33. Pterygium - The GEC-ESTRO Handbook of Brachytherapy

Pterygium

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 01/03/2023

7. CLINICAL TARGET VOLUME

The treatment decision should represent the consensus of a multidisciplinary specialist board. Regarding the Clinical Target Volume (CTV) it is recommended that the radiation oncologist evaluates the pterygiumbefore surgery to adequately choose the applicator size.

The CTV should include the entire surface of the resected lesion.

8. TECHNIQUE

Figure 1: Schematic drawing of the right eye.

The standard technique is using an epibulbar shaped strontium (strontium-90) applicator which is adapted with the treated volume as closely as possible. The applicator has a central radioactive disc and a non-radioactive rimof 2mm (figure 3). Standard treatment is under a local anaesthetic using eye drops containing procaine.The time of application is very important, treatment should be given as soon as possible after surgery (see section 10; Dose, dose rate and fractionation). The eyelids must be opened by a lid retractor. For better accessibility to the lesion, this spreader should be inserted carefully around the exterior region of the pterygium. The surface of the strontium-90 is brought into contact with the pterygium conjunctiva for a precise time which is calculated to deliver the prescribed dose (figure 4). Because of the limited variety in strontium-90 applicator sizes, amanual “radiation field-overlapping” technique was frequently practiced – resulting in higher toxicity rates because of the overdosed overlapping area. The radiation delivery time can be calculated based on thickness of the CTV, half-life of the source and date which we are using the applicator. Surface dose rate is normally about 1 Gy/s at the time of applicator manufacture. The surface dose rate in the central area of 4 mm diameter is reported in the calibration report [16]. Recently two other kinds of plaques have been proposed for pterygium, ruthenium-106 eye plaques, which are used for ocular melanoma and also other ophthalmic malignancies and phosphorus-32 in the same applicator shape as strontium-90. An advantage of phosphorus-32 applicators is that they can be used intraoperatively. However, long term results with this method are not yet published [17,18].

4. PATHOLOGY

The main histologic findings in a pterygium in layers from the surface include invading pterygium epithelial cells with proliferative features, squamous metaplasia, hyperplasia of goblet cells, underlying disrupted Bowman’s layer, stromal fibroblast and vessels, altered extracellular matrix (ECM) with accumulation of collagen and elastin fibers, and inflammatory infiltration [8].

5. WORK UP

A complete eye exam should be performed on all patients with pterygium focusing on assessment of visual acuity and changes in manifest refraction; corneal topography can help to determine the preoperative astigmatism and visual impact of pterygium. A series of typical clinical presentation is provided in figure 2.

6. INDICATIONS, CONTRA-INDICATIONS

Simple excision may result in a high recurrence rate, therefore following surgery adjuvant therapy should be considered [9]. Conjunctival-limbal autografting, as well as topical drugs, have proven to be effective choices with a reported recurrence rate of 5-10%. Adjuvant irradiation, delivered immediately after surgery, can effectively prevent pterygium recurrence and is an alternative to other adjuvant strategies such as cyclosporine, mitomycin C, 5-fluorouracil or bevacizumab [10,11,12,13]. Some authors advise radiotherapy alone for small pterygia and such a strategy has been successful in pterygia < 2mm horizontal length [14].

9. TREATMENT PLANNING

The dose prescription is always to the applicator surface which is assumed to be identical to the sclera/cornea surface. Dose per fraction and total dose at the applicator surface is decided, written and dated on the patient chart by the radiation oncologist on the case, then treatment time is generally calculated manually using a calculator based on the calibrated surface dose rate on the day of application (incorporating decay) by a physicist. Double checking data by another member of the physics team is strongly recommended. The target area should encompass the surgical