C Fox

On the sensitivity of patient-specific IMRT QA to MLC positioning errors

Accurate multileaf collimator (MLC) leaf positioning plays an essential role in the effective implementation of intensity modulated radiation therapy (IMRT). This work evaluates the sensitivity of current patient‐specific IMRT quality assurance (QA) procedures to minor MLC leaf positioning errors. Random errors of up to 2 mm and systematic errors of ±1mm and ±2mm in MLC leaf positions were introduced into 8 clinical IMRT patient plans (totaling 53 fields). Planar dose distributions calculated with modified plans were compared to dose distributions measured with both radiochromic films and a diode matrix. The agreement between calculation and measurement was evaluated using both absolute distance‐to‐agreement (DTA) analysis and γ index with 2%/2mm and 3%/3mm criteria. It was found that both the radiochromic film and the diode matrix could only detect systematic errors on the order of 2 mm or above. The diode array had larger sensitivity than film due to its excellent detector response (such as small variation, linear response, etc.). No difference was found between DTA analysis and γ index in terms of the sensitivity to MLC positioning errors. Higher sensitivity was observed with 2%/2mm than with 3%/3mm in general. When using the diode array and 2%/2mm criterion, the IMRT QA procedure showed strongest sensitivity to MLC position errors and, at the same time, achieved clinically acceptable passing rates. More accurate dose calculation and measurement would further enhance the sensitivity of patient‐specific IMRT QA to MLC positioning errors. However, considering the significant dosimetric effect such MLC errors could cause, patient‐specific IMRT QA should be combined with a periodic MLC QA program in order to guarantee the accuracy of IMRT delivery. PACS numbers: 87.50.Gi, 87.52.Df, 87.52.Px, 87.53.Dq, 87.53.Tf, 87.53.Kn, 87.56.Fc

Prediction of outcome of early ER+ breast cancer is improved using a biomarker panel, which includes Ki-67 and p53

Background:The aim of this study is to determine whether immunohistochemical (IHC) assessment of Ki67 and p53 improves prognostication of oestrogen receptor-positive (ER+) breast cancer after breast-conserving therapy (BCT). In all, 498 patients with invasive breast cancer from a randomised trial of BCT with or without tumour bed radiation boost were assessed using IHC.Methods:The ER+ tumours were classified as ‘luminal A’ (LA): ER+ and/or PR+, Ki-67 low, p53−, HER2− or ‘luminal B’ (LB): ER+ and/or PR+and/or Ki-67 high and/or p53+ and/or HER2+. Kaplan–Meier and Cox proportional hazards methodology were used to ascertain relationships to ispilateral breast tumour recurrence (IBTR), locoregional recurrence (LRR), distant metastasis-free survival (DMFS) and breast cancer-specific survival (BCSS).Results:In all, 73 patients previously LA were re-classified as LB: a greater than four-fold increase (4.6–19.3%) compared with ER, PR, HER2 alone. In multivariate analysis, the LB signature independently predicted LRR (hazard ratio (HR) 3.612, 95% CI 1.555–8.340, P=0.003), DMFS (HR 3.023, 95% CI 1.501–6.087, P=0.002) and BCSS (HR 3.617, 95% CI 1.629–8.031, P=0.002) but not IBTR.Conclusion:The prognostic evaluation of ER+ breast cancer is improved using a marker panel, which includes Ki-67 and p53. This may help better define a group of poor prognosis ER+ patients with a greater probability of failure with endocrine therapy.

Randomized, paired comparison of No-Sting Barrier Film versus sorbolene cream (10% glycerine) skin care during postmastectomy irradiation

PURPOSE Postmastectomy irradiation provides an excellent model for irradiated skin care practices because of the relatively uniform surface and radiation compared with other situations in which radiation-induced moist desquamation is common. We designed a study to test the effect of prophylactic 3M Cavilon No-Sting Barrier Film (No-Sting) on the rates of moist desquamation compared with sorbolene cream (with 10% glycerin). METHODS AND MATERIALS The irradiated chest wall was divided into medial and lateral halves. Sixty-one women were randomized to have No-Sting applied to either the medial or lateral half, with the alternate half treated with sorbolene. RESULTS For all patients, the skin toxicity, calculated as the area under the curve, mean No-Sting and sorbolene score was 8.1 vs. 9.2, respectively (p = 0.005, Wilcoxon signed rank test). The total number of weeks of moist desquamation for the 61 patients was 40 vs. 45, equating to a mean of 0.65 week vs. 0.74 week per patient in the No-Sting and sorbolene-treated areas, respectively. The rates of moist desquamation were 33% vs. 46% (p = 0.096, McNemars Exact test). For 58 fully assessable patients (minimum of 7 weekly observations), the area under the curve and rates of moist desquamation were significantly different statistically (p = 0.002 and 0.049, respectively). No statistically significant differences were noted in the pain scores. The pruritus scores were significantly reduced in the No-Sting area (area under the curve, p = 0.011). CONCLUSION No-Sting reduces the duration and frequency of radiation-induced moist desquamation.

Comparative analysis of 60Co intensity-modulated radiation therapy

In this study, we perform a scientific comparative analysis of using (60)Co beams in intensity-modulated radiation therapy (IMRT). In particular, we evaluate the treatment plan quality obtained with (i) 6 MV, 18 MV and (60)Co IMRT; (ii) different numbers of static multileaf collimator (MLC) delivered (60)Co beams and (iii) a helical tomotherapy (60)Co beam geometry. We employ a convex fluence map optimization (FMO) model, which allows for the comparison of plan quality between different beam energies and configurations for a given case. A total of 25 clinical patient cases that each contain volumetric CT studies, primary and secondary delineated targets, and contoured structures were studied: 5 head-and-neck (H&N), 5 prostate, 5 central nervous system (CNS), 5 breast and 5 lung cases. The DICOM plan data were anonymized and exported to the University of Florida optimized radiation therapy (UFORT) treatment planning system. The FMO problem was solved for each case for 5-71 equidistant beams as well as a helical geometry for H&N, prostate, CNS and lung cases, and for 3-7 equidistant beams in the upper hemisphere for breast cases, all with 6 MV, 18 MV and (60)Co dose models. In all cases, 95% of the target volumes received at least the prescribed dose with clinical sparing criteria for critical organs being met for all structures that were not wholly or partially contained within the target volume. Improvements in critical organ sparing were found with an increasing number of equidistant (60)Co beams, yet were marginal above 9 beams for H&N, prostate, CNS and lung. Breast cases produced similar plans for 3-7 beams. A helical (60)Co beam geometry achieved similar plan quality as static plans with 11 equidistant (60)Co beams. Furthermore, 18 MV plans were initially found not to provide the same target coverage as 6 MV and (60)Co plans; however, adjusting the trade-offs in the optimization model allowed equivalent target coverage for 18 MV. For plans with comparable target coverage, critical structure sparing was best achieved with 6 MV beams followed closely by (60)Co beams, with 18 MV beams requiring significantly increased dose to critical structures. In this paper, we report in detail on a representative set of results from these experiments. The results of the investigation demonstrate the potential for IMRT radiotherapy employing commercially available (60)Co sources and a double-focused MLC. Increasing the number of equidistant beams beyond 9 was not observed to significantly improve target coverage or critical organ sparing and static plans were found to produce comparable plans to those obtained using a helical tomotherapy treatment delivery when optimized using the same well-tuned convex FMO model. While previous studies have shown that 18 MV plans are equivalent to 6 MV for prostate IMRT, we found that the 18 MV beams actually required more fluence to provide similar quality target coverage.

Three-dimensional dose distribution of tangential breast irradiation: results of a multicentre phantom dosimetry study

BACKGROUND AND PURPOSE One aspect of good radiotherapeutic practice is to achieve dose homogeneity. Dose inhomogeneities occur with breast tangent irradiation, particularly in women with large breasts. MATERIALS AND METHODS Ten Australian radiation oncology centres agreed to participate in this multicentre phantom dosimetry study. An Alderson radiation therapy anthropomorphic phantom with attachable breasts of two different cup sizes (B and DD) was used. The entire phantom was capable of having thermoluminescent dosimeters (TLD) material inserted at various locations. Nine TLD positions were distributed throughout the left breast phantom including the superior and inferior planes. The ten centres were asked to simulate, plan and treat (with a prescription of 100 cGy) the breast phantoms according to their standard practice. Point doses from resultant computer plans were calculated for each TLD position. Measured and calculated (planning computer) doses were compared. RESULTS The dose planning predictability between departments did not appear to be significantly different for both the small and large breast phantoms. The median dose deviation (calculated dose minus measured dose) for all centres ranged from 2. 3 to 5.3 cGy on the central axis and from 2.1 to 7.5 cGy for the off-axis planes. The highest absolute dose was measured in the inferior plane of the large breast (128.7 cGy). The greatest dose inhomogeneity occurred in the small breast phantom volume (median range 93.2-105 cGy) compared with the large breast phantom volume (median range, 100.1-107.7 cGy). There was considerable variation in the use (or not) of wedges to obtain optimized dosimetry. No department used 3D compensators. CONCLUSION The results highlight areas of potential improvement in the delivery of breast tangent radiotherapy. Despite reasonable dose predictability, the greatest dose deviation and highest measured doses occurred in the inferior aspects of both the small and large breast phantoms.

Assessment of the setup dependence of detector response functions for mega-voltage linear accelerators

PURPOSE Accurate modeling of beam profiles is important for precise treatment planning dosimetry. Calculated beam profiles need to precisely replicate profiles measured during machine commissioning. Finite detector size introduces perturbations into the measured profiles, which, in turn, impact the resulting modeled profiles. The authors investigate a method for extracting the unperturbed beam profiles from those measured during linear accelerator commissioning. METHODS In-plane and cross-plane data were collected for an Elekta Synergy linac at 6 MV using ionization chambers of volume 0.01, 0.04, 0.13, and 0.65 cm3 and a diode of surface area 0.64 mm2. The detectors were orientated with the stem perpendicular to the beam and pointing away from the gantry. Profiles were measured for a 10 x 10 cm2 field at depths ranging from 0.8 to 25.0 cm and SSDs from 90 to 110 cm. Shaping parameters of a Gaussian response function were obtained relative to the Edge detector. The Gaussian function was deconvolved from the measured ionization chamber data. The Edge detector profile was taken as an approximation to the true profile, to which deconvolved data were compared. Data were also collected with CC13 and Edge detectors for additional fields and energies on an Elekta Synergy, Varian Trilogy, and Siemens Oncor linear accelerator and response functions obtained. Response functions were compared as a function of depth, SSD, and detector scan direction. Variations in the shaping parameter were introduced and the effect on the resulting deconvolution profiles assessed. RESULTS Up to 10% setup dependence in the Gaussian shaping parameter occurred, for each detector for a particular plane. This translated to less than a +/- 0.7 mm variation in the 80%-20% penumbral width. For large volume ionization chambers such as the FC65 Farmer type, where the cavity length to diameter ratio is far from 1, the scan direction produced up to a 40% difference in the shaping parameter between in-plane and cross-plane measurements. This is primarily due to the directional difference in penumbral width measured by the FC65 chamber, which can more than double in profiles obtained with the detector stem parallel compared to perpendicular to the scan direction. For the more symmetric CC13 chamber the variation was only 3% between in-plane and cross-plane measurements. CONCLUSIONS The authors have shown that the detector response varies with detector type, depth, SSD, and detector scan direction. In-plane vs. cross-plane scanning can require calculation of a direction dependent response function. The effect of a 10% overall variation in the response function, for an ionization chamber, translates to a small deviation in the penumbra from that of the Edge detector measured profile when deconvolved. Due to the uncertainties introduced by deconvolution the Edge detector would be preferable in obtaining an approximation of the true profile, particularly for field sizes where the energy dependence of the diode can be neglected. However, an averaged response function could be utilized to provide a good approximation of the true profile for large ionization chambers and for larger fields for which diode detectors are not recommended.

A computational implementation and comparison of several intensity modulated proton therapy treatment planning algorithms.

The authors present a comparative study of intensity modulated proton therapy (IMPT) treatment planning employing algorithms of three-dimensional (3D) modulation, and 2.5-dimensional (2.5D) modulation, and intensity modulated distal edge tracking (DET) [A. Lomax, Phys. Med. Biol. 44, 185-205 (1999)] applied to the treatment of head-and-neck cancer radiotherapy. These three approaches were also compared with 6 MV photon intensity modulated radiation therapy (IMRT). All algorithms were implemented in the University of Florida Optimized Radiation Therapy system using a finite sized pencil beam dose model and a convex fluence map optimization model. The 3D IMPT and the DET algorithms showed considerable advantages over the photon IMRT in terms of dose conformity and sparing of organs at risk when the beam number was not constrained. The 2.5D algorithm did not show an advantage over the photon IMRT except in the dose reduction to the distant healthy tissues, which is inherent in proton beam delivery. The influences of proton beam number and pencil beam size on the IMPT plan quality were also studied. Out of 24 cases studied, three cases could be adequately planned with one beam and 12 cases could be adequately planned with two beams, but the dose uniformity was often marginally acceptable. Adding one or two more beams in each case dramatically improved the dose uniformity. The finite pencil beam size had more influence on the plan quality of the 2.5D and DET algorithms than that of the 3D IMPT. To obtain a satisfactory plan quality, a 0.5 cm pencil beam size was required for the 3D IMPT and a 0.3 cm size was required for the 2.5D and the DET algorithms. Delivery of the IMPT plans produced in this study would require a proton beam spot scanning technique that has yet to be developed clinically.

Multileaf collimator characteristics and reliability requirements for IMRT Elekta system.

Understanding the characteristics of a multileaf collimator (MLC) system, modeling MLC in a treatment planning system, and maintaining the mechanical accuracy of the linear accelerator gantry head system are important factors in the safe implementation of an intensity-modulated radiotherapy program. We review the characteristics of an Elekta MLC system, discuss the necessary MLC modeling parameters for a treatment planning system, and provide a novel method to establish an MLC leaf position quality assurance program. To perform quality assurance on 40 pairs of individual MLC leaves is a time-consuming and difficult task. In this report, an effective routine MLC quality assurance method based on the field edge of a backup jaw as referenced in conjunction with a diode array as a radiation detector system is discussed. The sensitivity of this test for determining the relative leaf positions was observed to be better than 0.1 mm. The Elekta MLC leaf position accuracy measured with this system has been better than 0.3 mm.

An MLC calibration method using a detector array

PURPOSE The authors have developed a quantitative calibration method for a multileaf collimator (MLC) which measures individual leaf positions relative to the MLC backup jaw on an Elekta Synergy linear accelerator. METHODS The method utilizes a commercially available two-axis detector array (Profiler 2; Sun Nuclear Corporation, Melbourne, FL). To calibrate the MLC bank, its backup jaw is positioned at the central axis and the opposing jaw is retracted to create a half-beam configuration. The position of the backup jaws field edge is then measured with the array to obtain what is termed the radiation defined reference line. The positions of the individual leaf ends relative to this reference line are then inferred by the detector response in the leaf end penumbra. Iteratively adjusting and remeasuring the leaf end positions to within specifications completes the calibration. Using the backup jaw as a reference for the leaf end positions is based on three assumptions: (1) The leading edge of an MLC leaf bank is parallel to its backup jaws leading edge, (2) the backup jaw position is reproducible, and (3) the measured radiation field edge created by each leaf end is representative of that leafs position. Data from an electronic portal imaging device (EPID) were used in a similar analysis to check the results obtained with the array. RESULTS The relative leaf end positions measured with the array differed from those measured with the EPID by an average of 0.11+/-0.09 mm per leaf. The maximum leaf positional change measured with the Profiler 2 over a 3 month period was 0.51 mm. A leaf positional accuracy of +/-0.4 mm is easily attainable through the iterative calibration process. The method requires an average of 40 min to measure both leaf banks. CONCLUSIONS This work demonstrates that the Profiler 2 is an effective tool for efficient and quantitative MLC quality assurance and calibration.

SU‐GG‐T‐168: Measurement Errors Associated with Linear Accelerator Commissioning Data

Purpose:Linear accelerator(LINAC) commissioning employs 3‐D water phantoms (WP) to accurately assess LINAC dosimetric characteristics. The accuracy of the treatment planning system (TPS) modeling is based on the collected data. This investigation aims to assess the errors associated with the collection of commissioning data. Method and Materials: Possible sources of error within 3‐D WP measurements that were assessed are water tank mechanics, data acquisition, and chamber aspects. WP scanning software, mechanical measurements, and infra‐red motion tracking cameras were used to evaluate the mechanical performance of the WP. Continuous motion and point by point measurements were compared for a variety of standard detectors. Minor field size variations were scaled to the nominal field size for TPS import purposes using two methods, 1) geometric scaling of the whole profile or 2) geometric scaling of 80% of the field width. The detector orientation effects were evaluated for profiles and percentage depth dose. Results: WP positional accuracy and reproducibility was within 0.4mm and mechanical hysteresis within 0.46mm. The collection method impacted the results obtained for small detectors (<0.01cc) with the point measurement technique resulting in reduced noise level. However, the time scale increased considerably with the point by point technique. For field width scaling the 80% scaling method showed improved results compared to scaling the whole profile. The detector orientation showed variations in the build up region for small detectors and small variations were observed for penumbral widths of profile measurements. Conclusion: Error sources in data collection have been identified and quantified in this work. Detailed error analysis of experimental set up is presented. This work supported in part with Federal funds from the National Cancer Institute, Contract No HHSN261200522014C, and by Sun Nuclear Corporation.