ESTRO 37 Abstract book

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ESTRO 37

cm) remains challenging. For particle beam therapy with scanned pencil beam delivery dosimetric procedures have been established as well, but here the strong dependency of longitudinal dose profiles on density variation remains an issue. Due to intra- and inter-fractional changes treatment delivery verification with either imaging methods or in-vivo dosimetry is of outmost importance in external beam therapy and brachytherapy when performing small volume irradiations. SP-0011 Partial bladder irradiation with a focus on treatment planning and IGRT D. Eekhout 1 , M. Buijs 1 , M. Frantzen-Steneker 1 , F. Pos 1 , P. Remeijer 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Abstract text External Beam Radiotherapy (EBRT) with or without chemotherapy is the standard choice of treatment for inoperable patients with muscle invasive bladder cancer and for those patients who prefer a non-invasive treatment, or would like to retain their bladder. EBRT for bladder carcinoma consists of two options: either the whole bladder receives the full dose or the treatment is focused and the bladder tumor receives the full dose as part of the treatment: partial bladder RT. The advantage of the partial bladder RT is that a smaller volume is treated and therefore the healthy part of the bladder and organs at risk receive less dose . Partial bladder RT is complicated because of difficulties in reliable GTV definition and large inter- and intra-fraction organ motion. Several studies have shown that there is no consistency in bladder filling, resulting in large inter- fraction motion, with relatively large CTV to PTV margins. With the use of lipiodol and IGRT it is possible to reduce these margins and therefore reduce dose to the organs at risk. Since December 2012 patients are treated in our institute with partial bladder irradiation. The primary tumor is not visible on a CT scan. Therefore a few days before CT acquisition lipiodol is injected into the bladder wall around the tumor by a urologist during cystoscopy. A full bladder CT scan is acquired to optimally spare the residual bladder volume and used to delineate the CTV (visible tumor and lipiodol). The lipiodol injection, spot visibility and washout was investigated in our institute in 2009 [Pos et al.]. This study showed that the lipiodol spots are visible and remain visible throughout the whole course of EBRT and showed little washout (2 out of 40 patients). The size of the lipiodol spots varied from 1-4 mm and sometimes spread outside the bladder wall. However the lipiodol still provides additional information for tumor delineation (see fig. 1). The CTV to PTV margin with lipiodol is isotropic 1.5 cm and if the lipiodol is washed out, 2 cm. The dose to the PTV is 60Gy in 25 fractions and planned conform IRCU criteria using a 10 MV VMAT technique. The IGRT procedure consists of a daily online CBCT registration using a 3D shaped ROI around the CTV containing the lipiodol, using a grey value registration algorithm to measure and correct for inter-fraction motion during treatment. The online corrected systematic and random inter-fraction motion was analyzed for 65 patients with an average of 21 CBCT scans per patient. The results show a large inter-fraction motion due to the inconsistency in bladder filling (see table 1). In this lecture different treatment planning techniques will be demonstrated. Furthermore for partial bladder treatment, the use of lipiodol and IGRT, allows a reduction in the CTV to PTV margin and therefore reduces the dose to the organs at risk.

SP-0008 Abstract not received

Symposium: Partial organ treatments to minimise late radiation effects

SP-0009 Evidence for partial organ treatments R. Laing 1 1 Royal Surrey NHS Trust, Oncology, Guildford, United Kingdom Abstract text Partial organ treatment has been extensively investigated in breast cancer with over 18,000 patients randomised in 8 large trials. Whilst different techniques and patient selection were employed encouraging early results have been reported using different treatment techniques. The evidence in Prostate Cancer is much more limited with relatively little data with small patient numbers using a range of methods ranging from hemi-gland brachytherapy to ultra-focal treatments. This talk will describe the definitions used in partial organ treatments and the emerging data in Prostate Cancer as well as a brief summary of the current situation in Breast Cancer. SP-0010 Variation in dose distribution by different radiation modalities D. Georg 1 1 Medizinische Universität Wien, Division Medical Radiation Physics- Department of Radiation Oncology, Vienna, Austria Abstract text Partial organ irradiation has the potential to minimize late radiation effects, especially in light of the enormous technological progress made during the last decades. More specifically, computerized treatment plan optimization and image guided beam delivery of external beam therapy with photons, particle beams and brachytherapy enable such strategies. Both the geometric and dosimetric precision of today’s radiotherapy tools and equipment allows to precisely target small volumes with either radiation modality. On the other hand, the concept of partial organ/volume irradiation has been conceptually discussed in scientific literature for several years, but it did not find widespread clinical implementation so far. The reasons for that are likely manifold. The issue of advanced imaging for structure segmentation and the inherent inter- and inter-observer variations as well spatial resolution limitations of various imaging methods certainly contributes to the underutilization of available technology. Other issues might be related to dosimetric aspects. In this contribution the planning and dose delivery feasibility of small volume irradiation with advanced photon beam therapy such as volumetric modulated arc therapy (VMAT), intensity modulated particle therapy (IMPT) with scanned beam delivery as well as high dose rate brachytherapy (HDR-BT) is feasible and will be reviewed. In this context the basic differences in their dosimetric characteristics will be compared and inherent dose delivery uncertainties and the dose calculation uncertainties will be discussed. For brachytherapy the dosimetric and geometric challenges of small volume irradiation are inherent elements of the dose delivery method. With the adoption of more advanced dose calculation principles that also include tissue characteristics one of the remaining dosimetric uncertainties could be overcome in brachytherapy. For external beam therapy with advanced photon beam techniques dosimetry of very small fields (e.g. < 1 cm x 1