ESTRO 38 Abstract book

S46 ESTRO 38

7.ITV error : dose error at end of motion range associated with using ITV technique (Fig. 1B) 8.Localization error in the direction of motion (SI): dose distribution improperly aligned with target in direction of motion 9.Localization (SI) & ITV error: a combination of 3 & 4 above 10.Localization errors in multiple directions: dose distribution improperly aligned with target in directions other than direction of motion The number of failures in each category are listed in table 1, and dose profiles showing some of the phantom errors are illustrated in figure 1.

Conclusion Our 4DMC code accurately calculates dose delivered to a realistic breathing/deforming anatomy. This tool can be used for adaptive purposes to calculate the cumulative dose delivered to patients during treatments. 1 Gholampourkashi S., et al. Phys. Medica 51 , 2018 ; 81–90 2 Cherpak A., et al. Med. Phys. 36(5), 2009;1672–1679 3 Gholampourkashi S., et al. Med. Phys. 44(1), 2017;299– 310 OC-0091 Patterns of failures among Imaging and Radiation Oncology Core lung and spine phantom irradiations S. Edward 1 , H.A. Molineu 2 , P.E. Alvarez 2 , D. Followill 1 , S.F. Kry 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA ; 2 The University of Texas MD Anderson Cancer Center, IROC Houston Quality Assurance Center, Houston, USA Purpose or Objective To investigate and classify the reasons why institutions fail the Imaging and Radiation Oncology Core (IROC) SBRT spine and moving lung phantom (which are used to credential institutions for clinical trial participation). Material and Methods A total of 189 IROC phantom irradiation results (61 spine and 128 lung) irradiated between 2012-present were randomly selected for investigation. Those that failed to meet established IROC criteria for a satisfactory irradiation were categorized as failures. The reports of the failing irradiations, including point dose disagreement, dose profiles, and gamma analyses, were qualitatively analyzed by IROC physicists. Classes of failure patterns were created and used to categorize each instance. Results There were 34 phantom irradiation failures: 16 spine (26% of spine cases) and 18 lung (14% of lung cases). After individual analysis of each report, the phantom failures were classified as follows: Spine 1.Systematic dose : uniform overdosing or underdosing of the PTV 2.Dose fall-off region : dose error in the steep dose- gradient between the PTV and spinal cord (Fig. 1D) 3.OAR overdose : overdose of the spinal cord structure 4.Localization error: dose distribution improperly aligned with target Lung 5.Systematic dose : a uniform overdosing or underdosing of the PTV (all cases were underdosed) 6.Local dose failure : dose error in an isolated area of the plan

Table 1. Distribution of phantom failure categories

Fig. 1 Screenshots of lung and spine phantom dose profiles; pink represents the planned dose, blue is the measured dose. (A) shows an example of a lung phantom ITV error on the superior and inferior edges (direction of motion). (B) shows a spine phantom with an under dose in the dose fall off region. Conclusion There are two distinct patterns of failure between phantoms. A majority (81%) of the spine phantoms failed due to a systematic dose error. Most (78%) of the moving lung phantom failures were due to localization of the moving target. Both of these errors are clinically relevant and will likely manifest as errors in patient cases. This information can help guide the community in improving the quality of radiation therapy. OC-0092 Portal dosimetry of small unflattened beams A. Torres Valderrama 1 , I. Olaciregui-Ruiz 1 , P. González 1 , A. Mans 1 1 The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Unflattened fields allow higher dose per pulse and faster delivery, but dosimetric difficulties arise with shrinking field size: (i) Electron range can exceed the field size yielding loss of equilibrium. (ii) Volume averaging effects smooth the penumbra and reduce the signal on the central axis (CAX). (iii) Accurate positioning of detectors becomes more critical than in large fields. Using traditional dosimetry under such circumstances has led to radiation accidents with over-dosage and harm to patients [1]. IAEA’s code of practice TRS-483 [2] addresses these issues, providing output factors for several detector types, beam qualities and machine types. EPIDs have particular dosimetric characteristics requiring several corrections in order to match measurements of dose to water similar to those addressed in TRS-483. In this work we demonstrate that the back-projection algorithm in Ref. [3], after