Electron therapy Class 3: Clinical procedures

Transcription

1 Electron therapy Class 3: Clinical procedures Laurence Court Reference: Faiz M. Khan, The Physics of Radiation Therapy Slide acknowledgements: Karl Prado, Rebecca Howell, Kent Gifford, and Khan s book

2 Clinical procedures Total Skin Electron Therapy Total limb irradiation Electron Arc Therapy Total Scalp Bolus electron conformal therapy (custom bolus)

3 Total Skin Irradiation Used for treating superficial lesions that cover large areas of the body. Cutaneous T-cell lymphoma (mycosis fungoides) Low energy electron beams have rapid fall-off in dose beyond a shallow depth. Superficial lesions extending to about 1cm can be treated without exceeding bone marrow tolerance. Two categories of total skin irradiation. 1) Translational Technique 1) Translational Technique Patient lies horizontally (historical). 2) Large Field Technique Patient treated standing up.

4 Cutaneous T cell lymphoma Cutaneous T cell lymphoma (CTCL) is a slow growing form of cancer in which some of the body's white blood cells T-T lymphocytes (T-Cells). become malignant. These abnormal cells are drawn to the skin and some are deposited there. Cutaneous T cell lymphoma usually develops slowly over many years. In the early stages skin may develop dry, dark patches on the skin, sometimes itchy, sometimes not. Often, it is misdiagnosed as psoriasis or eczema at first and only recognized after several biopsies.

5 Cutaneous T Cell Lymphoma The most common type of CTCL is still sometimes known by its old name "mycosis fungoides". This name referred to mushroom fungus look of the skin only seen in advanced disease. If the disease progresses unchecked, raised growths may form on the skin after a period of years. If they become tumors, the risk increases that tumors will form in the lymph glands or other organs in the body.

6 Total Skin Irradiation Special Requirements Large treatment room Large SSDs (technique dependent) High Dose Rate Shorten treatment time Depending on the technique, total skin irradiation can be very lengthy. Good Ventilation, frequent air exchange. Significant ozone production from ionizing large volumes of air in the treatment room.

7 Total Skin Irradiation Large Field Technique Patient Treated standing up. Combination of broad beams produced by electron scattering at large SSDs (2 to 6m). Many different methods: Single beam Pair of Parallel beams Pairs of angled beams Pendulum arc Patient rotation From this slide on, we will only be referring to the

8 Total Skin Irradiation X-Ray Contamination = Limiting Factor Source Bremsstrahlung interactions in the exit window, scattering foil, ion chambers, collimators, air, and patient. Proportional to the number of treatment fields used since all fields contribute penetrating x-rays. x The cumulative dose due to the x-ray x component, measured at 10cm depth and averaged over patient volume for all fields typically ranges from 1-4% 1 of max electron dose. 4% generally unacceptable! Desirable levels <1% of total mean electron dose at d max. 36Gy would ideally have 0.36Gy x-ray x contamination dose.

9 Total Skin Irradiation The Stanford Technique Six Treatment Orientations AP/PA and 4 oblique (these orientations are achieved with different patient stances, shown on the next slide). Sequential Two day treatment cycle: : 3 orientations treated each day

10 Total Skin Irradiation The Stanford Technique The different orientations are achieved with different patient stances. This illustration shows the patient position stances for the anterior, posterior, and two of the angled treatment orientations. Each orientation is treated with a two component beam to cover entire length of the body.

11 Total Skin Irradiation The Scatter Plate Clear Lucite scatterer/energy degrader plate. Thickness 1cm Cross section 2m x 1m Location 20cm from patient Contributes to large angle scatter of the emergent electrons. IMPROVES dose uniformity especially on oblique body surfaces. But, also REDUCES penetration and dd falls off at shallower depth.

12 X-ray contamination Multiple fields leads to increase in x-ray dose Angled fields helps minimize this TG #30

13 Total Skin Irradiation Dose Distribution When multiple large beams are directed at the patient from different angles, the dd curve and d max shift toward the surface. Dose uniformity of 20% can be achieved over most of the body surface using 6-field 6 technique (even better with rotational technique). In-Vivo Dosimetry - TLDs used to measure doses in specific locations during course of treatment. Excessive dose regions (120%-130%) 130%) can occur in areas of sharp projections, curved surfaces, regions of multiple field overlap. Low dose regions occur when skin is shielded by other parts of the body or overlying body folds: Axillary folds, perineum, soles of feet Areas receiving less dose can be boosted

14 Total limb irradiation Superficial cancers of the limbs: Melanoma, lymphoma, Kaposi s s sarcoma Electrons to reduce bone dose Thin bolus may be used 90% moves closer to skin compared with single field Surface dose ~90% (higher than single field)

15 Multiple fields: change in PDD

16 Electron Arc Therapy Excellent dose distributions for treating superficial tumors along curved surfaces. Limbs Ribs Chest wall Useful in cases which tumor involves a large chest wall span and extends posteriorly beyond the midaxillary line. Conventional tangents would irradiate too much of underlying lung. Difficult technique, not done in many centers.

17 Electron arc therapy Two approaches: True continuous arc Pseudoarc Skin dose is lower than would be expected for fixed electron beams Commissioning: Skin collimation Angle effects

18 Arc: change in PDD

19 Comparison of arced narrow beam and arced broad beam dose distribution Narrow Broad Surface dose* decreases increases Therapeutic depth* increases decreases * Relative to non-arced beam

20 Total scalp Cancers involving skin of scalp to depth ~5mm No invasion of skull or brain Challenge: reasonably uniform dose to skin while sparing bone and brain Options: Electron-only only method 6 abutted low-energy e-e fields Junctions shifted 2cm halfway though course Photon/electron combination Angiosarcoma of the scalp. Fakih et al, JCO 19,

21 Mixed photon/electron total scalp treatments Parallel-opposed 6MV photon beams Matched pair of low-energy electron beams 3mm overlap to improve dose uniformity 1cm shift to feather junction Bolus to increase skin dose and protect brain Superior to electron-only only approach in dose uniformity and simplicity

22 Bolus electron conformal therapy Single electron beam with variable thickness bolus Shape distal 90% dose surface to conform to PTV Deliver minimal dose to underlying tissues Process: CT scan Delineate PTV Specification of e-e beam (field size, direction, energy) Bolus design Dose calculation Fabricate bolus (typically off-site) QA (CT with bolus, dose calc, comparison with planned dose) Treatment delivery Commercialized by.decimal

23 Perkins et al, Int. J. Radiation Oncology Biol. Phys., Vol. 51, No. 4, pp , 2001 Postmastectomy irradiation View of the postmastectomy chest wall demonstrating variation in chest wall anatomy. Note the inferior medial concavity (at the base of the postmastectomy scar) and the superior lateral convexity (at the apex of the postmastectomy scar). Outlined on the patient chest wall are the treatment field for chest wall electrons and the corresponding supraclavicular/axillary apex field. Field junctions were moved 0.5 cm twice during treatment to avoid excessive dose.

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25 Isodose distribution with 3D electron bolus

26 Parotid gland Postoperative RT Large surface gradients PTV thickness 1-5cm1 Conventional: 2 patched fields Kudchadker et al, jacmp 4 (2003)

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28 Treatment Combine 20MeV electron and 6MV x-ray x (4:1) Photons reduce risk of erythema and moist desquamation Skin recovered in 2 weeks. No disease at 6 months.

29 More custom bolus examples Posterior wall sarcoma (Low et al1995) Parotid (Kudchadker 2003)

30 References The Physics of Radiation Therapy 2 nd edition, Faiz Khan AAPM TG 70 (supplement to 25) AAPM TG 25 (electron therapy) AAPM TG 51 (output) AAPM TG 30 (TSI) Radiation Therapy Physics, 2 nd edition, William Hende and Geoffrey Ibbott

31 Raphex Question: T68, 2000 In Total Skin Electron Beam Therapy (TSET), multiple oblique beams do all of the following except: A. Improve uniformity of the skin dose. B. Decrease the effective depth of d max. C. Increase the uniformity of the target volume. D. Decrease the x-ray x dose.

32 Raphex Question: T52, 2001 Which of the following may be used in total skin electron treatment? Low energy electrons. 2. Multiple patient positions. 3. Beam scatterer or diffuser. 4. Boost fields. 5. Eye shields. A. 1,2,3 B. 1,3 C. 2,4 D. 1,3,5 E. 1,2,3,4,5

33 Raphex Question: T64, 2002 In total skin electron beam therapy, when a large Lucite screen, typically 1 cm thick, is placed in front of the patient, this is done in order to: A. Protect the patient from scattered radiation. B. Attenuate the Bremsstrahlung component of the beam. C. Increase dose uniformity. D. Increase Depth Dose. E. Reduce Skin Dose.

34 Raphex Question: T71, 2003 In total skin Electron beam therapy, multiple oblique beams do all of the following except: A. Improve uniformity of skin dose B. Decrease effective depth of d max. C. Increase dose uniformity in target volume D. Decrease whole body x-ray x dose.