SU-E-T-630: MCO-Informed VMAT Planning for Prostate Cancer.

PURPOSE: To develop a VMAT optimization procedure using information from Multi-Criteria Optimization of IMRT plans and to perform a treatment planning comparison for prostate cancer patients. METHODS: IMRT plans using Multi-Criteria Optimization (MCO), 6 MV photons, 20 and 7 treatment fields were generated for 10 prostate patients in the RayStation treatment planning system (Version 2.2.13, Raysearch Laboratories, Stockholm, Sweden). The prescription dose was 7560 cGy to the prostate PTV and 5796 cGy to the seminal vesicles, using a simultaneous integrated boost technique. The resulting DVH parameters of the 20 field IMRT-MCO plan were used as initial optimization parameters for VMAT planning. The initial VMAT plan for each patient was further optimized by adjusting the optimization objectives/constraints. Final plan quality was compared using a homogeneity index (HI) and D98 for PTV-prostate, V70 and V75 for anterior rectum and V70 for bladder. Moreover, delivery efficiency of VMAT and the 7 field MCO-IMRT plans was also evaluated. RESULTS: All plans fulfilled the standard clinical objectives. The average HI of the PTV-prostate was 0.11 for VMAT, 0.13 for 20 field IMRT-MCO and 0.12 for 7 field IMRT-MCO, respectively. Average D98 values were 7191, 7294 and 7305 cGy for VMAT, 20 field IMRT-MCO and 7 field IMRT-MCO, respectively. For organ-at-risk (OAR), V70 and V75 for anterior rectum and V70 for bladder were within 3%. Analysis of delivery efficiency shows the estimated delivery time of VMAT is less than 2 minutes, while it is 7 min for 7 field IMRT-MCO. CONCLUSIONS: MCO-informed VMAT optimization is a useful way to generate optimal VMAT plans. The resulting VMAT plan quality essentially matched the MCO-IMRT plan but with a shorter delivery time. Dose homogeneity of VMAT is slight superior compared to IMRT-MCO while the cold spots are slightly inferior. Furthermore, there is no clinically significant difference in OAR sparing. Funding support provided by NCI Federal Share Proton Beam Program Income Grant and Raysearch Laboratories.

Effects of organ motion on IMRT treatments with segments of few monitor units.

Interplay between organ (breathing) motion and leaf motion has been shown in the literature to have a small dosimetric impact for clinical conditions (over a 30 fraction treatment). However, previous studies did not consider the case of treatment beams made up of many few-monitor-unit (MU) segments, where the segment delivery time (1-2 s) is of the order of the breathing period (3-5 s). In this study we assess if breathing compromises the radiotherapy treatment with IMRT segments of low number of MUs. We assess (i) how delivered dose varies, from patient to patient, with the number of MU per segment, (ii) if this delivered dose is identical to the average dose calculated without motion over the path of the motion, and (iii) the impact of the daily variation of the delivered dose as a function of MU per segment. The organ motion was studied along two orthogonal directions, representing the left-right and cranial-caudal directions of organ movement for a patient setup in the supine position. Breathing motion was modeled as sin(x), sin4(x), and sin6(x), based on functions used in the literature to represent organ motion. Measurements were performed with an ionization chamber and films. For a systematic study of motion effects, a MATLAB simulation was written to model organ movement and dose delivery. In the case of a single beam made up of one single segment, the dose delivered to point in a moving target over 30 fractions can vary up to 20% and 10% for segments of 10 MU and 20 MU, respectively. This dose error occurs because the tumor spends most of the time near the edges of the radiation beam. In the case of a single beam made of multiple segments with low MU, we observed 2.4%, 3.3%, and 4.3% differences, respectively, for sin(x), sin4(x), and sin6(x) motion, between delivered dose and motion-averaged dose for points in the penumbra region of the beam and over 30 fractions. In approximately 5-10% of the cases, differences between the motion-averaged dose and the delivered 30-fraction dose could reach 6%, 8% and 10-12%, respectively for sin(x), sin4(x), and sin6(x) motion. To analyze a clinical IMRT beam, two patient plans were randomly selected. For one of the patients, the beams showed a likelihood of up to 25.6% that the delivered dose would deviate from the motion-averaged dose by more than 1%. For the second patient, there was a likelihood of up to 62.8% of delivering a dose that differs by more than 1% from the motion-averaged dose and a likelihood of up to approximately 30% for a 2% dose error. For the entire five-beam IMRT plan, statistical averaging over the beams reduces the overall dose error between the delivered dose and the motion-averaged dose. For both patients there was a likelihood of up to 7.0% and 33.9% that the dose error was greater than 1%, respectively. For one of the patients, there was a 12.6% likelihood of a 2% dose error. Daily intrafraction variation of the delivered dose of more than 10% is non-negligible and can potentially lead to biological effects. We observed [for sin(x), sin4(x), and sin6(x)] that below 10-15 MU leads to large daily variations of the order of 15-35%. Therefore, for small MU segments, non-negligible biological effects can be incurred. We conclude that for most clinical cases the effects may be small because of the use of many beams, it is desirable to avoid low-MU segments when treating moving targets. In addition, dose averaging may not work well for hypo-fractionation, where fewer fractions are used. For hypo-fractionation, PDF modeling of the tumor motion in IMRT optimization may not be adequate.

In-phantom dosimetry for the 13C(d,n)14N reaction as a source for accelerator-based BNCT.

The use of the 13C(d,n) 14N reaction at Ed=1.5 MeV for accelerator-based boron neutron capture therapy (AB-BNCT) is investigated. Among the deuteron-induced reactions at low incident energy, the 3C(d,n)14N reaction turns out to be one of the best for AB-BNCT because of beneficial materials properties inherent to carbon and its relatively large neutron production cross section. The deuteron beam was produced by a tandem accelerator at MIT's Laboratory for Accelerator Beam Applications (LABA) and the neutron beam shaping assembly included a heavy water moderator and a lead reflector. The resulting neutron spectrum was dosimetrically evaluated at different depths inside a water-filled brain phantom using the dual ionization chamber technique for fast neutrons and photons and bare and cadmium-covered gold foils for the thermal neutron flux. The RBE doses in tumor and healthy tissue were calculated from experimental data assuming a tumor 10B concentration of 40 ppm and a healthy tissue 10B concentration of 11.4 ppm (corresponding to a reported ratio of 3.5:1). All results were simulated using the code MCNP, a general Monte Carlo radiation transport code capable of simulating electron, photon, and neutron transport. Experimental and simulated results are presented at 1, 2, 3, 4, 6, 8, and 10 cm depths along the brain phantom centerline. An advantage depth of 5.6 cm was obtained for a treatment time of 56 min assuming a 4 mA deuteron current and a maximum healthy tissue dose of 12.5 RBE Gy.

Characterization of a soft X-ray source for intravascular radiation therapy.

PURPOSE: A soft X-ray device for intravascular radiation therapy of restenosis is characterized in terms of dose delivery for several artery configurations, including arteries with implanted stents, calcified plaque, and noncentered sources. METHODS AND MATERIALS: The Monte Carlo code MCNP4B was used to determine the X-ray fluence and energy spectra for 15, 20, and 30-kV X-ray source generating voltages. Dose as a function of distance was calculated under a variety of artery conditions. RESULTS: Calculated depth-dose profiles for the X-ray sources are within presumed artery dose tolerance limits for the range of generating voltages considered. Treatment times to deliver 8 Gy to the adventitia range from 2.7 minutes to 6.7 minutes for the 20-kV generating voltage and a 3-cm-long lesion, depending on the diameter of the artery. The does perturbation due to stent wires or calcified plaque is found to be more severe for the X-ray sources than for the radioactive sources. The effects of noncentering are found to be similar for radioactive sources and X-ray sources with generating voltages of 20 kV or higher. CONCLUSION: The results of this study indicate that soft X-ray sources are suitable candidates for intravascular radiation therapy over a wide range of artery sizes, tissue compositions, and stent configurations.

The feasibility of accelerator-based in vivo neutron activation analysis of nitrogen.

The feasibility of accelerator-based in vivo neutron activation analysis of nitrogen has been investigated. It was found that a moderated neutron flux from approximately 10 microA of 2.5 MeV protons on a 9Be target performed as well as, and possibly slightly better than the existing isotope-based approach in terms of net counts per unit subject dose. Such a system may be an attractive alternative to the widespread use of (238,239)Pu/Be or 252Cf neutron sources, since there is more flexibility in the energy spectrum generated by accelerator-based neutron sources. From a radiation safety standpoint, accelerators have the advantage in that they only produce radiation when in operation. Furthermore, an accelerator beam can be pulsed, to reduce background detected in the prompt-gamma measurement, and such a device has a wide range of additional biological and medical applications.

An investigation of the feasibility of gadolinium for neutron capture synovectomy.

Neutron capture synovectomy (NCS) has been proposed as a possible treatment modality for rheumatoid arthritis. Neutron capture synovectomy is a two-part modality, in which a compound containing an isotope with an appreciable thermal neutron capture cross section is injected directly into the joint, followed by irradiation with a neutron beam. Investigations to date for NCS have focused on boron neutron capture synovectomy (BNCS), which utilizes the 10B(n,alpha)7Li nuclear reaction to deliver a highly localized dose to the synovium. This paper examines the feasibility of gadolinium, specifically 157Gd, as an alternative to boron as a neutron capture agent for NCS. This alternative modality is termed Gadolinium Neutron Capture Synovectomy, or GNCS. Monte Carlo simulations have been used to compare 10B and 157Gd as isotopes for accelerator-based NCS. The neutron source used in these calculations was a moderated spectrum from the 9Be(p,n) reaction at a proton energy of 4 MeV. The therapy time to deliver the NCS therapeutic dose of 10000 RBE-cGy, is 27 times longer when 157Gd is used instead of 10B. The skin dose to the treated joint is 33 times larger when 157Gd is used instead of 10B. Furthermore, the impact of using 157Gd instead of 10B was examined in terms of shielded whole-body dose to the patient. The effective dose is 202 mSv for GNCS, compared to 7.6 mSv for BNCS. This is shown to be a result of the longer treatment times required for GNCS; the contribution of the high-energy photons emitted from neutron capture in gadolinium is minimal. Possible explanations as to the relative performance of 157Gd and 10B are discussed, including differences in the RBE and range of boron and gadolinium neutron capture reaction products, and the relative values of the 10B and 157Gd thermal neutron capture cross section as a function of neutron energy.

Development and construction of a neutron beam line for accelerator-based boron neutron capture synovectomy.

A potential application of the 10B(n, alpha)7Li nuclear reaction for the treatment of rheumatoid arthritis, termed Boron Neutron Capture Synovectomy (BNCS), is under investigation. In an arthritic joint, the synovial lining becomes inflamed and is a source of great pain and discomfort for the afflicted patient. The goal of BNCS is to ablate the synovium, thereby eliminating the symptoms of the arthritis. A BNCS treatment would consist of an intra-articular injection of boron followed by neutron irradiation of the joint. Monte Carlo radiation transport calculations have been used to develop an accelerator-based epithermal neutron beam line for BNCS treatments. The model includes a moderator/reflector assembly, neutron producing target, target cooling system, and arthritic joint phantom. Single and parallel opposed beam irradiations have been modeled for the human knee, human finger, and rabbit knee joints. Additional reflectors, placed to the side and back of the joint, have been added to the model and have been shown to improve treatment times and skin doses by about a factor of 2. Several neutron-producing charged particle reactions have been examined for BNCS, including the 9Be(p,n) reaction at proton energies of 4 and 3.7 MeV, the 9Be(d,n) reaction at deuteron energies of 1.5 and 2.6 MeV, and the 7Li(p,n) reaction at a proton energy of 2.5 MeV. For an accelerator beam current of 1 mA and synovial boron uptake of 1000 ppm, the time to deliver a therapy dose of 10,000 RBEcGy ranges from 3 to 48 min, depending on the treated joint and the neutron producing charged particle reaction. The whole-body effective dose that a human would incur during a knee treatment has been estimated to be 3.6 rem or 0.75 rem, for 1000 ppm or 19,000 ppm synovial boron uptake, respectively, although the shielding configuration has not yet been optimized. The Monte Carlo design process culminated in the construction, installation, and testing of a dedicated BNCS beam line on the high-current tandem electrostatic accelerator at the Laboratory for Accelerator Beam Applications at the Massachusetts Institute of Technology.