Intensity modulated proton therapy

Intensity modulated proton therapy (IMPT) implies the electromagnetic spatial control of well-circumscribed “pencil beams” of protons of variable energy and intensity. Proton pencil beams take advantage of the charged-particle Bragg peak—the characteristic peak of dose at the end of range—combined with the modulation of pencil beam variables to create target-local modulations in dose that achieves the dose objectives. IMPT improves on X-ray intensity modulated beams (intensity modulated radiotherapy or volumetric modulated arc therapy) with dose modulation along the beam axis as well as lateral, in-field, dose modulation. The clinical practice of IMPT further improves the healthy tissue vs target dose differential in comparison with X-rays and thus allows increased target dose with dose reduction elsewhere. In addition, heavy-charged-particle beams allow for the modulation of biological effects, which is of active interest in combination with dose “painting” within a target. The clinical utilization of IMPT is actively pursued but technical, physical and clinical questions remain. Technical questions pertain to control processes for manipulating pencil beams from the creation of the proton beam to delivery within the patient within the accuracy requirement. Physical questions pertain to the interplay between the proton penetration and variations between planned and actual patient anatomical representation and the intrinsic uncertainty in tissue stopping powers (the measure of energy loss per unit distance). Clinical questions remain concerning the impact and management of the technical and physical questions within the context of the daily treatment delivery, the clinical benefit of IMPT and the biological response differential compared with X-rays against which clinical benefit will be judged. It is expected that IMPT will replace other modes of proton field delivery. Proton radiotherapy, since its first practice 50 years ago, always required the highest level of accuracy and pioneered volumetric treatment planning and imaging at a level of quality now standard in X-ray therapy. IMPT requires not only the highest precision tools but also the highest level of system integration of the services required to deliver high-precision radiotherapy.The practice of proton radiotherapy covers 50 years since the first proton patient at the Berkeley Lawrence Livermore Laboratory (Berkeley, CA). In that period, a few post-research proton accelerators have been transformed into semi-clinical facilities and commenced treatments. One such facility at the Harvard Cyclotron Laboratory (Cambridge, MA) had a 160 MeV accelerator well suited for the treatment of cranial neoplasms¹ in parallel with similar practice in Sweden,² eyes³ and large field treatments.⁴ These sites were managed in three semi-independent clinical programmes that persist today at the F H Burr Proton Therapy Center at the Massachusetts General Hospital in Boston.The large field programme required the development of proton field scattering and energy modulation techniques to achieve uniform fields and spread-out Bragg peak modulated (SOBP) fields of constant penetration range and modulation. The large field programme was only possible after the introduction of CT to model these fields, with apertures and range compensators to control the lateral extent and penetration around the three dimensional (3D) target volume extent as identified on CT.^5,6 The fields were created by mechanical means, which allowed their early clinical use in the absence of electronic controls.The practice of SOBP proton radiotherapy required all the quality management features of modern radiotherapy: volumetric treatment planning, accurate immobilization and verification and on-treatment imaging. The practice of SOBP proton radiotherapy established the axiom of radiotherapy: accuracy improves healthy tissue dose avoidance and target coverage and higher target dose achieves cure. The promise and realization of cure was demonstrated in patients with otherwise incurable chordoma.^7,8 The practice of SOBP proton radiotherapy persists today, and most patients are still treated with SOBP fields.The primary proton beam out of an accelerator is, in the absence of scattering materials, a collimated well-circumscribed “pencil” beam and easily manipulated by electromagnetic means. The proton pencil beam allows dose modulation in the patient with four degrees of freedom: number of protons (NP) to control the local dose deposition, energy to control the local penetration and magnetic deflection to control the off-axis position. The size of the pencil beam is a fifth degree of freedom although not readily available. Spot size control would positively impact delivery efficiency, as “larger” spots can deliver more protons in vivo given safety constraints (see section on back-of-the-envelope calculations), albeit possibly with an increase of integral dose. The spot size is typically characterized by the gaussian width σ of the pencil beam lateral intensity distribution and quantified in air at the isocentre.Proton pencil beams thus have one (or two) more degrees of freedom, penetration dose modulation, compared with intensity modulated radiotherapy [IMRT or volumetric arc therapy (VMAT)] fields. Proton fields (at dose equilibrium) exhibit the charged-particle Bragg peak depth dose characterized by a sharp dose increase, the “spot” at the energy characteristic penetration range and absence of dose beyond this distal range. The full electromagnetic control of the heavy-charged-particle pencil beam combined with the Bragg peak and absence of distal dose makes pencil beam scanning (PBS) an easier and more powerful delivery system for modulated therapy compared with the mechanical multileaf collimators (MLCs) required in X-ray IMRT (or VMATs), as well as the creation of SOBP fields.We use the label “pencil beam (spot) scanning” (where “spot” refers to the location of the Bragg peak in the patient) for the technical mode of delivery and the label “intensity modulated proton therapy (IMPT)” for the clinical mode of PBS where each individual field is allowed to assume an arbitrary dose distribution, and only the full set of fields in the treatment fraction, as in IMRT, assumes the desired dose fraction distribution. Other clinical modes exist, but IMPT is simply the desired, although presumably the most challenging, goal of PBS and our focus here.Clinical PBS was systematically developed and applied at the Paul Scherrer Institute in Villigen Switzerland.^9,10 Their original clinical system consisted of a very compact isocentric gantry combined with a couch and a scanning system that scanned a single line of pencil beams (i.e. irradiating planes in the patient) and thus required patient movement to accommodate multiple planes. The gantry transported protons at a fixed set of constant energies, whose energy at the patient was modulated by a set of mechanical degraders. The system implemented full modulation of all pencil beam parameters, albeit by considerable mechanical means. This unique design demonstrates, amongst other things, the possible variability of delivery systems, although all modern systems employ near-complete electromagnetic modes to implement scanning. Nevertheless, modern system designs and choice of components will influence the technical and clinical quality of scanning.As stated, technical, physical and clinical challenges remain for the effective clinical deployment of IMPT. A pre-IMPT point–counterpoint argued that while IMPT may in-silico outperform IMRT, its expense and complexity exceeds that of IMRT and that of SOBP treatments.¹¹ A rebuttal¹² argued that IMPT will become generally available and its use necessary to fully exploit the dosimetric advantages of proton radiotherapy. Indeed, IMPT (or more precisely PBS) delivery technology is now standard and is, in fact, more cost-effective and simpler in terms of commissioning¹³ and operation compared with other delivery modes of proton radiotherapy. Overall costs, depending on accounting, are generally assumed to be twice that of IMRT and remain an issue.The sections below elaborate on these individual issues. We argue that clinical IMPT requires a system approach whereby the current (i.e. in X-ray radiotherapy) individuality of treatment management components must be integrated to achieve optimal performance. Optimal performance combined with exploitation of dosimetric advantages, in turn, can lead to an improved cost profile. The hypothesis is if IMRT is cost-effective in some end point (see, for example, Kohler et al¹⁴), then IMPT can exceed this cost-effectiveness criterion through additional dose advantages or through increased performance such as may be achieved through hypofractionation.     

Intensity modulated radiation therapy versus three dimensional conformal radiation therapy for treatment of high grade glioma: a radiobiological modeling study

Treatment of glioblastoma results in a median survival of 12 months. Radiation dose escalation trials for high grade gliomas have resulted in modest improvements in survival in selected patients with small peripheral tumors at the expense of normal brain toxicity. Neurotoxicity includes radiation necrosis but it is increasingly recognized that long-term survivors may develop neuro-cognitive deficits. Tumor control probability (TCP) and normal tissue complication probability (NTCP) are radiobiological models used to predict treatment outcomes. This study assesses the impact of radiation dose escalation from 59.6 Gy to 90 Gy on TCP and NTCP in ten patients planned with Three Dimensional Conformal Therapy (3DCRT) and Intensity Modulated Radiation Therapy (IMRT). No difference in TCP was observed between 3DCRT and IMRT at doses of 59.4 Gy and 90 Gy. However, dose escalation to 90 Gy resulted in about 25% relative TCP increase. Compared to 3DCRT, dose escalation with IMRT significantly reduced NTCP by 70% (10.75% v. 3.75%, respectively). As a result, highly conformal techniques are recommended to obviate radiation exposure of normal brain especially when radiation dose escalation is used. Further understanding of the molecular mechanisms underlying neurotoxicity will allow the development of more precise radiobiological predictive models and of approaches to prevent or treat radiation-induced brain damage.     

Volumetric modulated arc therapy: a review of current literature and clinical use in practice

Volumetric modulated arc therapy (VMAT) is a novel radiation technique, which can achieve highly conformal dose distributions with improved target volume coverage and sparing of normal tissues compared with conventional radiotherapy techniques. VMAT also has the potential to offer additional advantages, such as reduced treatment delivery time compared with conventional static field intensity modulated radiotherapy (IMRT). The clinical worldwide use of VMAT is increasing significantly. Currently the majority of published data on VMAT are limited to planning and feasibility studies, although there is emerging clinical outcome data in several tumour sites. This article aims to discuss the current use of VMAT techniques in practice and review the available data from planning and clinical outcome studies in various tumour sites including prostate, pelvis (lower gastrointestinal, gynaecological), head and neck, thoracic, central nervous system, breast and other tumour sites.     

Comparison of forward planned conformal radiation therapy and inverse planned intensity modulated radiation therapy for esthesioneuroblastoma

The purpose of this study was to compare dose distribution of inverse planned intensity modulated radiation therapy (IMRT) with that of conformal radiation therapy (SCRT) in the treatment of esthesioneuroblastoma, and to report initial clinical results. 13 patients with esthesioneuroblastoma were planned both with IMRT and SCRT using complete three-dimensional data sets. A target dose of 60 Gy was prescribed. We performed a detailed dose volume histogram analysis. Dose coverage was equal in both plans while dose distribution was more conformal to the target volume with IMRT. Mean and maximum dose of the brain stem, chiasm, optic nerves and orbits were lower using IMRT than SCRT. The reduction was significant regarding orbit and optic nerve (p<0.05). IMRT was superior in sparing of organs at risk compared with SCRT. The additional sparing by IMRT was positively correlated to the size of the target volume, which was evident with target volumes above 200 cm3. Treatment time was approximately 20 minutes per fraction using IMRT compared with 15 minutes per fraction using SCRT. We conclude that IMRT is both feasible and a valuable tool for more conformal dose distribution in the treatment of esthesioneuroblastoma and to spare organs at risk that are in critical relationship to the tumour. This advantage could be seen especially well in complex shaped target volumes above 200 cm3. Thus, using IMRT, risk of complications may be minimized and local tumour control may be increased.     

Stereotactic ablative radiation therapy for brain metastases with volumetric modulated arc therapy and flattening filter free delivery: feasibility and early clinical results

Aim

For selected patients with brain metastases (BMs), the role of stereotactic radiosurgery (SRS) or fractionated stereotactic radiotherapy (SFRT) is well recognized. The recent introduction of flattening filter free (FFF) delivery during linac-based SRS or SFRT allows shorter beam-on-time, improving patients’ comfort and facility workflow. Nevertheless, limited experiences evaluated the impact of FFF linac-based SRS and SFRT in BMs treatment. Aim of the current study was to analyze SRS/SFRT linac-based FFF delivery for BMs in terms of dosimetric and early clinical results.

Materials and methods

Patients with life expectancy >3 months, number of BMs <5, diameter <3 cm, and controlled or synchronous primary tumor received SRS/SFRT. The prescribed total dose and fractionation, based on BMs size and proximity to organs at risk, ranged from 15 Gy in 1 fraction to 30 Gy in 5 fractions. A FFF volumetric modulated arc therapy (VMAT) plan was generated with one or two coplanar partial arcs. Toxicity was assessed according to CTCAE v4.0.

Results

From April 2014 to February 2016, 45 patients (89 BMs) were treated with SRS/SFRT linac-based FFF delivery. The mean beam-on-time was 140 s for each lesion (range 90–290 s) and the average brain Dmean was 1 Gy (range 0.1–4.8 Gy). At the time of analysis, local control was reported in 93.2% (83/89 BMs). With a median follow-up time of 12 months (range 1–27 months), the median overall survival was 14 months and the 6-month overall survival was 77%. Finally, the median intracranial disease control was 11 months. Acute and late toxicities were acceptable without severe events (no adverse events ≥G2 were recorded).

Conclusions

These preliminary results highlighted the feasibility and safety of linac-based SRS/SFRT with FFF mode for BMs patients. A longer follow-up is necessary to confirm the efficacy of this treatment modality in BM patients.

     

Simultaneous integrated boost-intensity modulated radiation therapy for inoperable hepatocellular carcinoma

Purpose

The aim of this work was to evaluate the clinical efficacy and safety of simultaneous integrated boost-intensity modulated radiation therapy (SIB-IMRT) in patients with inoperable hepatocellular carcinoma (HCC).

Methods and materials

A total of 53 patients with inoperable HCC underwent SIB-IMRT using two dose-fractionation schemes, depending on the proximity of gastrointestinal structures. The 41 patients in the low dose-fractionation (LD) group, with internal target volume (ITV) Results Overall, treatment was well tolerated, with no grade >?3 toxicity. The LD group had larger sized tumors (median: 6 vs. 3.4 cm) and greater frequencies of vascular invasion (80.6 vs. 16.7?%) than patients in the HD group (p?p?=?0.039) and 2-year LPFS (85.7 vs. 59?%, p?=?0.119), RFS (38.1 vs. 7.3?%, p?=?0.063), and OS (83.3 vs. 44.3?%, p?=?0.037) rates than the LD group. Multivariate analysis showed that tumor response was significantly associated with OS.

Conclusion

SIB-IMRT is feasible and safe for patients with inoperable HCC.     

MRI扩散加权成像在鼻咽癌调强放疗疗效评价中的应用价值

目的 探讨磁共振扩散加权成像(diffusion weighted imaging,DWI)在鼻咽癌调强放射治疗中的临床应用价值.方法 回顾性分析经病理检查证实的60例初诊鼻咽癌患者,均行调强放射治疗,放射治疗前1周内和放射治疗50Gy时均行MRI及DWI检查,比较放疗前后MRI及DWI-MRI情况.结果 56例放疗后肿瘤病灶缩小,3例变化不明显,1例增大,与放疗前比较差异具有统计学意义(P＜0.05).放疗后弥散加权成像信号减低,表观弥散系数(apparent diffusion coefficient,ADC)值增大,与放疗前比较差异具有统计学意义(P＜0.05).52例颈部转移性淋巴结缩小.结论 MRI扩散加权成像可快速,灵敏显示鼻咽癌放疗前后的变化,对IMRT的疗效评价具有很好的临床应用价值.     

Leakage-Penumbra effect in intensity modulated radiation therapy step-and-shoot dose delivery

AIM: To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT). METHODS: Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle³ treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0^o with equal separation of 72^o, was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations. RESULTS: A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams. CONCLUSION: We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.     

EBT GAFCHROMICTM film dosimetry in compensator-based intensity modulated radiation therapy

The electron benefit transfer (EBT) GAFCHROMIC films possess a number of features making them appropriate for high-quality dosimetry in intensity-modulated radiation therapy (IMRT). Compensators to deliver IMRT are known to change the beam-energy spectrum as well as to produce scattered photons and to contaminate electrons; therefore, the accuracy and validity of EBT-film dosimetry in compensator-based IMRT should be investigated. Percentage-depth doses and lateral-beam profiles were measured using EBT films in perpendicular orientation with respect to 6 and 18 MV photon beam energies for: (1) different thicknesses of cerrobend slab (open, 1.0, 2.0, 4.0, and 6.0 cm), field sizes (5×5, 10×10, and 20×20 cm²), and measurement depths (D_max, 5.0 and 10.0 cm); and (2) step-wedged compensator in a solid phantom. To verify results, same measurements were implemented using a 0.125 cm³ ionization chamber in a water phantom and also in Monte Carlo simulations using the Monte Carlo N-particle radiation transport computer code. The mean energy of photons was increased due to beam hardening in comparison with open fields at both 6 and 18 MV energies. For a 20×20 cm² field size of a 6 MV photon beam and a 6.0 cm thick block, the surface dose decreased by about 12% and percentage-depth doses increased up to 3% at 30.0 cm depth, due to the beam-hardening effect induced by the block. In contrast, at 18 MV, the surface dose increased by about 8% and depth dose reduced by 3% at 30.0 cm depth. The penumbral widths (80% to 20%) increase with block thickness, field size, and beam energy. The EBT film results were in good agreement with the ionization chamber dose profiles and Monte Carlo N-particle radiation transport computer code simulation behind the step-wedged compensator. Also, there was a good agreement between the EBT-film and the treatment-planning results on the anthropomorphic phantom. The EBT films can be accurately used as a 2D dosimeter for dose verification and quality assurance of compensator-based C-IMRT.     

IMRT (intensity modulated radiation therapy): progress in technology and reimbursement.

For a new treatment technology to become widely accepted in today's healthcare environment, the technology must not only be effective but also financially viable. Intensity modulated radiation therapy (IMRT), a technology that enables radiation oncologists to precisely target and attack cancerous tumors with higher doses of radiation using strategically positioned beams while minimizing collateral damage to healthy cells, now meets both criteria. With IMRT, radiation oncologists for the first time have obtained the ability to divide the treatment field covered by each beam angle into hundreds of segments as small as 2.5 mm by 5 mm. Using the adjustable leaves of an MLC to shape the beam and by controlling exposure times, physicians can deliver a different dose to each segment and therefore modulate dose intensity across the entire treatment field. Development of optimal IMRT plans using conventional manual treatment planning methods would take days. To be clinically practical, IMRT required the development of "inverse treatment planning" software. With this software, a radiation oncologist can prescribe the ideal radiation dose for a specific tumor as well as maximum dose limits for surrounding healthy tissue. These numbers are entered into the treatment planning program which then calculates the optimal delivery approach that will best fit the oncologist's requirements. The radiation oncologist then reviews and approves the proposed treatment plan before it is initiated. The most recent advance in IMRT technology offers a "dynamic" mode or "sliding window" technique. In this more rapid delivery method, the beam remains on while the leaves of the collimator continually re-shape and move the beam aperture over the planned treatment area. This creates a moving beam that saturates the tumor volume with the desired radiation dose while leaving the surrounding healthy tissue in a protective shadow created by the leaves of the collimator. In the dynamic mode, an IMRT treatment session generally can be initiated and completed within the traditional 15-minute appointment window for radiation oncology clinics. In addition to being comforting for the patient, this rapid treatment delivery mode satisfies a key financial issue for hospitals and clinics by giving them the ability to handle high patient loads and achieve a more rapid return on their investment in an IMRT system. New IMRT reimbursement codes have been issued under the pass-through provisions of Medicare's Outpatient Prospective Payment System (OPPS), which authorize special or increased reimbursement levels for promising new developments in healthcare technology that previous reimbursement procedures did not address. These pass-through payments are generally applicable for defined periods during a promising new technology's early stage of adoption. In the case of codes G0174 and G0178, the effective period has been left open-ended. While the CMS adoption of these new IMRT reimbursement codes certainly paves the economic road for the diffusion of this technology by flattening out some of the economic obstacles, there are still bumps to overcome. The most obvious one is the investment in hardware and software that may be required. However, the added demands on staff and the cost of training cannot be ignored. IMRT is a treatment process involving FDA-approved medical devices, offering the hope of improved treatment outcomes with fewer complications for patients and higher reimbursement rates for hospital providers. By the end of the year 2001, there will probably be more than 75 hospitals with IMRT capabilities in place.     

Some limitations in the practical delivery of intensity modulated radiation therapy

The resolution characteristics of intensity modulated beam (IMB) profiles produced by milled compensators and by multileaf collimators (MLCs) are independently investigated with respect to the primary fluence. It is shown that both methods have different characteristics in the longitudinal and lateral direction and, as a consequence, the resolutions of the longitudinal and lateral delivered IMB profiles differ. For both methods, the restrictions are identified. For compensators, the maximum slopes in the machining process, which should not be exceeded, are quantified. For MLCs, emphasis is given to the direction perpendicular to leaf movement. A number of test modulations were created and the effect of different size MLCs on the intensity profile revealed that unacceptable errors can be introduced if the profiles are heavily modulated. The production of intensity modulated radiation therapy (IMRT) beams by both machined compensators or by MLCs is limited by physical constraints. Having identified these constraints, some steps should now be taken to accommodate them either in the objective function for the calculation of the beam profiles or in the delivery system.     

Dosimetric comparison of volumetric modulated arc therapy and intensity-modulated radiation therapy for pancreatic malignancies

Volumetric-modulated arc therapy (VMAT) has been previously evaluated for several tumor sites and has been shown to provide significant dosimetric and delivery benefits when compared with intensity-modulated radiation therapy (IMRT). To date, there have been no published full reports on the benefits of VMAT use in pancreatic patients compared with IMRT. Ten patients with pancreatic malignancies treated with either IMRT or VMAT were retrospectively identified. Both a double-arc VMAT and a 7-field IMRT plan were generated for each of the 10 patients using the same defined tumor volumes, organs at risk (OAR) volumes, dose, fractionation, and optimization constraints. The planning tumor volume (PTV) maximum dose (55.8 Gy vs. 54.4 Gy), PTV mean dose (53.9 Gy vs. 52.1 Gy), and conformality index (1.11 vs. 0.99) were statistically similar between the IMRT and VMAT plans, respectively. The VMAT plans had a statistically significant reduction in monitor units compared with the IMRT plans (1109 vs. 498, p < 0.001). In addition, the doses to the liver, small bowel, and spinal cord were comparable between the IMRT and VMAT plans. However, the VMAT plans demonstrated a statistically significant reduction in the mean left kidney V₂₅ (9.4 Gy vs. 2.3 Gy, p = 0.018), mean right kidney V₁₅ (53.4 Gy vs. 45.9 Gy, p = 0.035), V₂₀ (32.2 Gy vs. 25.5 Gy, p = 0.016), and V₂₅ (21.7 Gy vs. 14.9 Gy, p = 0.001). VMAT was investigated in patients with pancreatic malignancies and compared with the current standard of IMRT. VMAT was found to have similar or improved dosimetric parameters for all endpoints considered. Specifically, VMAT provided reduced monitor units and improved bilateral kidney normal tissue dose. The clinical relevance of these benefits in the context of pancreatic cancer patients, however, is currently unclear and requires further investigation.     

旋转调强与固定野逆向调强放疗在颅脑多发转移瘤中的剂量学比较研究

目的 比较旋转调强(RapidArc)与固定野调强(IMRT)放疗在颅脑多发转移瘤中的剂量学差异。方法 针对10例多发脑转移瘤患者分别设计3种放疗计划:固定野逆向调强(IMRT),RapidArc单弧旋转调强(RA1),双弧旋转调强(RA2)。在保证计划均满足临床要求前提下,分别比较3种计划的靶区剂量分布、危及器官及靶区外正常组织的受照剂量、机器跳数以及治疗时间,探讨其剂量学差异。结果 3种计划均满足临床要求,在靶区适形度和均匀性方面,RA2计划优于IMRT(Z＝-2.803、-2.094,P<0.05)和RA1(Z＝-2.448、-2.191,P＜0.05),RA1计划与IMRT计划差别不大。RA1、RA2计划中的双侧晶体、双侧眼球、脑干的最大剂量均显著低于IMRT(Z＝-2.803～-2.191,P <0.05)。RA2计划评估的双侧视神经最大剂量均显著低于IMRT(Z＝-2.293、-2.701,P<0.05)。RA1、RA2计划中的机器跳数相对于IMRT平均分别减少了43%和24%,缩短了治疗时间。结论 单弧和双弧旋转调强计划均可达到或优于IMRT计划的靶区剂量分布,能更好地降低部分危及器官的受照剂量,同时可以显著降低机器跳数和治疗实施时间。     

旋转调强与固定野逆向调强放疗在颅脑多发转移瘤中的剂量学比较研究

Objective To evaluate the performace of fixed field Intensity modulated radiation therapy (IMRT) and RapidArc in the radiotherapy for multiple intracranial metastases.Methods The clinical data of 10 patients with multiple intracranial metastases,8 male and 2 female,aged 65-73,were used to design 3 plans:fixed field IMRT,RapidArc with single Arc (RA1),and RapidArc with double Arc (Arc 2).Dose-volume-histogram analysis was used to compare dose results,monitor unit,and delivery time.Results All 3 plans met the clinical requirements.The best target conformity and homogeneity were observed in the RA2 plan (Z = -2.803,- 2.904,P ＜ 0.05) and there were no statistical differences between the IMRT plan and RA1 plan.The maximum doses to the lens,eyes,and brainstem of the two RapidArc plans were all significantly lower than those of the IMRT plan(Z = -2.803--2.191 ,P ＜0.05),and the maximum dose to the optic nerves of the RA2 plan was significantly lower than that of the IMRT plan (Z = -2.293,-2.701 ,P ＜0.05).Compared with the IMRT plan,the average monitor units of the RA1 and RA2 plans were reduced by 29% and 24%,respectively,and the delivery time of these plans were significantly shorter by 84% and 69%,respectively.Conclusions Compared to the IMRT plan,RapidArc plans with single or double Arcs show similar or better effects in the target dose distribution,reduction of irradiation doses on organs at risk and,moreover,significant decrease of the monitor units and delivery time.