IMAT-SIM: A new method for the clinical dosimetry of intensity-modulated arc therapy (IMAT)

Primo software as a tool for Monte Carlo simulations of intensity modulated radiotherapy: a feasibility study

Background

IMRT provides higher dose conformation to the target and dose sparing to surrounding tissues than 3DCRT. Monte Carlo method in Medical Physics is not a novelty to approach dosimetric problems. A new PENELOPE based code named PRIMO recently was published. The most intriguing features of PRIMO are the user-friendly approach, the stand-alone property and the built-in definition of different linear accelerators models. Nevertheless, IMRT simulations are not yet implemented.

Methods

A Varian Trilogy with a Millennium120 MLC and a Varian Novalis with 120HD MLC were studied. A RW3 multi-slab phantom was irradiated with Gafchromic films inserted between slabs. An Expression 10000XL scanner (Seiko Epson Corp., Nagano, Japan) was used to digitalize the films. PTW-Verisoft software using the global Gamma Function (2%, 2 mm) was used to compare simulated and experimental results.

The primary beam parameters were adjusted to best match reference data previously obtained in a water phantom. Static MLC simulations were performed to validate the MLC models in use. Two Dynamic IMRT preliminary tests were performed with leaves moving with constant and variable speed. A further test of an in phantom delivery of a real IMRT field allowed simulating a clinical-like MLC modulation.

Results

Simulated PDD, X- and Y-profiles in reference conditions showed respectively 100.0%, 100.0% and 99.4% of Gamma points < 1 (2%, 2 mm). Static MLC simulations showed 100.0% of Gamma points < 1 with the 120HD MLC and 99.1% with the Millennium compared with the scanned images.

The fixed speed test showed 99.5 and 98.9% of Gamma points < 1 respectively with two different MLC configuration-sampling algorithms when the 120HD MLC was used. The higher modulation MLC motion simulation showed 99.1% of Gamma points < 1 with respect to the experimental. This result depends on the number of the fields to reproduce the MLC motion, as well as calculation time. The clinical-like simulation showed 96.2% of Gamma points < 1 using the same analysis conditions.

Conclusions

The numerical model of the Varian Trilogy and Novalis in the PRIMO software was validated. The algorithms to simulate MLC motion were considered reliable. A clinical-like procedure was successfully simulated.

Patient QA systems for rotational radiation therapy: a comparative experimental study with intentional errors

PURPOSE

The purpose of the present study was to investigate the ability of commercial patient quality assurance (QA) systems to detect linear accelerator-related errors.

METHODS

Four measuring systems (Delta(4®), OCTAVIUS(®), COMPASS, and Epiqa™) designed for patient specific quality assurance for rotational radiation therapy were compared by measuring four clinical rotational intensity modulated radiation therapy plans as well as plans with introduced intentional errors. The intentional errors included increasing the number of monitor units, widening of the MLC banks, and rotation of the collimator. The measurements were analyzed using the inherent gamma evaluation with 2% and 2 mm criteria and 3% and 3 mm criteria. When applicable, the plans with intentional errors were compared with the original plans both by 3D gamma evaluation and by inspecting the dose volume histograms produced by the systems.

RESULTS

There was considerable variation in the type of errors that the various systems detected; the failure rate for the plans with errors varied between 0% and 72%. When using 2% and 2 mm criteria and 95% as a pass rate the Delta(4®) detected 15 of 20 errors, OCTAVIUS(®) detected 8 of 20 errors, COMPASS detected 8 of 20 errors, and Epiqa™ detected 20 of 20 errors. It was also found that the calibration and measuring procedure could benefit from improvements for some of the patient QA systems.

CONCLUSIONS

The various systems can detect various errors and the sensitivity to the introduced errors depends on the plan. There was poor correlation between the gamma evaluation pass rates of the QA procedures and the deviations observed in the dose volume histograms.

Hypo-fractionated IMRT for patients with newly diagnosed glioblastoma multiforme: a 6 year single institutional experience

OBJECTIVES

Glioblastoma (GBM) is the most common malignant primary brain tumour in adults. Surgery and radiotherapy constitute the cornerstones for the therapeutic management of GBM. The standard treatment today is maximal surgical resection followed by concomitant chemo-radiation therapy followed by adjuvant TMZ according to Stupp protocol. Despite the progress in neurosurgery, radiotherapy and oncology, the prognosis still results poor. In order to reduce the long time of standard treatment, maintaining or improving the clinical results, in our institute we have investigated the effects of hypo-fractionated radiation therapy for patients with GBM.

PATIENTS AND METHODS

Sixty-seven patients affected by GBM who had previously undergone surgical resection (total, subtotal or biopsy) were enrolled between October 2005 and December 2011 in a single institutional study of hypo-fractionated intensity modulated radiation therapy (IMRT) followed or not by adjuvant chemotherapy with TMZ (6-12 cycles). The most important eligibility criteria were: biopsy-proven GBM, KPS ≥ 60, age ≥ 18 years, no previous brain irradiation, informed consensus. Hypo-fractionated IMRT was delivered to a total dose of 25 Gy in 5 fractions prescribed to 70% isodose. Response to treatment, OS, PFS, toxicity and patterns of recurrence were evaluated, and sex, age, type of surgery, Karnofsky performance status, Recursive Partitioning Analysis (RPA) classification, time between surgery and initiation of radiotherapy were evaluated as potential prognostic factors for survival.

RESULTS

All patients have completed the treatment protocol. Median age was 64.5 years (range 41-82 years) with 31 females (46%) and 36 males (54%). Median KPS at time of treatment was 80. The surgery was gross total in 38 patients and subtotal in 14 patients; 15 patients underwent only biopsy. No grade 3-4 acute or late neurotoxicity was observed. With median follow-up of 14.9 months, the median OS and PFS were 13.4 and 7.9 months, respectively.

CONCLUSIONS

The hypo-fractionated radiation therapy can be used for patients with GBM, resulting in favourable overall survival, low rates of toxicity and satisfying QoL. Future investigations are needed to determine the optimal fractionation for GBM.

Independent verification of gantry angle for pre-treatment VMAT QA using EPID

We propose a method to incorporate independent verification of gantry angle for electronic portal imaging device (EPID)-based pre-treatment quality assurance (QA) of clinical volumetric modulated arc therapy (VMAT) plans. Gantry angle is measured using projections in the EPID of a custom phantom placed on the couch and the treatment plan is modified so as to be incident on the phantom with a portion of the beam that is collimated in the clinical plan. For our implementation, collimator and couch angles were set to zero and the inferior jaw and two most inferior multi-leaf collimator pairs were opened for the entire QA delivery. A phantom containing five gold coils was used to measure the gantry rotation through which each portal image was acquired. We performed the EPID QA for ten clinical plans and evaluated accuracy of gantry angle measurement, scatter incident on the imager due to the phantom, inter-image pixel linearity and inter- and intra-image noise. The gantry angle could be measured to within 0.0 ± 0.3° for static gantry and 0.2 ± 0.2° for arc acquisitions. Scatter due to the presence of the phantom was negligible. The procedure was shown to be feasible and adds gantry angle to the treatment planning parameters that can be verified by EPID-based pre-treatment VMAT QA.

Quality assurance of volumetric modulated arc therapy: evaluation and comparison of different dosimetric systems

PURPOSE

To compare and evaluate different dosimetric techniques and devices for the QA of VMAT plans created by two treatment planning systems (TPSs).

METHODS

A total of 50 VMAT plans were optimized for treatment of anatomical sites of various complexities by two TPSs which use rather different approaches to VMAT optimization. Dosimetric plan verifications were performed both as part of commissioning and as patient specific QA of clinical treatments. Absolute point doses were measured for all plans by a micro ion chamber inserted in a dedicated water-filled cylindrical phantom. Delivered dose distributions were verified by four techniques based on different detectors: radiographic and gafchromic films, two systems based on 2D diode arrays and an ion chamber array. Gamma index analysis with various tolerance levels (3%, 3 mm and 3%, 2 mm) was used to analyze differences between calculated and delivered doses. Sensitivity to possible delivery errors was also evaluated for three of the considered devices introducing +/-3 mm shifts along the three directions and a 3 degrees gantry offset.

RESULTS

Ion chamber measured point doses were within 3% of calculated ones for 48 out of 50 values. For delivered dose distribution, the average fraction of passed gamma values using 3% and 3 mm criteria was above 95% for both TPSs and all detectors except gafchromic film which yielded on average of 91.4%. For 49 out of 50 plans, a pass-rate above 94% was obtained by at least one of the four techniques. Shrinking the tolerance to 3% and 2 mm, the average pass-rate by all detectors (except film) was still above 95% for one of the two TPSs, but lower for the other one. The detector sensitivity to 3 mm shifts and to gantry angle offset was strongly plan and partially detector dependent: the obtained pass-rate reduction ranged from 2% to 30%.

CONCLUSIONS

The presented results for VMAT plans QA assess the reliability of the delivered doses for both TPSs. The slightly lower pass-rate obtained for one of the considered TPS can be attributed to a higher level of complexity of the optimized plans. The results by different dosimetric techniques are coherent, apart from a few measurements by gafchromic films. The detector sensitivity to delivery errors, being strongly plan dependent, is not easy to evaluate.

Intensity-modulated arc therapy: principles, technologies and clinical implementation

Intensity-modulated arc therapy (IMAT) was proposed by Yu (1995 Phys. Med. Biol. 40 1435-49) as an alternative to tomotherapy. Over more than a decade, much progress has been made. The advantages and limitations of the IMAT technique have also been better understood. In recent years, single-arc forms of IMAT have emerged and become commercially adopted. The leading example is the volumetric-modulated arc therapy (VMAT), a single-arc form of IMAT that delivers apertures of varying weights with a single-arc rotation that uses dose-rate variation of the treatment machine. With commercial implementation of VMAT, wide clinical adoption has quickly taken root. However, there remains a lack of general understanding for the planning of such arc treatments, as well as what delivery limitations and compromises are made. Commercial promotion and competition add further confusion for the end users. It is therefore necessary to provide a summary of this technology and some guidelines on its clinical implementation. The purpose of this review is to provide a summary of the works from the radiotherapy community that led to wide clinical adoption, and point out the issues that still remain, providing some perspective on its further developments. Because there has been vast experience in IMRT using multiple intensity-modulated fields, comparisons between IMAT and IMRT are also made in the review within the areas of planning, delivery and quality assurance.