In the past decades cure rates in paediatric oncology have been increased. Especially in those cohorts with high cure rates and long-term survivors all efforts should be made with regard to the reduction of acute and chronic therapy related morbidity. Most authors report on LINAC-based procedures with a simple technique using ap/pa portals, although other attempts at treating the patient over the whole axis using HT, as we did, have been published [17, 18]. The concept of HT for TBI offers advantages over conventional LINAC-based TBI with higher conformality due to the 360° of beam application versus the standard fixed beam approach (Figure 1). Further it allows for individual sparing of organs at high risk of radiation induced toxicity such as the lungs. Since the main goal of this study was to establish HT in TBI at our institute we used the same fractionation and dose constraints as in our standard LINAC-based TBI protocol only adding a minimum dose for the lungs to prevent underdosing and thus a possible increase in disease relapses. Highly conformal lung sparing could be achieved with mean lung doses of no more than 10 Gy. Overall morbidity during treatment was low corresponding to only mild grade 1–2 side effects which is in line with the results seen by other groups such as Schultheiss et al. , grade 3 – 4 side effects were not observed . No lung toxicity was observed and is still not reported on with up to 15 months of follow-up.
A disadvantage of HT is the limited translation length of the table, allowing irradiable PTV lengths of approximately 145 cm. All patients exceeding 145 cm body length need a solution concerning the irradiation technique for the lower part of the body. The interruption of treatment and shift of the patient from one machine to another implies discomfort for the patient and is a potential source of QA-problems. That is why we decided to use a “single-machine”-solution and devised separate lower body plans for patients with exceeding body length. Other groups kept the legs of patients with exceeding body length in a folded position in a vac-loc bag , others are using ap/pa portals of a Linac for TBI and are applying Helical Tomotherapy Total Marrow Irradiation only as a boost .
We could show that TBI using HT is an interdisciplinary challenge, but feasible as a one-machine-solution even for patients with body length exceeding 145 cm.
Planning constraints derived from conventional LINAC-based TBI could be fulfilled. We opted for a minimal lung dose of 8 Gy to prevent underdosing and thus increasing possible relapse rates . We verified calculated doses by TLD-measurements on defined localizations during treatment on the patients. Measured doses generally corresponded well to the calculated doses. No under-dosing was observed. Dose maxima within the PTV were acceptable. Set-up errors or patient movement during treatment of up to 1 cm were anticipated by using a double PTV concept and creating a second PTV (PTV2) of 10 mm around the patient, thereby creating a safety margin where dose would be deposited in case the patient moved.
A reduction of overall treatment time compared to our standard LINAC-based TBI approach could not be achieved. Necessary patient preparation with image guidance and correction for set-up errors, sedation or anesthesia where necessary added to the complexity of the procedure and called for increased man-power with not only radiation oncologists, physicists and radiographers but sometimes also paediatric oncologists, anesthesists and their personnel overseeing the treatment. The elevated stress level for the patients caused by the strict immobilization, the claustrophobic interior of the bore and the loud machine noise made up some of the draw-backs of the procedure.
Technical difficulties, this mainly being MLC-errors, further prolonged overall treatment time. Excess maintenance could only partly compensate for the problem thus clearly leaving room for improvement in technical reliability.