PD-0345 PITCH CHOICE FOR OFF-AXIS BLOCKED TARGETS TREATED WITH TOMOTHERAPY

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Purpose/Objective: Optimization of radiotherapy treatment plans is commonly based on physical absorbed dose, using dose volume parameters and a quadratic objective function (OF). The clinical impact of the delivered absorbed dose, however, is not described explicitly with these parameters. NTCP models are used to predict normal tissue complications using patient specific information and three dimensional dose distributions. Therefore, application of NTCP functions directly in the OF may result in plans with less complication probability than with the quadratic OF. The aim of this study was to demonstrate treatment planning optimization using multivariable NTCP models. Materials and Methods: Recently, different NTCP models for salivary dysfunction (xerostomia) and swallowing dysfunction (when swallowing liquids, soft- and solid food and choking when swallowing) were developed at our department. The NTCP models were implemented in a research version of the Pinnacle treatment planning system, extending the IMRT optimization function to a NTCP-based OF. IMRT plans for the head-and-neck region (for three patients) were constructed with two optimization methods, the quadratic- and NTCPbased OF. For both methods identical target- and critical volumes were used in the objective list. Using the quadratic OF, specific dosevolume requirements needed to be assigned to each objective. Using the NTCP-based OF, the NTCP models were assigned to the corresponding organs at risk in the objective list without the need for further dose-volume requirements. All plans were evaluated using dose-volume-histograms of the target volumes and NTCP values for the organs at risk. Pareto sets were used to compare plans optimized with both optimization methods. Pareto sets projected the relative volume of target under-dosage (PTVD<95%) against calculated salivary- and swallowing dysfunction NTCP values. Additionally, the lowest NTCP value at PTVD<95% = 0.02 was derived from each Pareto set. Results: For one patient, Pareto sets (250 plans per set) acquired with both optimization methods are shown in figure 1. Estimated NTCP values for swallowing dysfunction at PTVD<95% = 0.02 decreased by 0.3 to 9.4 % using the NTCP-based optimization function. This was not the case for NTCP for xerostomia, where the mean dose of the contralateral parotid gland (model parameter) could not be reduced due to convergence problems during optimization.

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Purpose/Objective: Rotational IMRT promises superior treatment efficiency by similar or increased treatment plan quality compared to static gantry IMRT. HybridArc (HA) is a new planning strategy provided by Brainlab® which combines a dynamic conformal arc (DCA) with static IMRT beams. In this study this new approach was benchmarked against Elekta’s® VMAT solution in terms of plan quality, treatment efficiency and dosimetric accuracy. Materials and Methods: CT data sets and contours of three primary prostate patients were used to create plans with HA and VMAT. For both Monte Carlo dose calculation was applied in the respective TPS (iPlan and Monaco). The prescribed dose to the PTV was 78Gy and we aimed for a maximum dose of less than 110%, as well as a D95% of 74.1Gy. In-house constraints were used for rectum and bladder. HA was evaluated at two different DCA vs. IMRT beam weightings, i.e. 70/30 (HA7030) and 30/70 (HA3070). To make a fair comparison between the three planning approaches (HA7030, HA3070, VMAT) pareto optimal fronts were created using the D95% for PTV; D50% for bladder; and D50% and D10% for rectum. Treatment efficiency was evaluated assessing total number of monitor units (MU), static IMRT segments (just for HA) and beam on time of clinically acceptable plans. The Delta4™ system was used for dosimetric verification of 36 plans (12 plans each). If 90% of points had a gamma value smaller than one (GPR – gamma pass rate) the accuracy was considered as acceptable (DTA -3mm, dose deviation - 3%). Results: In total 206 treatment plans were created. 110 out of those fulfilled our objectives and were therefore considered as clinical acceptable. For all three patients D10% pareto optimal fronts for rectum were superior for VMAT compared to HA7030 and HA3070. For rectum D50% the advantage was diminished and for bladder D50% both HA approaches were superior to VMAT in two patients. Comparing the two weighting schemes for HA, HA3070 was either equal or superior to HA7030 in all pareto fronts. The median treatment duration for HA7030 plans was 476s with 450 MU. Median number of segments for the HA IMRT part were 70. The numbers for HA3070 were 525 s and 464 MU with 73 segments. VMAT was highly efficient with a median irradiation time of 176s and median MU of 434. For VMAT all twelve irradiated plans had a GPR of higher than 96%. For HA 2 out of 24 plans had a GPR of 89%, but all other plans were above 92%. Conclusions: In summary treatment plan quality was superior for VMAT except for fronts created for the D50% of the bladder. The higher the weighting of IMRT beams in HA the longer the treatment duration, but unexpectedly it didn´t decrease significantly with a higher arc weighting. This was due to the fact that the segment number was quite similar. VMAT treatment efficiency was counterbalanced by longer planning times. Dosimetric deviations were acceptable with minor advantages for the investigated VMAT implementation. PD-0345 PITCH CHOICE FOR OFF-AXIS BLOCKED TARGETS TREATED WITH TOMOTHERAPY D. O'Doherty1, M. Lenane1, A. Geater1, S. Tudor1 1 Addenbrooke's Hospital, Medical Physics Box 152, Cambridge, United Kingdom

Conclusions: We achieved to incorporate an IMRT plan optimization using multivariable NTCP models into a commercial treatment planning system. Since the NTCP models contain explicit information over a whole dose range, no specific dose-volume requirements are required as input to the objectives, simplifying the planning procedure. The NTCP for swallowing dysfunction was considerably reduced. However, further research should be conducted to improve the NTCP for salivary dysfunction. PD-0344 HYBRIDARCTM - EFFICIENT MIXTURE OF DYNAMIC ARC AND STATIC IMRT? COMPARISON TO VMAT IN TERMS OF PLAN QUALITY & EFFICIENCY M. Stock1, R. Dreindl2, D. Georg1 1 Medical University of Vienna, Department of Radiotherapy & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 2 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria

Purpose/Objective: The magnitude of the thread effect on tomotherapy plans depends on the off-axis distance of the PTV and the degree to which the plan has blocked structures. When both features are present, such as in limb sarcoma plans, there is a potential for a thread effect that reduces PTV coverage below the 95% isodose. It should be possible to calculate a pitch suited to these circumstances from geometric principles. Materials and Methods: In order to calculate the ideal pitch for an off-axis target with many directionally blocked structures, we can approximate the situation to the limiting case in which the treatment is blocked such that beams are allowed from just one direction. The ideal pitch can then be calculated geometrically. To minimise the thread effect, beams should abut at the position of the target, necessitating the longitudinal displacement between beams to be FW*(85cm-x)/(n*85cm), where FW is the field width used, x is the distance off-axis towards the source position and n =1, 2, 3 etc.. As the longitudinal displacement between beams on adjacent rotations is equal to p*FW, where p is the pitch of the plan, it can be seen that the pitch required is equal to (85cm-x)/(n*85cm). Structures were created on 2 offset cheese phantom CT datasets in order to simulate 2 sarcoma plans with off-axis PTVs with centres 14 and 19cm from the centre of the bore. For the 19cm off-axis PTV 4 plans were created in the Tomotherapy Treatment Planning System

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(TPS), one with a standard pitch used in planning of 0.43, one with a calculated 2nd ideal (n=2) pitch of 0.388, and two with pitches slightly higher or lower than the calculated ideal (0.418 & 0.358). 3 plans were created for the 14cm off axis PTV using the standard (0.43), the 2nd ideal (0.418) and non-ideal pitch (0.388). Upon calculation of theses plans coronal dose profiles through the centre of the PTV were created to indicate the magnitude of expected thread effect. Delivery Quality Assurance plans were then created for the 19cm off-axis PTV plans and coronal film dose planes were acquired on the treatment unit. The resulting film scans were converted to dose and coronal dose profiles were created through the centre of the PTV. Results: Magnitude of Thread Effect (from peak to trough) through 19cm off-axis PTV Pitch

TPS Profiles

Film Plane Profiles

0.430 (standard)

5.2%

4.7%

0.418 (non-ideal)

3.7%

3.7%

0.388 (2nd ideal)

1.3%

2.0%

0.358 (non-ideal)

3.8%

6.0%

The dose profiles through the 14cm off-axis PTV on the TPS indicate a less pronounced difference in thread effect with a magnitude 4.2% for the non-ideal pitch of 0.388, falling to 3.2% for the standard pitch of 0.43 and 2.3% for the 2nd ideal pitch of 0.418. Conclusions: Results from both the treatment planning system and film measurement strongly indicate that the magnitude of the thread effect can be reduced by selecting the ideal pitch based on the offaxis distance of the PTV. In the case of the 19cm off-axis PTV plans, the TPS profiles indicate a reduction in thread effect of 3.9% from standard planning pitch to ideal pitch and for the film profiles the thread effect is reduced by 2.7% from standard pitch to ideal. PD-0346 A NOVEL SOFTWARE TOOL TO GUIDE DOSIMETRISTS TO EFFICIENTLY GENERATE HIGH QUALITY PROSTATE IMRT TREATMENT PLANS Y. Wang1, L. Incrocci1, A. Zolnay1, H. Joosten1, S. Breedveld1, T. McNutt2, B. Heijmen1, S. Petit1 1 Erasmus Medical Center Rotterdam, Department of Radiation Oncology, Rotterdam, The Netherlands 2 Johns Hopkins University, Department of Radiation Oncology and Molecular Radiation Sciences, Baltimore MD, USA Purpose/Objective: Using the current commercial IMRT treatment planning solutions, it is often difficult to determine whether an IMRT treatment plan can be further optimized in terms of normal tissue sparing. Consequently, time is often wasted trying to improve an IMRT plan that cannot be further improved, while other patients might be treated with unnecessary high doses to the organs at risk (OAR). The goal of this study was to develop a model that can predict, based only on the anatomy of the patient, the lowest achievable IMRT dose to an OAR. The predictions can be used as a benchmark to assess the quality of an IMRT plan. Materials and Methods: The lowest achievable dose to an OAR depends on its proximity to the planning target volume (PTV). A database of 48 trial prostate cancer patients, treated with IMRT, was queried to find the lowest achieved dose to the rectum of all patients with less favorable PTV-rectum configurations than a new patient. This minimal dose should also be achievable for the new patient. For 10 randomly selected patients, the minimal dose to the highest 35% of the rectum (D35) was predicted in this way. To test the accuracy of the predictions, the patients were replanned with the same beam angles using an in-house developed, fully automated, multi-criteria optimization algorithm that uses a priority wishlist as input. The first priority was to push the rectum dose as low as possible without aggravating the dose to the PTV and the other organs. The new plans were compared to the original plans to determine how much can be gained in treatment plan quality when the treatment planner would be aware of what is achievable in terms of dose to the rectum Results: Even though all the original plans fulfilled the dose constraints defined by the trial protocol, our model predicted that the D35 could be, on average, 6 Gy (range: 0 – 16 Gy) lower than for the original plans. After replanning the predicted D35 could indeed be achieved for all patients, without increasing the dose to rest of the

rectum, or to any part of the bladder, the anus or the hips, without losing PTV coverage and without a considerable decrease in conformity (4%). The D35 of the new plan was even lower than predicted, on average 6 Gy (range: 0 – 11 Gy). Conclusions: This method can predict the achievable dose to the rectum for prostate cancer patients treated with IMRT. This prediction can be used to determine the quality of an IMRT plan compared to the best relevant plan in the database of prior patients. For each patient the predictions were realistic, achievable and for most patients considerably lower than with the initial, clinical plan. We used D35, but the method is suitable for any dose-volume constraint. The differences in D35 between the predictions and the new plans, underlines the importance of high quality plans in the database. This method can guide dosimetrists to increase treatment planning efficiency and generate better IMRT plans with considerably less dose to the rectum. PD-0347 EXTENDED PENUMBRA REDUCTION FOR NORMAL TISSUE SPARING IN PROTON THERAPY OF INTRACRANIAL TUMOR M. Bubula1, U. Jelen2, F. Ammazzalorso2, U. Weber3, K. Zink1, R. Engenhart-Cabillic2, A. Wittig2 1 University Medical Center Gießen and Marburg, Department of Radiotherapy and Radiation Oncology, Marburg, Germany 2 University of Marburg, Department of Radiotherapy and Radiation Oncology, Marburg, Germany 3 Roen-Klinikum AG, Partikeltherapiezentrum, Marburg, Germany Purpose/Objective: Proton beams undergo larger spread at the nozzle than carbon ion beams, especially at lower therapeutic energies. The geometry of treatment rooms in combined proton-carbon fixed-nozzle facilities for raster scanning typically represents a trade-off between technical solutions in beam delivery, flexibility of patient positioning and clinical requirements. This may result in a relatively large nozzleto-patient distance and in laterally broader pencil beams at the isocenter with consequent reduction of the achievable conformity. Therefore, at our facility, the use of the Extended Penumbra Reduction (EPR) has been planned. This method enables treatment planning and delivery at reference points closer to the nozzle than the room's isocenter. The purpose of this planning study was to assess the potential benefits of the EPR in terms of treatment conformity and dose reduction to normal tissue in proton treatment of intracranial tumors. Materials and Methods: 12 patients with intracranial lesions close to the base of the scull, who were treated at our institution with stereotactic photon radiotherapy, were chosen for this treatment planning study. For each case, an intensity-modulated proton therapy (IMPT) treatment plan was created with a beam setup attainable at a fixed-nozzle facility, i.e. allowing isocentric table rotations. In all cases a dose of 60 GyRBE over 30 fractions was prescribed to the planning target volume (PTV). Each plan was optimized twice, applying different vacuum-window-to-isocenter distances: 140 cm (standard room isocenter, no EPR) and 100 cm (EPR 40 cm). Each pair of dose distributions was compared in terms of dose-volume histograms (DVHs) and selected dosimetric quality indicators. Results: No statistically significant difference in PTV coverage was observed, while, by means of the EPR, a reduction of the dose to healthy structures was achieved for all patients. For Dmax the reduction was significant for the whole brain (p<0.05), but nonsignificant (p>0.05) for single organs-at-risk (OARs) like brainstem, N. optici, hippocampi, temporal lobes. Significantly lower Dmean and V10% were achieved in the whole brain and in all OARs (p<0.001). Also significant was the reduction of V50% in the hippocampi and temporal lobes (p<0.001) and of V83% in both temporal lobes (p<0.05). Conclusions: The study shows the potential advantage of the EPR in proton treatment of intracranial tumors in terms of statistically significant normal brain tissue sparing and integral dose reduction. As the EPR affects the lateral beam penumbra, a clinically relevant benefit can be expected in selected situations only i.e. for OARs directly adjacent to the beam channel, and hence is dependent on the degrees of freedom in beam setup. PD-0348 PROTON AND CARBON ION IMPT TREATMENT PLANS IN RASTER SCANNING ION BEAM THERAPY M. Ellerbrock1, B. Ackermann1, N. Chaudhri1, S. Ecker1, K. Henkner1, H. Prokesch1, M. Winter1, O. Jaekel1