10.3 Radiation Therapy, I-131-MIBG, and Proton Beam

Radiation is local control with a beam or a targeted isotope. CPHON TCO III.B.6 tests whether you can explain photons versus protons, get a toddler through daily anesthesia and immobilization, name acute versus late effects, nurse craniospinal fields and total body irradiation (TBI), and run an iodine-131 metaiodobenzylguanidine (I-131-MIBG) isolation room without inventing an official isolation-day number.

Photons versus protons: Bragg peak and developing tissue

Photon (x-ray) beams deposit dose on the way in and on the way out. Exit dose is the pediatric problem: cochlea, heart, lungs, thyroid, and contralateral breast sit in the path of yesterday's technology.

Proton beams deposit most of their energy at a finite depth—the Bragg peak—then stop. That is why protons are used to spare developing tissue: cochlear sparing in central-nervous-system and head-and-neck fields, cardiac and lung sparing in mediastinal or left-chest fields, and contralateral-breast and gonad sparing when anatomy allows. Protons are a delivery method, not a different diagnosis. They reduce—they do not erase—growth delay, endocrinopathy, neurocognitive change, or secondary malignancy. A family who hears "protons mean no late effects" has been mistaught. Not every child who would benefit can travel to a proton center; COG protocols specify when protons are preferred versus allowed. Do not teach that photons are malpractice.

A 6-year-old receiving craniospinal radiation for medulloblastoma is the classic Bragg-peak conversation: spare cochlea, heart, and thyroid as much as geometry allows, still plan hearing tests, growth-hormone surveillance, and school supports.

Daily treatment: immobilization, marks, anesthesia, NPO

Radiation is fractionated: small daily doses over days to weeks. Children must lie still. Immobilization uses thermoplastic masks, vac-locks, and custom cushions. Tattoos or skin marks are millimeters that aim the beam; do not scrub them off as dirt, and teach families not to apply unapproved lotions that smear the field. Some centers use temporary marks rather than permanent ink; either way, the marks stay until radiation oncology says otherwise.

A 3-year-old cannot hold a mask for 20 minutes: daily anesthesia (often propofol-based) is common. That means nothing by mouth (NPO) each treatment morning per anesthesia rules, an intravenous or port access plan, recovery-room time, and a family schedule that is a second job. Do not invent a single national NPO hour as ONCC fact; follow anesthesia. Missed fractions because "the child ate cereal" delay local control. Hydrate and plan calories around the NPO window; do not let a toddler go from radiation recovery onto an empty-stomach chemotherapy day without a nutrition plan.

Skin in the field is irradiated skin: wash gently, no scented alcohol wipes, no heating pads, and use only the radiation-oncology–approved moisturizer. A 5-year-old in a CSI mask who must be NPO at dawn is living this section, not a worksheet about linear accelerators.

Acute effects

Somnolence syndrome after cranial radiation is a child who, days to a couple of weeks after the course, sleeps excessively, is irritable, and looks "not themselves" without a new mass on emergency imaging. It is usually self-limited. Teach it so families do not hear only "the tumor is back." Still notify—because infection, endocrine crisis, and true progression also cause sleepiness. A 7-year-old who sleeps through meals two weeks after cranial radiation needs that differential, not a shrug and not an automatic declaration of relapse.

Late effects: the reason pediatric radiation is rationed

Developing bone in the field grows less—asymmetric spine after flank radiation, short sitting height after CSI, hypoplasia of the treated face. Endocrine failure follows hypothalamic-pituitary or thyroid dose: growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, gonadotropins. Neurocognitive change (processing speed, attention) after brain dose needs 504 or individualized education program planning. Secondary malignancy (breast, thyroid, sarcoma, brain) is a decades-long risk; teach surveillance, not a made-up percentage. Infertility follows gonadal or hypothalamic dose and TBI; fertility-preservation conversations belong before the first fraction when feasible. Photons versus protons change the probability, not the counseling duty.

Craniospinal radiation and TBI

Craniospinal irradiation (CSI) treats the whole neuraxis for tumors that seed cerebrospinal fluid (classic: medulloblastoma in children old enough to receive it). The brain field is matched to a spine field. Nursing specifics: nausea is often worse than for a small local field—give antiemetics before the fraction; bone marrow in the spine field drops counts, so coordinate complete blood counts with chemotherapy; watch skin at match lines and the gluteal fold; expect mucositis and esophagitis from junctions or exit dose. Very young children: delay or avoid CSI when protocols allow, because myelination is incomplete. Do not teach that every infant brain tumor receives CSI on the day of diagnosis.

TBI is a systemic radiation conditioning tool for some HSCT regimens (selected leukemias, some immunodeficiencies). It is not routine for solid tumors. Acute effects are skin, parotitis, diarrhea, mucositis, and pancytopenia on top of conditioning. Late effects are growth, cataracts, endocrine failure, infertility, and secondary cancer. Nurse TBI like a radiation-plus-transplant problem: mouth care, skin, and a fertility footnote that should have been written before day −7. The dedicated HSCT chapter owns graft and veno-occlusive-disease detail; here the point is that TBI is radiation the whole body sees.

I-131-MIBG: radiation safety is the nursing plan

I-131-MIBG is a norepinephrine-analog radiopharmaceutical taken up by MIBG-avid neuroblastoma. It is not first-line therapy for a tiny perinatal adrenal mass (see the neuroblastoma chapter). When it is used for refractory avid disease, the child becomes a radiation source.

Principles—follow radiation-safety orders; do not memorize a single isolation-day number as ONCC fact. Isolation length depends on administered activity, excretion, and the radiation safety officer's measurements.

• Thyroid blockade with potassium iodide starts before the dose so free iodide does not irradiate the thyroid. Missed iodide is a preventable late effect. It belongs on the medication administration record like any other time-critical drug.
• Radiation isolation in a designated room. Time, distance, and shielding protect staff and caregivers. Pregnant visitors are typically excluded. Caregiver exposure limits and film badges or electronic dosimeters are not optional etiquette.
• Contamination control: urine, stool, vomit, sweat, and saliva are radioactive. Gloves, gowns, shoe covers, and designated waste. Spills are radiation events, not housekeeping tickets.
• Catheters and diapers are radioactive waste. Many programs place a urinary catheter so urine is a closed, measurable source instead of soaked diapers. If diapers are used, they go in radioactive waste, not the regular trash or home laundry. Do not rinse radioactive diapers in the family bathroom.
• Myelosuppression can be severe; some children have stem-cell backup. Hydration to flush excreted isotope follows the protocol.
• Teach the family that hugging time is prescribed by physics, not by how much they miss the child. Video and the doorway are love.

A 5-year-old in a CSI mask who must be NPO for anesthesia, a 7-year-old sleepy two weeks after cranial radiation, and a toddler behind an MIBG door with potassium iodide on the MAR and diapers in the radioactive barrel are the three pictures. Photons versus Bragg-peak protons, daily immobilization, acute skin/mucosa/somnolence, late growth/endocrine/cognition/second cancer, CSI and TBI, and MIBG isolation without a fake day count will carry Domain III.B.6.