PTV Definition Research Articles

RADT-06. ENCOURAGING OUTCOMES AFTER CRANIO-SPINAL IRRADIATION (CSI) WITH MODERN IMAGE GUIDED INTENSITY MODULATED PROTON THERAPY (IMPT) (IMPT-CSI) IN CHILDREN AND YOUNG ADULTS (AYA)- AN INSTITUTION EXPERIENCE

Abstract BACKGROUND Outcomes of IMPT-CSI in children and adolescent-young adults (AYA) treated at our institute over a period of 5 years was analyzed MATERIALS AND METHODS 92 children/AYA were treated by IMPT-CSI. PTV definition of the spine for skeletally immature patients was modified to only the vertebral body without isotropic margins. All plans were multifield optimized for 5 mm robustness (cranio-caudal) & 2.5% range uncertainty. The treatment set-up was confirmed by both orthogonal radiograph and CBCT. Toxicity was graded as per RTOG criteria. RESULTS Median CSI dose of 35GyE in 21 fractions was given to cohort with 74% males, median age of 7 years (2-39 years), consisting of Medulloblastoma (58%), other Embryonal tumours (20%), recurrent Ependymoma (10%), GCT (10%) and others (2%) with 18% being reirradiation and under sedation in 25%. Concurrent chemotherapy was given in 40%. The median V98% for clinical target volumes and V95% for PTVs of the brain, spine and craniospinal were >97%. Isodose 95% covered the cribriform plate completely and optic nerves keeping Dmax to the lens <3.9 GyRBE, critical OARs (mean of the cohort- Cochlea Dmax 28GyE, midline mucosa 23.2GyE, Esophagus 23.2GyE and gonads (testes and ovaries- 0GyE and 0.4GyE). The online set-up verification showed translational and rotational deviation within 2 mm and 0.5 in 94% and 97% of the cases. Radiation dermatitis was Grade 2-40%; Gr3-10%. Neutropenia was Grade 3 in 30%; few needed G-CSF. Less than 10% developed >grade 3 significant weight loss/ anorexia/ mucositis. At median F/U 20 months (6–60m), 12 patients have progressed with PFS of 78% & OS of 87%, best in GCT and worst in Ependymoma (recurrent), 100% vs. 55% (p<0.014) CONCLUSION Outcomes of IMPT-CSI in children and young adults, showed very encouraging dosimetry, toxicity profile and long-term survival.

Assessment of Central Nervous System Lymphoma Based on CXCR4 Expression In Vivo Using 68Ga-Pentixafor PET/MRI.

F-FDG PET is limited for assessment of central nervous system lymphoma (CNSL) due to physiologic tracer accumulation in the brain. We prospectively evaluated the novel PET tracer Ga-pentixafor, which targets the C-X-C chemokine receptor 4 (CXCR4), for lesion visualization and response assessment of CNSL. Seven CNSL patients underwent Ga-pentixafor PET/MRI with contrast enhancement (CE-MRI) and diffusion-weighted sequences. The accuracy of Ga-pentixafor PET for CNSL lesion detection relative to the CE-MRI reference standard was determined. Standardized uptake values (SUVmean and SUVmax), PET-based (PTV) and MRI-based (VOLMRI) tumor volumes, and apparent diffusion coefficients (ADCs) were assessed, and correlation coefficients were calculated. Three SUVmax thresholds (41%, 50%, and 70%) were evaluated for PTV definitions (PTV41%, PTV50%, and PTV70%) and tested against VOLMRI using paired sample t tests. Twelve Ga-pentixafor PET/MRI examinations (including 5 follow-up scans) of 7 patients were evaluated. Ga-pentixafor PET demonstrated 18 lesions, all of which were confirmed by CE-MRI; there were no false-positive lesions on PET (accuracy, 100%). PTV41% showed the highest concordance with lesion morphology, with no significant difference compared with VOLMRI (mean difference, -0.24 cm; P = 0.45). The correlation between ADCmean and SUVmean41% (r = 0.68) was moderate. Changes in PTV41% on follow-up PET/MRI showed the same trend as VOLMRI changes, including progression of 1 lesion each in patient 1 (+456.0% PTV41% and +350.8% VOLMRI) and patient 3 (+110.4% PTV41% and +85.1% VOLMRI). Ga-pentixafor PET may be feasible for assessment and follow-up of CNSL. Future studies need to focus on testing its clinical value to distinguish between glioma and CNSL, and between radiation-induced inflammation and viable residual tumor.

ESTRO ACROP guidelines for target volume definition in pancreatic cancer

Despite of the predominant role of chemotherapy and surgery in pancreatic ductal adenocarcinoma (PDAC), radiotherapy (RT) still has a place in multimodal management of this disease where local tumour sequelae are fatal in about 40% of the patients. RT (chemoradiotherapy and stereotactic body radiotherapy) is used and investigated in the non-metastatic setting as part of definitive treatment strategies, in (neo)adjuvant settings and for locally recurrent disease. The ACROP committee was delegated by ESTRO to recommend target volume delineation for these clinical situations. The guidelines of this document are a result of a structured evaluation of the best available evidence by a panel of international experts in the field. Guidance for treatment planning including diagnostic imaging is provided. Recommendations are given for GTV delineation. The role and the definition of CTV volumes are critically discussed. Aspects of motion management and patient positioning are taken into account for PTV definition. Furthermore, aspects of delineation of organs at risk and of dose constraints are described in both, standard and hypofractionated, settings. This guideline has the purpose to support standardised and optimised processes of RT treatment planning for both, clinical practice and prospective studies.

Variability in spine radiosurgery treatment planning - results of an international multi-institutional study.

BackgroundThe aim of this study was to quantify the variability in spinal radiosurgery (SRS) planning practices between five international institutions, all member of the Elekta Spine Radiosurgery Research Consortium.MethodsFour institutions provided one representative patient case each consisting of the medical history, CT and MR imaging. A step-wise planning approach was used where, after each planning step a consensus was generated that formed the basis for the next planning step. This allowed independent analysis of all planning steps of CT-MR image registration, GTV definition, CTV definition, PTV definition and SRS treatment planning. In addition, each institution generated one additional SRS plan for each case based on intra-institutional image registration and contouring, independent of consensus results.ResultsAveraged over the four cases, image registration variability ranged between translational 1.1 mm and 2.4 mm and rotational 1.1° and 2.0° in all three directions. GTV delineation variability was 1.5 mm in axial and 1.6 mm in longitudinal direction averaged for the four cases. CTV delineation variability was 0.8 mm in axial and 1.2 mm in longitudinal direction. CTV-to-PTV margins ranged between 0 mm and 2 mm according to institutional protocol. Delineation variability was 1 mm in axial directions for the spinal cord. Average PTV coverage for a single fraction18 Gy prescription was 87 ± 5 %; Dmin to the PTV was 7.5 ± 1.8 Gy averaged over all cases and institutions. Average Dmax to the PRV_SC (spinal cord + 1 mm) was 10.5 ± 1.6 Gy and the average Paddick conformity index was 0.69 ± 0.06.ConclusionsResults of this study reflect the variability in current practice of spine radiosurgery in large and highly experienced academic centers. Despite close methodical agreement in the daily workflow, clinically significant variability in all steps of the treatment planning process was demonstrated. This may translate into differences in patient clinical outcome and highlights the need for consensus and established delineation and planning criteria.

OC-0380: Moving away from binary definition of PTVs: a novel probabilistic approach to PTV definition

OC-0380: Moving away from binary definition of PTVs: a novel probabilistic approach to PTV definition

SP-0024: GEC-ESTRO guidelines for target definition in breast brachytherapy

GEC ESTRO first published guidelines for HDR prostate brachytherapy in 2005. Since then there has been a considerable expansion in knowledge and use of the technique as a result of which an update has now been produced by the UROGEC group of GEC ESTRO, published in Radiotherapy and Oncology. These are evidence-based consensus guidelines from the group resulting from rigorous appraisal of the literature and discussions between experienced practitioners in this area. Recommendations for patient selection, treatment facility, implant technique, dose prescription and dosimetry reporting are given. The indications for HDRBT in dose escalation schedules with external beam are wide ranging with all patients having localized disease eligible for this technique.Exclusion criteria are few encompassing patients medically unfit for the procedure and those with significant urinary outflow symptoms. It is recognised that both single step ultrasound based and two step CT or MR-based technique scan be used. There remain areas of uncertainty in prostate HDR brachytherapy reflected in the guidelines. These include the optimal dose prescription, organ at risk constraints, PTV definition, evaluation of implant quality and the role of HDR monotherapy and salvage in prostate cancer. The use of these guidelines will ensure that HDR brachytherapy in prostate cancer can be practiced effectively and safely. The main indication at present is dose escalation with external beam but in the future HDR monotherapy, focal therapy and treatment of recurrence will be important areas of development.

Dynamic Target Definition: A novel approach for PTV definition in ion beam therapy

PurposeTo present a beam arrangement specific approach for PTV definition in ion beam therapy. Materials and methodsBy means of a Monte Carlo error propagation analysis a criteria is formulated to assess whether a voxel is safely treated. Based on this a non-isotropical expansion rule is proposed aiming to minimize the impact of uncertainties on the dose delivered. ResultsThe method is exemplified in two cases: a Head and Neck case and a Prostate case. In both cases the modality used is proton beam irradiation and the sources of uncertainties taken into account are positioning (set up) errors and range uncertainties.It is shown how different beam arrangements have an impact on plan robustness which leads to different target expansions necessary to assure a predefined level of plan robustness. The relevance of appropriate beam angle arrangements as a way to minimize uncertainties is demonstrated. ConclusionsA novel method for PTV definition in on beam therapy is presented. The method show promising results by improving the probability of correct dose CTV coverage while reducing the size of the PTV volume. In a clinical scenario this translates into an enhanced tumor control probability while reducing the volume of healthy tissue being irradiated.

Influence of Patient Setup and Target Delineation on Air Cavity Tomotherapy Dosimetry

Large air cavity is commonly seen in the radiation therapy treatment planning of head and neck cancer. It is often unclear how much cavity should be included in PTV definition for planning and its consequence on delivered dose with the presence of patient setup uncertainty. The aim of this study is 1) determine if three different methods of PTV delineation at an air cavity interface yield differences in overall tomotherapy plan quality and treatment efficiency, and 2) analyze how patient setup uncertainty affects the delivered dose for these three target definitions.

SU-E-J-24: Image-Guidance Using Cone-Beam CT for Stereotactic Body Radiotherapy (SBRT) of Lung Cancer Patients: Bony Alignment or Soft Tissue Alignment?

To assess the reliability of soft tissue alignment by comparing pre- and post-treatment cone-beam CT (CBCT) for image guidance in stereotactic body radiotherapy (SBRT) of lung cancers. Our lung SBRT procedures require all patients undergo 4D CT scan in order to obtain patient-specific target motion information through reconstructed 4D data using the maximum-intensity projection (MIP) algorithm. The internal target volume (ITV) was outlined directly from the MIP images and a 3-5 mm margin expansion was then applied to the ITV to create the PTV. Conformal treatment planning was performed on the helical images, to which the MIP images were fused. Prior to each treatment, CBCT was used for image guidance by comparing with the simulation CT and for patient relocalization based on the bony anatomy. Any displacement of the patient bony structure would be considered as setup errors and would be corrected by couch shifts. Theoretically, as the PTV definition included target internal motion, no further shifts other than setup corrections should be made. However, it is our practice to have treating physicians further check target localization within the PTV. Whenever the shifts based on the soft-tissue alignment (that is, target alignment) exceeded a certain value (e.g. 5 mm), a post-treatment CBCT was carried out to ensure that the tissue alignment is reliable by comparing between pre- and post-treatment CBCT. Pre- and post-CBCT has been performed for 7 patients so far who had shifts beyond 5 mm despite bony alignment. For all patients, post CBCT confirmed that the visualized target position was kept in the same position as before treatment after adjusting for soft-tissue alignment. For the patient population studied, it is shown that soft-tissue alignment is necessary and reliable in the lung SBRT for individual cases.

Evaluation of Dose Volume Histograms for Rectal and Prostate Cancer Patients Undergoing CBCT Based IGRT

Advances in radiotherapy treatment planning led to improved local tumor control and better healthy tissues sparing. Without image guidance at the time of therapy (IGRT), changes in position, shape and intra-fractional motions may prevent desired dose from being precisely delivered to the planning target. The rapid fall-off in dose outside the target in highly conformal radiotherapy techniques impose demand for accurate target localization at the time of the treatment to avoid underdosing of disease. Daily kV cone-beam computed tomography (CBCT) acquisition can provide accurate 3D volumetric patient's anatomy prior to every treatment fraction and it is suitable for adaptive corrections of treatment positioning variations. This study retrospectively assesses dose volume histograms (DVHs) with and without IGRT employing CBCT. We also report on additional radiation skin dose to patients during CBCT. Pre-treatment 3D imaging of 20 patients in treatment position was acquired using Pelvis CBCT protocol on on-board imaging (OBI) system (v. 1.4 Varian Medical Systems, Palo Alto) on a daily basis and twice a week for prostate and rectum cancer patients respectively. The obtained reconstructed CBCT images were co-registered with planning CT data using bony landmarks for rectal patients and prostate soft tissue visibility and prostate contours for prostate cancer patients. Obtained shifts were used in the Eclipse Treatment planning system to mimic the non-IGRT guided treatment. We calculated the dose distribution for original and simulated plan and obtained DVHs for targets and organs at risk were compared. Prior to CBCT scan, film pieces were taped on patient skin. Films were scanned before and after irradiation with the Epson Expression 10000XL document scanner in reflective mode. Response of the reference film dosimetry system was calibrated in terms of air kerma in air. Our preliminary results show small difference in PTV coverage for rectum patients with better sparing of the bladder in high dose region. For prostate patients, DVH of simulated plan shows 5% worse PTV coverage when compared to the original plan while the bladder sparing increases significantly. Measured anterior dose during CBCT amounts to 1.60 cGy in larger and up to 3.17 cGy in smaller patients, while the lateral dose ranges from 1.63 cGy in larger and up to 2.68 cGy in smaller patients. Our preliminary data indicate better sparing of bladder in high dose regions, and possible margin reduction for PTV definition if daily CBCT was used. This could lead to better tumor control and decrease NTCP, but the benefits must be weighted against the dose delivered to patients during CBCT.

Stereotactic Body Radiotherapy for Primary Management of Early-Stage, Low- to Intermediate-Risk Prostate Cancer: Report of the American Society for Therapeutic Radiology and Oncology Emerging Technology Committee

Stereotactic Body Radiotherapy for Primary Management of Early-Stage, Low- to Intermediate-Risk Prostate Cancer: Report of the American Society for Therapeutic Radiology and Oncology Emerging Technology Committee

SU‐GG‐T‐482: Effects of Normal Lung Delineation Using PTV Vs. GTV On NTCP and Vx for Lung Cancer Cases Planned with and Without Tissue Inhomogeneity Correction

Purpose: There is no clear consensus as to how to define normal lung volume; clinicians subtract either PTV or GTV from total lung volume. This study quantifies the dosimetric and biological differences between these definitions for radiotherapy planning with and without inhomogeneity corrections. Method and Materials: In this study, ten lung cancer patient cases with prescription doses (PD) 50–70Gy (average 62Gy) were evaluated. Dosimetric plans were generated using XIO‐CMS software. Complex (4–6 fields) 3D conformal radiation therapy plans with and without tissue heterogeneity corrections were compared. NTCPs were computed for three scenarios; respiratory pneumonitis (RP) endpoint of SWOG grade ⩾2RP (TD50=30.8Gy, n=0.99, m=0.37; NTCP1), SWOG ⩾1RP (TD50=28Gy, n=0.87, m=0.18; NTCP2), and radiographic & symptomatic pneumonitis (TD50=21.9Gy, n=0.8, m=0.37; NTCP3). Normal lung for NTCP was defined in two ways: [(left lung + right lung) − PTV], and [(left lung + right lung) − GTV]. Volume (Vx) of normal lung receiving 5Gy (V5), 13Gy (V13), 20Gy (V20) and 30Gy (V30) were also investigated. Results: For inhomogeneity non‐corrected plans: average PTV dose coverage was 97% (92.1–101.7%) of PD; with −PTV definition, average V5=41.3% (24.3–70.2%), V13=26.4% (15.9–36.8%), V20=22.2% (13.1–32.4%), V30=18.3% (10.9–27.8%), NTCP1=6.5% (2–16.5%), NTCP2=1.3% (0.01–8.6%), NTCP3=23.3% (6.7–51.2%); and with −GTV definition, average V5=43.2% (25.6–71.3%), V13=28.7% (17.9–39.2%), V20=24.7% (15.1–34.9%), V30=21% (13–30.5%), NTCP1=8.8% (2.6–20.7%), NTCP2=3.5% (0.02–16.3%), NTCP3=31% (9.7–60.8%). For inhomogeneity corrected plans: average PTV dose coverage was 99.6% (93.1–107.6%); with −PTV definition, average V5=43.7% (25.3–74%), V13=27.9% (17–42.7%), V20=23.2% (14.7–33.7%) and V30=19% (11.2–28.5%), NTCP1=7.8% (2.2–19.9%), NTCP2=2.5% (0.01–14.1%), NTCP3=27.9% (7.4–58.2%); and with −GTV definition, average V5=45.5% (26.5–75%), V13=30.2% (19.5–44.5%), V20=25.7% (16.1–36.1%), V30=21.7% (13.6–31.1%), NTCP1=10.7% (2.8–24.9%), NTCP2=6% (0.03–24.9%), NTCP3=36.5% (10.7–67.8%). Discussion and Conclusion: Results revealed that for heterogeneity non‐corrected plans, relative increase (with respect to −PTV definition) of Vxs were 4.7%, 8.9%, 11.3%, 14.7%, and NTCPs were 32.6%, 193.4%, 32.8%; and similar increases were observed for heterogeneity corrected plans.

SU-FF-I-08: Geometrical CT Image Deformation Generated On Moving Targets

Introduction:CT generates high resolution anatomical images. However, scanning moving targets, like lungtumors, may create geometrical deformations in shape, volume and position. The objective of this work was to study the geometrical changes produced on CTimages scanning a known moving target. Material and Methods: A motorized phantom capable of variable oscillatory movement with a platform to attach different radio opaque volumes was constructed (prism, cube, pyramid and sphere). A CTGELight Speed was used to scan the volume during oscillatory movement. Axial and helical images were acquired with different CT parameters, X‐ray tube rotation speed and couch speed. Then, images were acquired with a fixed tube speed and different frequency and amplitude of phantom movement. Analysis of target deformation was done on a Varian Eclipse v.7.3 TPS. Center of mass (CM) displacement, volume deformation and area versus position were compared between static and moving targets. Results: For fixed oscillation parameter with different X‐ray tube speed, the average and maximum CM displacement were 6 and 9mm for axial and 5 and 9mm for helical scans and volume deformation were 6% and 18% for axial and 1% and 31% for helical scans. For fixed X‐ray tube speed and variable oscillatory parameter target movement the average and maximum CM displacement were 6mm, 6mm, 3mm and 2mm respectively. Average and maximum deformation for prism, cube, pyramid and sphere were 8.3% – 44%, 0.4% – 10%, 5.8% – 9.8% and 1.6% – 6.6% respectively. A mathematical predictive model can be established for set conditions. Conclusions: Displacement and deformity varies independently with X‐ray tube rotation speed and oscillatory movement. Movement may produce inaccuracy for PTV definition.

SU‐FF‐T‐91: An IMRT Planning Technique for Head‐And‐Neck Cancers That Utilizes Direct Aperture Optimization

Purpose: IMRT can play an important role in the irradiation of head and neck tumors traditionally treated by lateral fields matched with an anterior supraclivicular field. However, due to the complex PTV geometry, these IMRT plans result in large numbers of segments leading to inefficient deliveries. We have developed an alternative IMRT planning technique utilizing Direct Aperture Optimization (DAO) to streamline the planning process and provide significant efficiency gains. Method and Materials: The process begins with the placement of traditional 3D conformal fields (laterals and anerior superclavicular). Next, the dose is calculated with this beam arrangement. The 90% isodose line is converted into a PTV with surrounding critical structures (e.g. spinal cord, parotid glands, posterior medial neck region) excluded from the PTV definition. The resulting PTV serves as the target for IMRT planning. For our planning technique, we have used the DAO algorithm in the Prowess Panther planning system. DAO plans generally result in significantly fewer segments as compared with those generated by traditional IMRT planning techniques. This is of critical importance since traditional IMRT plans for these cases have excessively long treatment times. Using DAO allows practical treatment times without sacrificing plan quality. Results: Fifteen patients were planned and treated with this technique. Seven equispaced beams were used in each. The objectives were PTV dose conformity and low dose to any avoidance regions. The spinal cord limit was 40Gy. For the DAO plans, treatment times ranged from 9 to 17 minutes on an Elekta SL20 acclerator. For corresponding plans produced using Pinnacle3, treatment times ranged from 30 to 45 minutes. Conclusions: An IMRT treatment technique for head and neck cancer has been devised. This technique removes field matching and allows the initial 50Gy to be delivered with a single plan. Using DAO provides significant reductions in treatment times.

Comparaison simulation classique–simulation virtuelle au cours d'une irradiation mammaire : étude prospective sur 14 patientes

To compare conventional 2D simulation and virtual simulation on 14 patients with breast cancer. Patients were simulated for treatment using standard procedure. They subsequently underwent CT scan in the treatment position. The CTV was defined as breast tissue. The PTV was obtained by adding a 3D margin of 1 cm around CTV. Organs at risk (lungs and heart) were outlined. Ballistics and dose distribution obtained with the two planning methods were compared. With conventional simulation, 95% of CTV received 95% of the dose prescribed. Virtual simulation significantly improved dosimetric coverage of PTV without increasing irradiation volume of lung and heart. In 2D simulation, using three slices allowed optimisation by adjusting wedge angle. The five-slice plan was a much better predictor of the maximum dose regions when compared to the three-slice plan. Using entire CT data didn't give any benefit. Variations in CTV delineation and PTV definition limit interest of virtual simulation. In classic simulation, a 5 CT slice-plan can be used to optimise dose distribution.

1052 poster Critical appraisal of CTV and PTV definitions by matching of simulator (SF) and portal films (PF) in prostate cancer patients (pts)

1052 poster Critical appraisal of CTV and PTV definitions by matching of simulator (SF) and portal films (PF) in prostate cancer patients (pts)

Assessment of the accuracy of a conventional simulation for radiotherapy of head and skull base tumors.

In this prospective study we investigated the absolute accuracy of the conventional simulation in head and skull base tumors. 41 isocenters in 40 consecutive patients with tumors of the head and skull base were included. In all cases a rigid stereotactic mask system was used for non-invasive fixation. The stereotactic ("calculated") coordinates of the isocenter were defined by the treatment planning computer. Each patient underwent a physical simulation using exclusively anatomical reference points to define the "preliminary" isocenter. The displacement between its coordinates and those of the stereotactic target point was recorded in X-, Y- and Z-direction with help of the targeting device, and the spatial error was calculated. Additionally, the patients were stratified by basal or calvarial tumor site to estimate the importance of the basal bone structures in the simulation accuracy. The influence of the learning effect on simulation accuracy was also determined. The results showed an accuracy of set-up at the linac within 1 mm in all three directions as calculated from orthogonal portal films. Mean shift of the isocenter coordinates obtained from physical simulation compared to the calculated stereotactic coordinates was 2.15 mm, 2.54 mm, and 2.69 mm for X-, Y-, and Z-direction, respectively. Mean spatial displacement amounted 5.06 mm, and the median was 4.50 mm. No significant difference could be noted between basal and calvarial location of the isocenter. A significant "learning effect" was observed with a decrease in spatial shift with increasing patient numbers. This effect was stronger in basal lesions, whereas calvarial lesions showed only a minor, insignificant effect. In conclusion, a physical simulation requires a safety margin of 5 mm in PTV definition in addition to other factors, e.g. organ movement.

Stereotactic radiotherapy of extracranial targets: CT-simulation and accuracy of treatment in the stereotactic body frame

Background and purpose: Evaluation of set-up accuracy and analysis of target reproducibility in the stereotactic body frame (SBF), designed by Blomgren and Lax from Karolinska Hospital, Stockholm. Different types of targets were analyzed for the risk of target deviation. The correlation of target deviation to bony structures was analyzed to evaluate the value of bones as reference structures for isocenter verification. Materials and methods: Thirty patients with 32 targets were treated in the SBF for primary or metastatic peripheral lung cancer, liver metastases, abdominal and pelvic tumor recurrences or bone metastases. Set-up accuracy and target mobility were evaluated by CT-simulation and port films. The contours of the target at isocenter level, bony structures and body outline were compared by matching the CT-slices for treatment planning and simulation using the stereotactic coordinates of the SBF as external reference system. The matching procedure was performed by using a 3D treatment planning program. Results: Set-up accuracy represented by bony structures revealed standard deviations (SD) of 3.5 mm in longitudinal, 2.2 mm in anterior–posterior and 3.9 mm in lateral directions. Target reproducibility showed a SD of 4.4 mm in longitudinal, 3.4 mm ap and 3.3 mm in lateral direction prior to correction. Correlation of target deviation to bones ranged from 33% (soft tissue targets) to 100% (bones). Conclusion: A security margin of 5 mm for PTV definition is sufficient, if CT simulation is performed prior to each treatment to correct larger target deviations or set-up errors. Isocenter verification relative to bony structures is only safe for bony targets but not for soft tissue targets.

Improving the accuracy of localisation in the radiotherapy treatment of head and neck, and brain cancer: some initial findings

Aims: To investigate the impact on localisation of utilising contrast enhanced computed tomography (CT) scans and the formal input of a radiologist in the radiotherapy planning process.Method: Ten head and neck / brain patients had pre- and post-contrast CT scans in the treatment position. Over several months, their unenhanced and enhanced scans were re-contoured by the original oncologist, and a radiologist. These new contours were compared to the original unenhanced contours and differences in contour volume, geographical position and tolerance doses on the associated PTVs were evaluated.Results: The use of contrast lead to significant differences in the size of GTVs. Mean differences in GTVs of 32.8 % were significant at p=0.01. No significant impact on the position of the contour centre was noted. The impact of the radiologist lead to large differences in GTV (mean 20.5 %), but large SDs meant this result was not statistically significant. The contouring precision of the oncologist showed no significant difference for GTVs and PTVs.Conclusions: The use of contrast when planning the radiotherapy treatment for head and neck / brain patients was found to lead to significant differences in GTV size, a lesser effect on PTV definition and little impact on the position of the contour centre. It may have important implications for multi-phase treatments where the GTV (rather than the PTV) is targeted for boost doses. Differences due to the input of a radiologist appear to be considerable and require further investigation when additional patient numbers have been acquired to improve precision.

Transcranial sonography: integration into target volume definition for glioblastoma multiforme

Purpose: Recent studies indicate that transcranial sonography (TCS) reliably displays the extension of malignant brain tumors. The effect of integrating TCS into radiotherapy planning for glioblastoma multiforme (GBM) was investigated herein. Methods and Materials: Thirteen patients subtotally resected for GBM underwent TCS during radiotherapy planning and were conventionally treated (54 to 60 Gy). Gross tumor volumes (GTVs) and stereotactic boost planning target volumes (PTVs, 3-mm margin) were created, based on contrast enhancement on computed tomography (CT) only (PTV CT) or the combined CT and TCS information (PTV CT+TCS). Noncoplonar conformal treatment plans for both PTVs were compared. Tumor progression patterns and preoperative magnetic resonance imaging (MRI) were related to both PTVs. Results: A sufficient temporal bone window for TCS was present in 11 of 13 patients. GTVs as defined by TCS were considerably larger than the respective CT volumes: Of the composite GTV CT+TCS (median volume 42 ml), 23%, 13%, and 66% (medians) were covered by the overlap of both methods, CT only and TCS only, respectively. Median sizes of PTV CT and PTV CT+TCS were 34 and 74 ml, respectively. Addition of TCS to CT information led to a median increase of the volume irradiated within the 80% isodose by 32 ml (median factor 1.51). PTV CT+TCS volume was at median 24% of a “conventional” MRI(T2)-based PTV. Of eight progressions analyzed, three and six occurred inside the 80% isodose of the plans for PTV CT and for PTV CT+TCS, respectively. Conclusion: Addition of TCS tumor volume to the contrast-enhancing CT volume in postoperative radiotherapy planning for GBM increases the treated volume by a median factor of 1.5. Since a high frequency of marginal recurrences is reported from dose-escalation trials of this disease, TCS may complement established methods in PTV definition.