ESTRO 2020 Abstract Book

S785 ESTRO 2020

Material and Methods Datasets of 52 patients (27 females and 25 males) with median age of 15 years (range 6 -17) were obtained. Thirty and twenty-two patients were treated with 3D-CRT and VMAT, respectively. Low dose (14.4-25.6 Gy) involved field radiotherapy following chemotherapy (COPP/ABV) was delivered to the mediastinum according to AIEOP 2004 HD study protocol. Heart, thyroid, breast, lungs and spinal cord were identified as organs at risk (OARs). Provided that the dose distribution in the target volume was similar between the two treatments, dose-volume histograms (DVHs) were evaluated and compared for OARs. Results Mean dose to the heart was 3.12 Gy (3D-CRT) and 2.72 Gy (VMAT) (p=0.33), respectively; the same feature for thyroid was 10.84 Gy (3D-CRT) and 8.75 Gy (VMAT) (p=0.07), respectively; among the female population, VMAT resulted in a better sparing of breast volumes receiving intermediate or high doses compared to 3D-CRT (3D-CRT Dmax left and right breast: 16.31 Gy and 15.21 Gy versus VMAT Dmax left and right breast: 10.46 Gy and 10.28 Gy, p=0.03). No significant difference between the two techniques was observed in terms of mean dose received to both the breasts and the lungs. Dmax for spinal cord was 18.04 Gy (3D-CRT) and 13.29 Gy (VMAT), respectively (p= 0.0005). Conclusion VMAT resulted in a substantial sparing of thyroid and spinal cord. Mean doses to heart, breasts and lungs did not significantly differ between the two techniques, while VMAT allowed a better sparing of breast volumes receiving intermediate/ high doses potentially involved in the risk of second malignancies.As lymphoma therapy continues to evolve, with an emphasis on treatment reduction, radiation oncologists should use at best the available tools to minimize the dose to organs at risk. An individual approach considering age, gender, and finality of RT has been strongly recommended. PO-1467 Comparison of planning techniques in randomised radiotherapy trials for localised prostate cancer I. Gleeson 1 1 Addenbrooke's Hospital - Oncology Centre, Medical Physics, Cambridge, United Kingdom Purpose or Objective Moderate hypofractionated radiotherapy remains standard of care in many countries as SBRT data continues to mature. Heterogeneity exists across trials for techniques in localised prostate radiotherapy. Most trials report similarly high efficacy albeit with different toxicity. This work compares radiotherapy techniques used in the randomised external beam trials CHHiP, CHHiP IGRT sub- study, PACE B, PIVOTALboost arm A (PIV B) and PROFIT. Material and Methods 5 patients were planned by the same planner using 6 MV VMAT following trial protocols CHHiP, CHHiP IGRT sub study, PACE B, PIVOTAL boost arm A and PROFIT. Target volumes were grown as per their respective trial protocol in Table 1 and one set of OAR were used for all plans which was outlined as per CHHiP. Plans were prescribed 60 Gy in 20 fractions except PACE plans which was 62 Gy in 20 fractions. Target coverage was achieved and doses to the rectum, bladder and penile bulb were compared using one- way ANOVA Bonferroni’s multiple pairwise comparison test.

Minimum isodose coverage (Gy)

Low Risk (LR)

Moderate (MR)/Intermedia te risk (IR)

Dos e (Gy)

Protocol

Prostat e and SV base + 10mm Prostat e and SV base + 6mm Prostat e + 10mm/ 5 post Prostat e + 6mm / 3 post Prostat e + 5mm /0 post Prostat e + 3mm /0 post Prostat e + 10mm/ 7 post Prostat e + 5mm

Prostate + SV +10mm

CHHiP PTV1

48 45.6

CHHiP IGRT PTV1

Prostate + SV +6mm

48 45.6

Prostate

+

CHHiP PTV2

57.6 54.6

10mm/5 post

CHHiP IGRT PTV2

Prostate + 6mm /3 post

57.6 54.6

Prostate + 5 mm /0 post

CHHiP PTV3

60 57

CHHiP IGRT PTV3

Prostate + 3 mm /0 post

60 57

Prostate + proximal 1cm SV + 10 mm/7 post + proximal 1cm SV + 5mm + proximal 1cm SV + 3mm Prostate + SV + 6mm Prostate Prostate

CTV V60≥99%PT V V57≥99%

PROFIT PTV60

60

PACE PTV62

B

62 D98≥58.9G y

PIVOTALboo st PTV60 PIVOTALboo st PTV47

N/A

60 D98≥57Gy

N/A

47 D98≥44.65 Gy

Results Comparing the techniques showed more variations for the rectum then bladder. Lower doses (V24.6 – V40.8 Gy) to the rectum were more favourable for PACE and CHHiP IGRT plans. PROFIT and PACE plans tended to have highest rectal V57 – V60 doses compared with others (p ≤ 0.05). Bladder doses did not differ significantly across techniques (max dose volume change of 6.1%). Statistical significant bladder differences were seen only for PROFIT plans having higher bladder V60 than others and PACE IR plans higher than CHHiP IGRT (p ≤ 0.05). Penile bulb doses (Dmean, D2 %) were significantly higher (about twice as high) in CHHiP and PROFIT plans compared to the other techniques with their smaller margins.