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ESTRO 37

Conclusion No significant differences were found in dose delivery even though large gantry acceleration were present during the treatment. Large dose per fraction treatments can be delivered in less number of arcs. Nevertheless, the MLC errors caused by these accelerations suggest that some kind of dose delivery verification is mandatory.

and health economic reasons are drivers for ever- increasing hypo-fractionation. Initial studies approximately halved the conventional number of 35-40 Gy fractions (e.g. HYPRO trial 19 × 3.4 Gy, CHHiP 20 × 3 Gy) but still showed considerable GI and GU toxicity in both arms. In contrast, recent trials on stereotactic regimes in predominantly low risk cancer patients (e.g. pHART3, 5 × 7 Gy), yield excellent biochemical disease- free survival and in particular very limited side effects . At the same time they indicate that toxicity may be quite sensitive to the exact choice of margin, delivery technique and fractionation (e.g. pHART6, 5 × 8 Gy). The focal boosting up to 95 Gy of intraprostatic GTVs defined on multi-parametric MRI, delivered in a conventional schedule, is being explored in the FLAME trial. After a two year follow-up no increase in ≥2 GU and GI toxicity with respect to the 77 Gy control arm was found. The ideas of focal boosting for intermediate to high risk prostate cancer and extreme hypo-fractionation have been combined in the recent Hypo-FLAME trial, which delivers 5 × 7 Gy to the entire prostate and 5 × 10 Gy to the GTV (overall treatment time 4 weeks). Because of the small margins (4 mm CTV-PTV) and steep dose gradients, extra care was taken to avoid large prostate rotations and intrafraction motions. Of course, it still needs to be proven that such radical fractionation approaches are not inferior, or possibly even superior to more conventional fractionation in larger trials. Nevertheless, initial early toxicity seems promising and spurs the thought of applying further hypo-fractionation with even smaller margins and shorter overall treatment times. Key to this development is the introduction of MR- guided RT. The technical possibilities of continuous MR online imaging during dose delivery, automatic image segmentation and ultra-fast dose re-optimization available in the MR linac environment would allow to minimize the impact of organ deformation, prostate rotation and intrafraction motion, and may potentially lead to the limit of ‘one-stop curation’. However, care must be taken that such approaches will not lead to more frequent and severe adverse effects. The steps already taken on this road, as well as the remaining hurdles will be discussed. SP-0147 Preserving bladder function with brachytherapy for muscle-invasive bladder carcinoma B. Pieters 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Abstract text Surgical therapy for muscle-invasive bladder cancer (MIBC) involves radical cystectomy resulting in loss of bladder function. In selected cases with limited tumor invasion in superficial muscle layers a trans urethral resection (TURB) can be considered. However, about half of the patients will experience a local relapse with a disease-specific mortality of 47%. Alternative bladder sparing therapies were developed with the great advantage that bladder function will be preserved. Standard treatments are trimodality therapies either as TURB, external beam radiotherapy (EBRT) and concomitant chemotherapy or TURB, EBRT, and brachytherapy. Selection to be treated with brachytherapy is a solitary tumor, less than 5 cm, T2 (limited T3) and not located in the bladder neck. Patients undergoing bladder conservative therapies would likely being happy because of retaining their bladder function. However, these patients will continue to be at risk for developing bladder recurrences, either non-muscle- invasive or muscle-invasive. Local recurrence rate is 20%. Of these recurrences about 40-50% are non-muscle- invasive for which can be treated with repeated conservative therapy (TURB with or without bladder instillations). Reported bladder function preservation

Symposium: How to minimise toxicity in pelvic malignancies in the era of precision radiotherapy

SP-0145 Hematologic toxicity during RT for pelvic malignancies: how to reduce it? P. Franco 1 , F. Arcadipane 2 , S. Martini 3 , G.C. Iorio 3 , U. Ricardi 3 1 Ospedale Molinette University of Turin, Department of Oncology- Radiation Oncology, Torino, Italy 2 AOU Citta' della Salute e della Scienza, Department of Oncology- Radiation Oncology, Turin, Italy 3 University of Turin, Department of Oncology- Radiation Oncology, Turin, Italy Abstract text Hematologic toxicity is an important side effect occurring in patients affected with pelvic malignancies, undergoing combined radio-chemotherapy, with consistent clinical meaningfulness. Since more than a half of bone marrow is comprised within the pelvic region, the radiation dose received by this functional compartment is crucial. Modern imaging modalities may provide a useful tool to identify bone marrow and new delivery technology may enhance the radiation oncologist’s possibility to selectively spare these structures, potentially decreasing acute hematologic toxicity profile in this setting. Correlation between dose to pelvic structures and acute hematologic toxicity has been studied in several oncological settings, mainly on a retrospective frame. Different dose metrics were found to be correlated including mean doses and different points within the dose-volume histogram ranging from low to medium-high doses. Several imaging modalities were used to identify bone marrow both morphological and functional. Several clinical endpoints were used. In general, accounting for bone marrow during the treatment planning process may be important to decrease the acute hematologic toxicity profile during concurrent chemo-radiation in patients affceted with pelvic malignancies. The most appropriate strategy to address this issue need further investigation and deserve validation in a prospective clinical framework. SP-0146 High precision prostate radiotherapy: a bigger bang for your buck H. De Boer 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Abstract text Over the past decade, the delivery of external beam radiotherapy (EBRT) to the prostate has underwent rapid evolution in terms of fractionation, localization and precision. Improved target definition on MRI as well as increased treatment delivery precision, mainly using kV/MV imaging of gold fiducial markers and conebeam- CT, have allowed substantial planning margin reductions, down to 4-5 mm. The optimization of IMRT and VMAT techniques has further increased the conformality of the dose distribution to the target. These developments go hand in hand with, and have enabled the increased application of localized dose escalation to dominant intra-prostatic lesions (focal boost) and hypo- fractionation. Prostate tumour biology, patient comfort