IMRT Delivery Techniques Research Articles

Evaluation of Two Intensity Modulated Radiotherapy Techniques for Total Body Irradiation: Initial Clinical Experience From an Ongoing Pilot Study

A multi-institutional pilot study is initiated to compare two IMRT delivery techniques (volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT)) for total body irradiation as conditioning regimen in patients undergoing hematopoietic stem cell transplant with an updated lung dose limit based on recently published clinical data on lung toxicities. We report comparison and initial clinical experience in dosimetric plan quality, patient setup correction techniques, and dose delivery efficiency.We reviewed treatment planning and delivery record for the initial seven patients that were prospectively enrolled in this study. TBI was given to each patient in 8 fractions at 165 cGy per fraction. Patient height ranged from 154.9 to 193.5 cm. The planning target volume (PTV) is the whole body excluding the lung volume. The total lung and 15 additional normal organs were included in plan optimization and dosimetric evaluation. In planning, mean lung dose (MLD) limit was 8 Gy and maximum dose (Dmax) to normal organs was required to be < 130% of the prescribed dose with a recommended goal of < 115%. Four patients received TBI with HT and the other three on a conventional Linac. Patients treated with HT had an upper body HT plan with a rotating gantry using a 5-cm jaw and a lower extremity HT plan using static AP/PA HT fields. Patients treated on a conventional Linac had an upper body VMAT plan using 9 to 10 VMAT fields and a lower body plan with either VMAT fields or static AP/PA fields. The virtual bolus technique was used in VMAT plans to reduce dose variation due to setup uncertainty. The VMAT fields used a 6 MV photon beam with a 90° collimator angle and had an overlap of at least 2 cm between adjacent fields along the longitudinal direction. All the plans were normalized so that 85% of the PTV received at least 13.2 Gy in total dose.In the upper body HT plans the average PTV dose was 13.6 Gy (range: 13.5 - 13.7 Gy), the MLD averaged at 7.71 Gy (range: 7.21 - 7.95 Gy), and Dmax was < 115% of prescribed dose for at least 14 normal organs. In the upper-body VMAT plans the average PTV dose was 13.95 Gy (range: 13.86 - 14.02 Gy), the MLD averaged at 7.12 Gy (range: 6.85 - 7.63 Gy), and Dmax was < 115% of prescribed dose with 1 to 9 normal organs. The Dmax limit of 130% of prescribed dose was achieved in all the plans. In HT treatments, an MVCT scan was used for daily patient setup corrections in both the upper and lower body treatments and the overall treatment time per fraction was 1.23 ± 0.21 hours (range: 0.90 - 1.88 hours). In VMAT treatments, both cone beam CT scans and orthogonal kilovoltage images were used for daily patient setup on a conventional Linac and the overall treatment time per fraction was 1.39 ± 0.20 hours.Both the HT and VMAT plans achieved adequate lung sparing under our new institutional lung dose limit in TBI. VMAT TBI is clinically feasible and could provide access to more patients for TBI treatments. The two IMRT techniques also allow selective sparing of additional organs in TBI treatments.

Assessing the out-of-field dose calculation accuracy by eclipse treatment planning system in sliding window IMRT of prostate cancer patients

AimThe objective of this study was to evaluate out-of-field dose distribution calculation accuracy by the Anisotropic Analytical Algorithm (AAA), version 13.0.26, in Eclipse TPS, (Varian Medical Systems, Palo Alto, Ca, USA) for sliding window IMRT delivery technique in prostate cancer patients. Materials and methodsProstate IMRT plans with nine coplanar were calculated with the AAA Eclipse treatment planning system. To assess the accuracy of dose calculation predicted by the Eclipse in normal tissue and OARs located out of radiation field areas, including the rectum, bladder, right and left head of the femur, absolute organ dose value, and dose distribution were measured using the Delta4+ IMRT phantom. ResultsIn the out-of-field areas, underestimation of −0.66% in organs near the field edge to −39.63% in organs far from the field edge (2.5 and 7.3 cm respectively) occurred in the TPS calculations. The percentage of dose deviation for the femoral heads was 95.7 on average while for the organ closer to the target (rectum) it was 79.81. ConclusionsAAA dosimetry algorithm (used in Eclipse TPS) showed poor dose calculation in areas beyond the treatment fields border where underestimation varies with the distance from the field edges. A significant underestimation was found for the AAA algorithm in the sliding window IMRT technique (P-value > 0.05).

Comprehensive Dosimetric Evaluation of a Biology-Guided Radiotherapy Machine in Treatment Plans for Brain, Lung, Head and Neck, Esophagus, and Prostate Malignancies

A new biology-guided radiotherapy (BgRT) delivery machine is currently under development. It combines a compact 6 MV linear accelerator and a binary multileaf collimator with PET, kVCT, and MV imaging systems on the same ring gantry that rotates continuously at 60 rotations per minute while the patient is translated through the bore with small increments. We aimed to perform inter-comparison of treatment plans for typical clinical disease sites between the new BgRT machine and existing IMRT delivery techniques. To ensure objective comparison, biology guidance was not assumed in planning for the new system and the same planning margins were used for each case with all the delivery techniques. We retrospectively retrieved one clinical external-beam radiotherapy treatment plan for each of the five disease sites: intracranial lesion for stereotactic radiosurgery (SRS), early-stage lung lesion for stereotactic body radiotherapy (SBRT), head and neck, esophagus, and prostate with seminal vesicle and pelvic lymph node treatments. Both a helical tomotherapy (HT) plan (using a 1-cm or 2-cm jaw size) and a volumetric modulated arc therapy (VMAT) plan (using 0.5-cm MLC or 0.25-cm MLC leaves) were generated for each clinical case. A treatment plan (the prototype plan) for the new system was generated for each case using either a 1-cm jaw size for the intracranial SRS and lung SBRT plans, and a 2-cm jaw size for the other sites. Dosimetric constraints in national radiotherapy treatment protocols (RTOG-0813, NRG-HN001, RTOG-1010, and RTOG-0534) were used to facilitate dosimetric comparison of treatments plans. The prototype plans met all the dosimetric constraints in all disease sites. The relative strengths with each delivery technique differ with respect to the disease site. With the head & neck case, while the VMAT and HT plans shows similar normal organ dose, the prototype plan achieved comparable PTV dose coverage and lower dose to most critical organs including a 40% reduction in parotid mean dose compared to the HT and the VMAT plans. With the intracranial SRS case, the VMAT plan achieved slightly better dose conformity and faster dose drop-off compared to the HT and prototype plans, while all the plans achieved comparable dose to the normal brain and other organs-at-risk. With the lung SBRT case, both the VMAT and the prototype plans achieve similar dose to the normal lung volume and other organs. In both the prostate and esophagus cases, mixed results were observed with one technique achieving better sparing to certain organs. While all the delivery technique met given dosimetric constraints in most disease sites, the prototype system was shown to achieve better normal organ sparing with the head & neck site, while the VMAT technique with smaller MLC leaves was more suitable for intracranial SRS. Actual dosimetric measurements will follow to verify dose delivery accuracy with the new BgRT machine.

Dosimetric Evaluation of Treatment Plans for a Biology-Guided Radiation Therapy System in Treatment of Nasopharyngeal Cancer

This is an evaluation of a prototype treatment planning system (TPS) for a new biology-guided radiotherapy (BgRT) delivery system currently under development. The system combines a compact 6 MV linear accelerator and binary multileaf collimator with PET, kVCT, and MV imaging systems on the same ring gantry that rotates continuously at 60 rotations per minute while the patient is translated through the bore. We investigated the plan quality for treatment of nasopharyngeal patients without using PET guidance by comparing the plans with those using existing IMRT delivery techniques. We retrospectively retrieved 5 clinical helical tomotherapy (HT) plans using a 2.5 cm jaw size and 3 clinical step-and-shoot IMRT plans with seven co-planar 6-MV fields that were used to treat nasopharyngeal patients according to the NRG-HN001 protocol, each with 3 or 4 PTVs at different prescription dose levels given in 33 equal fractions. A treatment plan (the prototype plan) using the prototype TPS was generated for each case, which models a binary delivery system that fires at 51 fixed gantry angles with each leaf fully opened or closed. Dose modulation is achieved with fast gantry rotation, couch moving in small increments, and each firing position being visited multiple times in the same axial plane. Volumetric modulated arc therapy (VMAT) plans using two coplanar 6 MV full arc fields were also generated for all 8 cases, where maximum sparing of organs at risk (OARs) rather than PTV dose homogeneity was emphasized during optimization. Paired t-tests were used to compare dosimetric parameters of PTVs and 21 OARs used in plan optimization. Each treatment plan achieved the required PTV dose coverage as in the protocol, with each PTV receiving at least 95% coverage by the corresponding prescription dose. Compared to the 3 step-and-shoot IMRT plans, the prototype plans gave lower dose to all the OARs, showing average reduction in the OAR dosimetric parameters in the range of -7.0% to -76.9%. Compared to the 5 HT plans, the prototype plans showed significantly higher dose heterogeneity to the PTVs, comparable maximum dose to the mandible, temporal lobes and brachial plexus, and lower dose to the other OARs with statistical significance. The VMAT plans achieved lower dose to some OARs with higher dose heterogeneity compared to the HT plans. Compared to the VMAT plans, the prototype plans had comparable PTV dose heterogeneity and comparable or lower dose to the OARs. The average mean dose to the parotid glands were 30.3±10.3 Gy, 22.3±4.7 Gy, and 18.8±2.9 Gy in the HT, VMAT, prototype plans, respectively. Based on the beam modulation design of the BgRT delivery system, the prototype TPS could achieve sharp dose gradient outside the PTVs in radiotherapy plans for nasopharyngeal cancer patients with comparable or lower dose to a number of OARs compared to existing IMRT delivery techniques.

Comparison of Stereotactic Body Radiotherapy Delivery Techniques for Early-Stage Lung Cancer Using Lung Toxicity Modeling

Purpose: Lung toxicity is a primary side effect in stereotactic radiotherapy (SBRT) for early-stage non-small cell lung cancer (NSCLC). We aimed to use a set of radiobiological models to evaluate and compare modern IMRT delivery techniques with three-dimensional conformal techniques for SBRT treatment of NSCLC in terms of lung toxicity, and aimed to compare the results from different radiobiologcal models. Methods: Ten early-stage NSCLC patients treated with SBRT were retrospectively selected. Five treatment plans were generated to deliver 50 Gy in five fractions to the planning target volume for each case: a helical tomotherapy (HT) plan, two three-dimensional cofnromal radiotherapy (3D-CRT) plans using 6-MV and 10-MV photon beams respectively, and two volumetric modulated arc therapy (VMAT) plans using one and two arc fields respectively. The lung RDV was calculated with three parallel functional sub-unit (FSU) models and two normal tissue complication probability (NTCP) models. Results: Both the HT and VMAT plans showed significantly higher contralateral mean lung dose and lower ipsilateral mean lung dose compared to the 3D-CRT plans. There was no statistically significant difference in terms of lung toxicities between the IMRT and 3D-CRT techniques using either the FSU models or the NTCP models. Based on both the FSU and the NTCP models, there was strong correlation between lung toxicity and the mean lung dose in SBRT treatment plans. Conclusions: Based on both the NTCP and parallel FSU models, both IMRT and traditional 3D-CRT delivery techniques could achieve comparable lung sparing inn SBRT treatment of early-stage lung cancer. However, the validity of the radiobiological model results should be checked by clinical data.

SU‐F‐T‐539: Dosimetric Comparison of Volumetric Modulated Arc Therapy and Intensity Modulated Radiation Therapy for Whole Brain Hippocampal Sparing Radiation Therapy Treatments

Purpose:To compare the treatment plan quality and dose gradient near the hippocampus between VMAT (RapidArc) and IMRT delivery techniques for whole brain radiation therapy.Methods:Fifteen patients were evaluated in this retrospective study. All treatments were planned on Varian Eclipse TPS, using 3‐Arc VMAT and 9‐Field IMRT, following NRG Oncology protocol NRG‐CC001 guidelines evaluated by a single radiation oncologist. Prescribed doses in all plans were 30 Gy delivered over 10 fractions normalized to a minimum of 100% of the dose covering 95% of the target volume. Identical contour sets and dose‐volume constraints following protocol guidelines were also applied in all plans. A paired t‐test analysis was used to compare VMAT and IMRT plans.Results:NRG‐CC001 protocol dose‐volume constraints were met for all VMAT and IMRT plans. For the planning target volume (PTV), the average values for D2% and D98% were 6% lower and 4% higher in VMAT than in IMRT, respectively. The average mean and maximum hippocampus doses in Gy for VMAT vs IMRT plans were (11.85±0.81 vs. 12.24±0.56, p=0.10) and (16.27±0.78 vs. 16.59±0.71, p=0.24), respectively. In VMAT, the average mean and maximum chiasm doses were 3% and 1% higher than in IMRT plans, respectively. For the left optic nerve, the average mean and maximum doses were 10% and 5% higher in VMAT than in IMRT plans, respectively. These values were 12% and 3% for the right optic nerve. The average percentage of dose gradient around the hippocampus in the 0–5mm and 5–10mm abutted regions for VMAT vs. IMRT were (4.42%±2.22% /mm vs. 3.95%±2.61% /mm, p=0.43) and (4.54%±1.50% /mm vs. 4.39%±1.28% /mm, p=0.73), respectively.Conclusion:VMAT plans can achieve higher hippocampus sparing with a faster dose fall‐off than IMRT plans. Though statistically insignificant, VMAT offers better PTV coverage with slightly higher doses to OARs.

Dosimetric comparison of Acuros XB with collapsed cone convolution/superposition and anisotropic analytic algorithm for stereotactic ablative radiotherapy of thoracic spinal metastases.

The aim of this study is to compare the recent Eclipse Acuros XB (AXB) dose calculation engine with the Pinnacle collapsed cone convolution/superposition (CCC) dose calculation algorithm and the Eclipse anisotropic analytic algorithm (AAA) for stereotactic ablative radiotherapy (SAbR) treatment planning of thoracic spinal (T‐spine) metastases using IMRT and VMAT delivery techniques. The three commissioned dose engines (CCC, AAA, and AXB) were validated with ion chamber and EBT2 film measurements utilizing a heterogeneous slab‐geometry water phantom and an anthropomorphic phantom. Step‐and‐shoot IMRT and VMAT treatment plans were developed and optimized for eight patients in Pinnacle, following our institutional SAbR protocol for spinal metastases. The CCC algorithm, with heterogeneity corrections, was used for dose calculations. These plans were then exported to Eclipse and recalculated using the AAA and AXB dose calculation algorithms. Various dosimetric parameters calculated with CCC and AAA were compared to that of the AXB calculations. In regions receiving above 50% of prescription dose, the calculated CCC mean dose is 3.1%–4.1% higher than that of AXB calculations for IMRT plans and 2.8%–3.5% higher for VMAT plans, while the calculated AAA mean dose is 1.5%–2.4% lower for IMRT and 1.2%–1.6% lower for VMAT. Statistically significant differences (p<0.05) were observed for most GTV and PTV indices between the CCC and AXB calculations for IMRT and VMAT, while differences between the AAA and AXB calculations were not statistically significant. For T‐spine SAbR treatment planning, the CCC calculations give a statistically significant overestimation of target dose compared to AXB. AAA underestimates target dose with no statistical significance compared to AXB. Further study is needed to determine the clinical impact of these findings.PACS number: 87.55.D‐, 87.53.Ly

Patient-specific QA using 4D Monte Carlo phase space predictions and EPID dosimetry

The goal of this review is to outline a solution for patient-specific QA of VMAT, IMRT, and other complex treatment delivery techniques. This solution has been developed in direct response to clinical needs, in order to allow our institution to offer VMAT to all patients who could potentially benefit from this advanced technique. To date, over 2500 VMAT patient plans and approximately 1000 IMRT patient plans have been verified by this method in Vancouver, while 40 other institutions worldwide have expressed interest in, or are already at various stages of implementing, this process. The addition of EPID in vivo dosimetry (i.e. data acquired during the patient treatment) and associated Monte Carlo predictions amounts to introducing a 'measurement component' in this QA process, which is currently mandated by the regulatory framework in some European countries, or for billing purposes in the USA. The fully automated, patient-specific, Monte Carlo based QA process described here is fast, maximally efficient in terms of departmental resources, and capable of simulating any plan in a single run, regardless of its complexity.

Hybrid Modulated Arc Therapy (HMAT) on an Artiste Linac

Purpose/Objective(s): Pulsed low dose rate radiation therapy (PLDR) has been investigated in a number of clinical trials for the management of recurrent cancers and radiation resistant malignancies based on the in vitro and in vivo experimental findings indicating that many tumor cell lines exhibited hyper radio-sensitivities (HRS) at small doses and enhanced normal tissue repair at low dose rates. This retrospective study aimed to develop treatment planning strategies for PLDR treatment of recurrent cancers utilizing advanced delivery techniques including IMRT and VMAT. Materials/Methods: The current PLDR treatment protocol requires that the daily 2Gy dose be delivered in 10 x 0.2Gy sub-fractions (pulses) with a 3min interval. To take advantage of low-dose HRS the mean dose to the target volume is set at 0.2Gy and the maximum dose is limited to 0.4Gy per pulse. The 3min interval is selected for clinical practicality; PLDR is primarily for palliative treatment, which should be delivered within a reasonable time for recurrent patients. Sixty patients were randomly selected from our patient database for the following body sites: head and neck, breast, thorax, abdomen and pelvis. Practical planning strategies/ guidelines were developed for IMRT and VMAT delivery techniques: (1) use 10 beam angles for IMRT and 10 arcs for VMAT (2-arc plan to be delivered 5 times) with each angle/arc as a pulse; (2) set the mean dose (0.2Gy) and maximum dose (0.4Gy) to the target per pulse as hard constraints (no hard constraints to the OAR); (3) select optimal beam/arc angles to avoid OARs; and (4) add reference structures in or around target/ OARs to reduce maximum dose to the target and OARs. IMRT, VMAT and 3DCRT plans were generated for these patients and compared. Results: Target dose heterogeneity is intrinsic for PLDR with 3DCRT when each pulse is delivered using one single field due to difference in beam attenuation and scattering. Further dose variation is introduced with IMRT due to lateral intensity modulation. A single arc can minimize the difference in the target dose due to multi-angle incidence although the delivery time is longer than 3DCRT and IMRT. Following the planning guidelines developed in this work, all PLDR treatment plans using IMRT and VMAT met the dosimetry requirements of the PLDR protocol. The mean dose was 0.20Gy 0.01Gy and the maximum dose was <0.3Gy for 75% of fields/arcs, <0.35Gy for 98% of fields/arcs and all < 0.4Gy. In comparison with 3DCRT, IMRT, and VMAT exhibited significantly improved target dose conformity and OAR dose sparing. Conclusions: Both IMRT and VMAT can be used for PLDR treatment of recurrent cancers with superior target dose conformity and critical structure sparing. The planning strategies/guidelines developed in this work are practical for IMRT/VMAT treatment planning to meet the dosimetry requirements of the PLDR protocol. Author Disclosure: C. Ma: None. M. Lin: None. K. Wei: None. J. Li: None. P. Wang: None. J. Lang: None.

Pulsed Low-Dose-Rate Radiation Therapy (PLDR) for Recurrent Cancers: Treatment Planning Strategies for IMRT and VMAT

Purpose/Objective(s): Pulsed low dose rate radiation therapy (PLDR) has been investigated in a number of clinical trials for the management of recurrent cancers and radiation resistant malignancies based on the in vitro and in vivo experimental findings indicating that many tumor cell lines exhibited hyper radio-sensitivities (HRS) at small doses and enhanced normal tissue repair at low dose rates. This retrospective study aimed to develop treatment planning strategies for PLDR treatment of recurrent cancers utilizing advanced delivery techniques including IMRT and VMAT. Materials/Methods: The current PLDR treatment protocol requires that the daily 2Gy dose be delivered in 10 x 0.2Gy sub-fractions (pulses) with a 3min interval. To take advantage of low-dose HRS the mean dose to the target volume is set at 0.2Gy and the maximum dose is limited to 0.4Gy per pulse. The 3min interval is selected for clinical practicality; PLDR is primarily for palliative treatment, which should be delivered within a reasonable time for recurrent patients. Sixty patients were randomly selected from our patient database for the following body sites: head and neck, breast, thorax, abdomen and pelvis. Practical planning strategies/ guidelines were developed for IMRT and VMAT delivery techniques: (1) use 10 beam angles for IMRT and 10 arcs for VMAT (2-arc plan to be delivered 5 times) with each angle/arc as a pulse; (2) set the mean dose (0.2Gy) and maximum dose (0.4Gy) to the target per pulse as hard constraints (no hard constraints to the OAR); (3) select optimal beam/arc angles to avoid OARs; and (4) add reference structures in or around target/ OARs to reduce maximum dose to the target and OARs. IMRT, VMAT and 3DCRT plans were generated for these patients and compared. Results: Target dose heterogeneity is intrinsic for PLDR with 3DCRT when each pulse is delivered using one single field due to difference in beam attenuation and scattering. Further dose variation is introduced with IMRT due to lateral intensity modulation. A single arc can minimize the difference in the target dose due to multi-angle incidence although the delivery time is longer than 3DCRT and IMRT. Following the planning guidelines developed in this work, all PLDR treatment plans using IMRT and VMAT met the dosimetry requirements of the PLDR protocol. The mean dose was 0.20Gy 0.01Gy and the maximum dose was <0.3Gy for 75% of fields/arcs, <0.35Gy for 98% of fields/arcs and all < 0.4Gy. In comparison with 3DCRT, IMRT, and VMAT exhibited significantly improved target dose conformity and OAR dose sparing. Conclusions: Both IMRT and VMAT can be used for PLDR treatment of recurrent cancers with superior target dose conformity and critical structure sparing. The planning strategies/guidelines developed in this work are practical for IMRT/VMAT treatment planning to meet the dosimetry requirements of the PLDR protocol. Author Disclosure: C. Ma: None. M. Lin: None. K. Wei: None. J. Li: None. P. Wang: None. J. Lang: None.

Acute Hematologic and Mucosal Toxicities in Head and Neck Cancer Patients Undergoing Chemoradiotherapy: A Comparison of 3D-CRT, IMRT, and Helical Tomotherapy

IMRT and helical tomotherapy for head and neck cancer (HNC) treatment are associated with higher doses to certain non-target tissues than traditional static beam techniques. We hypothesized that this may lead to higher acute mucosal and hematologic toxicities. This analysis was limited to 178 patients receiving ≥60 Gy with concurrent weekly cisplatin. Radiation delivery used 3D-CRT in 41 patients (23%), conventional IMRT in 56 patients (31%), and helical tomotherapy in 81 patients (46%). Acute mucositis rates, weekly hematologic parameters, and ability to deliver planned chemotherapy cycles were examined for each patient during their course of chemoradiotherapy. Analysis showed patients were well balanced with regard to sex, age, and stage. Treatment time, as assessed by delivered monitor units, varied significantly between the 3D-CRT (median = 502), IMRT (median = 1087), and tomotherapy (median = 6757) cohorts. Acute mucositis grades did not significantly differ between the three subsets. Through six weeks of chemoradiotherapy, the median decline in hemoglobin was 15.6%, the median decline in platelets was 30.6%, and the median decline in leukocytes was 51.5%, but these drops were not significantly different between treatment cohorts. Chemotherapy was discontinued or held secondary to hematologic toxicity in 12% of 3D-CRT patients, 5% of IMRT patients and 15% of tomotherapy patients (p = 0.14). In conclusion, HNC patients undergoing high dose radiation with concurrent weekly cisplatin chemotherapy, the longer beam-on times and larger volumes of low-to-moderate radiation doses to non-target tissues associated with modern IMRT delivery techniques do not appear to result in increased acute hematologic or mucosal toxicities.

SU‐E‐T‐696: Registry Based Framework for Plan Quality Improvement: A Head‐And‐Neck Radiotherapy Application Improving Simultaneous Integrated Boost Treatments

Purpose: To systemically evaluate the dosimetric plan quality and consistency for multiple IMRT delivery techniques to further improve guidelines for head‐and‐neck (H&amp;N) irradiation. Method: One hundred consecutive H&amp;N patients from our patient registry were selected and analyzed retrospectively. Simultaneous integrated boost (SIB) IMRT treatment were generated with the same prescription dose of 69.3/62.7/56.1Gy on TomoTherapy(Tomo, Accuray), TrueBeam(TB, Varian) using fixed‐field IMRT(TB_IMRT) or RAPIDARC(TB_RAPIDARC) or Siemens Oncor (Siemens_IMRT, Siemens). The contouring was performed by the same physicians delineating targets and normal structures. The standard in house dose‐volume objectives were used as planning guideline for all cases for all techniques. A Matlab based dose‐volume extraction and analysis tool was developed to export dose‐volume endpoints for each patient. A more specific guideline was proposed for future planning purpose. Results: While all techniques yielded comparable target coverage, the doses to the critical structures differed. In average, the percent volumes receiving greater than 26Gy (V26 and standard deviation) for the total parotid were 37.8±0.1, 46.1±0.2, 44.3±0.2 and 41.8±12.9%, the V65 for mandible were 4.7±5.6, 6.3±11.1, 3.0±5.3 and 8.0±7.3 % with the mean mandible doses of 42.5±6.1, 41.4±8.2, 37.4±10.7 and 46.6±8.7 Gy for Tomo, TB_IMRT, TB_RAPIDARC and Siemens_IMRT plans respectively. ANOVA analyses showed significant difference for the maximum dose of cord but no significant difference was found for other selected structures among four IMRT techniques. Based on current dataset, the projected constraints (at 95% confidence level) for future H&amp;N IMRT plans for cord maximum doses were established at 34.0±2.9, 42.7±2.5, 43.3±2.3, and 45.4±2.4Gy for the evaluated techniques, respectively. The refined dosimetric constraints may improve plan quality and consistency. Conclusion: Patient registry based processes enable easy access of systematic dosimetric monitoring of the plan quality and consistency. The self‐evolving real‐time dosimetric evaluation and ensuing guidelines will help to continuously improve plan quality, resulting in better clinical outcome.

Poster - Thur Eve - 33: A comprehensive analysis of the effect of respiratory motion on the delivery of IMRT to advanced stage non-small cell lung cancer.

The goal of this work was to quantify the interplay effect of various IMRT delivery techniques in the treatment of Stage III non-small cell lung cancer. Five patients with significant tumour motion were retrospectively planned on the average 4D-CT dataset with eight different IMRT techniques: three Tomotherapy techniques with different beam-widths, two step-and-shoot (SS-IMRT) with different complexity, one sliding-window (SW-IMRT), and two VMAT techniques (RapidArc and SmartArc). Each plan was calculated on a delivery verification phantom that was mounted on a programmable respiratory motion platform and delivered under the following motion conditions: 1) Static; 2) sinusoidal with 4 different amplitudes; 3) Real Patient Breathing. A standard 3%/3mm gamma analysis compared the sum of all 30 fractions to their corresponding 60Gy/30fx plan. One-way ANOVA was conducted for respiratory motion amplitude and IMRT modality, separately. There were no significant differences amongst the modalities at any amplitude level. However, for individual modalities, there were significant differences amongst different amplitudes except for Tomo-2.5cm (p=0.260). Post-hoc Tukey tests determined that detectable significant differences amongst any motion level, including real-patient breathing, were observed when compared to the 20mm amplitude for all modalities except Tomo-2.5cm and SmartArc. SW-IMRT showed significant differences at 15mm when compared to both static (p=0.033) and 5mm (p=0.008). All methods except for RapidArc averaged out to clinically acceptable gamma pass rates up to 15mm. In conclusion, for motion levels above 15mm, the interplay effect can be clinically unacceptable. However, the interplay effect at these motion levels does not appear to be modality dependent.

Evaluating the Commissioning and QA of Intensity Modulated Arc Therapy using a Benchmark IMRT Planning Suite

Volumetric Intensity Modulated Arc Therapy (IMAT) is often implemented using a planning and delivery system that has already been commissioned for static beam IMRT. This study is designed to provide benchmark data for commissioning IMAT on these dual-purpose systems. The primary aim of this phase of the study was to assess whether an existing test suite could be used for evaluating IMAT commissioning and QA, with an emphasis on comparisons to traditional IMRT delivery techniques.

Volumetric modulated arc therapy (VMAT) vs. serial tomotherapy, step-and-shoot IMRT and 3D-conformal RT for treatment of prostate cancer

Introduction Volumetric modulated arc therapy (VMAT), a complex treatment strategy for intensity-modulated radiation therapy, may increase treatment efficiency and has recently been established clinically. This analysis compares VMAT against established IMRT and 3D-conformal radiation therapy (3D-CRT) delivery techniques. Methods Based on CT datasets of 9 patients treated for prostate cancer step-and-shoot IMRT, serial tomotherapy (MIMiC), 3D-CRT and VMAT were compared with regard to plan quality and treatment efficiency. Two VMAT approaches (one rotation (VMAT1x) and one rotation plus a second 200° rotation (VMAT2x)) were calculated for the plan comparison. Plan quality was assessed by calculating homogeneity and conformity index (HI and CI), dose to normal tissue (non-target) and D 95% (dose encompassing 95% of the target volume). For plan efficiency evaluation, treatment time and number of monitor units (MU) were considered. Results For MIMiC/IMRT MLC/VMAT2x/VMAT1x/3D-CRT, mean CI was 1.5/1.23/1.45/1.51/1.46 and HI was 1.19/1.1/1.09/1.11/1.04. For a prescribed dose of 76 Gy, mean doses to organs-at-risk (OAR) were 50.69 Gy/53.99 Gy/60.29 Gy/61.59 Gy/66.33 Gy for the anterior half of the rectum and 31.85 Gy/34.89 Gy/38.75 Gy/38.57 Gy/55.43 Gy for the posterior rectum. Volumes of non-target normal tissue receiving ⩾70% of prescribed dose (53 Gy) were 337 ml/284 ml/482 ml/505 ml/414 ml, for ⩾ 50% (38 Gy) 869 ml/933 ml/1155 ml/1231 ml/1993 ml and for ⩾ 30% (23 Gy) 2819 ml/3414 ml/3340 ml/3438 ml /3061 ml. D 95% was 69.79 Gy/70.51 Gy/71,7 Gy/71.59 Gy/73.42 Gy. Mean treatment time was 12 min/6 min/3.7 min/1.8 min/2.5 min. Conclusion All approaches yield treatment plans of improved quality when compared to 3D-conformal treatments, with serial tomotherapy providing best OAR sparing and VMAT being the most efficient treatment option in our comparison. Plans which were calculated with 3D-CRT provided good target coverage but resulted in higher dose to the rectum.

Some considerations concerning volume-modulated arc therapy: a stepping stone towards a general theory

In this paper it is formally shown that the dynamic multileaf collimator (MLC) IMRT delivery technique remains valid if the MLC is supported on a 1D moving platform. It is also shown that, in such circumstances, it is always time preferable to deliver overlapping modulating fields as a single swept field rather than as separate fields. The most general formulism is presented and then related to simpler equations in limiting cases. The paper explains in detail how a ‘small-arc approximation’ can be invoked to relate the 1D linear theory to the MLC-on-moving-platform-(gantry) delivery technique involving rotation therapy and known as volume-modulated arc therapy (VMAT). It is explained how volume-modulated arc therapy delivered with open unmodulated fields and which can deliver conformal dose distributions can be interpreted as an IMRT delivery. The (Elekta adopted) term VMAT will be used in a generic sense to include a similar (Varian) method known as RapidArc. Approximate expressions are derived for the ‘amount of modulation’ possible in a VMAT delivery. This paper does not discuss the actual VMAT planning but gives an insight at a deep level into VMAT delivery. No universal theory of VMAT is known in the sense that there is no theory that can predict precisely the performance of a VMAT delivery in terms of the free parameters available (variable gantry speed, variable fluence-delivery rate, set of MLC shapes, MLC orientation, number of arcs, coplanarity versus non-coplanarity, etc). This is in stark contrast to the situation with several other IMRT delivery techniques where such theoretical analyses are known. In this paper we do not provide such a theory; the material presented is a stepping stone on the path towards this.

SU-FF-T-152: Comparison Between Fixed Gantry Angle Intensity Modulated Radiotherapy and Intensity Modulated Arc Therapy for Head-And-Neck Cancers

Purpose: To compare the dose distributions resulting form fixed gantry angle IMRT and IMAT (dynamic arc) optimization for head‐and‐neck tumors (HN). Materials and Methods: Four (HN) cases were retrospectively investigated. Pinnacle3 (v. 8.1x) was used to generate treatment plans for an IMAT and a 9‐field IMRT delivery techniques. The targets were the planning target volume (PTV) and the lymph nodes (LN). The organs at risk (OARs) were the spinal cord (cord), the cricopharyngeus, the esophagus and the parotid glands (parotids). The IMAT and IMRT plan cross‐comparison was based on dose indices used as objectives in the optimization process. The evaluated dose indices were D95 for PTV and LN and D1 for the cord and average doses for the parotids, the cricopharyngeus and the esophagus. The standard deviations of the dose (expressed as a fraction of the prescription dose) over the PTV and the LN were also tallied. The highest priority in the optimization was given to target coverage. Results: The dose indices for the PTV and the LN differed by less than 1% and 3% respectively. A reduction in OAR dose was possible in the IMRT plans while maintaining the dose standard‐deviation below 3% (used as a dose uniformity criterion) for all targets. The IMAT plans resulted in higher dose indices/average doses to the OARs by as much as 50%. The IMAT dose standard deviation in all patients for the LN and in one patient for the PTV was greater than 3%. Conclusions: A better balance between dose uniformity and dose coverage was found in the IMRT plans. In addition the IMRT plans yielded lower doses to the OARs. We hypothesize that allowing for variable gantry speed in the IMAT optimization will result in IMAT plans which are at least as good as the fixed gantry angle IMRT plans.

SU-FF-T-503: Biological Effect of Different IMRT Delivery Techniques: SMLC, DMLC and Helical TomoTherapy®

Purpose: Intensity‐Modulated Radiotherapy is delivered using a variety of techniques with differing temporal dose characteristics. Spatial dose metrics are generally used to evaluate treatment plan quality, however, use of this information alone neglects effects of the significant differences in dose delivery duration and dose accumulation patterns, both of which can impact cell survival. This study uses the Linear‐Quadratic model with dose protraction corrections to evaluate the biological effectiveness of different IMRT delivery techniques: SMLC (step‐and‐shoot), DMLC (sliding window), and helical tomotherapy (HT) for the treatment of prostate and head/neck sites. Method and Materials: The temporal dose pattern was measured to calculate the protraction factor and Biologically Equivalent Dose (BED) was calculated for a range of repair half‐times and α/β ratios. The treatment BED is compared against the target prescription to evaluate loss in biological effectiveness. In the case of a conventional prescription, the loss in biological effectiveness was evaluated using Tumor Control Probability (TCP) data. Results: Our analysis finds HT to be biologically superior to SMLC and DMLC because of reduced dose accumulation times for individual target‐volume elements. With SMLC or DMLC delivery, the expected loss in BED for a conventional prostate prescription can be as high as 3%, leading to a maximum predicted 10% reduction in TCP. Furthermore, use of hypo‐fractionation can lead to BED losses up to 5%, raising even greater concerns about clinical consequences. For HT and conventional fractionation, the predicted BED and TCP losses are less than 1% and 2.5%, respectively. For head/neck tumors the larger α/β ratio results in smaller losses in biological effectiveness (∼1%) which are clinically less significant. The possible range of losses can be larger if smaller repair half‐times and small α/β ratios are considered. Conclusion: Temporal dose delivery patterns are an important component in determining the biological effects of IMRT treatment.

MO‐D‐BRB‐04: Mixed Integer Models for Elucidating the Tradeoff Between Treatment Time and Plan Quality in VMAT Delivery

Purpose: Rotational IMRT delivery techniques, such as volumetric modulated arc therapy (VMAT), are alternatives to standard IMRT delivery, claiming faster delivery and equally good dose distributions. In reality there is a cost to faster delivery times. The purpose of this investigation is to understand the tradeoff between plan quality and treatment time in rotational delivery by use of optimization models. Method and Materials: We study a simplified VMAT model for a 2D phantom and construct an exact mixed integer optimization model for it. The model includes treatment time as a constraint, and otherwise does not specify how the delivery is to be done, allowing for either VMAT or IMRT solutions. The model is too large to solve, but we trap the optimal solution between tight upper and lower bounds with additional models. The lower bound solutions are used to warm start the upper bound solutions. We construct Pareto surfaces for the joint tradeoffs of treatment time, tumor maximum dose, and organ at risk mean dose. Results: Upper bounds are within 10% of lower bound solutions. This allows us to conclusively demonstrate that 1) the treatment time constraint becomes more important when high intensity modulation is required to achieve optimal dose distributions, and 2) finding a good feasible VMAT plan is benefitted (by ∼5% in objective value) by warmstarting using the fluence modulation required at each angular location. Conclusion: For problems where the optimal dose distribution has non‐convex iso‐dose lines (to avoid nearby critical structures), the high modulation necessary to achieve such plans translates into longer treatment times. In these cases, when allowed treatment time is small, the optimal solutions show fluence being delivered at every angle — VMAT. If treatment time is allowed to be larger, IMRT plans are found to be optimal.

Real-time motion-adaptive delivery (MAD) using binary MLC: I. Static beam (topotherapy) delivery

Intra-fraction target motion hits the fundamental basis of IMRT where precise target positions are assumed. Real-time motion compensation is necessary to ensure that the same dose is delivered as planned. Strategies for conventional IMRT delivery for moving targets by dynamic multi-leaf collimators (MLC) tracking are well published. Binary MLC-based IMRT, such as TomoTherapy®, requires synchronized motion of MLC, the couch and the gantry, which suggests a unique motion management strategy. Thanks to its ultra-fast leaf response and fast projection rate, real-time motion compensation for binary MLC-based IMRT is feasible. Topotherapy is a new IMRT delivery technique, which can be implemented in commercial helical TomoTherapy® machines using only fixed gantry positions. In this paper, we present a novel approach for TopoTherapy delivery that adjusts for moving targets without additional hardware and control requirement. This technique uses the planned leaf sequence but rearranges the projection and leaf indices. It does not involve time-consuming operations, such as reoptimization. Unlike gating or breath-hold-based methods, this technique can achieve nearly a 100% duty cycle with little breath control. Unlike dynamic MLC-based tracking methods, this technique requires neither the whole target motion trajectory nor the velocity of target motion. Instead, it only requires instantaneous target positions, which greatly simplifies the system implementation. Extensive simulations, including the worst-case scenarios, validated the presented technique to be applicable to relatively regular or mild irregular respirations. The delivered dose conforms well to the target, and significant margin reduction can be achieved provided that accurate, real-time tumor localization is available.