ESTRO 2021 Abstract Book

S450

ESTRO 2021

dissemination at presentation. The use of front-line immune checkpoint inhibitors with or without chemotherapy in NSCLC patients depends on the expression of PD-L1 in the tumor and on patients’ comorbidities. For tumors with high PD-L1 expression, systemic therapy with PD1 or PD-L1 inhibitors results in long-term benefit in approximately 30% of patients. Current research efforts are aimed to further improve response rates and survival by multiple checkpoint blockade, such as CTLA4 or TIGIT. A number of bispecific antibodies with unique farmacological properties are in early clinical development with great potential in NSCLC. Front-line or second-line chemotherapy with or without anti-angiogenic agents remains a viable option for NSCLC patients with particular clinical or molecular characteristics. Radiation oncologists should be aware of the advances in systemic therapies of NSCLC to make the best choices regarding integration of radiotherapy in the multimodality treatment strategy. Examples of frequent clinical dillemas include front-line radiotherapy vs. targeted therapy of brain metastases or integration of radiotherapy and immunotherapy in oligometastatic NSCLC.

SP-0569 Shared-decision making in the treatment of lung cancer TBC

Debate: Rectal spacers for prostate radiotherapy - The farther the better?

SP-0570 For the motion B. Vanneste 1 1 MAASTRO, Radiation Oncology, Maastricht, The Netherlands

Abstract Text Prostate Cancer is the most common cancer among males in the Western world, with a lifetime risk of 1 in 8. High-dose radiotherapy is one of the treatment options. The outcomes are high, however the quality of life may decrease mainly due changes in bowel function. Radiation proctitis is still a problem: grade 2 or more gastrointestinal toxicity is described in 10 to 20 % of treated patients, especially in the area of dose-escalation which revealed a higher biochemical disease free survival (1, 2). Since the ano-rectal complex is closely located to the prostatic gland the PTV-margin partially overlaps with the anterior ano-rectal wall, such that the latter is included in the high-dose volume. Several investigators have demonstrated that minimizing the radiation dose to this volume of the ano-rectal structure reduces the risk for late rectal bleeding, and other rectal complaints. More precisely, when 20% and 15% of the ano-rectal volume receives a dose of at least 70 and 75 Gy (in 2 Gy fractions), the risk of developing Grade 2 and Grade 3 late rectal bleeding is <15% and <10% (3). Despite the ability of modern dose delivery techniques to administer highly conformal dose distributions with very steep dose gradients at the rim of the PTV, this still remains a significant risk. The inherent problem is that the high-dose volume of the PTV partially overlaps with the ano- rectum, no matter how steep the dose gradient is. Therefore, the only option to prevent rectal volumes from being exposed to high radiation doses is to artificially increase the distance between the prostate and the ano- rectal complex. Several devices have been developed to achieve a better sparing of the rectal structures: hydrogels, hyaluronic acids, human collagen, and saline filled balloons. Implantable rectum spacers are cost-effective, which is reported in 3 published analysis. Two papers revealed the benefits of a spacer to decrease rectal toxicities due to lower medical interventions needed of acute and late side effects in patients implanted with spacers (4,5). Another analysis demonstrated the increased cost effectiveness of SBRT in comparison with conventional EBRT (6). However, several parameters are important to fulfill a complete cost-effectiveness analysis, and add these models with clinical input could further optimize these results. Although if you can select patients upfront on high risk on rectal toxicity, the cost effectiveness will further increase (7). Several parameters are described in the literature that are prone for toxicity exacerbation, like IBD (8). Until now, one randomized trial is published and revealed no complications concerning implantation procedure, with a significant reduction in mean rectal V70 (3% vs 12%), resulting in a significant reduction in late rectal toxicity: 2% vs 7% in the spacer and control groups, respectively (8). Concerning a decrease of bowel quality of life of 10 point decline at 15 months, 11% and 21% are reported in the spacer and control groups, respectively. Furthermore one meta-analysis is performed consisting this randomized trial with 6 cohort studies comparing men who received a hydrogel spacer vs men who did not receive a spacer (controls) prior to prostate radiotherapy (9). The review consisted of 1011 men (486 spacers and 525 controls), with a median duration follow-up of 26 months. The mean perirectal separation distance was 11.2 mm. A significant reduction in mean rectal V70 was observed: 3% vs 10%, resulting in a significant reduction in late rectal toxicity: 1% vs 6% in the spacer and control groups, respectively. Changes in bowel-related quality of life were greater in the spacer group in late follow-up: a mean difference of 5.4 points are observed. Currently, more hypofractionation (SBRT) and dose escalation are performed in prostate cancer radiotherapy, that could increase the role of spacing. In the recently published FLAME trial, a further dose escalation above 80 Gy resulted in higher biochemical relapse free survival of 92% in comparison with 85% without dose escalation. However, cumulative incidence of late GI toxicity grade 2 or more was still 13% in the boost arm. So further increase of dose will be difficult without use of spacer devices. Further research has to be conducted to confirm this. References: (1) Heemsbergen, et al. Radiother and Oncol, 2014;110:104–109. (2) Al-Mamgani, et al. IJROBP, 2008;72:980- 988. (3) Michalski, et al. IJROBP, 2013; 87(5):932–938. (4) Levy, et al. PRO, 2019;9,172-179. (5) Vanneste, et al. Radiother and Oncol, 2015;114:276–281. (6) Hutchinson, et al. Urologic Oncol, 2016;34:19–26. (7) Vanneste, et al. Radiother and Oncol, 2016;121:118–123. (8) Vanneste, et al. CTRO, 2021; 27:121-125. (9) Mariados, et al. IJROBP, 2015;92(5):971-7. (10) Miller, et al. JAMA, 2020;3(6):208221. (11) Kerkmeijer, et al. JCO 2021;39(7):787-796.