planned with ADC were compared with delivered dose distributions as determined by Monte Carlo simulations. A total of 50 patients were investigated in this analysis with 10 patients per site for 5 treatment sites (head and neck, lung, breast, prostate, liver). Differences were evaluated using dosimetric indices based on a dose-volume histogram analysis, a γ-index analysis, and estimations of TCP. RESULTS We found that ADC overestimated the target doses on average by 1% to 2% for all patients considered. The mean dose, D95, D50, and D02 (the dose value covering 95%, 50% and 2% of the target volume, respectively) were predicted within 5% of the delivered dose. The γ-index passing rate for target volumes was above 96% for a 3%/3 mm criterion. Differences in TCP were up to 2%, 2.5%, 6%, 6.5%, and 11% for liver and breast, prostate, head and neck, and lung patients, respectively. Differences in normal tissue complication probabilities for bladder and anterior rectum of prostate patients were less than 3%. CONCLUSION Our results indicate that current dose calculation algorithms lead to underdosage of the target by as much as 5%, resulting in differences in TCP of up to 11%. To ensure full target coverage, advanced dose calculation methods like Monte Carlo simulations may be necessary in proton therapy. Monte Carlo simulations may also be required to avoid biases resulting from systematic discrepancies in calculated dose distributions for clinical trials comparing proton therapy with conventional radiation therapy.

Range verification of passively scattered proton beams based on prompt gamma time patterns

We propose a proton range verification technique for passive scattering proton therapy systems where spread out Bragg peak (SOBP) fields are produced with rotating range modulator wheels. The technique is based on the correlation of time patterns of the prompt gamma ray emission with the range of protons delivering the SOBP. The main feature of the technique is the ability to verify the proton range with a single point of measurement and a simple detector configuration. We performed four-dimensional (time-dependent) Monte Carlo simulations using TOPAS to show the validity and accuracy of the technique. First, we validated the hadronic models used in TOPAS by comparing simulations and prompt gamma spectrometry measurements published in the literature. Second, prompt gamma simulations for proton range verification were performed for the case of a water phantom and a prostate cancer patient. In the water phantom, the proton range was determined with 2 mm accuracy with a full ring detector configuration for a dose of ~2.5 cGy. For the prostate cancer patient, 4 mm accuracy on range determination was achieved for a dose of ~15 cGy. The results presented in this paper are encouraging in view of a potential clinical application of the technique.

Mapping 15O Production Rate for Proton Therapy Verification

PURPOSE This work was a proof-of-principle study for the evaluation of oxygen-15 ((15)O) production as an imaging target through the use of positron emission tomography (PET), to improve verification of proton treatment plans and to study the effects of perfusion. METHODS AND MATERIALS Dynamic PET measurements of irradiation-produced isotopes were made for a phantom and rabbit thigh muscles. The rabbit muscle was irradiated and imaged under both live and dead conditions. A differential equation was fitted to phantom and in vivo data, yielding estimates of (15)O production and clearance rates, which were compared to live versus dead rates for the rabbit and to Monte Carlo predictions. RESULTS PET clearance rates agreed with decay constants of the dominant radionuclide species in 3 different phantom materials. In 2 oxygen-rich materials, the ratio of (15)O production rates agreed with the expected ratio. In the dead rabbit thighs, the dynamic PET concentration histories were accurately described using (15)O decay constant, whereas the live thigh activity decayed faster. Most importantly, the (15)O production rates agreed within 2% (P>.5) between conditions. CONCLUSIONS We developed a new method for quantitative measurement of (15)O production and clearance rates in the period immediately following proton therapy. Measurements in the phantom and rabbits were well described in terms of (15)O production and clearance rates, plus a correction for other isotopes. These proof-of-principle results support the feasibility of detailed verification of proton therapy treatment delivery. In addition, (15)O clearance rates may be useful in monitoring permeability changes due to therapy.

Evaluation of permanent alopecia in pediatric medulloblastoma patients treated with proton radiation

BackgroundTo precisely calculate skin dose and thus to evaluate the relationship between the skin dose and permanent alopecia for pediatric medulloblastoma patients treated with proton beams.MethodsThe dosimetry and alopecia outcomes of 12 children with medulloblastoma (ages 4-15 years) comprise the study cohort. Permanent alopecia was assessed and graded after completion of the entire therapy. Skin threshold doses of permanent alopecia were calculated based on the skin dose from the craniospinal irradiation (CSI) plan using the concept of generalized equivalent uniform dose (gEUD) and accounting for chemotherapy intensity. Monte Carlo simulations were employed to accurately assess uncertainties due to beam range prediction and secondary particles.ResultsIncreasing the dose of the CSI field or the dose given by the boost field to the posterior fossa increased total skin dose delivered in that region. It was found that permanent alopecia could be correlated with CSI dose with a threshold of about 21 Gy (relative biological effectiveness, RBE) with high dose chemotherapy and 30 Gy (RBE) with conventional chemotherapy.ConclusionsOur results based on 12 patients provide a relationship between the skin dose and permanent alopecia for pediatric medulloblastoma patients treated with protons. The alopecia risk as assessed with gEUD could be predicted based on the treatment plan information.

Two-Dimensional Prompt Gamma Measurement Simulation for In Vivo Dose Verification in Proton Therapy: A Monte Carlo Study

Abstract In proton therapy, accurate verification of in vivo dose distribution is necessary to ensure not only the safety of the patient but also the success of the treatment itself. It has been shown, both by Monte Carlo simulations and by limited experiments, that the proton beam range in a patient can be accurately determined by measuring the distribution of the prompt gammas generated from proton-induced nuclear interactions. In the present study, a two-dimensional (2-D) prompt gamma detection system incorporating a 51 (longitudinal) × 21 (lateral) detector array was designed and tested by Monte Carlo simulations using the MCNPX code. Additionally, the detection probability of the prompt gammas per primary proton was calculated for different proton energies. Despite the increase of the beam dispersion effect and background gammas with the increase of the proton energy, our simulation results clearly showed that it is possible to measure the 2-D distribution of prompt gammas up to 150 MeV using the 2-D prompt gamma detection system.

Experimental Test of Double-Layer Method for Industrial SPECT

Abstract In industrial-type single-photon-emission computed tomography (SPECT) systems, the use of relatively large detectors and collimators for effective detection of high-energy gammas significantly limits imaging performance, primarily because of insufficient measurement points. In the present study, a simple but very effective image-quality improvement method, the double-layer method, was tested. In this method, two layers of identical SPECT systems are employed in order to increase the number of measurement points and, thereby, improve the image quality. For experimentation, the two identical detector layers were arranged for 30 deg of rotation with respect to each other. The results showed that the double-layer method indeed significantly improves the image quality of the industrial SPECT system, substantially reducing errors in source size and location for both low-energy (99mTc) and high-energy (113mIn) gamma sources.

Determination of Optimal Energy Window for Measurement of Prompt Gammas from Proton Beam by Monte Carlo Simulations

The range of a proton beam in a patient can be determined by scanning the distribution of the prompt gammas emitted from the beam passage. However, this method suffers from a high level of background gammas, especially in the case of high energy proton beams. The present study determined the optimal energy window for selective measurement of the prompt gammas, effectively discriminating background gammas, for a prompt gamma scanning system. To that end, the energy spectra of the prompt and background gammas were calculated by transporting the protons and other secondary particles with MCNPX. A detailed analysis of these spectra revealed that the optimal energy window is 4–10 MeV. The application of the energy window to simulated and measured data confirmed that the range of a proton beam in a patient can be determined much more accurately by employing the optimal energy window.

Effective dose evaluation of NORM-added consumer products using Monte Carlo simulations and the ICRP computational human phantoms

The aim of this study is to evaluate the potential hazard of naturally occurring radioactive material (NORM) added consumer products. Using the Monte Carlo method, the radioactive products were simulated with ICRP reference phantom and the organ doses were calculated with the usage scenario. Finally, the annual effective doses were evaluated as lower than the public dose limit of 1mSv y(-1) for 44 products. It was demonstrated that NORM-added consumer products could be quantitatively assessed for the safety regulation.