ESTRO 2021 Abstract Book

S533

ESTRO 2021

C. Onal 1 , G. Ozyigit 2 , E. Oymak 3 , O.C. Guler 1 , P. Hurmuz 2 , B. Tilki 2 , M. Reyhan 4 , M. Tuncel 5 , F. Akyol 2 1 Bakent University Faculty of Medicine, Department of Radiation Oncology, Adana, Turkey; 2 Hacettepe University Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey; 3 Iskenderun Gelisim Hospital, Division of Radiaition Oncology, Hatay, Turkey; 4 Hacettepe University Faculty of Medicine, Department of Nuclear Medicine, Ankara, Turkey; 5 Baskent University Faculty of Medicine, Department of Nuclear Medicine, Adana, Turkey Purpose or Objective Defining the extent of disease spread with imaging modalities is crucial for therapeutic decision-making and definition of treatment. This study aimed to investigate whether clinical parameters and nomograms predict prostate-specific membrane antigen (PSMA)-positive lymph node metastasis in treatment-naïve nonmetastatic prostate cancer (PC) patients. Materials and Methods The clinical data of 443 PC patients (83.3% high-risk and 16.7% intermediate-risk) were retrospectively analyzed. The inclusion criteria were treatment-naïve PC with a GS of ≥7, a 68 Ga-PSMA-PET/CT referral for staging, and intermediate- or high-risk disease per D’Amico staging. Receiver operating characteristic (ROC) curves with areas under the curve (AUC) were generated to evaluate the accuracy of clinical parameters (prostate-specific antigen [PSA], T stage, Gleason score [GS], International Society of Urological Pathology [ISUP] grade) and nomograms (Roach formula [RF], Yale formula [YF], and a new formula [NF]) in predicting lymph node metastasis. The AUCs of the various parameters and clinical nomograms were compared using ROC and precision-recall curves. Results A total of 443 patients were retrospectively analyzed, and 288 lymph node metastases were identified in 121 patients (27.3%) using 68 Ga-PSMA-PET/CT. Most PSMA-avid lymph node metastases occurred in external or internal iliac lymph nodes (142; 49.3%). Other metastatic lymph nodes were located in the obturator (54; 18.8%), common iliac (38; 13.2%), presacral (12; 4.2%), perirectal (18; 6.2%), and para-aortic (24; 8.3%) regions.The clinical T stage, serum PSA level, GS, and ISUP grade showed similar accuracy in predicting PSMA- positive metastasis, with AUC values ranging from 0.675 to 0.704. The median risks for lymph node metastasis according to the RF, YF, and NF were 31.3% (range: 12.3–100%), 22.3% (range: 4.7–100%), and 40.5% (range: 12.3–100%), respectively. The AUC values calculated in the PR curve analysis for the RF (0.497; 95% confidence interval [CI]: 0.427–0.578) and YF (0.504; 95% CI: 0.436–0.579) were higher than that of the NF (0.443; 95% CI: 0.417–0.563; Figure 2). The F1 score representing the mean of the precision and recall values was 0.563 for both the RF and the YF, which was higher than that of the NF. Conclusion The clinical T stage, PSA, GS, and ISUP grade are independent predictors of PSMA-positive lymph node metastasis. The RF and YF can be used to identify patients who can benefit from 68 Ga-PSMA-PET/CT for the detection of lymph node metastasis. Together with nomograms, 68 Ga-PSMA-PET/CT images help to localize PSMA-positive lymph node metastases and can thus assist in RT planning.

Tuesday 31 August 2021

Teaching lecture: The 4 Rs of radiobiology revisited in hypofractionated radiotherapy

SP-0663 The 4 Rs of radiobiology revisited in hypofractionated radiotherapy M. Joiner 1 1 Wayne State University School of Medicine, Department of Oncology, Detroit, USA Abstract Text

Better Physics, just within this 21 st century, now enables us to deliver radiation to a target volume with accuracy better than 1 mm. Given this accuracy, why fractionate at all? If we can put dose only on the cancer, and extremely little on critical normal tissue, then surely just give a high single-dose to that cancer, and job done. Local tumor control is 100% with minimal toxicity. If only. Two linked issues keep Biology (radiobiology) in the clinical game. First, our ability to identify, localize, and immobilize anatomy and pathology does not yet correspond with this sub-millimeter accuracy of radiotherapy delivery. Second, even if that imaging resolution is reached it could still not detect occult disease. Consequently, unless the cancer is truly isolated, which it sometimes may be e.g. in organ-confined early-stage prostate cancer, it is always necessary to “degrade” the treatment plan by defining a CTV and PTV into which the radiation delivery is expanded. This inevitably imposes a risk of normal-tissue radiotoxicity, therefore we must use fractionation to minimize that risk. Traditionally, fractionation has been carried out with doses close to 2 Gy per fraction. In fractionation, the Linear-Quadratic (LQ) model describes the relationship between total dose and dose per fraction, for isoeffect. A lower α/β value indicates a steeper relationship. Generally late-reacting normal tissues exhibit lower α/β and early-reacting normal tissues exhibit higher α/β . A hypothesis for these different α/β values for early- and late-reacting tissues, is that a naturally low α/β for a cell population is smoothed out to a higher value as the sum of the responses of different proliferative subpopulations. Malignancies can have lower or higher α/β value depending on tumor type. In some malignancies, notably human prostate and breast, clinical data indeed indicate a low α/β , which might also reflect more uniformity in response perhaps more characteristic of lower proliferative or early-stage disease. Thus in breast and prostate cancers hypofractionation, arbitrarily defined as a dose per fraction >2.2 Gy, has become standard of care. In early- stage non-small cell lung cancers a higher α/β is seen, similar to early-reacting normal tissue, but these isolated malignancies can still be more effectively controlled with radical hypofractionation which indicates the picture is more complex than a simple LQ-brush alone can paint.