Plan Quality Indices Research Articles

MRI-Related Geometric Distortions in Stereotactic Radiotherapy Treatment Planning: Evaluation and Dosimetric Impact.

In view of their superior soft tissue contrast compared to computed tomography, magnetic resonance images are commonly involved in stereotactic radiosurgery/radiotherapy applications for target delineation purposes. It is known, however, that magnetic resonance images are geometrically distorted, thus deteriorating dose delivery accuracy. The present work focuses on the assessment of geometric distortion inherent in magnetic resonance images used in stereotactic radiosurgery/radiotherapy treatment planning and attempts to quantitively evaluate the consequent impact on dose delivery. The geometric distortions for 3 clinical magnetic resonance protocols (at both 1.5 and 3.0 T) used for stereotactic radiosurgery/radiotherapy treatment planning were evaluated using a recently proposed phantom and methodology. Areas of increased distortion were identified at the edges of the imaged volume which was comparable to a brain scan. Although mean absolute distortion did not exceed 0.5 mm on any spatial axis, maximum detected control point disposition reached 2 mm. In an effort to establish what could be considered as acceptable geometric uncertainty, highly conformal plans were utilized to irradiate targets of different diameters (5-50 mm). The targets were mispositioned by 0.5 up to 3 mm, and dose–volume histograms and plan quality indices clinically used for plan evaluation and acceptance were derived and used to investigate the effect of geometrical uncertainty (distortion) on dose delivery accuracy and plan quality. The latter was found to be strongly dependent on target size. For targets less than 20 mm in diameter, a spatial disposition of the order of 1 mm could significantly affect (>5%) plan acceptance/quality indices. For targets with diameter greater than 2 cm, the corresponding disposition was found greater than 1.5 mm. Overall results of this work suggest that efficacy of stereotactic radiosurgery/radiotherapy applications could be compromised in case of very small targets lying distant from the scanner’s isocenter (eg, the periphery of the brain).

Multi-centre audit of VMAT planning and pre-treatment verification

Background and purposeWe performed a multi-centre intercomparison of VMAT dose planning and pre-treatment verification. The aims were to analyse the dose plans in terms of dosimetric quality and deliverability, and to validate whether in-house pre-treatment verification results agreed with those of an external audit. Materials and methodsThe nine participating centres encompassed different machines, equipment, and methodologies. Two mock cases (prostate and head and neck) were planned using one and two arcs. A plan quality index was defined to compare the plans and different complexity indices were calculated to check their deliverability. We compared gamma index pass rates using the centre’s equipment and methodology to those of an external audit (global 3D gamma, absolute dose differences, 10% of maximum dose threshold). Log-file analysis was performed to look for delivery errors. ResultsAll centres fulfilled the dosimetric goals but plan quality and delivery complexity were heterogeneous and uncorrelated, depending on the manufacturer and the planner’s methodology. Pre-treatment verifications results were within tolerance in all cases for gamma 3%-3mm evaluation. Nevertheless, differences between the external audit and in-house measurements arose due to different equipment or methodology, especially for 2%-2mm criteria with differences up to 20%. No correlation was found between complexity indices and verification results amongst centres. ConclusionsAll plans fulfilled dosimetric constraints, but plan quality and complexity did not correlate and were strongly dependent on the planner and the vendor. In-house measurements cannot completely replace external audits for credentialing.

Dosimetric and radiobiological comparison of TG-43 and Monte Carlo calculations in 192Ir breast brachytherapy applications

PurposeTo investigate the clinical significance of introducing model based dose calculation algorithms (MBDCAs) as an alternative to TG-43 in 192Ir interstitial breast brachytherapy. Materials and methodsA 57 patient cohort was used in a retrospective comparison between TG-43 based dosimetry data exported from a treatment planning system and Monte Carlo (MC) dosimetry performed using MCNP v. 6.1 with plan and anatomy information in DICOM-RT format. Comparison was performed for the target, ipsilateral lung, heart, skin, breast and ribs, using dose distributions, dose-volume histograms (DVH) and plan quality indices clinically used for plan evaluation, as well as radiobiological parameters. ResultsTG-43 overestimation of target DVH parameters is statistically significant but small (less than 2% for the target coverage indices and 4% for homogeneity indices, on average). Significant dose differences (>5%) were observed close to the skin and at relatively large distances from the implant leading to a TG-43 dose overestimation for the organs at risk. These differences correspond to low dose regions (<50% of the prescribed dose), being less than 2% of the prescribed dose. Detected dosimetric differences did not induce clinically significant differences in calculated tumor control probabilities (mean absolute difference <0.2%) and normal tissue complication probabilities. ConclusionWhile TG-43 shows a statistically significant overestimation of most indices used for plan evaluation, differences are small and therefore not clinically significant. Improved MBDCA dosimetry could be important for re-irradiation, technique inter-comparison and/or the assessment of secondary cancer induction risk, where accurate dosimetry in the whole patient anatomy is of the essence.

Cranio-spinal irradiation of pediatric patients using volumetric modulated ARC therapy

Introduction Cranio-spinal irradiation (CSI) used for the management of certain primary brain tumours is one of the most challenging processes in radiation planning and delivery. Volumetric modulated arc therapy (VMAT), a recent advantage of radiotherapy technology can provide highly conformal radiation dose distributions around complex target volumes, while minimizing the dose to adjacent organs at risk (OARs), in an efficient way (i.e. reasonable treatment delivery times comparable to 3D conformal radiotherapy). Purpose To report the treatment of cranio-spinal irradiation with volumetric modulated arc therapy in comparison to the commonly used conventional 3D conformal radiotherapy (3D-CRT). Materials and methods Five children were treated with CSI using VMAT on an Elekta Versa linear accelerator. Multiple arc arrangements with two isocentres were used. Supine position was utilized for all patients. Plan quality and DVH-indices used for plan evaluation and approval were compared to corresponding values derived using 3D conformal plans. Results VMAT plans were found superior in all criteria clinically used for plan evaluation and acceptance including dose distributions, plan quality indices such as target coverage and dose homogeneity as well as doses to OARs and thus VMAT was used for patient treatments. Conclusion The use of VMAT provides excellent target coverage and sparing of the adjacent OARs, with absorbed doses well within the published pediatric dose constraints, in relatively short treatment time (less than 5 min) and thus being superior in treating patients requiring CSI compared to the currently used 3D-CRT.

TU-FG-201-10: Quality Management of Accelerated Partial Breast Irradiation (APBI) Plans

Purpose: Since 2008, over 700 patients received high dose rate (HDR) APBI treatment at Virginia Hospital Center. The complexity involved in the planning process demonstrated a broad variation between patient geometry across all applicators, in relation to anatomical regions of interest. A quality management program instituting various metrics was implemented in March 2013 with the goal of ensuring an optimal plan is achieved for each patient. Methods: For each plan, an in-house complexity index, geometric conformity index, and plan quality index were defined. These indices were obtained for all patients treated. For patients treated after the implementation, the conformity index and quality index were maximized while other dosimetric limits, such as maximum skin and rib doses, were strictly kept. Subsequently, all evaluation parameters and applicator information were placed in a database for cross-evaluation with similar complexity. Results: Both the conformity and quality indices show good correlation with the complexity index. They decrease as complexity increases for all applicators. Multi lumen type balloon applicators demonstrate a minimal advantage over single lumen applicators in increasingly complex patient geometries, as compared to SAVI applicators which showed considerably greater advantage in these circumstances. After the implementation of the in-house planning protocol, there is a direct improvement of quality for SAVI plans. Conclusion: Due to their interstitial nature, SAVI devices show a better conformity in comparison to balloon-based devices regardless of the number of lumens, especially in complex cases. The quality management program focuses on optimizing indices by utilizing prior planning knowledge based on complexity levels. The database of indices assists in decision making and has subsequently aided in balancing the experience level among planners. This approach has made APBI planning more robust for patient care, with a measurable improvement in the plan quality.

MO-FG-202-02: Automated Plan Quality Assurance Integrated with Eclipse Using Varian's ESAPI Interface

Purpose: Treatment plan compliance to physician's intent is still assessed by hand in clinical practice by visually comparing prescription details against corresponding plan values. We report on a script that uses the Varian's programming interface to automate the scoring of treatment plan dose values to the physician's prescribed constraints. Methods: The Eclipse Scripting Advance Programming Interface (ESAPI) is a FDA-approved scripting environment written in C# that allows programming access to a patient's record in Aria and Eclipse. With the ESAPI, it is possible to create customized add-on software libraries that can be installed in the planning system as a pull-down menu item. We used the flexibility of ESAPI to parse plan quality metrics from the prescription and compare them automatically against the corresponding plan values. The script can check fractionation as well as minimum, mean and maximum doses to an organ specified in the prescription, DVHs metrics such as volume or dose coverage for both targets and critical organs, conformity indexes specific to radio surgery and biological indexes such as EUD, TCP and NCTP. Lists of generally accepted constraints limits, such as QUANTEC or clinical trials constraints are implemented in the script and can be invoked to automatically fill the prescription. Results: The script interprets the prescription and compares plan quality indices parsed from it against the counterpart values extracted from the plan. A coloring scheming is employed to automatically mark with green, yellow or red plan quality metrics that meet, are borderline fail (10%) or fail the prescription criteria. While manually performing this task can take 30 minutes for a complex head and neck case with many constraints, the script takes approximately 1 minute. Conclusion: The script provided easy to understand feedback on quality metrics in a simple and practical solution that significantly streamline plan double-checks in clinical practice.

Multi-catheter interstitial brachytherapy for partial breast irradiation: an audit of implant quality based on dosimetric evaluation comparing intra-operative versus post-operative placement.

PurposeThe use of multicatheter interstitial brachytherapy (MIB) for accelerated partial breast irradiation (APBI) in early breast cancer (EBC) patients outside the trial setting has increased. Hence, there is a need to critically evaluate implant quality. Moreover, there is a scarcity of reports using an open cavity technique. We report the dosimetric indices of open and closed cavity MIB techniques.Material and methodsThe dosimetric parameters of 60 EBC patients treated with MIB (open and closed cavity) who underwent three dimensional, computerized tomography (CT) based planning for APBI from November 2011 to July 2015 were evaluated. Coverage Index (CI), Dose Homogeneity Index (DHI), Conformity Index (COIN), Plan Quality Index (PQI), and Dose Non-uniformity Index (DNR) were assessed.ResultsForty-one patients underwent open cavity and 19 patients underwent closed cavity placement of brachytherapy catheters. The median number of planes was 4 and median number of needles was 20. Median dose was 34 Gy with dose per fraction of 3.4 Gy, given twice a day, 6 hours apart. The D90 of the cavity and clinical target volume (CTV) were 105% and 89%, respectively. The median doses to the surgical clips were greater than 100%. The median CI of the cavity and CTV was 0.96 and 0.82, respectively. The DHI and COIN index of the CTV was 0.73 and 0.67. There were no significant differences in the dosimetric parameters based on whether the technique was done open or closed.ConclusionsCritical evaluation of the dosimetric parameters of MIB-APBI is important for optimal results. While the open and closed techniques have similar dosimetry, our institutional preference is for an open technique which eases the procedure due to direct visualization of the tumor cavity.

SU‐E‐T‐654: Quantifying Plan Quality Can Effectively Distinguish Between Competing Equivocal IMRT Prostate Plans

Purpose:The purpose of this study was to create a prostate IMRT plan quality index (PQI) that may be used to quantitatively compare competing plans using a methodology that mimics physician preference. This methodology allows planners to choose between plans with equivocal characteristics, prior to physician scrutiny.Methods:An observer study was conducted to gather data from 3 radiation oncology physicians who ranked a set of 20 patients (each with 5 plans). The rankings were used to optimize a PQI that combined weighted portions of the rectum, bladder, and planning target volume DVHs, such that the relative PQI mimicked physician rankings as best as possible. Once optimized, a test study assessed the PQI by comparison to physician rankings in a new set of 25 patients (each with 4 plans). The physician group in the test study included 6 physicians, 5 of whom were not included in the modeling study. PQI scores were evaluated against the physicians’ rank list using Spearman rank correlation.Results:The optimized plan quality index combined the following DVH features: high dose regions above 40Gy/60Gy (rectum/bladder), organ weightings, and PTV shoulder coverage. Mean correlation of the PQI vs. physicians’ rankings in the modeling study was 0.507 (range: 0.345–0.706). By comparison, the mean correlation between physicians was 0.301 (range: 0.242–0.334). The mean correlation of the PQI vs. physician rankings in test study was 0.726 (range: 0.416–0.936), indicating robustness of the PQI by virtue of producing similar results to the modeling study. Intra‐physician correlation was 0.564 (range: 0.398–0.689).Conclusion:The correlation coefficients of the PQI vs. physicians were similar to the correlation coefficients of the physicians with each other, implying that the PQI developed in this work shows promise in reflecting physician clinical preference when selecting between competing, dosimetrically equivocal plans.

SU‐E‐T‐60: A Plan Quality Index in IMRT QA That Is Independent of the Acceptance Criteria

Purpose:In IMRT QA, plan quality evaluation is made based on pass rate under preset acceptance criteria, mostly using gamma‐values. This method is convenient but, its Result highly depends on what the acceptance criteria are and suffers from the lack of sensitivity in judging how good the plan is. In this study, we introduced a simple but effective plan quality index of IMRT QA based on dose difference only to supplement such shortcomings, and investigated its validity.Methods:The proposed index is a single value which is calculated mainly based on point‐by‐point comparison between planned and measured dose distributions, and it becomes “1” in an ideal case. A systematic evaluation was performed with one‐dimensional test dose distributions. For 3 hypothetical dose profiles, various displacements (in both dose and space) were introduced, the proposed index was calculated for each case, and the behavior of obtained indices was analyzed and compared with that of gamma evaluation. In addition, the feasibility of the index was assessed with clinical IMRT/VMAT/SBRT QA cases for different sites (prostate, head &amp; neck, liver, lung, spine, and abdomen).Results:The proposed index showed more robust correlation with the amount of induced displacement compared to the gamma evaluation method. No matter what the acceptance criteria are (e.g., whether 3%/3mm or 2%/2mm), it was possible to clearly rank every case with the proposed index while it was difficult to do with the gamma evaluation method.Conclusion:IMRT plan quality can be evaluated quantitatively by the proposed index. It is considered that the proposed index would provide useful information for better judging the level of goodness of each plan and its Result is independent of the acceptance criteria.This work was supported by the Radiation Technology R&amp;D program (No. 2013M2A2A7043498) and the Mid‐career Researcher Program (2014R1A2A1A10050270) through the National Research Foundation of Korea funded by the Ministry of Science, ICT&amp;Future Planning.

Comparing the quality of passively-scattered proton and photon tomotherapy plans for brain and head and neck disease sites

We compare the quality of photon IMRT (helical tomotherapy) with classic proton plans for brain, head and neck tumors, in terms of target dose uniformity and conformity along with organ-at-risk (OAR) sparing.Plans were created for twelve target volumes among eight cases. All patients were originally planned and treated using helical tomotherapy. Proton plans were generated using a passively-scattered beam model with a maximum range of 32 g cm−2 (225 MeV), range modulation in 0.5 g cm−2 increments and range compensators with 4.8 mm milling tool diameters. All proton plans were limited to two to four beams. Plan quality was compared using uniformity index (UI), conformation number (CN) and a EUD-based plan quality index (fEUD).For 11 of the 12 targets, UI was improved for the proton plan; on average, UI was 1.05 for protons versus 1.08 for tomotherapy. For 7 of the 12 targets, the tomotherapy plan exhibited more favorable CN. For proximal OARs, the improved dose conformity to the target volume from tomotherapy led to a lower maximum dose. For distal OARs, the maximum dose was much lower for proton plans. For 6 of the 8 cases, near-total avoidance for distal OARs provided by protons leads to improved fEUD. However, if distal OARs are excluded in the fEUD calculation, the proton plans exhibit better fEUD in only 3 of the 8 cases.The distal OAR sparing and target dose uniformity are generally better with passive-scatter proton planning than with photon tomotherapy; proton therapy may be preferred if the clinician deems those attributes critical. However, tomotherapy may serve equally as well as protons for cases where superior target dose conformity from tomotherapy leads to plan quality nearly identical to or better than protons and for cases where distal OAR sparing is not concerning.

An efficient Volumetric Arc Therapy treatment planning approach for hippocampal-avoidance whole-brain radiation therapy (HA-WBRT)

An efficient and simple class solution is proposed for hippocampal-avoidance whole-brain radiation therapy (HA-WBRT) planning using the Volumetric Arc Therapy (VMAT) delivery technique following the NRG Oncology protocol NRG-CC001 treatment planning guidelines. The whole-brain planning target volume (PTV) was subdivided into subplanning volumes that lie in plane and out of plane with the hippocampal-avoidance volume. To further improve VMAT treatment plans, a partial-field dual-arc technique was developed. Both the arcs were allowed to overlap on the in-plane subtarget volume, and in addition, one arc covered the superior out-of-plane sub-PTV, while the other covered the inferior out-of-plane subtarget volume. For all plans (n = 20), the NRG-CC001 protocol dose-volume criteria were met. Mean values of volumes for the hippocampus and the hippocampal-avoidance volume were 4.1cm3 ± 1.0cm3 and 28.52cm3 ± 3.22cm3, respectively. For the PTV, the average values of D2% and D98% were 36.1Gy ± 0.8Gy and 26.2Gy ± 0.6Gy, respectively. The hippocampus D100% mean value was 8.5Gy ± 0.2Gy and the maximum dose was 15.7Gy ± 0.3Gy. The corresponding plan quality indices were 0.30 ± 0.01 (homogeneity index), 0.94 ± 0.01 (target conformality), and 0.75 ± 0.02 (confirmation number). The median total monitor unit (MU) per fraction was 806MU (interquartile range [IQR]: 792 to 818MU) and the average beam total delivery time was 121.2 seconds (IQR: 120.6 to 121.35 seconds). All plans passed the gamma evaluation using the 5-mm, 4% criteria, with γ > 1 of not more than 9.1% data points for all fields. An efficient and simple planning class solution for HA-WBRT using VMAT has been developed that allows all protocol constraints of NRG-CC001 to be met.

Is high–dose rate RapidArc-based radiosurgery dosimetrically advantageous for the treatment of intracranial tumors?

In linac-based stereotactic radiosurgery (SRS) and radiotherapy (SRT), circular cone(s) or conformal arc(s) are conventionally used to treat intracranial lesions. However, when the target is in close proximity to critical structures, it is frequently quite challenging to generate a quality plan using these techniques. In this study, we investigated the dosimetric characteristics of using high–dose rate RapidArc (RA) technique for radiosurgical treatment of intracranial lesions. A total of 10 intracranial SRS/SRT cases previously planned using dynamic conformal arc (DCA) or cone-based techniques have been included in this study. For each case, 3 treatment plans were generated: (1) a DCA plan with multiple noncoplanar arcs, (2) a high–dose rate RA plan with arcs oriented the same as DCA (multiple-arc RA), and 3) a high–dose rate RA plan with a single coplanar arc (single-arc RA). All treatment plans were generated under the same prescription and similar critical structure dose limits. Plan quality for different plans was evaluated by comparing various dosimetric parameters such as target coverage, conformity index (CI), homogeneity index (HI), critical structures, and normal brain tissue doses as well as beam delivery time. With similar critical structure sparing, high–dose rate RA plans can achieve much better target coverage, dose conformity, and dose homogeneity than the DCA plans can. Plan quality indices CI and HI, for the DCA, multiple-arc RA, and single-arc RA techniques, were measured as 1.67 ± 0.39, 1.32 ± 0.28, and 1.38 ± 0.30 and 1.24 ± 0.11, 1.10 ± 0.04, and 1.12 ± 0.07, respectively. Normal brain tissue dose (V12Gy) was found to be similar for DCA and multiple-arc RA plans but much larger for the single-arc RA plans. Beam delivery was similar for DCA and multiple-arc RA plans but shorter with single-arc RA plans. Multiple-arc RA SRS/SRT can provide better treatment plans than conventional DCA plans, especially for complex cases.

Multicentre validation of IMRT pre-treatment verification: Comparison of in-house and external audit

Background and purposeWe performed a multicentre intercomparison of IMRT optimisation and dose planning and IMRT pre-treatment verification methods and results. The aims were to check consistency between dose plans and to validate whether in-house pre-treatment verification results agreed with those of an external audit. Materials and methodsParticipating centres used two mock cases (prostate and head and neck) for the intercomparison and audit. Compliance to dosimetric goals and total number of MU per plan were collected. A simple quality index to compare the different plans was proposed. We compared gamma index pass rates using the centre’s equipment and methodology to those of an external audit. ResultsWhile for the prostate case, all centres fulfilled the dosimetric goals and plan quality was homogeneous, that was not the case for the head and neck case. The number of MU did not correlate with the plan quality index. Pre-treatment verifications results of the external audit did not agree with those of the in-house measurements for two centres: being within tolerance for in-house measurements and unacceptable for the audit or the other way round. ConclusionsAlthough all plans fulfilled dosimetric constraints, plan quality is highly dependent on the planner expertise. External audits are an excellent tool to detect errors in IMRT implementation and cannot be replaced by intercomparison using results obtained by centres.

A new plan quality index for dose painting radiotherapy.

Dose painting radiotherapy is considered a promising radiotherapy technology that enables more targeted dose delivery to tumor rich regions while saving critical normal tissues. Obviously, dose painting planning would be more complicated and hard to be evaluated with current plan quality index systems that were developed under the paradigm of uniform dose prescription. In this study, we introduce a new plan quality index, named “index of achievement (IOA)” that assesses how close the planned dose distribution is to the prescribed one in a dose painting radiotherapy plan. By using voxel‐based comparison between planned and prescribed dose distributions in its formulation, the index allows for a single‐value evaluation regardless of the number of prescribed dose levels, which cannot be achieved with the conventional indices such as conventional homogeneity index. Benchmark calculations using patient data demonstrated feasibility of the index not only for contour‐based dose painting plans, but also for dose painting by numbers plans. Also, it was shown that there is strong correlation between the new index and conventional indices, which indicates a potential of the new index as an alternative to conventional ones in general radiotherapy plan evaluation.PACS number: 87.55.D‐

Treatment Planning Methods in High Dose Rate Interstitial Brachytherapy of Carcinoma Cervix: A Dosimetric and Radiobiological Analysis

Treatment planning is a trial and error process that determines optimal dwell times, dose distribution, and loading pattern for high dose rate brachytherapy. Planning systems offer a number of dose calculation methods to either normalize or optimize the radiation dose. Each method has its own characteristics for achieving therapeutic dose to mitigate cancer growth without harming contiguous normal tissues. Our aim is to propose the best suited method for planning interstitial brachytherapy. 40 cervical cancer patients were randomly selected and 5 planning methods were iterated. Graphical optimization was compared with implant geometry and dose point normalization/optimization techniques using dosimetrical and radiobiological plan quality indices retrospectively. Mean tumor control probability was similar in all the methods with no statistical significance. Mean normal tissue complication probability for bladder and rectum is 0.3252 and 0.3126 (P = 0.0001), respectively, in graphical optimized plans compared to other methods. There was no significant correlation found between Conformity Index and tumor control probability when the plans were ranked according to Pearson product moment method (r = −0.120). Graphical optimization can result in maximum sparing of normal tissues.

SU‐E‐T‐686: A New Plan Quality Index for Dose‐Painting Radiotherapy

Purpose: In dose‐painting by numbers, conventional plan quality indices such as homogeneity index (HI) and dose‐standard‐deviation (STD) have limitations in application. This study aims to introduce new indices for evaluating treatment plans of this radiotherapy technique. Methods: Deviation‐from‐prescription‐index (DFPI) was formulated as the volume‐weighted average of the deviation between prescription dose and planned dose. Two sub‐indices, index‐of‐hotness (IOH) and index‐of‐coldness (IOC) were also defined specifically for selected voxels of which planned dose was higher and lower than the prescribed dose, respectively. The new indices were first tested for 9 IMRT/VMAT plans having 3 different dose prescription levels. For comparison purpose, 4 conventional indices (three types of HI and STD) and difference between V98 and V110 [V110 + (100− V98)] were calculated in each prescription region, and then averaged. Using the Spearman rank correlation test, the relationship was investigated between the DFPI‐based ranks and the composite ranks based on all of 5 conventional indices. The new indices were also evaluated in 9 theoretically‐generated plans having a continuously varying function of dose prescription where the conventional indices are not applicable. Results: The plan quality ranks according to DFPI were found to be highly correlated with that from the composite of conventional indices (correlation coefficient = 0.95, where 1 means perfect correlation). It was also demonstrated that the new indices are applicable to dose painted plans in a continuously varying dose prescription. Conclusion: It was shown that the new plan quality indices formulated with voxel‐based prescription dose can be appropriately used for dose‐painting radiotherapy. The indices allow for a single‐value evaluation regardless of the number of prescribed dose levels, which cannot be achieved using the conventional indices.

P-0150 - Intensity Modulated Versus Conformal Radiotherapy for Anal Cancer: Comparison of Plan Quality and Acute Toxicity Using a Novel Plan Comparator

Background: Chemoradiation is standard of care for anal cancers with organ preservation rates of approximately 70%. Toxicity due to dose to organs at risk (OAR) has a significant impact on quality of life and can lead to inferior outcomes through treatment delays. Intensity-modulated radiotherapy (IMRT) can reduce doses to OAR in comparison with conformal radiotherapy (CRT), reducing toxicity and potentially allowing for dose escalation. We compare planning target volume (PTV) dose homogeneity, plan quality and OAR dose between IMRT and CRT plans along with acute toxicities. Methods: Nine randomly selected patients received chemoradiation for anal cancer. 5 received CRT and 4 IMRT. A second plan was created retrospectively for each patient, such that each had a CRT and an IMRT plan (Eclipse version 10.0). Dose volume histogram (DVH) data was exported to a novel plan comparison matrix to calculate and compare plan quality - specifically plan quality index (PQI), homogeneity index (HI), conformality index (CI), merit (M) and penalty (P). Doses to OAR were obtained from the DVH. Acute toxicity information was obtained from case notes. Results: 6/9 patients were stage IIIB, receiving full prescription dose to a nodal volume in addition to the primary tumour. All patients had a maximum prescribed dose of 50.4Gy with dose delivered to 1% volume ranging from 52.2Gy to 54.9Gy. All patients received concurrent chemotherapy (mitomycin C 6-12mg/m2 D1 and capecitabine 600-625mg/m2 D1-14 and D22-36). Dose homogeneity (HI) was superior with CRT for all high dose PTV volumes. Doses to all OAR (small bowel, bladder, genitalia, iliac crests, and femoral heads) were lower with IMRT. All plans had M of >0.99. PQI and CI were superior for all IMRT plans and P was better for the IMRT plan for 8/9 patients, suggesting a significant improvement in the therapeutic ratio for treating anal cancers using IMRT. The only patient who experienced G3 skin toxicity was receiving CRT. No other G3 toxicity was encountered and no treatment delays occurred. Conclusion: OAR dose was uniformly lower with IMRT, most notably to external genitalia, and correspondingly significant skin toxicity was not seen in IMRT patients. The relative sparing of critical structures implies potential for dose escalation. In spite of inferior HI with IMRT, no local recurrences have occurred. Longer term follow up data is needed to evaluate local control and survival with IMRT versus CRT. Quantitative plan quality comparisons were possible through the use of a novel, evidence-based matrix, which could be used to compare plan quality when evaluating newer techniques such as volumetric modulated arc therapy in the treatment of anal cancers.

MO-A-137-05: Assessment and Prediction of Treatment Plan Quality Through Integrated Planning Script

Purpose: To identify plan quality prognostic features that can be used to improve treatment plan quality and intra-and inter-planner consistency using a self-evolving institutional treatment planning workflow that employs treatment-planning scripts. Methods: Institution-specific plan quality analysis software was developed using scripting integrated with a commercial treatment planning system (TPS). During treatment plan review, the script automatically extracted a set of representative quantities derived from dose volume histograms (DVH). The quantities were compared against national published standards and institution-specific criteria generated from statistics of accumulated planning data to assist in plan evaluation or further optimization. After plan acceptance, the plan quality indices and patient specific parameters, such as DVH endpoints and geometric features, were extracted by the script and saved into an evolving treatment plan database to update the existing institution-specific planning statistics. Results: A treatment-planning database was constructed using a group of prostate patients (n=48) and following the proposed workflow. Dosimetric indices and geometric features were exacted and analyzed. For example, the PTV and OAR overlap volumes consistently provided good prognostic power for plan quality prediction. Retrospective analysis of the prostate planning database showed a consistent PTV volume coverage (PTV-V100 of 94.9±1.09%). No significant difference in PTV volume coverage was found for different prescription levels or target size. A conformity index (V50/VPTV) of 3.51±0.26 was achieved for prostate-only treatment plans. Strong correlations (R>0.8) were observed between OAR dose volume indices (Rectum V70Gy, V65Gy and Bladder V70Gy, V65Gy) and the OAR and PTV overlapping volumes. Conclusion: The feasibility of automatic treatment plan quality aggregation, analysis and consistency improvement using planning scripts integrated into the treatment planning system was demonstrated in this pilot study. It is being incorporated into the clinical routine in our institute to determine its full utility.

SU‐E‐T‐423: Effect of X‐Ray Source Spot Size Modeling in AcurosXB Dose Calculation Algorithm On VMAT Based SBRT Treatments

Purpose: Aim of this study is to evaluate AcurosXB(AXB) dose calculation algorithm dosimetrically with different X‐ray source spot size modeling and its clinical effect on Volumetric Modulated Arc Therapy(VMAT) based Stereotactic Body Radiotherapy(SBRT). Methods: 10 SBRT cases were purposely selected to analyze the effect of source spot size for smaller field size. All the cases were planned in Varian Eclipse(V10.0) treatment planning system(TPS) which uses AXB algorithm for VMAT dose computation. For this study source spot sizes varied from 0.5mm, 1.0mm, 1.5mm and 2.0mm in X as well as Y‐direction in the beam data configuration. SBRT plans were optimized using Progressive Resolution Optimizer III(PRO‐III) and dose was calculated by AXB with different source spot sizes. Plan quality index such as conformity index(CI) and gradient measure for PTV and for normal tissues volume of 90% and 25% prescribed dose were used for analysis. VMAT plans were delivered with Varian Clinac‐iX and 2D‐planner dose was measured using iba iMatrixx 2D array in Multicube phantom. Gamma (3mm/3%) analyses were performed between measured and predicted doses for different spot sizes. Results: Conformity index value for PTV were found to be 1.16±0.21, 1.14±0.20, 1.13±0.2 &amp; 1.08±0.17 and gradient measure were 1.45±0.26, 1.46±0.26, 1.47±0.25 &amp; 1.52±0.27 for spot size of 0.5mm, 1.0mm, 1.5mm &amp; 2.0mm respectively. Volume of normal tissue receiving 90% of prescribed dose for 2mm spot size was 2.7% more in compare to 0.5mm spot size. Gamma analysis(3mm/3%) were found to be 98.77±1.03%, 98.96±0.91%, 99.18±0.76% &amp; 99.43±0.60% for spot size 0.5mm, 1.0mm, 1.5mm &amp; 2.0mm respectively. Conclusion: The Result showed that spot size configuration of 1.5–2.0mm were matching very closely with our measurements. Based on the variation in the PTV coverage and dose to normal tissues it is highly recommended to fine tune spot size parameters for VMAT based SBRT.

SU-E-CAMPUS-T-04: Plan Quality of Proton Vs. Photon IMRT for Whole Ventricle Irradiation

Purpose: To assess whether photon IMRT is an alternative to protons for whole ventricle irradiation (WVI), we compare dosimetric plans of helical tomotherapy with passively-scattered protons for WVI based upon target dose uniformity and conformity and organ-at-risk (OAR) sparing. Methods: Six representative pediatric patients treated for CNS disease were evaluated retrospectively. A 3 mm isotropic expansion of the ventricles' contour was designated as the PTV and prescribed to 24 Gy. Among the OARs considered were the temporal lobes, optic chiasm, optic nerves, inner ears, and lenses. Tomotherapy plans were generated using the TomoTherapy planning system v. 4.2 (Accuray, Inc.) with the longitudinal jaw size of 2.5 cm and the pitch of 0.287. Three-field proton plans were created using CMS XiO v. 4.70 (Elekta, Inc.), using a passively-scattered proton beam model with maximum range of 32 g/cm2 (225 MeV), range modulation in 0.5 g/cm2 increments, and Lucite range compensators. Plan quality was compared using a series of parameters including conformity index (CI), uniformity index (UI), and a EUD-based plan quality index (fEUD), a composite value calculated using the generalized EUD of all targets and OARs. Results: Among the 6 patients, proton plans exhibited slightly better CI for 4 cases and slightly better UI for 5 cases. If all OARs are considered, the proton plans had slightly better fEUD (0.20 vs. 0.17). This was attributed to the total avoidance of the eyes and lenses among the proton plans. If the eyes and lenses are excluded, the average fEUD is equal to 0.20 for both planning techniques. Conclusion: For WVI, if the sparing of distal ROIs such as the eyes and lenses is a concern, classical proton planning is a more favorable choice. Otherwise, the overall dose to the target and proximal OARs are comparable for both planning techniques.