Background. Health promotion interventions are needed during adolescent cancer treatment to facilitate the acquisition of good health practices as patients transition to survivorship. Although meta-analytical evidence supports the health benefits of exercise in the context of childhood cancer, there is scant data focusing solely on adolescents.

Hypothesis and objectives. The investigators hypothesise that an inhospital exercise intervention combined with lifestyle counselling during treatment for adolescent cancer will provide several health benefits, particularly related to the cardiometabolic profile. Thus, the investigators will study the effects of an inhospital exercise intervention combined with lifestyle--including diet--counselling along the duration of treatment (neoadjuvant \[solid tumours\]/intense chemotherapy \[leukaemias\], expected median duration 5-6 months) on several health-related variables.

Setting and Methods. Participants will be recruited from 3 hospitals in Madrid (Spain). Inclusion criteria: male/female aged 12-19 years, newly diagnosed with a malignant extracranial tumour; not having received any type of therapy--except surgery--at the time of diagnosis; adequate health status (Karnofsky/Eastern Cooperative Oncology Group scale score ≥50/2); to understand Spanish language and to provide written informed consent. The investigators will recruit ≥136 participants and conduct a randomised controlled trial (1:1 ratio randomisation with a block on gender and tumour type \[leukaemias/lymphomas\]). In addition to usual care, the control group will be informed of the benefits of a healthy lifestyle. The intervention group will follow a physical exercise and lifestyle counselling program. The exercise intervention will be performed in the hospital gymnasium (3 sessions/week of aerobic and resistance exercises), except for neutropenic phases--during which time sessions will be performed in the patients' ward--and will also include inspiratory muscle training (5 days/week). Health counselling will include a psychological intervention (1 session/week) based on motivational interviewing techniques, guidance by a nutritionist (2 sessions/month), and support sessions (1/month) with survivors (≥5-year survivorship) who will share their experiences with the study participants. The following outcomes will be assessed at baseline (diagnosis), end of treatment, and at 3-month follow-up in all participants: echocardiographydetermined left ventricular function (primary outcome); and blood pressure, blood lipids, dual-energy X-ray absorptiometry-determined body composition (fat \[including visceral adipose tissue\]/lean mass, bone mineral content/density), accelerometry-determined physical activity, dietary recall determined energy intake, cardiorespiratory fitness, muscle strength, functional mobility, health-related quality of life, cancer-related fatigue, stress coping, anxiety, depression, clinical variables (survival, treatment tolerability, hospitalisation length, infections), and potential biological underpinnings of exercise multisystemic benefits (metabolic \[glucose homeostasis indicators, high-sensitivity C-reactive protein\] and inflammatory \[cytokine panel\] markers, plasma proteome, gut microbiome, and immune function \[lymphocyte subpopulations, natural killer cell cytotoxicity\]) (secondary outcomes).