ESTRO 37 Abstract book

S292

ESTRO 37

You need data for this, and the best kind is not sampled. On the other side, worker satisfaction improves when people know they’ve not made a mistake. It’s effective and good practise to provide feedback to your department: “well done, we’re within x%”. For all these reasons and more, we need the significant, useful data that patient specific QA provides. SP-0560 Against the motion M. Schwarz 1 1 Centro di Protonterapia, Protontherapy, Trento, Italy Abstract text Patient specific quality assurance(PSQA) may in principle take several different forms, but the introduction of Intensity Modulated Radiation Therapy(IMRT) did promote and made popular quite a specific interpretation of what PSQA means. It is this interpretation, and not the general concept of PSQA, that deserves plenty of criticism. Current PSQA typically consists in a)recalculating the planned dose on a simplified geometry; b)generating 2D dose map(s) by measuring the dose on that simplified geometry and c)deciding on the acceptability of the treatment by comparing calculations vs. measurements with a dedicated metric (e.g. the gamma analysis). Is this process “patient specific”? Very little. The only specific component which is verified is whether the treatment head model in the TPS does not fail for the irradiation pattern of that patient. Is this process ”quality assurance” of the treatment workflow? Very little. From the process perspective, we are only verifying the consistency of information between treatment planning and delivery system. Is the metric used to assess the plan quality -correlated to clinically relevant indices? There are quite some indications that this is not the case. -sensitive enough to point out possible causes of disagreement? Questionable at least. -sensitive enough to detect systematic equipment errors that can be easily corrected (e.g. in MLC positioning)? Most likely not. Are there guidelines, or commonly agreed criteria, for the conditions under which individual patient specific QA can be discontinued? No. Were recent major accidents due to lack of PSQA? Probably not. Could they have been prevented by much simpler checks? Yes. Can PSQA as currently performed be a hurdle to improvements of radiotherapy techniques? Yes. In 2018, the chance that the best way to detect a clinically significant error is via a measurement in a water equivalent phantom and gamma analysis is small at best. Then why are we doing all this? Because the software and hardware of early days IMRT was not entirely up to the challenge, a proper commissioning was impossible, so we accepted the idea of doing continuous commissioning and called it “Patient specific QA”. The process then became ingrained in our way of working, and we’re struggling to stop it or to change it into something better. Is there a way out? Yes, there are a few options.The most pragmatic approach is to: Get back to accurate TPS commissioning, thus ending “patient specific” measurements much earlier than now; Rely on accurate equipment QA to detect (small) systematic errors that can be corrected; Use treatment delivery information (e.g. via logfiles) or

add-on tools (e.g. transmission chambers) to perform workflow QA and avoid large errors; Last but not least, use the time we saved to work and improve overall treatment quality.

Symposium: Focus on site - BREAST

SP-0561 The Clinical Impact of DIBH For Breast Radiotherapy G. Lawler 1 , M. Leech 2 1 Beacon Hospital, Clinical Trials Research Unit- Radiotherapy Department Level -1, Dublin 18, Ireland 2 Trinity College Dublin, Radiation Therapy, Dublin, Ireland To assess if deep inspiration breath-hold (DIBH) treatment for left breast radiotherapy patients, resulted in dose sparing for organs-at-risk compared to free breathing (FB) treatment. Consequently, to evaluate if breath-hold amplitude depths could be correlated with observed dose reductions. Background Historically, left breast cancer patients treated with radiotherapy experienced improved cancer survival however, mortality rates did not decrease. This was directly related to long term treatment side effects, mainly chronic cardiac (e.g. coronary artery disease, ischaemia, myocardial infarction) and less so, pulmonary complications (e.g. pneumonitis), induced from radiotherapy treatment. Today, left breast cancer patients continue to be at an increased risk of cardiac complications induced from treatment, particularly compared to right breast patients. DIBH could potentially reduce dose to organs-at-risk without compromising target dose, hence potentially reducing complication incidence and improving patients’ quality of life and overall survival. Materials & Methods FB and DIBH CT planning scans obtained using Varian RPM Gating software for n=28 left breast/left chest wall +/- left supraclavicular patients, treated between January 2008-December 2013 were retrospectively re-contoured and re-planned. Organs-at-risk included the lungs, left lung, heart and left anterior descending coronary artery (LADCA). 6MV field-in-field tangential technique was used and occasionally low weighted 15MV beams also, to achieve improved dose coverage and homogeneity. Mono- isocentric technique was used for all supraclavicular patients, n=4. Cardiac shielding, without target dose compromise, was performed on all 56 plans. Maximum plan dose was kept within 1% agreement between FB and DIBH plans for comparative purposes. Varian Eclipse software was used to plan all patients and Anisotropic Analytical Algorithm (AAA) used for dose calculations. Statistical analysis of treatment plans was then performed. Results: No dose benefits or disadvantages were established for the combined lungs or ipsi-lateral lung using DIBH over FB technique. However, the smaller volumes of heart receiving large doses, D10%- D50% heart volume inclusive, received significant dose reductions using DIBH ranging from 27%-12.7% respectively. Maximum heart dose, Dmax was relatively reduced by 34.5% (Mean=41.81Gy, SD=3.963Gy FB vs Mean=27.39Gy, SD=12.393Gy DIBH, p<0.000) while mean heart dose, Dmean achieved a 32.6% reduction (Mean=1.817Gy, SD=0.627Gy FB vs Mean=1.224Gy, SD=0.344Gy DIBH, p<0.000). The heart was completely removed from the treatment field using DIBH in 28.6%, n=8 participants. The LADCA also experienced significant reductions in Abstract text Aim