ESTRO 2020 Abstract Book

S412 ESTRO 2020

Abstract text Sexual dysfunction is often unrecognized, underestimated and untreated. Sexual dysfunction is one of the more common consequences of cancer treatment. Sexual dysfunction in cancer patients may result from biological, psychological and social factors. Biological factors such as anatomic alterations (rectum or penile amputation), physiological changes (hormonal status) may preclude normal sexual functioning even when sex desire is intact. Negative emotional states such as anxiety, depression, anger may also disrupt sexual activity. Prostate cancer is the most frequent non-skin male malignancy in Western countries. Radiation therapy is together with radical prostatectomy the most effective treatment for localized disease. Both external-beam radiotherapy (EBRT) and brachytherapy can be offered as curative options. Published rates of erectile dysfunction (ED) following EBRT vary from 7–80%, and after brachytherapy this percentage may be as high as 60%. The etiology of ED after radiation of prostate cancer is multi- factorial. Vascular, neurogenic and psychogenic factors are often reported, though a vascular mechanism seems more likely to be involved. Sildenafil and tadalafil are effective to treat ED in about half of the patients after radiotherapy. Testicular cancer affects more often young men in their fertile and sexually active life. In our survey, about 20% of the patients reported less sexual interest, pleasure and activity since treatment, and this was not significantly correlated with age. Though, these percentages were not different from an age-matched control group of men without testicular cancer. Fifteen per cent of the patients had ED. Sixty-two per cent found their body had changed after treatment. In general, reported sexual function soon after treatment indicates high levels of sexual dysfunction that tend to improve over time. More than half of testicular cancer patients reported that their body image had changed after treatment. Carcinoma of the penis is a rare malignancy. Most patients can still enjoy a sexual life if laser treatment is used, but more invasive procedures reduce this likelihood. The stability of sexual function in husbands and wives of cancer patients suggests that the sexual problems in these patients are caused by the emotional and medical impact of the illness as by the stress in the couple’s relationship. Patients need to be adequately counselled on the effects of cancer treatment on their sexual life and relationship, about the different treatment possibilities and reassured of being able to enjoy a normal sexual life. Unfortunately, sexual counselling has not become a routine part of oncology care in most hospitals, but this should be routinely provided. SP-0727 Dosimetry and QA for MR-Linacs B. Van Asselen 1 , S.L. Hackett 1 , S.J. Woodings 1 , B.W. Raaymakers 1 , J.W.H. Wolthaus 1 1 university Medical Center Utrecht, Department Of Radiation Oncology, Utrecht, The Netherlands Abstract text The excellent visualization of soft-tissue with MRI allows direct visualization of the tumor when applied during the delivery of radiotherapy. Several MRI guided systems have been developed, which combine MRI with an accelerator: the MRidian (Viewray Technolgies Inc, USA) and the Unity (Elekta AB, Sweden). The latter system was Teaching Lecture: Dosimetry and QA for MR-Linacs

developed together with the UMC Utrecht. In high- precision MRI-guided radiotherapy, the treatment dose is delivered in a magnetic field. In both designs the dose is delivered in a transverse magnetic field of respectively 0.35 T and 1.5 T for the MRIdian and Unity systems. In a magnetic field, dose deposition is affected by the high magnetic field and therefore dosimetry and QA guidelines for conventional radiotherapy do not apply. When the dose is delivered in a magnetic field, the Lorenz force will change the trajectories of the high energy electrons generated by the megavoltage radiation. The effect on dose distribution the depends on the magnetic field strength, its direction relative to the treatment beam and the beam energy. This can result in a change in the build-up region and shifted penumbra. Changes can also be observed in the dose distribution near interfaces of two materials with different densities, particularly near tissue- air boundaries as electrons can be curved back into the tissue (electron-return-effect). Electrons exiting the tissue can be captured by the magnetic field and will spiral outside the beam along the magnetic field lines (the electron streaming effect). The influence of the magnetic field also affects the reading of various detectors used for reference dosimetry, acceptance and commissioning, regular quality assurance (QA) and patient QA. The change in reading of a detector depends on the field strength, orientation relative to the photon beam and the magnetic field and the presence of layers of air between build-up material and detector. For reference dosimetry in a magnetic field, two issues need to be considered: the change in local dose due to the change in electron trajectories and the influence of the magnetic field on the detector reading. Ideally the change in detector reading due to the magnetic field would be proportional to the change in local dose. This is however not always the case, therefore the currently used formalisms have to be adapted to correct for the influence of the magnetic field. To avoid any possible effects of air gaps around detectors, measurements should preferably performed in water. The performance of waterproof farmer type ionization chambers in magnetic fields has been investigated thoroughly. Correction factors have been derived for the magnetic field in various geometries and orientations to obtain absolute dose measurements. Other detectors such as a diamond detector have also been investigated for use in magnetic fields. To evaluate dose distributions of clinical plan delivery, patient specific quality assurance can be performed using various dedicated 3D detectors arrays (Delta4, ArcCHECK, Octavius, Martix). The performance of a dedicated MRI compatible versions of such systems have been evaluated. The devices generally perform equally well in a 1.5 T magnetic field compared to the conventional linac use but require a recalibration of the detectors. Besides these detectors film dosimetry has been used for plan QA in MRI guided radiotherapy for it high spatial resolution. The effects of the magnetic field on film dosimetry have also been investigated. Since the MR-Linac aims for online adaptive radiotherapy, online plan QA needs to be developed additional to the offline QA procedures and devices. At the MRI-linac, an IMRT plan will be created online based on the daily anatomy while the patient is on the treatment couch. Therefore, individual plan QA via measurements can’t be performed. New methods have been developed for online plan QA such as fast independent dose calculation algorithms and plan parameter checks. After twenty years of research the magnetic field effects