The argon 532 nm (green) laser is particularly useful for very small retinoblastoma tumors (1.5 mm or less) or for focal consolidation after at least one cycle of systemic chemotherapy. As with other uses of retinal photocoagulation, focal consolidation should not be attempted if the retina containing the lesion is detached. We have found that the argon laser midrange visible (532 nm) wavelength is more readily absorbed in the nonpigmented retinoblastoma tissue than the longer wavelength infrared 810 nm diode laser, which becomes a factor in thick tumors or those occurring over calcified scars. The margins of the treated zone when using the argon laser are also easier to control than with the diode laser. Its main disadvantage when compared to the diode laser is the small spot size. Care must be taken to increase the power density judiciously within the small spot to avoid tumor dissemination or hemorrhages. Tumor disruption may occur in a small spot if the power out of the laser exceeds 700–800 mw for more than 0.3–0.4 s.

The 532 nm green laser is available as a tabletop solid state laser with an indirect laser delivery system that works best for transpupillary laser applications under general anesthesia. The desirable end point for the ophthalmologist is a gentle white spot generated at the boundary between normal retina and tumor edge (Fig. 10.1). The power is initially set between 250 and 350 mw for 0.3–0.5 s. Laser burns are initially placed at the edge of the lesion, half-on and half-off the tumor. The power and/or burn duration is slowly increased until a clear reaction is achieved. Punctate hemorrhage within the treatment spot is a sign that the power density is near the maximum tolerated levels.

Once the appropriate power level is set, the edge of the tumor is treated with overlapping burns to establish the perimeter of the lesion. Subsequently the entire lesion should be treated with burns having the same overlap. In the central thicker portions of the tumor, a visible whitening reaction following treatment may not be present. However, neither the power nor the burn duration should be increased once the parameters have been established.

Typically the treatment is repeated every 3–4 weeks, immediately before the next cycle of chemotherapy. A 2–4 week interval can be adopted if the course of intravenous chemotherapy has been completed. Edge tumor recurrence may appear if the laser consolidation process was insufficient, typically within the first 6 months after the last laser session (rarely up to 2 years) (Fig. 10.2).

When photocoagulation is used on retinoblastoma lesions and the patient subsequently receives planned systemic chemotherapy (e.g., carboplatin) within 24 h, two tumor-destroying mechanisms may come into play. The first and the most important is direct tumor cell destruction generated by temperatures in excess of 65–70 °C within the treatment spot. The second mechanism occurs in the “donut” or ring of tissue extending for several millimeters outside the laser spot. Heat radiating out from the central spot increases the temperature to the thermotherapy range between 45 and 60 °C. In this region, there is a synergism with the carboplatin, assuming an adequate level of carboplatin is achieved in the tumor. To take advantage of the latter mechanism, we typically perform laser treatment within 24 h of the next cycle of intravenous chemotherapy.

In Los Angeles, our treatment protocol requires that each lesion be treated completely with laser on at least three occasions, 2–4 weeks apart. In our experience, even a flat chorioretinal scar achieved after one laser session cannot be considered sterilized. Larger lesions should be lasered at sequential examinations (minimum of three sessions) until the regressed tumor is either flat (type IV scar) or completely calcified (type I). Fleshy type II regression should be lasered until you achieve a type I/IV scar or until you begin to notice retinal contraction. Small areas of type II regression may be left untreated immediately adjacent to the optic nerve or fovea, although the risk for tumor recurrence is always higher with type II vs. type I (or type IV) scars.

The 810 nm diode laser is most effective when there is intact RPE beneath the tumor. Through a 28D lens, the indirect ophthalmoscope delivery system offers a spot size of 0.35 mm (small spot) and 1.4 mm (large spot). We typically use the large spot indirect system, which provides improved safety and convenience for treating larger tumors versus the argon laser. Safety comes from the reduced likelihood of concentrated power intensity in a small spot creating tumor dissemination. The larger spot size as compared with argon saves treatment time, thus conferring convenience. It is also our impression that the depth of treatment with the diode laser is greater than the argon laser. However, it is more difficult to control the size of the burn with the diode laser, and care is warranted near the optic nerve and fovea.

The delivery technique is similar to that with argon green laser photocoagulation. The entire tumor is treated with overlap of the spots similar to that described above for the argon 532 nm laser. The initial power settings with the diode laser, especially when the large spot is used, are somewhat higher than for the argon laser. We generally select an initial setting of 300 mw for peripheral and macular lesions undergoing primary therapy and 400–500 mw for large tumors undergoing chemoreduction. If there is little color change induced at the initial power settings for these larger tumors, it is possible to increase the power up to 800 mw, provided that the surgeon is carefully monitoring for complications. The power settings will vary for each patient and for each tumor because of the degree of pigmentation underlying the lesion(s). As with the argon laser, both the power and burn duration can be increased incrementally until the appropriate end point is reached. We typically leave the duration of the treatment on the longest setting (9,000 ms) and set the interval to 50 ms; with these parameters the laser is essentially being used in continuous mode and the surgeon can control the duration of each spot treatment with the foot pedal. Using the diode laser in this manner allows the surgeon to use the diode laser for photocoagulation (1–10 s) or possibly thermotherapy (30–50 s).