ESTRO 36 Abstract Book

S902 ESTRO 36 _______________________________________________________________________________________________

The DVCs were then used as limits such that the dose that could be delivered would result in the tightest constraint being just met. Therefore, the dose for that fraction was increased or decreased to ensure that the DVC was on the tolerance limit. The impact of the dose escalation was then evaluated using TCP and NTCP. Results Thirteen of the patients investigated could have received net higher doses during their treatment without exceeding their OAR DVCs. In the remaining 18 patients, only 20 fractions out of 257 would allow an increase in dose while staying below the DVC limits. The rectum was the limiting structure in 97 % of fractions. The largest individual increase possible for a given fraction was 87.4 cGy. If all changes were made, the maximum accumulated net increase in dose possible for any patient was 13.58 Gy, assuming the imaged fractions were representative of the patients’ entire treatment and scaling to a full treatment. This corresponded to an increase in TCP and rectal NTCP of 13.7 % and 13.6 % respectively. Table 1 shows the results for the 13 patients. Conclusion Adapting the dose to be delivered to the patient on a fraction-by-fraction basis has the potential to allow for significant dose escalation while staying within institutional DVCs, significantly increasing TCP. This could be particularly useful in the hypofractionation approach to treatments. [1] Physica Medica, 32(4):618–624, 2016. EP-1661 Adaptive strategy to accommodate anatomical changes during RT in oesophageal cancer patients T. Nyeng 1 , M. Nordsmark 2 , L. Hoffmann 1 1 Aarhus University Hospital, Medical Physics, Aarhus C, Denmark 2 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Purpose or Objective During chemoradiotherapy (chemoRT) in oesophageal cancer (EC), some patients show large interfractional anatomical changes. These changes may affect the dose distribution adversely, demanding adaptation of the treatment plan. The aim of this study was to investigate a decision support system for treatment adaptation based on daily cone-beam CT (CBCT) scans. Material and Methods Twenty consecutive patients treated with chemoRT for oesophageal and gastro-oesophageal junction cancer were setup to the spinal cord with a tolerance of 5mm using daily CBCT scans. On CBCT, mediastinal structures are barely visible. Therefore, a surrogate structure (SS) was used to evaluate the actual target position. The SS was generated by indicating the borders between dense tissue nearby the clinical target volume (CTV) and lung tissue or air, see Fig1. Geometrical changes above 3mm in the tissue defined by the SS were registered by the radiation therapists (RTTs) for each fraction. Additionally, the RTTs noted changes of the base line diaphragm position above 5mm, the mediastinum above 5mm, the body contour above 10mm, and the shoulder blades above 10mm. Three consecutive registrations in any category triggered an adaptation of the treatment plan, requiring a new CT-scan with IV contrast. Targets and organs at risk were re- delineated, based on deformably propagated contours from the planning CT-scan. We recalculated the original treatment plan on the new CT-scan to evaluate the consequences of the observed anatomical changes.

EP-1660 Improvement in tumour control probability by adapting dose to daily OAR DVCs D. Foley 1 , B. McClean 1 , P. McBride 1 1 St Luke's Reseach Oncology Network, Physics, Dublin, Ireland Purpose or Objective A technique using analysis of on-board CBCT images to adapt the dose to the target on a fraction-by-fraction basis was developed. This new approach involves using the upper limit of dose volume constraints (DVCs) as the objective to be met at each fraction by tracking and accumulating dose voxels. The aim was to adapt the dose per fraction such that it was optimised each day without any organ at risk (OAR) DVCs being exceeded. The impact on tumour control probability (TCP) and normal tissue complication probability (NTCP) was evaluated. Material and Methods 31 patients who underwent prostate treatment were retrospectively investigated for this study. Initial VMAT plans consisting of 2 arcs were designed to deliver 74 Gy in 37 fractions of 2 Gy each to the target. The patients had on-board CBCT scans taken prior to treatment for between 9 and 33 fractions (436 in total). An in-house registration algorithm based on phase correlation[1] was used to retrospectively register CBCT images to the planning CT to determine the transformations and deformations in patients’ anatomy. This allowed the original plan to be recalculated on the registered CT image that provided the position of the target and OARs for that fraction. By tracking individual voxels throughout treatment, the dose was accumulated and the DVHs and DVC values were determined for each fraction.