Neck Intensity-modulated Radiation Therapy Research Articles

Adaptive Planning in Intensity-Modulated Radiation Therapy for Head and Neck Cancers: Single-Institution Experience and Clinical Implications

Anatomic changes and positional variability during intensity-modulated radiation therapy (IMRT) for head and neck cancer can lead to clinically significant dosimetric changes. We report our single-institution experience using an adaptive protocol and correlate these changes with anatomic and positional changes during treatment. Twenty-three sequential head and neck IMRT patients underwent serial computed tomography (CT) scans during their radiation course. After undergoing the planning CT scan, patients underwent planned rescans at 11, 22, and 33 fractions; a total of 89 scans with 129 unique CT plan combinations were thus analyzed. Positional variability and anatomic changes during treatment were correlated with changes in dosimetric parameters to target and avoidance structures between planning CT and subsequent scans. A total of 15/23 patients (65%) benefited from adaptive planning, either due to inadequate dose to gross disease or to increased dose to organs at risk. Significant differences in primary and nodal targets (planning target volume, gross tumor volume, and clinical tumor volume), parotid, and spinal cord dosimetric parameters were noted throughout the treatment. Correlations were established between these dosimetric changes and weight loss, fraction number, multiple skin separations, and change in position of the skull, mandible, and cervical spine. Variations in patient positioning and anatomy changes during IMRT for head and neck cancer can affect dosimetric parameters and have wide-ranging clinical implications. The interplay between random positional variability and gradual anatomic changes requires careful clinical monitoring and frequent use of CT- based image-guided radiation therapy, which should determine variations necessitating new plans.

Poster — Thur Eve — 52: Head and Neck IMRT Complexity Characterization and Prediction of Deliverability Using the Modulation Complexity Score

Intensity modulated radiation therapy (IMRT) is often referred to as being highly complex, but the definition of complexity can be varied. A single metric, the modulation complexity score (MCS), has previously been developed to quantify IMRT complexity. The purpose of this study is to evaluate the use of MCS in the characterization of segmentation complexity and beam deliverability in the context of head and neck IMRT. Fifty treatment plans were evaluated by calculating the MCS per beam. Patient‐specific measurements were obtained as part of the standard IMRT QA process using a diode array and retrospective analysis was completed using various gamma analysis criteria. The MCS, as well as single beam parameters (e.g. number of MU or control points), were compared to dosimetric results. 375 individual treatment beams were analyzed, with an average MCS of 0.245 (range: −0.338 – 0.754) and 125 MU on average (range: 32 to 295). All beams had >90% of diodes passing the standard gamma analysis (3%/3mm) with an average of 98%. There was a linear relationship between the number of MU and the MCS score (r2=0.75). The relationship between pass rate and complexity (characterized by MCS or MU) is not simple, however it may be possible to predict good dosimetric results based on the plan complexity. In conjunction with the ability to compile complexity statistics for specific treatment sites or protocols, MCS could impact the radiation therapy process at many points, including during planning, plan evaluation and QA.

Analysis of peripheral doses for base of tongue treatment by linear accelerator and helical TomoTherapy IMRT

The purpose of this study was to compare the peripheral doses to various organs from a typical head and neck intensity‐modulated radiation therapy (IMRT) treatment delivered by linear accelerator (linac) and helical TomoTherapy.Multiple human CT data sets were used to segment critical structures and organs at risk, fused and adjusted to an anthropomorphic phantom. Eighteen contours were designated for thermoluminescent dosimeter (TLD) placement. Following the RTOG IMRT Protocol 0522, treatment of the primary tumor and involved nodes (PTV70) and subclinical disease sites (PTV56) was planned utilizing IMRT to 70 Gy and 56 Gy. Clinically acceptable treatment plans were produced for linac and TomoTherapy treatments. TLDs were placed and each treatment plan was delivered to the anthropomorphic phantom four times.Within 2.5 cm (one helical TomoTherapy field width) superior and inferior to the field edges, normal tissue doses were on average 45% lower using linear accelerator. Beyond 2.5 cm, the helical TomoTherapy normal tissue dose was an average of 52% lower. The majority of points proved to be statistically different using the Student's t‐test with p<0.05. Using one method of calculation, probability of a secondary malignancy was 5.88% for the linear accelerator and 4.08% for helical TomoTherapy.Helical TomoTherapy delivers more dose than a linac immediately above and below the treatment field, contributing to the higher peripheral doses adjacent to the field. At distances beyond one field width (where leakage is dominant), helical TomoTherapy doses are lower than linear accelerator doses.PACS number: 87.50.cm Dosimetry/exposure assessment

SU‐GG‐J‐65: CBCT Image Guidance for Head and Neck IMRT Radiation Therapy—A Study on the Tumor Coverage and Critical Structure Avoidance

Purpose: Cone beam computed tomography (CBCT) is becoming more prevalently used for image‐guided radiation therapy (IGRT). The ideal CBCT frequency and protocol will limit patient imaging dose, while insuring the accuracy of the radiation therapy (RT). We herein examine the effects of a possible CBCT imaging protocol on the delivered RT. Methods and Materials: The intensity modulated radiation therapy (IMRT) plans for 6 head and neck cancer patients undergoing IGRT with CBCT were retrospectively studied. The CBCT protocol called for daily IGRT for the first treatment week, followed by twice‐weekly imaging for the remaining RT fractions. The statistics of the IGRT patient set‐up shifts throughout the treatment course was used to create the treatment dose for non‐IGRT days using our in‐house TPS PlanUNC. The cumulative dose from the entire treatment course is the sum of the non‐IGRT daily doses and the planned daily doses for IGRT days. The resulting cumulative doses to the clinical target volume (CTV) and critical structures were examined.Results: The delivered treatment plans maintained the CTV treatment planning goals for 6 patients. Incorporating the IGRT shift statistics caused the CTV of one patient to be under dosed. The hotspots for the optic chiasm, cord, and brainstem were all within the treatment goals, and the absolute doses were within 5% of the planned RT. The dose constraints for the larynx and parotids were all maintained in the delivered treatment plans.Conclusions: The proposed CBCT protocol is sufficient for accurate RT delivery to the target volumes and critical structures of head and neck IMRT patients.

Comprehensive IMRT plus weekly cisplatin for advanced head and neck cancer: The University of Wisconsin experience

We retrospectively examined the treatment efficacy and toxicity profile of intensity-modulated radiotherapy (IMRT) plus concurrent weekly cisplatin chemotherapy in patients with locoregionally advanced head and neck squamous cell carcinoma (HNSCC). A total of 57 patients with stage III or IV HNSCC were treated with IMRT and concurrent weekly cisplatin (dosed at 30 mg/m(2)) between November 2001 and May 2007. The median prescription dose to the gross tumor volume was 70 Gy (using 2.0-2.2 Gy daily fractions). In-field tumor control at 2 years was 89.1%, locoregional control was 85.5%, and overall survival was 86.9%. The median radiation dose delivered was 70 Gy. The mean dose intensity of cisplatin administered was 25.7 mg/m(2)/week. Comprehensive head and neck IMRT to 70 Gy delivered with weekly cisplatin chemotherapy (30 mg/m(2)) is feasible and generally well tolerated.

IMRT and IGRT in head and neck cancer: Have we delivered what we promised?

Intensity-modulated radiation therapy (IMRT) is a revolutionary new paradigm that aims at improving the therapeutic ratio by increasing the dosegradient between target tissues and surrounding normal structures thereby offering probability of better loco-regional control with decreased risk of complications. IMRT is relatively intolerant to set-up uncertainties, warranting periodic image-guidance, making Image-Guided Radiation Therapy (IGRT) a natural corollary to IMRT. There are several challenges associated with the planning, delivery, and quality assurance of the IMRT and IGRT processes that must be addressed to realize the full potential of such exciting and promising technology. Given the complexities involved, it is quite intuitive to understand that IMRT and IGRT are resource-intensive, demanding increased labor, rigour, and expenses too. Other disadvantages associated with high-precision techniques include potentially increased risk of marginal failures, decreased dose homogeneity, and an increase in total body dose with increased risk of secondary carcinogenesis. The aim of this review is to define the role of IMRT and IGRT in contemporary head and neck oncologic practice through a critical appraisal of pertinent literature. Despite relatively short follow-up and limited clinical outcomes data, the weight of evidence suggests that loco-regional control is not inferior (either comparable or even better) and toxicity lesser with IMRT resulting in potentially improved quality-of-life, prompting the widespread adoption of such technology in community practice. Ongoing clinical trials in head and neck IMRT are currently addressing issues to optimize the IMRT process, adopting functional imaging for dose-painting, and incorporating adaptive re-planning strategies to further improve outcomes.

Pre-treatment verification of intensity modulated radiation therapy plans using a commercial electronic portal dosimetry system

We commissioned a commercially available portal dosimetry system for quality assurance of intensity modulated radiation therapy (IMRT) treatment plans. The system included gamma analysis software to compare the measured and predicted fluence maps from individual IMRT fields. The portal dosimetry system was tested using six head and neck IMRT patient plans, and we demonstrated that the accuracy of the alignment of measured and predicted images improved by retracting and repositioning the electronic portal imaging device (EPID) at each new gantry angle. The mean gamma score (fraction of pixels passing the gamma criteria) for the six test plans (after initial testing and using the EPID retracting and repositioning method) was 0.987 (2SD = 0.018), using gamma criteria of a dose difference of 2% of the maximum field dose and 2 mm distance to agreement. The mean gamma score was 0.989 (2SD = 0.017) for 24 head and neck IMRT patient plans carried out with portal dosimetry. Using gamma criteria of 2% maximum field dose and 2 mm distance to agreement, a gamma score tolerance of 0.980 is a useful way of highlighting only those fields requiring further analysis. Portal dosimetry is a quick way of assessing individual field fluence distributions and can be integrated into an IMRT quality assurance programme.

Effect of Brain Stem and Dorsal Vagus Complex Dosimetry on Nausea and Vomiting in Head and Neck Intensity-Modulated Radiation Therapy

Intensity-modulated radiation therapy (IMRT) is becoming the treatment of choice for many head and neck cancer patients. IMRT reduces some toxicities by reducing radiation dose to uninvolved normal tissue near tumor targets; however, other tissues not irradiated using previous 3D techniques may receive clinically significant doses, causing undesirable side effects including nausea and vomiting (NV). Irradiation of the brainstem, and more specifically, the area postrema and dorsal vagal complex (DVC), has been linked to NV. We previously reported preliminary hypothesis-generating dose effects associated with NV in IMRT patients. The goal of this study is to relate brainstem dose to NV symptoms. We retrospectively studied 100 consecutive patients that were treated for oropharyngeal cancer with IMRT. We contoured the brainstem, area postrema, and DVC with the assistance of an expert diagnostic neuroradiologist. We correlated dosimetry for the 3 areas contoured with weekly NV rates during IMRT. NV rates were significantly higher for patients who received concurrent chemotherapy. Post hoc analysis demonstrated that chemoradiation cases exhibited a trend towards the same dose-response relationship with both brainstem mean dose ( p = 0.0025) and area postrema mean dose ( p = 0.004); however, both failed to meet statistical significance at the p ≤ 0.002 level. Duration of toxicity was also greater for chemoradiation patients, who averaged 3.3 weeks with reported Common Terminology Criteria for Adverse Events (CTC-AE), compared with an average of 2 weeks for definitive RT patients ( p = 0.002). For definitive RT cases, no dose-response trend could be ascertained. The mean brainstem dose emerged as a key parameter of interest; however, no one dose parameter (mean/median/EUD) best correlated with NV. This study does not address extraneous factors that would affect NV incidence, including the use of antiemetics, nor chemotherapy dose schedule specifics before and during RT. A prospective study will be required to depict exactly how IMRT dose affects NV.

Dosimetric verification of brain and head and neck intensity-modulated radiation therapy treatment using EDR2 films and 2D ion chamber array matrix

The evaluation of the agreement between measured and calculated dose plays an essential role in the quality assurance (QA) procedures of intensity-modulated radiation therapy (IMRT). The purpose of this study is to compare performances of the two dosimetric systems (EDR2 and I'matriXX) in the verification of the dose distributions calculated by the TPS for brain and head and neck dynamic IMRT cases. The comparison of cumulative fluence by using Kodak extended dose rate (EDR2) and I'matriXX detectors has been done for the evaluation of 10 brain, 10 head and neck IMRT cases treated with 6 MV beams. The parameter used to assess the quality of dose calculation is the gamma-index (g -index) method. The acceptance limits for g calculation we have used are 3% and 3 mm respectively for dose agreement and distance to agreement parameters. Statistical analyses were performed by using the paired, two-tailed Student t-test, and P< 0.01 is kept as a threshold for the significance level. The qualitative dose distribution comparison was performed using composite dose distribution in the measurement plane and profiles along various axes for TPS vs. EDR2 film and TPS Vs I'matriXX. The quantitative analysis between the calculated and measured dose distribution was evaluated using DTA and g-index. The percentage of pixels matching with the set DTA and g values are comparable for both with EDR2 film and I'matriXX array detectors. Statistically there was no significant variation observed between EDR2 film and I'matriXX in terms of the mean percentage of pixel passing g for brain cases (98.77 +/- 1.03 vs 97.62 +/- 1.66, P = 0.0218) and for head and neck cases (97.39 +/- 2.13 vs 97.17 +/- 1.52%, P = 0.7404). Due to simplicity and fast evaluation process of array detectors, it can be routinely used in busy departments without compromising the measurement accuracy.

Megavoltage versus kilovoltage image guidance for efficiency and accuracy in head and neck IMRT

AbstractAccurate patient positioning is vitally important in intensity modulated radiation therapy (IMRT) for head and neck (H&amp;N) cancer. The introduction of kilovoltage (kV) on-board imaging (OBI) at our centre was anticipated to improve the accuracy and efficiency of H&amp;N IMRT patient position verification over traditional megavoltage (MV) electronic portal imaging (EPI). This study compares these imaging systems with a phantom accuracy study and retrospective analysis of imaging workload in H&amp;N IMRT at our centre. Six therapists performed online evaluation of phantom images, and residual positional errors for each system were recorded. The largest residual error was 1 mm for OBI and 3 mm for EPI. The estimated improvement in residual error in OBI over EPI was 0.57 mm (95% confidence interval 0.33–0.81 mm), suggesting treatment staff would be better able to detect set-up deviations with OBI. Electronic treatment records of 20 H&amp;N IMRT patients (10 verified daily with MV EPI and 10 with kV OBI) were analysed. Mean imaging session duration was 9.51 min for EPI and 9.76 min for OBI. Analysis found no evidence of an effect on duration due to the imaging system used for this subset of patients (p = 0.664).

Dosimetric Comparison of Split Field and Fixed Jaw Techniques for Large IMRT Target Volumes in the Head and Neck

Some treatment planning systems (TPSs), when used for large-field (>14 cm) intensity-modulated radiation therapy (IMRT), create split fields that produce excessive multiple-leaf collimator segments, match-line dose inhomogeneity, and higher treatment times than nonsplit fields. A new method using a fixed-jaw technique (FJT) forces the jaw to stay at a fixed position during optimization and is proposed to reduce problems associated with split fields. Dosimetric comparisons between split-field technique (SFT) and FJT used for IMRT treatment is presented. Five patients with head and neck malignancies and regional target volumes were studied and compared with both techniques. Treatment planning was performed on an Eclipse TPS using beam data generated for Varian 2100C linear accelerator. A standard beam arrangement consisting of nine coplanar fields, equally spaced, was used in both techniques. Institutional dose-volume constraints used in head and neck cancer were kept the same for both techniques. The dosimetric coverage for the target volumes between SFT and FJT for head and neck IMRT plan is identical within ±1% up to 90% dose. Similarly, the organs at risk (OARs) have dose-volume coverage nearly identical for all patients. When the total monitor unit (MU) and segments were analyzed, SFT produces statistically significant higher segments (17.3 ± 6.3%) and higher MU (13.7 ± 4.4%) than the FJT. There is no match line in FJT and hence dose uniformity in the target volume is superior to the SFT. Dosimetrically, SFT and FJT are similar for dose-volume coverage; however, the FJT method provides better logistics, lower MU, shorter treatment time, and better dose uniformity. The number of segments and MU also has been correlated with the whole body radiation dose with long-term complications. Thus, FJT should be the preferred option over SFT for large target volumes.

Evaluation of the impact of dental artefacts on intensity-modulated radiotherapy planning for the head and neck

Background and purpose High density materials create severe artefacts in the computed tomography (CT) scans used for radiotherapy dose calculations. Increased use of intensity-modulated radiotherapy (IMRT) to treat oropharyngeal cancers raises concerns over the accuracy of the resulting dose calculation. This work quantifies their impact and evaluates a simple corrective technique. Materials and methods Fifteen oropharyngeal patients with severe artefacts were retrospectively planned with IMRT using two different CT/density look-up tables. Each plan was recalculated using a corrected CT dataset to evaluate the dose distribution delivered to the patient. Plan quality in the absence of dental artefacts was similarly assessed. A range of dosimetric and radiobiological parameters were compared pre- and post-correction. Results Plans using a standard CT/density look-up table (density ⩽1.8 g/cm 3) revealed inconsistent inter-patient errors, mostly within clinical acceptance, although potentially significantly reducing target coverage for individual patients. Using an extended CT/density look-up table (density ⩽10.0 g/cm 3) greatly reduced the errors for 13/15 patients. In 2/15 patients with residual errors the CTV extended into the severely affected region and could be corrected by applying a simple manual correction. Conclusions Use of an extended CT/density look-up table together with a simple manual bulk density correction reduces the impact of dental artefacts on head and neck IMRT planning to acceptable levels.

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

Dosimetry audit for a multi-centre IMRT head and neck trial

Background and purpose PARSPORT was a multi-centre randomised trial in the UK which compared Intensity-Modulated Radiotherapy (IMRT) and conventional radiotherapy (CRT) for patients with head and neck cancer. The dosimetry audit goals were to verify the plan delivery in participating centres, ascertain what tolerances were suitable for head and neck IMRT trials and develop an IMRT credentialing program. Materials and methods Centres enrolling patients underwent rigorous quality assurance before joining the trial. Following this each centre was visited for a dosimetry audit, which consisted of treatment planning system tests, fluence verification films, combined field films and dose point measurements. Results Mean dose point measurements were made at six centres. For the primary planning target volume (PTV) the differences with the planned values for the IMRT and CRT arms were −0.6% (1.8% to −2.4%) and 0.7% (2.0% to −0.9%), respectively. Ninety-four percent of the IMRT fluence films for individual fields passed gamma criterion of 3%/3 mm and 75% of the films for combined fields passed gamma criterion 4%/3 mm (no significant difference between dynamic delivery and step and shoot delivery). Conclusions This audit suggests that a 3% tolerance could be applied for PTV point doses. For dose distributions tolerances of 3%/3 mm on individual fields and 4%/3 mm for combined fields are proposed for multi-centre head and neck IMRT trials.

Intra-cavitary dosimetry for IMRT head and neck treatment using thermoluminescent dosimeters in a naso-oesophageal tube

Complex intensity-modulated radiation therapy (IMRT) treatment plans require rigorous quality assurance tests. The aim of this study was to independently verify the delivered dose inside the patient in the region of the treatment site. A flexible naso-gastric tube containing thermoluminescent dosimeters (TLDs) was inserted into the oesophagus via the sinus cavity before the patient's first treatment. Lead markers were also inserted into the tube in order that the TLD positions could be accurately determined from the lateral and anterior–posterior electronic portal images taken prior to treatment. The measured dose was corrected for both daily linac output variations and the estimated dose received from the portal images. The predicted dose for each TLD was determined from the treatment planning system and compared to the measured TLD doses. The results comprise 431 TLD measurements on 43 patients. The mean measured-to-predicted dose ratio was 0.988 ± 0.011 (95% confidence interval) for measured doses above 0.2 Gy. There was a variation in this ratio when the measurements were separated into low dose (0.2–1.0 Gy), medium dose (1.0–1.8 Gy) and high dose (>1.8 Gy) measurements. The TLD-loaded, naso-oesophageal tube for in vivo dose verification is straightforward to implement, and well tolerated by patients. It provides independent reassurance of the delivered dose for head and neck IMRT.

Impact of interobserver variability in delineating clinical target volumes (CTV) for head and neck intensity modulated radiation therapy (IMRT): Potential for a geographical miss

e17013 Background: Despite modern day imaging techniques and guidelines for delineation of the clinical target volume, there remains significant inter-observer variability in delineating the CTV. With the use of IMRT, the target volume receives a significant tumoricidal dose while the regions just outside the target receive unpredictable doses. In this report, the dose to the region just outside the planning target volume (PTV) (defined as volume of uncertainty [VOU]), presumed to represent the regions subject to maximum inter-observer variability, was studied. Methods: The IMRT plans of 12 patients with head and neck cancer were used to determine the dose just outside the high-risk CTV by growing volumes around CTV with 3 mm, 5 mm, and 7 mm margins. These volumes were edited at regions close to skin/air and bone. PTVs were subsequently grown using the same margins as used in the original plans. With the Boolean operations, each of these volumes was subtracted from the existing PTV to generate the volumes of uncertainty (VOU) in 3 dimensions. The dose to these VOUs was analyzed. D95, D90 and median dose which are the doses received by 95%, 90%, and 50% of the target volume respectively were studied. Results: The median prescribed dose was 68 Gy (60 Gy-72 Gy). The median percentage D95 for 3mm, 5mm and 7mm VOU was 82.5% ± 4.95, 77.25% ± 5.53, and 69% ± 6.93, respectively. The median percentage D90 for these VOU's was 87.7% ± 3.53, 83.2% ± 4.61, and 79% ± 4.5, respectively. The median dose to each of these VOU”s was 96% ± 1.6, 94.5% ± 1.95, and 92.5% ± 1.85 respectively. Conclusions: This study documents that the volumes of uncertainty surrounding the PTV, which could contain subclinical disease, in fact receive a significant amount of RT dose. Hence, despite a large amount of evidence for inter-observer variability in target delineation for head and neck cancer,the majority of locoregional recurrences are within the high dose region and not marginal failures. No significant financial relationships to disclose.

Accuracy requirements for head and neck intensity-modulated radiation therapy based on observed dose response of the major salivary glands

Background and Purpose We estimated accuracy requirements for dose and position of the major salivary glands in head and neck intensity-modulated radiotherapy (IMRT) based on the dose response characteristics of the glands obtained by using the sigmoidal dose response model. Materials and methods Dose response of the parotid and submandibular salivary glands was determined for 25 head and neck cancer patients treated by IMRT. Individual salivary gland functions were assessed by scintigraphy before and 6 months after radiotherapy. Accuracy requirements were estimated by using the maximal slope of the fitted dose response model and average value of the dose gradients within the glands. In addition, systematic and random set-up errors were estimated for each patient by at least weekly portal imaging. We investigated the changes in the salivary gland mean doses ( D mean) that would have occurred without correction of patient positioning. This was done by shifting the planned isocenter according to the obtained systematic set-up error and by recalculating the dose distribution in treatment planning system (TPS). Results The maximal slope and D 50 values of the dose response model were −0.041 1/Gy and 30.4 Gy, respectively. The results suggested that spared fraction of individual salivary gland function can be estimated with an accuracy of ±10%, if actual D mean of the gland is within ±2.4 Gy with the planned value. On the average, this was achieved with maximal systematic positional 3D shift of 3.0 mm for the parotid glands and 2.7 mm for the submandibular glands. The magnitude of systematic 1D set-up errors was 1.7 ± 1.3 mm (mean ± SD) while that of systematic 3D errors was 3.4 ± 1.6 mm. The SD of random set-up errors was 1.5 mm. The magnitude of D mean shifts due to set-up errors was 1.5 ± 1.4 Gy. The steepness of dose gradients within the glands was 0.8 ± 0.5 Gy/mm in the most critical direction (toward the glands). Conclusions When substantial part of salivary gland function is intended to be spared in head and neck IMRT, narrow dosimetric and positional tolerances should be adopted for the major salivary glands due to steep dose response curve obtained for the glands.

Evaluation of the Delta4 phantom for IMRT and VMAT verification

The Delta4 diode array phantom (Scandidos, Uppsala, Sweden) was evaluated for verification of segmental intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) on an Elekta linear accelerator (Crawley UK). The device was tested for angular sensitivity by irradiating it from 36 different gantry angles, and the responses of the device to various step-and-shoot segment doses and dose rates were evaluated using an ionization chamber as a comparison. The phantom was then compared with ionization chamber and film results for two prostate and pelvic nodes IMRT plans, two head and neck IMRT plans and two lung VMAT plans. These plans were calculated using Pinnacle3 (Philips Radiation Oncology Systems, Madison, WI). The uniformity of angular response was better than 0.5% over the range of gantry angles. The uniformity of response of the Delta4 to different segment monitor units and dose rates was better than 0.5%. The assessment of the IMRT and VMAT plans showed that the Delta4 measured a dose within 2.5% of the ionization chamber, and compared to film recorded a slightly larger region (range −2% to +7%) agreeing with the planned dose to within 3% and 3 mm. The Delta4 is a complex device and requires careful benchmarking, but following the successful completion of these measurements, the Delta4 has been introduced into clinical use.

Accelerated fractionation head and neck intensity‐modulated radiation therapy and concurrent chemotherapy in the community setting: Safety and efficacy considerations

This article reviews our community cancer center's experience treating head and neck cancer primarily with accelerated fractionation intensity-modulated radiation therapy (IMRT), with or without concurrent chemotherapy, focusing on acute toxicity and efficacy. Fifty-two patients treated with IMRT at the Penrose Cancer Center between 2002 and 2007 constitute the cohort. The majority (75%) received an accelerated, altered fractionation regimen, typically concomitant boost to 7200 cGy. Concurrent chemotherapy was delivered to 32 (62%). The median follow-up was 24 months. The 2-year actuarial rates of local control, regional control, and distant metastasis-free survival were 100%, 91%, and 94%, respectively. Relapse-free survival and overall survival at 2 years were 89% and 91%, respectively. Overall, 32 of 52 patients (62%) experienced at least 1 type of grade 3 or 4 acute toxicity. Accelerated fractionation IMRT, with or without chemotherapy, can be given safely and effectively in a community cancer center setting.

An energy fluence‐convolution model for amorphous silicon EPID dose prediction

In this work, an amorphous silicon electronic portal imaging device (a-Si EPID) dose prediction model based on the energy fluence model of the Pinnacle treatment planning system Version 7 (Philips Medical Systems, Madison, WI) is developed. An energy fluence matrix at very high resolution (< 1 mm) is used to incorporate multileaf collimator (MLC) leaf effects in the predicted EPID images. The primary dose deposited in the EPID is calculated from the energy fluence using experimentally derived radially dependent EPID interaction coefficients. Separate coefficients are used for the open beam energy fluence component and the component of the energy fluence transmitted through closed MLC leaves to each EPID pixel. A spatially invariant EPID dose deposition kernel that describes both radiative dose deposition, central axis EPID backscatter, and optical glare is convolved with the primary dose. The kernel is further optimized to give accurate EPID penumbra prediction and EPID scatter factor with changing MLC field size. An EPID calibration method was developed to reduce the effect of nonuniform backscatter from the support arm (E-arm) in a calibrated EPID image. This method removes the backscatter component from the pixel sensitivity (flood field) correction matrix retaining only field-specific backscatter in the images. The model was compared to EPID images for jaw and MLC defined open fields and eight head and neck intensity modulated radiotherapy (IMRT) fields. For the head and neck IMRT fields with 2%, 2 mm criteria 97.6 +/- 0.6% (mean +/- 1 standard deviation) of points passed with a gamma index less than 1, and for 3%, 3 mm 99.4 +/- 0.4% of points were within the criteria. For these fields, the 2%, 2 mm pass score reduced to 96.0 +/- 1.5% when backscatter was present in the pixel sensitivity correction matrix. The model incorporates the effect of MLC leaf transmission, EPID response to open and MLC leakage dose components, and accurately predicts EPID images of IMRT fields. Removing the backscatter component of the pixel sensitivity matrix correction reduces the effect of nonuniform E-arm backscatter.