Rotational Intensity-modulated Radiotherapy Research Articles

Squamous-cell carcinoma of the anus: progress in radiotherapy treatment.

Chemoradiotherapy is the standard-of-care treatment of squamous-cell carcinoma of the anus (SCCA), and this has not changed in decades. Radiation doses of 50-60 Gy, as used in many phase III trials, result in substantial late morbidities and fail to control larger and node-positive tumours. Technological advances in radiation therapy are improving patient outcomes and quality of life, and should be applied to patients with SCCA. Modern techniques such as intensity-modulated radiotherapy (IMRT), rotational IMRT, image-guided radiotherapy using cone-beam CT, and stereotactic techniques have enabled smaller margins and highly conformal plans, resulting in decreased radiation doses to the organs at risk and ensuring a shorter overall treatment time. In this Perspectives article, the use of novel approaches to target delineation, optimized radiotherapy techniques, adaptive radiotherapy, dose-escalation with external-beam radiotherapy (EBRT) or brachytherapy, and the potential for modified fractionation are discussed in the context of SCCA.

Rotational Intensity Modulated Radiation Therapy in Complex Adjuvant Breast and Nodes Irradiation

Rotational Intensity Modulated Radiation Therapy in Complex Adjuvant Breast and Nodes Irradiation

Prostate Dose-painting Radiotherapy and Radiobiological Guided Optimisation Enhances the Therapeutic Ratio

AimsTo describe the treatment of 11 patients with radiobiologically guided dose-painting radiotherapy and report on toxicity. Materials and methodsBoost volumes were identified with functional magnetic resonance imaging scans in 11 patients with high-risk prostate cancer. Patients were treated using a dose-painting approach; the boost dose was limited to 86 Gy in 37 fractions, while keeping the rectal normal tissue complication probability to 5–6%. Rotational intensity-modulated radiotherapy was used with daily image guidance and fiducial markers. ResultsThe median dose to the prostate (outside the boost volume) and urethra was 75.4 Gy/37 fractions (range 75.1–75.8 Gy), whereas the median boost dose was 83.4 Gy (range 79.0–87.4 Gy). The tumour control probability (TCP) (Marsden model) increased from 71% for the standard plans to 83.6% [76.6–86.8%] for the dose-painting boost plans. The mean (Lyman-Kutcher-Burman) normal tissue complication probability for rectal bleeding was 5.2% (range 3.3–6.2%) and 5.2% for faecal incontinence (range 3.6–7.8%). All patients tolerated the treatment well, with a low acute toxicity profile. At a median follow-up of 36 months (range 24–50) there was no grade 3 late toxicity. Two patients had grade 2 late urinary toxicity (urethral stricture, urinary frequency and urgency), one patient had grade 1 and one grade 2 late rectal toxicity. The mean prostate-specific antigen at follow-up was 0.81 ng/ml after stopping hormone therapy; one patient relapsed biochemically at 32 months (2.70 ng/ml). ConclusionsThe toxicity for this radiobiological guided dose-painting protocol was low, but we have only treated a small cohort with limited follow-up time. The advantages of this treatment approach should be established in a clinical trial.

PO-0677: Rotational intensity Modulated Radiation Therapy in complex adjuvant breast and nodes irradiation

pZ.028), receive chemotherapy (84% vs. 67%, p<.001), and receive Herceptin (39% vs. 23%, pZ.005). The 7-year LRR-free survival for the entire cohort was 92.4% (95% CI 88.7-94.9). EIC+ patients had a significantly higher 7-year LRR rate (14.7% vs. 5.2%, pZ.019), as did younger (age 50) patients (14.1% vs. 3.7%, p<.001). After adjustment for age, chemotherapy receipt, and Herceptin receipt, the presence of EIC remained significant for increased LRR (HR 2.34; 95% CI 1.08-5.06; pZ.031). Conclusion: EIC was a prominent pathologic feature seen in 24% of patients with HER2+ breast cancer treated with breast conserving therapy. Despite the achievement of negative surgical margins in the vast majority of patients, EIC was prognostic for LRR. These results must be confirmed in larger cohorts of HER2+ patients treated with anti-HER2 therapies. Author Disclosure: N. Ohri: None. C.T. Siu: None. A. Eaton: None. A.Y. Ho: None.

EP-1553: Dosimetry study of two rotational intensity-modulated radiotherapy in esophageal cancer

EP-1553: Dosimetry study of two rotational intensity-modulated radiotherapy in esophageal cancer

Is linac-based volumetric modulated arc therapy better than helical tomotherapy in the radiotherapy of nasopharyngeal carcinoma?

Volumetric arc therapy (VMAT) and helical tomotherapy (HT) are currently the two state-of-the-art rotational intensity-modulated radiotherapy (IMRT). When compared with conventional IMRT, linac-based VMAT can shorten the treatment time while HT has been claimed to produce a better dosimetric outcome. The aim of our study was to evaluate whether the more “efficient” VMAT can produce a comparable or better dosimetric outcome when compared with HT for the treatment of nasopharyngeal carcinoma (NPC) patients. Sixteen primary NPC patients treated by HT were recruited. In addition to the original HT plans, two-arc VMAT plans using the same patient data and dose requirements were generated for each patient. The dosimetric evaluation of the plans included the comparison of conformity index (CI), homogeneity index (HI), dose to 2 and 98 % volume (D 2 and D 98) of the planning target volume (PTV) of the NP, and left and right lymphatic regions. For the organs at risk (OARs), maximum dose or D 2 and mean dose were used for serial and parallel organs, respectively. Student’s paired t test or Wilcoxon signed-rank test was used to check for significant difference. The dose to the NP target was similar between the two techniques. For the nodal targets, the VMAT plans produced higher mean D 2 and lower HI values. For the OARs, the spinal cord and spinal cord doses were higher in the VMAT plans than in the HT plans, whereas the opposite happened in the optic chiasm and pituitary gland. The doses to the brain stem, lens, and optic nerve did not show a significant difference. Both systems delivered satisfactory dose distributions and kept the dose to OARs below their tolerance. However, if the parotid gland toxicity is a concern in the treatment, HT would be more favorable because it is relatively more effective in sparing the parotid gland. Otherwise, the linac-based double-arc VMAT would be recommended because of its shorter treatment time.

A multi-institutional dosimetry audit of rotational intensity-modulated radiotherapy

BackgroundRotational IMRT (VMAT and Tomotherapy) has now been implemented in many radiotherapy centres. An audit to verify treatment planning system modelling and treatment delivery has been undertaken to ensure accurate clinical implementation. Material and methods34 institutions with 43 treatment delivery systems took part in the audit. A virtual phantom planning exercise (3DTPS test) and a clinical trial planning exercise were planned and independently measured in each institution using a phantom and array combination. Point dose differences and global gamma index (γ) were calculated in regions corresponding to PTVs and OARs. ResultsPoint dose differences gave a mean (±sd) of 0.1±2.6% and 0.2±2.0% for the 3DTPS test and clinical trial plans, respectively. 34/43 planning and delivery combinations achieved all measured planes with >95% pixels passing γ<1 at 3%/3mm and rose to 42/43 for clinical trial plans. A statistically significant difference in γ pass rates (p<0.01) was seen between planning systems where rotational IMRT modelling had been designed for the manufacturer’s own treatment delivery system and those designed independently of rotational IMRT delivery. ConclusionsA dosimetry audit of rotational radiotherapy has shown that TPS modelling and delivery for rotational IMRT can achieve high accuracy of plan delivery.

Helical tomotherapy in breast cancer treatment

SUMMARY Helical tomotherapy (HT) is a technique of rotational intensity-modulated radiation therapy allowing dose escalation and normal tissue sparing for various types of cancer. Dosimetric studies of HT in breast cancers showed an improvement in dose homogeneity, conformity and a decrease in high doses of organs at risk compared with standard techniques. On the other hand, HT increases low dose of organs at risk, which in rare cases can increases the risk of secondary cancer. The benefits of HT seem to be greater in complex target volumes in which dose coverage is difficult with standard techniques (funnel chest, tumor in the inner quadrant, obese patients). Indications for breast HT should be personalized and selected according to the expected benefit for every patient.

SU-E-T-113: Dose Distribution Using Respiratory Signals and Machine Parameters During Treatment

Purpose: Volumetric modulated arc therapy (VMAT) is a rotational intensity-modulated radiotherapy (IMRT) technique capable of acquiring projection images during treatment. Treatment plans for lung tumors using stereotactic body radiotherapy (SBRT) are calculated with planning computed tomography (CT) images only exhale phase. Purpose of this study is to evaluate dose distribution by reconstructing from only the data such as respiratory signals and machine parameters acquired during treatment. Methods: Phantom and three patients with lung tumor underwent CT scans for treatment planning. They were treated by VMAT while acquiring projection images to derive their respiratory signals and machine parameters including positions of multi leaf collimators, dose rates and integrated monitor units. The respiratory signals were divided into 4 and 10 phases and machine parameters were correlated with the divided respiratory signals based on the gantry angle. Dose distributions of each respiratory phase were calculated from plans which were reconstructed from the respiratory signals and the machine parameters during treatment. The doses at isocenter, maximum point and the centroid of target were evaluated. Results and Discussion: Dose distributions during treatment were calculated using the machine parameters and the respiratory signals detected from projection images. Maximum dose difference between plan and in treatment distributionmore » was −1.8±0.4% at centroid of target and dose differences of evaluated points between 4 and 10 phases were no significant. Conclusion: The present method successfully evaluated dose distribution using respiratory signals and machine parameters during treatment. This method is feasible to verify the actual dose for moving target.« less

Direct aperture optimization using an inverse form of back-projection.

Direct aperture optimization (DAO) has been used to produce high dosimetric quality intensity‐modulated radiotherapy (IMRT) treatment plans with fast treatment delivery by directly modeling the multileaf collimator segment shapes and weights. To improve plan quality and reduce treatment time for our in‐house treatment planning system, we implemented a new DAO approach without using a global objective function (GFO). An index concept is introduced as an inverse form of back‐projection used in the CT multiplicative algebraic reconstruction technique (MART). The index, introduced for IMRT optimization in this work, is analogous to the multiplicand in MART. The index is defined as the ratio of the optima over the current. It is assigned to each voxel and beamlet to optimize the fluence map. The indices for beamlets and segments are used to optimize multileaf collimator (MLC) segment shapes and segment weights, respectively. Preliminary data show that without sacrificing dosimetric quality, the implementation of the DAO reduced average IMRT treatment time from 13 min to 8 min for the prostate, and from 15 min to 9 min for the head and neck using our in‐house treatment planning system PlanUNC. The DAO approach has also shown promise in optimizing rotational IMRT with burst mode in a head and neck test case.PACS number: 87.55.D‐

Dose reconstruction using respiratory signals and machine parameters during treatment in stereotactic body radiotherapy

Volumetric modulated arc therapy (VMAT) is a rotational intensity-modulated radiotherapy (IMRT) technique capable of acquiring projection images during treatment. The purpose of this study was to reconstruct the dose distribution from respiratory signals and machine parameters acquired during stereotactic body radiotherapy (SBRT). The treatment plans created for VMAT-SBRT included the constraint of 1 mm/degree in multileaf collimator (MLC) for a moving phantom and three patients with lung tumors. The respiratory signals were derived from projection images acquired during VMAT delivery, while the machine parameters were derived from machine logs. The respiratory signals and machine parameters were then linked along with the gantry angle. With this data, the dose distribution of each respiratory phase was calculated on the planned four-dimensional CT (4D CT). The doses at the isocenter, the point of max dose and the centroid of the target were compared with those of the corresponding plans. In the phantom study, the maximum dose difference between the plan and "in-treatment" results was -0.4% at the centroid of the target. In the patient study, the difference was -1.8 ± 0.4% at the centroid of the target. Dose differences of the evaluated points between 4 and 10 phases were not significant. The present method successfully reconstructed the dose distribution using the respiratory signals and machine parameters acquired during treatment. This is a feasible method for verifying the actual dose for a moving target.

Stereotactic Body radiation therapy for liver tumors with or without rotational intensity modulated radiation therapy

BackgroundTo evaluate the feasibility and efficacy of Stereotactic body radiation therapy (SBRT) for primary liver lesions and liver metastases treated with linear accelerators with or without rotational Intensity Modulated RadioTherapy (IMRT).MethodsPatients with either hepatocellular carcinoma, cholangiocarcinoma or metastatic liver lesions who had one to three lesions treated with SBRT in a single institution were retrospectively reviewed. Tumor response was evaluated according to EASL criteria 3 months after SBRT completion using MRI and/or abdominal CT scan. Responses were categorised as: Stable Disease (SD), Partial Response (PR), Complete Response (CR), Local Progression or Distant Progression in cases of new intra-hepatic lesions out-of-field or extra-hepatic metastases. Local Control (LC), Progression Free Survival (PFS), Overall Survival (OS) and treatment-related toxicities are reported.ResultsBetween 2007 and 2012, 20 patients with a total of 24 lesions were treated with SBRT. Fourteen patients presented hepatocellular carcinoma (HCC), the others had either metastatic lesions from colorectal cancer (CRC) or cholangiocarcinoma. The median diameter of the lesions was 23 mm (5–98).The dose per fraction ranged from 6 to 20 Gy with a median total dose of 60 Gy (range: 36–60 Gy). The dose was prescribed to the 80% isodose line covering the PTV.The median follow-up was 24 months (15.7-29.7).The actuarial LC rate was 78% for patients with HCC and 83% for those with adenocarcinoma and cholangiocarcinoma. Median OS was 37 months and OS rates were 83% at 12 and 24 months for HCC and 100% for adenocarcinoma. PFS was 54% for HCC and 50% for other types of tumors at 24 months.Acute grade 3–4 toxicities occurred in 2 patients; a small proportion of the other patients experienced grade 1 or 2 toxicities.ConclusionsSBRT provides excellent local control with minimal side effects in selected patients.

The potential of helical tomotherapy in the treatment of head and neck cancer.

A decade after its first introduction into the clinic, little is known about the clinical impact of helical tomotherapy (HT) on head and neck cancer (HNC) treatment. Therefore, we analyzed the basics of this technique and reviewed the literature regarding HT's potential benefit in HNC. The past two decades have been characterized by a huge technological evolution in photon beam radiotherapy (RT). In HNC, static beam intensity-modulated radiotherapy (IMRT) has shown superiority over three-dimensional conformal RT in terms of xerostomia and is considered the standard of care. However, the next-generation IMRT, the rotational IMRT, has been introduced into the clinic without any evidence of superiority over static beam IMRT other than being substantially faster. Of these rotational techniques, HT is the first system especially developed for IMRT in combination with image-guided RT. HT is particularly promising for the treatment of HNC because its sharp dose gradients maximally spare the many radiosensitive organs at risk nearby. In addition, HT's integrated computed tomography scan assures a very precise dose administration and allows for some adaptive RT. Because HT is specifically developed for IMRT in combination with (integrated) image-guidance, it allows for precise dose distribution ("dose painting"), patient setup, and dose delivery. As such, it is an excellent tool for difficult HNC irradiation. The literature on the clinical results of HT in HNC all show excellent short-term (≤2 years) results with acceptable toxicity profiles. However, properly designed trials are still warranted to further substantiate these results.

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

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

Multicenter Collaborative Quality Assurance Program for the Province of Ontario, Canada: First-Year Results

The objective of this work was to develop a collaborative quality assurance (CQA) program to assess the performance of intensity modulated radiation therapy (IMRT) planning and delivery across the province of Ontario, Canada. The CQA program was designed to be a comprehensive end-to-end test that can be completed on multiple planning and delivery platforms. The first year of the program included a head-and-neck (H&N) planning exercise and on-site visit to acquire dosimetric measurements to assess planning and delivery performance. A single dosimeter was used at each institution, and the planned to measured dose agreement was evaluated for both the H&N plan and a standard plan (linear-accelerator specific) that was created to enable a direct comparison between centers with similar infrastructure. CQA program feasibility was demonstrated through participation of all 13 radiation therapy centers in the province. Planning and delivery was completed on a variety of infrastructure (treatment planning systems and linear accelerators). The planning exercise was completed using both static gantry and rotational IMRT, and planned-to-delivered dose agreement (pass rates) for 3%/3-mm gamma evaluation were greater than 90% (92.6%-99.6%). All centers had acceptable results, but variation in planned to delivered dose agreement for the same planning and delivery platform was noted. The upper end of the range will provide an achievable target for other centers through continued quality improvement, aided by feedback provided by the program through the use of standard plans and simple test fields.

Patterns-of-Care for Thoracic Stereotactic Body Radiotherapy among Practicing Radiation Oncologists in the United States

Radiation oncologists were surveyed to assess practice patterns in the use of stereotactic body radiotherapy (SBRT) for lung cancer. A customized patterns-of-care survey, consisting of 18 questions and two clinical scenarios, was e-mailed to 136 academic radiation oncologists and 768 community practitioners to evaluate the technical basis and delivery parameters associated with SBRT. A total of 117 surveys were evaluable. The cited delivery techniques included: static noncoplanar beams (48%), intensity-modulated radiotherapy (41%), rotational intensity-modulated radiotherapy (47%), dynamic conformal arcs (7%), and small-beam delivery with fiducial tracking (24%), with 46% using multiple techniques. The immobilization methods included: stereotactic frame (10%), alpha cradle or vacuum-lock system (52%), wingboard (3%), stereotactic frame with an alpha cradle or vacuum-lock system (11%); combination of devices (14%), or no immobilization (9%). Abdominal compression was used by 51% and respiratory gating by 31%. For a peripheral T1N0 tumor, the preferred doses included: 25 to 34 Gy in one fraction (1%); 54 to 60 Gy in three fractions (56%), 48 to 50 Gy in four fractions (18%), and 50 to 60 Gy in five fractions (25%). For a centrally located T1N0 tumor, 58% recommended SBRT outside a clinical protocol, with recommended doses ranging from 40 to 60 Gy in three to 10 fractions. The recommended interval to first surveillance imaging ranged from 6 weeks or lesser (32%) to 25 weeks or more (2%). Considerable variation exists for thoracic SBRT with regard to dose selection, fractionation, immobilization, planning, management of central lesions, and surveillance. Ongoing prospective evaluation is recommended to identify best practices and provide continual process improvement.

Development of a novel treatment planning test for credentialing rotational intensity-modulated radiotherapy techniques in the UK

The increasing use of tomotherapy and volumetric-modulated arc therapy in UK centres will result in more centres choosing to use this technology in a clinical trial setting. The Radiotherapy Trials Quality Assurance (RTTQA) group has developed a new procedure to integrate into the UK intensity-modulated radiotherapy (IMRT) credentialing programme to cover rotational IMRT delivery techniques. A planning test [three-dimensional treatment planning system (3DTPS)] was designed specifically for rotational IMRT techniques. The feasibility of using this test in the credentialing programme for rotational IMRT was validated by 10 experienced UK centres. The study included five centres using Varian RapidArc™ (RA) (Varian Medical Systems, Milpitas, CA), two using Elekta VMAT™ (VMAT) (Elekta Inc., Norcross, GA) and three using helical tomotherapy (HT) plans. Centres were asked to carry out their own in-house quality assurance (QA) for the plans submitted for this study. A survey was sent out to centres aiming to gather information on their experience in undertaking the exercise and their QA results. All centres fulfilled the primary goal by achieving the dose constraints of the primary planning target volume and organ at risk. Seven centres (three RA, one VMAT and three HT plans) were able to fulfil the secondary goal. Among those seven centres, three centres (two RA and one VMAT plans) achieved the tertiary goal. The results of the survey indicated that the 3DTPS test is a clinically relevant and practical planning test to be used. A planning test for rotational therapy techniques was developed for the RTTQA IMRT credentialing programme. This study validated the feasibility of a 3DTPS test to be used as part of a credentialing programme for rotational IMRT techniques in the UK.

SU-E-T-154: Online Dose Verification with Gafchromic Film for Fixed-Gantry and Rotational Intensity Modulated Radiation Therapy: A Phantom Study.

The patient specific quality assurance (QA) measurements for fixed-gantry and rotational intensity modulated radiation therapy (IMRT and VMAT/RapidArc) are usually performed on a homogeneous phantom prior to the treatment. The purpose of this study is to develop an online method to verify the delivered dose to the patient on the treatment day. An anthropomorphic (Rando) head phantom was immobilized in treatment position with a thermoplastic mask to simulate a real patient. A sheet of gafchromic film (EBT2) was sandwiched between a 1-cm-thick solid water slab, which was fixed to the Type-S extension board, and the patient's head hold (a pillow used here). The CT images of the Rando phantom were acquired and exported to the treatment planning systems. One step-and-shot fixed-gantry IMRT plan and one RapidArc plan were generated and the dose distributions on the film plane were calculated. The two plans were delivered to the patient (Rando phantom in this study) in the treatment position on a Varian Trilogy linear accelerator with two new films. The films were scanned, and the measurements were compared with the planned doses. The composite dose distributions measured on the film plane were the actual delivered dose for the treatment. The comparison between the measurement and planned dose profiles shows an agreement within 3% because of the good reproducibility of phantom positioning. Gamma pass rates (using 3mm and 3% criteria) for the IMRT and RapidArc plan were found to be 95% and 94%, respectively. The phantom study has demonstrated the feasibility of using gafchromic film for online dose verification. This simple method takes into account the patient heterogeneity and the treatment associated uncertainties such as setup error, intrafraction motions and machine related variations. It can be implemented as an online physics and/or clinical QA tool without taking additional machine time.

TU‐G‐BRA‐06: Motion Adaptive Rotational Intensity Modulated Radio Therapy (MA‐RIMRT) with Image Guidance

Purpose: Commercial intensity‐modulated‐arc‐therapy (IMAT) implementations rely on a 100% beam‐on duty‐cycle to achieve high efficiency. With moving targets, real‐time tracking or gating may be applied. Tracking is technically challenging and gating reduces efficiency by lowering the duty‐cycle. By combining burst‐mode rIMRT (in which all dose is administered while the gantry is within a few degrees of a beam optimization point [OP]), with a motion pre‐emption strategy and intrafractional imaging, we demonstrate an efficient implementation of motion‐adaptive rIMRT. Methods: 1. An rIMRT plan is created with 36–72 OPs. A gantry rotating at 6 degrees/s takes 3–6s to reach successive OPs. This co‐incides with the natural breathing period. 2. A Fourier‐descriptor‐based trajectory model is fitted to target features identified in pre‐treatment CBCT projections. The model predicts feature location at any combination of gantry angle and breathing phase. 3. The model sets the gantry speed to place the gantry at the next beam OP just in time as the target arrives at the planned position. In‐line projection images are acquired immediately before and during beam delivery. Treatment ensues only if the target position is correct (verification completes within 50ms, so maximum misdelivery possible at 2000MU/min is within 2MU). In cases of irregular breathing, the system gracefully defaults to gated‐IMRT. Results: For a MA‐rIMRT delivery of 750MU at 2000MU/min, treatment duration increases from 154s (regular rIMRT) to 199s (5s respiration cycle). Conclusions: With predictable patient breathing, MA‐rIMRT delivery times are competitive with conventional IMAT. High treatment accuracy is easily achieved, since there is no tracking lag and since the beam shape is static during delivery. Contemporary linacs rely on non‐realtime communication systems (typically Ethernet) to control time‐critical elements such as the MLC. Since such variable‐delay systems pose formidable problems in terms of predictive control, our approach offers a pragmatic alternative to tracking. Siemens Medical Solutions USA

Static and rotational step-and-shoot IMRT treatment plans for the prostate: A risk comparison study

For certain clinical applications, rotational intensity modulated radiation therapy (R-IMRT) techniques such as volumetric modulated arc therapy (VMAT) are capable of improved target dose coverage and shorter delivery time when compared to static, step-and-shoot IMRT. The authors performed a risk comparison study for two standard step-and-shoot IMRT plans and two step-and-shoot Rotational IMRT plans that were designed specifically for a Siemens Primus linear accelerator. CT images of a RANDO phantom were used to generate R-IMRT and static IMRT plans. One simple and one complex prostate cases were created to investigate these techniques. The R-IMRT plans consisted of 72 single-segmented 6 MV beams, equally spaced with beam angle separations of 5°. The static IMRT plans employed seven multisegmented 6 MV beams. Both types of plans were optimized in Pinnacle(3) with the direct machine parameter optimization (DMPO) algorithm using the same set of optimization objectives. The plans were delivered to a RANDO phantom and thermoluminescent diode (TLD) dose measurements were performed at various locations throughout the phantom. Risk coefficients and organ weightings as defined by International Commission on Radiological Protection (ICRP) Publication 103 were used to calculate the resulting effective doses to various organs at risk, as well as the overall risk estimate for both techniques. For the simple prostate case, the R-IMRT technique provided a higher dose to organs at risk within the CT volume and a lower overall peripheral dose to remaining organs. The R-IMRT plan had a risk estimate of 4.56% when compared to the IMRT risk of 4.78%. For the complex prostate case, there was no significant difference in the lifetime risks of the IMRT (5.73%) and R-IMRT (5.74%) plans. R-IMRT is an approximation to VMAT and it was found that there is no clinically significant difference between lifetime risk estimates between R-IMRT and standard seven-beam IMRT for the prostate.